Abstract:
A magnetooptical recording method for recording data using a bit having upward-magnetization and a bit having downward-magnetization on a recording layer of a magnetooptical-recording medium comprises the steps of using, as the medium, a multilayered magnetic recording medium consisting of a first layer having a perpendicular magnetic anisotropy acting as a recording layer, and a second layer having a perpendicular magnetic anisotropy acting as a reference layer; moving the medium; applying an initial field so that, before recording, the direction of magnetization of the recording layer is left unchanged and that of the reference layer is aligned either upward or downward; radiating two adjacent laser beams, i.e., a leading beam and a trailing beam onto the medium; the leading beam being an erasing beam of low level which is not modulated in principle; the trailing beam is a writing beam which is pulse modulated between high level and basis level in accordance with binary data to be recorded; and the basis level being able to have an intensity equal to or lower than the low level and to be zero level; when the trailing laser beam is radiated, applying a bias field to the irradiated portion; and when the intensity of the pulse-modulated laser beam is at high level, forming one of the bit having upward-magnetization, and when the leading low-level laser beam is radiated, forming the other bit.

Description:
This is a continuation of application Ser. No. 449,337 filed Dec. 12, 1989, which is a continuation of application Ser. No. 353,389 filed May 17, 1989 (both abandoned), which is a division of application Ser. No. 104,421 filed Oct. 5, 1987 (now U.S. Pat. No. 4,855,975, issued Aug. 8, 1989). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetooptical recording method and a magnetooptical recording apparatus and a medium used therefor and, more particularly, to an over write capable magnetooptical recording method used two beams, and over write capable magnetooptical recording apparatus and an over write capable medium. 
     2. Related Background Art 
     In recent years, many efforts have been made to develop an optical recording/reproduction method, an optical recording apparatus and a medium used therefor, which can satisfy various requirements including high density, large capacity, high speed access, and high recording/reproduction speed. 
     Of various optical recording/reproduction methods, the magnetooptical recording/reproduction method is most attractive due to its unique advantages that information can be erased after use and new information can be written thereon. 
     A recording medium used in a magnetooptical recording/reproduction method has a perpendicular magnetic layer or layers as a recording layer. The magnetic layer comprises, for example, amorphous GdFe, GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like. Concentric or spiral tracks are formed on the recording layer, and data is recorded on the tracks. Note that in this specification, one of the &#34;upward&#34; and &#34;downward&#34; directions of magnetization with respect to a film surface is defined as an &#34;A direction&#34; and the other one is defined as a &#34;non-A direction&#34;. Data to be recorded is binarized in advance, and is recorded by a bit (B 1 ) having &#34;A-directed&#34; magnetization and a bit (B 0 ) having &#34;non-A-directed&#34; magnetization. These bits B 1  and B 0  correspond to &#34;1&#34; and &#34;0&#34; levels of a digital signal, respectively. However, in general, the direction of magnetization of the recording tracks can be aligned in the &#34;non-A direction&#34; by applying a strong bias field before recording. This processing is called &#34;initialization&#34;. Thereafter, the bit (B 1 ) having &#34;A-directed&#34; magnetization is formed on the tracks. Data is recorded in accordance with the presence/absence and/or a bit length of the bit (B 1 ). 
     Principle of Bit Formation 
     In the bit formation, a characteristic feature of a laser, i.e., excellent coherence in space and time, is effectively used to focus a beam into a spot as small as the diffraction limit determined by the wavelength of the laser light. The focused light is radiated onto the track surface to write data by producing bits less than 1 μm in diameter on the recording layer. In the optical recording, a recording density up to 10 8  bit/cm 2  can be theoretically attained, since a laser beam can be concentrated into a spot with a size as small as its wavelength. 
     As shown in FIG. 1, in the magnetooptical recording, a laser beam L is focused onto a recording layer 1 to heat it, while a bias field (Hb) is externally applied to the heated portion in the direction opposite to the initialized direction. A coersivity Hc of the locally heated portion is decreased below the bias field (Hb). As a result, the direction of magnetization of that portion is aligned in the direction of the bias field (Hb). In this way, reversely magnetized bits are formed. 
     Ferromagnetic and ferrimagnetic materials differ in the temperature dependencies of the magnetization and Hc. Ferromagnetic materials have Hc which decreases around the Curie temperature and allow data recording based on this phenomenon. Thus, data recording in ferromagnetic materials is referred to as Tc recording (Curie temperature recording). 
     On the other hand, ferrimagnetic materials have a compensation temperature, below the Curie temperature, at which magnetization (M) becomes zero. The Hc abruptly increases around this temperature and hence abruptly decreases outside this temperature. The decreased Hc is cancelled by a relatively weak bias field (Hb). Namely, recording is enabled. This process is called Tcomp. recording (compensation point recording). 
     In this case, however, there is no need to adhere to the Curie point or temperatures therearound, and the compensation temperature. In other words, if a bias field (Hb) capable of cancelling a decreased Hc is applied to a magnetic material having the decreased Hc at a predetermined temperature higher than a room temperature, recording is enabled. 
     Principle of Reading 
     FIG. 2 illustrates the principle of data reading based on the magnetooptical effect. Light is an electromagnetic wave with an electromagnetic-field vector normally emanating in all directions in a plane perpendicular to the light path. When light is converted to linearly polarized beams (Lp) and radiated onto a recording layer (1), it is reflected by or passes through the recording layer (1). At this time, the plane of polarization rotates according to the direction of magnetization (M). This phenomenon is called the magnetic Kerr effect or magnetic Faraday effect. 
     For example, if the plane of polarization of the reflected light rotates through θ k  degrees for &#34;A-directed&#34; magnetization, it rotates through -θ k  degrees for the &#34;non-A-directed&#34; magnetization. Therefore, when the axis of an optical analyzer (polarizer) is set perpendicular to the plane inclined at -θ k , the light reflected by &#34;non-A-direction&#34; magnetized bit (B 0 ) cannot pass through the analyzer. On the contrary, a product (X sin 2θ k ) 2  of the light reflected by a bit (B 1 ) magnetized along the &#34;A direction&#34; passes through the analyzer and becomes incident on a detector (photoelectric conversion means). As a result, the bit (B 1 ) magnetized along the &#34;A direction&#34; looks brighter than the bit (B 0 ) magnetized along the &#34;non-A direction&#34;, and the detector produces a stronger electrical signal for the bit (B 1 ). The electrical signal from the detector is modulated in accordance with the recorded data, thus reading the data. 
     SUMMARY OF THE INVENTION 
     In order to re-use a recorded medium, (i) the medium must be re-initialized by an initializing device, (ii) an erasing head as well as a recording head must be added to a recording apparatus, or (iii) recorded data must be erased using a recording apparatus or an erasing apparatus as preliminary processing. 
     Therefore, in the conventional magnetooptical recording method, an over-write operation, which allows new data recording regardless of the presence/absence of the recorded data, is impossible. 
     If the direction of a bias field (Hc) can be freely changed between the &#34;A direction&#34; and &#34;non-A direction&#34;, an over-write operation is possible. However, the direction of the bias field (Hb) cannot be changed at high speed. For example, when a bias field (Hb) applying means is a permanent magnet, the direction of the magnet must be mechanically reversed. In this case, it is impossible to rotate the direction of the magnet at high speed. Similarly, when the bias field (Hb) applying means is an electromagnet, the direction of such a large current cannot be reversed at high speed. 
     It is, therefore, a first object of the present invention to provide a magnetooptical recording method capable of over writing by modulating light without changing the direction of the bias field (Hb). 
     It is a second object of the present invention to provide a magnetooptical recording apparatus capable of over writing. 
     It is a third object of the present invention to provide a magnetooptical recording medium capable of over writing. 
     According to the present invention, there is provided a magnetooptical recording method, in which data is recorded on a recording layer of a magnetooptical recording medium using a bit having upward-magnetization and a bit having downward-magnetization, the method comprising the steps of: 
     (a) using, as the medium, a multilayered magnetic recording medium consisting of a first layer having a perpendicular magnetic anisotropy acting as a recording layer, and a second layer having a perpendicular magnetic anisotropy acting as a reference layer; 
     (b) moving said medium; 
     (c) applying an initial field so that, before recording, the direction of magnetization of the first layer is left unchanged, and that of the second layer is aligned either upward or downward; 
     (d) after the direction of magnetization of the second layer is aligned, applying a leading erasing laser beam at low level onto a medium to align either upward or downward the direction of magnetization of the first layer utilizing the magnetization of the second layer; 
     (e) after the directions of magnetization of the first and second layers are aligned, applying a trailing writing laser beam at high level onto the medium while pulse modulating a beam intensity in accordance with binary data to be recorded; 
     (f) when the laser beam is radiated, applying a bias field to the irradiated portion; and 
     (g) if the direction of magnetization of the first layer is aligned either upward or downward when the erasing laser beam is radiated, forming, in the first layer, a bit having a direction of magnetization opposite to that of the first layer when the writing laser beam is at high level. 
     According to the present invention, since erasing is performed by the leading beam and writing is performed by the trailing beam, the process is not, strictly speaking, &#34;over write&#34;. However, since the time difference between erasing and writing is very short (e.g., 5×10 -8  sec), the method of the present invention can be called &#34;over write capable&#34;. 
     In the method of the present invention, two laser beams, i.e., erasing and writing laser beams, are used. The writing laser beam is pulse modulated based on data to be recorded. However, this procedure itself has been performed in the conventional magnetooptical recording method, and a means for pulse modulating the beam intensity based on two-valued or binary data to be recorded is a known means. For example, see &#34;THE BELL SYSTEM TECHNICAL JOURNAL&#34;, Vol. 62 (1983) pp. 1923-1936. In contrast to this, the erasing beam is not modulated in principle. 
     A characteristic feature of the present invention is (1) an intensity of the writing laser beam, at &#34;high level&#34;, which is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; based on data, and (2) an intensity of the erasing laser beam, at &#34;low level&#34;, which is not modulated in principle. The &#34;basis level&#34; may be equal to or lower than the &#34;low level&#34;, and may be &#34;zero level&#34;. 
     The direction of magnetization of the first layer is left unchanged and that of the second layer is aligned in the &#34;A direction&#34; beforehand by an initial field Hini. Thereafter, the leading erasing laser beam of a low intensity is radiated to align the direction of magnetization of the first layer in the &#34;A direction&#34; (or &#34;non-A direction)&#34; regardless of the presence of a bias field (Hb) by utilizing the magnetization of the second layer. Then, when the beam intensity of the trailing writing laser beam which is modulated based on data is high level, the &#34;A-directed&#34; magnetization of the second layer is temporarily reversed to the &#34;non-A direction&#34; by the bias field (Hb). A bit having the &#34;non-A-directed&#34; magnetization (or &#34;A-directed&#34; magnetization) is formed in the first layer by the &#34;non-A-directed&#34; magnetization of the second layer. As a result, when the beam intensity of the writing laser beam is not the high level (i.e., the basis level), the bit having the &#34;A-directed&#34; magnetization (or &#34;non-A-directed&#34; magnetization) is left in the first layer. 
     Even if only one magnetic field applying means is used, two bits having different directions of magnetization can be formed to allow an over write operation. In addition, even if two laser beams are used, they can be adjacent to each other, e.g., 2 μm to 5 mm (when the beam spot size is 1 μm), and hence, a single head can be used. 
     In this specification, if expressions ooo (or xxx) appear, ooo outside the parentheses in the first expression corresponds to ooo in the subsequent expressions ooo (or xxx), and vice versa. 
     As is well known, even if recording is not peformed, a laser beam is often turned on at very low level* in order to, for example, access a predetermined recording position on the medium. When the laser beam is also used for reading, the laser beam is often turned on at an intensity of the very low level*. In this invention, the intensity of at least one of the laser beams may be set at this very low level*. However, low level for forming a bit is higher than the very low level*. Therefore, the output waveform of the laser beams of the present invention is as shown in FIG. 5. 
     According to the present invention, there is provided a magnetooptical recording apparatus capable of over writing, comprising: 
     (a) means for moving a magnetooptical recording medium; 
     (b) initial field applying means; 
     (c) a leading erasing laser beam at low level; 
     (d) a trailing writing laser beam at high level; 
     (e) means for pulse modulating, based on binary data to be recorded, a beam intensity of the writing laser beam to obtain (1) high level that sets the medium at a temperature T H  suitable for forming one of a bit having upward magnetization and a bit having downward magnetization, and to obtain (2) basis level lower than the low level; and 
     (f) bias field applying means, which can also serve as the initial field applying means. 
     The modulating means is available by partially modifying a conventional beam modulating means if high and basis levels of the beam intensity are provided. Such a modification would be easy for those skilled in the art. 
     In addition, according to the present invention, there is provided an over-write capable multilayered magnetooptical recording medium consisting of a first layer having a perpendicular magnetic anisotropy as a recording layer, and a second layer having a perpendicular magnetic anisotropy as a reference layer. 
     The present invention is divided into first and second aspects. In both the aspects, the recording medium has a multilayered structure, which is divided into two layers, as shown in FIG. 7A. 
     The first layer is the recording layer, which exhibits high coersivity at room temperature and has low reversing temperature. The second layer is the reference layer, which exhibits low coersivity at room temperature and has a higher reversing temperature than the first layer. Both the layers comprise perpendicular magnetic layers. Note that each of the first and second layers can comprise a multilayered structure. If necessary, a third layer can be interposed between the first and second layers. In addition, a clear boundary between the first and second layers need not be formed, and one layer can be gradually converted into the other layer. 
     In the first aspect, the coersivity of a recording layer 1 is represented by H C1  ; that of a reference layer 2, H C2  ; the Curie temperature of the recording layer 1, T C1  ; that of the reference layer 2, T C2  ; the room temperature, T R  ; the temperature of the recording medium when a low level laser beam is radiated, T L  ; that when a high level laser beam is radiated, T H  ; a coupling field applied to the recording layer 1, H D1  ; and a coupling field applied to the reference layer 2, H D2 . In this case, the recording medium satisfies the following Formula 1, and satisfies Formulas 2 to 5 at the room temperature. 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.HFormula 1 
    
     
         H.sub.C1 &gt;H.sub.C2 +|H.sub.D1 ∓H.sub.D2 |Formula 2 
    
     
         H.sub.C1 &gt;H.sub.D1                                         Formula 3 
    
     
         H.sub.C2 &gt;H.sub.D2                                         Formula 4 
    
     
         H.sub.C2 +H.sub.D2 &lt;|Hini.|&lt;H.sub.C1 ±H.sub.D1Formula 5 
    
     In the above formula, symbol &#34;≈&#34; means &#34;equal to&#34; or &#34;substantially equal to&#34;. In addition, of double signs ± and ∓, the upper sign corresponds to an A (antiparallel) type medium, and the lower sign corresponds to a P (parallel) type medium (these media will be described later). Note that the P type medium includes a ferromagnetic material medium. 
     The relationship between the coersivity and the temperature is as shown in the graph of FIG. 6. Referring to FIG. 6, the thin curve represents the characteristics of the recording layer 1 and the bold curve represents those of the reference layer 2. 
     Therefore, when an initial field (Hini.) is applied to the recording medium at room temperature, the direction of magnetization of the reference layer 2 is reversed without reversing that of the recording layer 1, according to Formula 5. When the initial field (Hini.) is applied to the recording layer before recording, the reference layer 2 can be magnetized in the &#34;A direction&#34; (in the drawings, the &#34;A direction&#34; is indicated by an upward arrow &#34; &#34;, and the &#34;non-A direction&#34; is indicated by a downward arrow &#34; &#34;). If the initial field (Hini.) is decreased to zero, the direction of magnetization   of the reference layer 2 can be left unchanged without being re-reversed, according to Formula 4. 
     FIG. 7B schematically shows a state wherein only the reference layer 2 is magnetized in the &#34;A direction&#34;   immediately before recording. 
     Referring to FIG. 7B, the direction of magnetization in the recording layer 1 represents previously recorded data. Since the direction of magnetization in the recording layer 1 does not change the basic operation mechanism, it is indicated by X in the following description. The table in FIG. 7B is modified as shown in Condition 1 in FIG. 8 for the sake of simplicity. 
     Principle of Erasing 
     In Condition 1 in FIG. 8, before radiation of the writing laser beam, the erasing laser beam is radiated on the medium. The beam has an intensity corresponding to low level. The low-level laser beam is radiated onto the medium to increase the medium temperature to T L . Since T L  is near the Curie temperature T C1 , magnetization of the recording layer 1 disappears completely or almost completely. However, since T L  is below the Curie temperature T C2 , magnetization of the reference layer 2 does not disappear (Condition 2 L  in FIG. 8). In Condition 2 L , although the bias field (Hb) is unnecessary, it cannot be turned on or off at high speed. Therefore, the bias field (Hb) is left applied inevitably. 
     However, since the coersivity H C2  is maintained high, the direction of magnetization of the reference layer 2 will not be reversed due to the field Hb. Since the medium is moving, a given irradiated portion is immediately separated from the laser beam and is cooled by air. As cooling progresses, magnetization of the recording layer 1 appears. The direction of magnetization is influenced by that of the reference layer 2 due to the magnetic coupling force. As a result, magnetization   (the P type medium) or   (the A type medium) appears in accordance with the type of the medium. This magnetization is not changed even at the room temperature (Condition 3 L  in FIG. 8). 
     In this manner, since the laser beam at low level is not modulated based on data, magnetization of the first layer is aligned in the &#34;A direction&#34; or &#34;non-A direction&#34;. In other words, the preceding data is erased. Thus, the leading laser beam at low level is called an erasing laser beam. 
     Principle of Writing (Recording) 
     In contrast to this, the trailing laser beam is called a writing laser beam. The writing laser beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; based on binary data. In Condition 1 in FIG. 9, when the writing laser beam of high level is radiated on the medium, the medium temperature is increased to T H . Since T H  is higher than the Curie temperature T C1 , magnetization of the recording layer 1 disappears. In addition, since T H  is near the Curie temperature T H2 , magnetization of the reference layer 2 also disappears completely or almost completely (Condition 2 H  in FIG. 9). The bias field (Hb) in the &#34;A direction&#34; or &#34;non-A direction&#34; is applied to the medium in accordance with the type thereof. The bias field (Hb) can be a stray field from the medium itself. For the sake of simplicity, assume that the bias field (Hb) in the &#34;non-A direction&#34; is applied to the medium. Since the medium is moving, a given irradiated portion is immediately separated from the laser beam and is cooled by air. When the temperature of the medium is decreased under the presence of the field Hb, the direction of magnetization of the reference layer 2 is reversed to the &#34;non-A direction&#34; based on the field Hb (Condition 3 H  in FIG. 9). 
     When the medium is further cooled and the medium temperature is decreased below T C1 , magnetization of the recording layer 1 appears again. In this case, the direction of magnetization of the recording layer 1 is influenced by that of the reference layer 2 due to a magnetic coupling (exchange coupling) force. As a result, magnetization   (the P type medium) or   (the A type medium) is formed in accordance with the type of the medium, as shown in Condition 4 H  in FIG. 9. 
     When the writing laser beam is at basis level lower than the high level, the beam intensity must be set so that the medium temperature does not increase to T H . In some cases, the basis level may be equal to or higher or lower than the &#34;low level&#34;, or equal to &#34;very low level&#34; or &#34;zero level&#34;. 
     When the writing laser beam is only modulated, a bit (B 0 ) (or (B 1 )) having &#34;non-A-directed&#34;   (or &#34;A-directed&#34;  ) magnetization can be formed. A portion between the above bits has the &#34;A-directed&#34; (&#34;non-A-directed&#34;  ) magnetization, and can be regarded as the bit (B 1 ) (or (B 0 )). In this manner, recording can be performed in the first layer. 
     Note that the size of theleading beam (to be called a spot size) &#34;on a medium, in particular, on a magnetic thin film irradiation surface&#34; is preferably larger than the trailing beam. This is because the leading beam erases the previously recorded bits so as not to leave a nonerased portion around a beam spot. 
     FIG. 10A summarizes the above descriptions. 
     Note that the recording medium normally has a disk shape, and is rotated during recording. For this reason, a recorded portion (bit) is again influenced by the initial field (Hini.) during a single rotation. As a result, the direction of magnetization of the reference layer 2 is aligned along the original &#34;A direction&#34;  . (see FIG. 10B). However, at the room temperature, magnetization of the reference layer can no longer influence that of the recording layer 1, and the recorded data can be held. 
     If linearly polarized light is radiated onto the recording layer 1, since light reflected thereby includes data, data can be reproduced as in the conventional magnetooptical recording medium. 
     θ k  of the second layer is sometimes larger than that of the first layer. In this case, a C/N ratio is undesirably decreased. In the reproduction mode, a reproducing field H R  in a direction opposite to Hini. can be applied in advance to recover a state immediately after recording. Bits in a state of   in a P type medium, and bits in a state of   in an A type medium are in an unstable state because a magnetic wall is present therebetween. Thus, if the field H R  of an appropriate intensity is selected, only the unstable bits can be reversed to recover an original stable state (a state before being influenced by Hini. immediately after recording). FIG. 10C show the recovered state. 
     In this manner, since data in the first layer can be re-transferred to the second layer, a linearly polarized light beam can be radiated onto the second layer to reproduce a beam reflected thereby, resulting in a high C/N ratio. 
     A perpendicular magnetic film constituting the recording layer 1 and the reference layer 2 is selected from the group consisting of (1) crystalline or amorphous ferromagnetic or ferrimagnetic materials having the Curie temperature and no compensation temperature, and (2) crystalline or amorphous ferrimagnetic materials having both the compensation temperature and the Curie temperature. 
     The first aspect utilizing the Curie temperature has been described. In contrast to this, the second aspect of the present invention utilizes decreased coersivity H C  at a predetermined temperature exceeding the room temperature. The second aspect uses a temperature T S1  at which the recording layer 1 is magnetically coupled to the reference layer 2, in place of the temperature T C1  in the first aspect. In addition, instead of the temperature T C2 , a temperature T S2  at which the reference layer 2 is reversed under the influence of the field Hb is used. Thereby, the second aspect can provide the same effect as in the first aspect. 
     In the second aspect, the coersivity of the recording layer 1 is represented by H C1  ; that of the reference layer 2, H C2  ; a temperature at which the recording layer 1 is magnetically coupled to the reference layer 2, T S1  ; a temperature at which the direction of magnetization of the reference layer 2 is reversed upon influence of the field Hb, T S2  ; room temperature, T R  ; a temperature of the medium when a low-level laser beam is applied thereto, T L  ; a temperature of the medium when a high-level laser beam is applied thereto, T H  ; a coupling field applied to the recording layer 1, H D1  ; and a coupling field applied to the reference layer 2, H D2 . In this case, the recording medium satisfies the following Formula 6, and satisfies Formulas 7 to 10 at the room temperature. 
     
         T.sub.R &lt;T.sub.S1 ≈T.sub.L &lt;T.sub.S2 ≈T.sub.HFormula 6 
    
     
         H.sub.C1 &gt;H.sub.C2 +|H.sub.D1 ∓H.sub.D2        Formula 7 
    
     
         H.sub.C1 &gt;H.sub.D1                                         Formula 8 
    
     
         H.sub.C2 &gt;H.sub.D2                                         Formula 9 
    
     
         H.sub.C2 +H.sub.D2 &lt;|Hini.|&lt;H.sub.C1 ±H.sub.D1Formula 10 
    
     In the above formulas, upper signs of double signals ± and ∓ correspond to an A (antiparallel) type medium, and lower signs correspond to a P (parallel) medium (these media will be described later). 
     In the first and second aspects, the recording medium is constituted by the recording layer 1 and the reference layer 2, each of which preferably comprises an amorphous ferrimagnetic material selected from transition metal (e.g., Fe, Co)--heavy rare earth metal (e.g., Gd, Tb, Dy, and the like) alloy compositions. 
     When the recording layer 1 and the reference layer 2 are both selected from the transition metal--heavy rare earth metal alloy compositions, the direction and level of magnetization appearing outside the alloy are determined by the relationship between the direction and level of spin of transition metal atoms (to be referred to as TM hereinafter) and those of heavy rare earth metal atoms (to be referred to as RE hereinafter) inside the alloy. For example, the direction and level of TM spin are represented by a dotted vector  , those of RE spin are indicated by a solid vector ↑, and the direction and level of magnetization of the alloy as a whole are represented by a double-solid vector  . In this case, the vector   is represented by a sum of vectors   and ↑. However, in the alloy, the vectors   and ↑ are directed in the opposite directions due to the mutual effect of the TM spin and the RE spin. Therefore, when these vectors are equal to each other, the sum of vectors   and ↑ or the sum of vectors ↓ and   is zero (i.e., the level of magnetization appearing outside the alloy is zero). The alloy composition making the sum of vectors zero is called a compensation composition. When the alloy has another composition, it has a strength equal to a difference between the strengths of both the spins, and has a vector (  or  ) having a direction equal to that of the larger vector. Magnetization of this vector appears outside the alloy. For example, vectors ↑  correspond to a vector  , and vectors ↑  correspond to a vector  . 
     When one of the strengths of the vectors of the RM spin and RM spin is larger than the other, the alloy composition is referred to as &#34;oo rich&#34; named after the larger spin name (e.g., RE rich). 
     The recording layer 1 and the reference layer 2 can be classified into TM rich and RE rich compositions. Therefore, if the composition of the recording layer 1 is plotted along the ordinate and that of the reference layer 2 is plotted along the abscissa, the types of the recording media according to the present invention can be classified into four quadrants, as shown in FIG. 11. The P type medium described previously belongs to Quadrant I and III, and the A type medium belongs to Quadrant II and IV. Referring to FIG. 11, the intersection (origin) of the abscissa and the ordinate represents the compensation composition of both the layers. 
     In view of a change in coersivity versus a change in temperatures, a certain alloy composition has characteristics wherein the coersivity temporarily increases infinitely and then abruptly decreases before a temperature reaches the Curie temperature (at which the coersivity is zero). The temperature corresponding to the infinite coersivity is called a compensation temperature (Tcomp.). No compensation temperature is present between the room temperature and the Curie temperature in the TM rich alloy composition. The compensation temperature below the room temperature is nonsense in the magnetooptical recording, and hence, it is assumed in this specification that the compensation temperature is present between the room temperature and the Curie temperature. 
     If the first and second layers are classified in view of the presence/absence of the compensation temperature, the recording medium can be classified into four types. The recording medium in Quadrant I includes all the four types of media. The graphs of FIGS. 12A to 12D respectively show the relationship between the coersivity and the temperature of the four types of media. Note that thin curves represent characteristics of the recording layer 1 and bold curves represent those of reference layer 2. 
     When the recording layer 1 and the reference layer 2 are classified in view of their RE or TM rich characteristics and in view of the presence/absence of the compensation temperature, they can be classified into 9 classes. 
     
                       TABLE 1______________________________________  Quadrant I    First Layer: Second Layer:Class    RE rich      RE rich     Type______________________________________1        Tcomp.       Tcomp.      12        No Tcomp.    Tcomp.      23        Tcomp.       No Tcomp.   34        No Tcomp.    No Tcomp.   4______________________________________  Quadrant II    First Layer: Second Layer:Class    RE rich      TM rich     Type______________________________________5        Tcomp.       No Tcomp.   36        No Tcomp.    No Tcomp.   4______________________________________  Quadrant III    First Layer: Second Layer:Class    TM rich      TM rich     Type______________________________________7        No Tcomp.    No Tcomp.   4______________________________________  Quadrant IV    First Layer: Second Layer:Class    TM rich      RE rich     Type______________________________________8        No Tcomp.    Tcomp.      29        No Tcomp.    No Tcomp.   4______________________________________ 
    
     The principle of the method of the present invention will be described in detail using a specific medium No. 1 belonging to Class 1 (P type, Quadrant I, Type 1) shown in Table 1. 
     The medium No. 1 satisfies of Formula 11: 
     
         T.sub.R &lt;Tcomp.1&lt;T.sub.C1 ≈T.sub.L ≈Tcomp.2&lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 13 shows this relation. Note that thin curves indicate the first layer, and bold curves indicate the second layer. Those identifications are the same in the following graphs. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 12. The medium No. 1 satisfies Formula 12 at the T R . 
     
         H.sub.C1 &gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2) 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 15. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 13 and 14. The medium No. 1 satisfies Formulas 13 and 14. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 13 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 14 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 15 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 12 to 14 at the T R  is aligned along the &#34;A direction&#34; (↑ ) by the Hini. which satisfies Formula 15. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 14). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↑. 
     The principle of erasing will now be described with reference to FIG. 14. 
     Erasing Mode 
     In Condition 1, the medium temperature is increased to T L  upon irradiation of the low-level of the leading unmodulated laser beam. Since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 L ). 
     In Condition 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the recording layer 1 is influenced by the RE and RM spins (↑ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. As a result, magnetization of ↑  i.e.,  , without regard to the bias field ↑Hb, appears in the layer 1 (Condition 3 L ). Since the temperature in Condition 3 L  is higher than the temperature Tcomp.1, the TM spin is larger than the RE spin. 
     When the medium temperature i s decreased below the temperature Tcomp.1, the relationship between the RE and TM spins of the first layer is reversed (↑ →↑↓) in the same manner as in the high-temperature cycle. As a result, the magnetization of the layer 1 is (Condition 4 L ). 
     Condition 4 L  is maintained even if the medium temperature is decreased to the room temperature. In this way, the direction of magnetization of the first layer is aligned in the &#34;A-direction&#34;  , and preceding data is erased. 
     Next, the principle of recording will now be described with reference to FIG. 15. 
     Recording Mode 
     In Condition 1, when the medium temperature is increased to T L  upon irradiation of the high-level of the trailing beam, since the temperature T L  is substantially equal to the curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation further continues, the temperature of the medium increases accordingly. When the temperature of the medium slightly exceeds the temperature Tcomp.2 of the reference layer 2, the relationship between the strengths of the vectors is reversed (↑ →↑ ) although the directions of the RE and TM spins remain the same. For this reason, the direction of magnetization of the layer 2 is reversed to be along the &#34;non-A direction&#34;   (Condition 3 H ). 
     However, since the coersivity H C2  is still high at this temperature, the direction of magnetization of the layer 2 will not be reversed by the field Hb (↑). When the temperature further increases and reaches the temperature T H , the temperature of the layer 2 substantially corresponds to the Curie temperature T C2 , and magnetization of the layer 2 also disappears (Condition 4 H ). 
     In Condition 4 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization appears in the layer 2. In this case, magnetization   (↓ ) is generated by ↑Hb (Condition 5 H ). However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. 
     When the temperature of the medium further decreases below the temperature Tcomp.2, the relationship between the strengths of the vectors is reversed (↓ →↓↑) although the directions of the RE and TM spins remain the same. As a result, the direction of magnetization of the alloy as a whole is reversed from   to the &#34;non-A direction&#34; (Condition 6 H ). 
     In Condition 6 H , since the temperature of the medium is higher than the temperature T C1 , magnetization of the layer 1 has not yet appeared. In addition, since the coersivity H C2  at this temperature is high, the direction of magnetization   of the layer 2 cannot be reversed by the field ↑Hb. 
     When the temperature of the medium decreases and is slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. Since the temperature of the layer 1 is higher than the temperature Tcomp.1, the TM spin is larger than the RE spin, and hence, magnetization of ↓ , i.e., appears in the layer 1. This state is Condition 7 H . 
     When the temperature of the medium is decreased from the temperature in Condition 7 H  and is below the temperature Tcomp.1, the relationships between the strengths of the RE and TM spins of the layer 1 is reversed (↓ →↓ ). As a result, magnetization of   appears (Condition 8 H ). 
     Then, the temperature of the medium decreases from the temperature in Condition 8 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, Condition 8 H  is maintained without reversing the direction of magnetization of the layer 1 by the field ↑Hb. In this way, bit formation in the &#34;non-A direction&#34;   is completed. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;non-A-directed&#34; ( )bits. 
     The principle of the method of the present invention will be described in detail using a specific medium No. 2 belonging to Class 2 (P type, Quadrant I, Type 2) shown in Table 1. 
     The medium No. 2 satisfies Formula 16: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L ≈Tcomp.2&lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 16 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 17. The medium No. 2 satisfies Formula 17 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2) 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 20. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 18 and 19. The medium No. 2 satisfies Formulas 18 and 19. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 18 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 19 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 20 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 17 to 19 at the T R  is aligned along the &#34;A direction&#34; (↑ ) by the Hini. which satisfies Formula 20. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 17). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↑. 
     The principle of erasing will now be described with reference to FIG. 17. 
     Erasing Mode 
     In Condition 1, the medium temperature is increased to T L  upon irradiation of the nonmodulated leading low-level laser beam. Since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 L ). 
     In Condition 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the recording layer 1 is influenced by the RE and TM spins (↑ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. As a result, magnetization of ↑ , i.e.,  , appears in the layer 1 (Condition 3 L ). 
     Condition 3 L  is maintained even if the medium temperature is decreased to the room temperature. As a result, recording layer 1 is magnetized in the &#34;A direction&#34;   regardless of previously recorded bits and the recorded data is erased. 
     Next, the principle of recording will now be described with reference to FIG. 18. 
     Recording Mode 
     In Condition 1, when the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation further continues, the temperature of the medium increases accordingly. When the temperature of the medium slightly exceeds the temperature Tcomp.2 of the reference layer 2, the relationship between the strengths of the vectors is reversed (↑ →↑ ) although the directions of the RE and TM spins remain the same. For this reason, the direction of magnetization of the layer 2 is reversed to the &#34;non-A direction&#34;   (Condition 3 H ). 
     However, since the coersivity H C2  is still high at this temperature, the direction of magnetization of the layer 2 will not be reversed by the field Hb (↑). When the temperature further increases and reaches the temperature T H , the temperature of the layer 2 substantially corresponds to the Curie temperature T C2 , and magnetization of the layer 2 also disappears (Condition 4 H ). 
     In Condition 4 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization appears in the layer 2. In this case, magnetization   (↓ ) is generated by ↑Hb (Condition 5 H ). However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. 
     When the temperature of the medium further decreases below the temperature Tcomp.2, the relationship between the strengths of the vectors is reversed (↓ →↓ ) although the directions of the RE and TM spins remain the same. As a result, the direction of magnetization of the alloy as a whole is reversed from   to the &#34;non-A direction&#34;   (Condition 6 H ). 
     In Condition 6 H , since the temperature of the medium is higher than the temperature T C1 , magnetization of the layer 1 has not yet appeared. In addition, since the coersivity H C2  at this temperature is high, the direction of magnetization   of the layer 2 cannot be reversed by the field ↑Hb. 
     When the temperature of the medium decreases and is slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. Thus, magnetization of ↓ , i.e.,   appears in the layer 1. This state is Condition 7 H . 
     Then, the temperature of the medium decreases from the temperature in Condition 7 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, Condition 7 H  is maintained without reversing the direction of magnetization of the layer 1 by the field ↑Hb. In this way, bit formation in the &#34;non-A direction&#34;   is completed. 
     As a result, when the trailng writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;non-A-directed&#34; ( ) bits. 
     The principle of the method of the present invention will be described in detail using a specific medium No. 3 belong to Class 3 (P type, Quadrant I, Type 3) shown in Table 1. 
     the medium No. 3 satisfies Formula 21: 
     
         T.sub.R &lt;Tcomp.1&lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 19 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 22. The medium No. 3 satisfies Formula 22 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2) 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 25. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 23 and 24. The medium No. 3 satisfies Formulas 23 and 24. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 23 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 24 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 25 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 22 to 24 at the T R  is aligned along the &#34;A direction&#34; (↑ ) by the Hini. which satisfies Formula 25. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 20). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↓. 
     The principle of erasing will be described with reference to FIG. 20. 
     Erasing Mode 
     In Condition 1 immediately before recording, the medium temperature is increased to T L  upon irradiation of the leading low-level laser beam. Since temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears. However, at this temperature, since the coersivity H C2  of the layer 2 is sufficiently high, magnetization of the layer 2 will not be reversed by the bias field ↓Hb (Condition 2 L ). 
     In Condition 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the respective spins of the recording layer 1 are influenced by the RE and TM spins (↑ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. As a result, magnetization of ↑ , i.e.,  , appears in the layer 1. In this case, since the medium temperature is higher than the temperature Tcomp.1, the TM spin is larger than the RE spin (Condition 3 L ). 
     When the medium temperature is further decreased below the temperature Tcomp.1, the relationship between the strengths of the RE and TM spins of the layer 1 is reversed in the same manner as in the high-temperature cycle (↑ →↑ ). As a result, magnetization of  , without regard to the bias field ↓Hb, appears in the layer 1 (Condition 4 L ). 
     Condition 4 L  is maintained even if the medium temperature is decreased to the room temperature. In this way, the direction of magnetization of the layer 1 is aligned in the &#34;A direction&#34;  , and recorded data is erased. 
     Next, the principle of recording will now be described with reference to FIG. 21. 
     Recording Mode 
     In Condition 1, when the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation further continues, since the temperature T H  of the medium is substantially equal to the temperature T C2 , magnetization of the layer 2 also disappears (Condition 3 H ). 
     In Condition 3 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization appears in the layer 2. In this case, magnetization   (↓ ) is generated by ↓Hb. However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 4 H . 
     When the temperature of the medium decreases and is slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. Since the temperature of the medium is higher than the temperature Tcomp.1, the TM spin is larger than the RE spin (↓ ). As a result, magnetization of appears in the layer 1 (Condition 5 H ). 
     When the medium temperature further decreases from the temperature in Condition 5 H  and is below the temperature Tcomp.1, the relationship between the strengths of the TM and RE spins of the layer 1 is reversed (↓ →↓ ). For this reason, the direction of magnetization of the layer 1 is reversed to the &#34;non-A direction&#34; (Condition 6 H ). 
     Then, the temperature of the medium decreases from the temperature in Condition 6 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, magnetization of the layer 1 is stably maintained. In this way, bit formation in the &#34;non-A direction&#34;  is completed. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;non-A-directed&#34; ( ) bits. 
     The principle of the method of the present invention will be described in detail using a specific medium No. 4 belonging to Class 4 (P type, Quadrant I, Type 4) shown in Table 1. 
     The medium No. 4 satisfies Formula 26: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 22 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 27. The medium No. 4 satisfies Formula 27 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2) 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall enrgy. 
     At this time, a condition for the Hini. is represented by Formula 30. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 28 and 29. The medium No. 4 satisfies Formulas 28 and 29. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 28 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 29 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 30 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 27 to 29 at the T R  is aligned along the &#34;A direction&#34; (↑ ) by the Hini. which satisfies Formula 30. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 23). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↓. 
     The principle of erasing will be described with reference to FIG. 23. 
     Erasing Mode 
     In Condition 1, the medium temperature is increased to T L  upon irradiation of the nonmodulated leading low-level laser beam. Since the temperature T L  exceeds the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears. However, at this temperature, since the coersivity H C2  of the layer 2 is sufficiently high, magnetization of the layer 2 will not be reversed by the bias field ↓Hb. This state is Condition 2 L . 
     In Condition 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the respective spins of the recording layer 1 are influenced by the RE and TM spins (↑ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. As a result, magnetization of ↑ , i.e.,  , without regard to the bias field ↓Hb, appears in the layer 1. This state is Condition 3 L . 
     Condition 3 L  is maintained even if the medium temperature is decreased to the room temperature. 
     As a result, the direction of magnetization of the first layer is aligned in the &#34;A direction&#34;  , and the recorded data is erased. 
     Next, the principle of recording will now be described with reference to FIG. 24. 
     Recording Mode 
     When the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation further continues and the medium temperature is equal to the temperature T H , since the temperature T H  of the layer 2 is substantially equal to the Curie temperature T C2 , magnetization of the layer 2 also disappears. This state is Condition 3 H . 
     In Condition 3 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization of the layer 2 appears. In this case, magnetization   (↓ ) is generated by ↓Hb. However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 4 H . 
     When the temperature of the medium decreases and is slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. As a result, magnetization of ↓ , i.e.,   appears in the layer 1. This state is Condition 5 H . 
     Then, the temperature of the medium decreases from the temperature in Condition 5 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, magnetization of the layer 1 is stably maintained. In this way, bit formation in the &#34;non-A direction&#34;  is completed. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;non-A-directed&#34; ( ) bits. 
     The principle of the method of the present invention will be described in detail using a specific medium No. 5 belonging to Class 5 (A type, Quadrant II, type 3) shown in Table 1. 
     The medium No. 5 satisfies Formula 31: 
     
         T.sub.R &lt;Tcomp.1&lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 25 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layers 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 32. The medium No. 5 satisfies Formula 32 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)| 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 35. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 33 and 34. The medium No. 5 satisfies Formulas 33 and 34. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 33 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 34 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 35 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 32 to 34 at the T R  is aligned along the &#34;A direction&#34; (↓↑) by the Hini. which satisfies Formula 35. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 26). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↓. 
     The principle of erasing will be described with reference to FIG. 27. 
     Erasing Mode 
     In Condition 1, the medium temperature is increased to T L  upon irradiation of the nonmodulated leading low-level laser beam. Since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears. However, at this temperature, since the coersivity H C2  of the layer 2 is sufficiently high, magnetization of the layer 2 will not be reversed by the bias field ↓Hb (Condition 2 L ). 
     When the beam radiation is completed in Condition 2 L , the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the respective spins of the recording layer 1 are influenced by the RE and TM spins (↓ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. As a result, magnetization of ↓ , i.e.,  , without regard to bias field ↓Hb, appears in the layer 1. In this case, since the medium temperature is higher than the temperature Tcomp.1, the TM spin is larger than the RE spin (Condition 3 L ). 
     When the medium temperature is decreased below the temperature Tcomp.1, the relationship between the strengths of the RE and TM spins of the layer 1 is reversed as well as the high-temperature cycle (↓ →↓ ). As a result, the magnetization of the layer 1 is (↓ ) (Condition 4 L ). 
     Condition 4 L  is maintained even if the medium temperature is decreased to the room temperature. 
     As a result, the direction of magnetization of the first layer is aligned in the &#34;non-A direction&#34;  , and the recorded data is erased. 
     Next, the principle of recording will be described with reference to FIG. 27. 
     Recording Mode 
     When the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation further continues and the medium temperature is equal to the temperature T H , since the temperature T H  of the layer 2 is substantially equal to the Curie temperature T C2 , magnetization of the layer 2 also disappears (Condition 3 H ). 
     In Condition 3 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization of the layer 2 appears. In this case, magnetization   (↑ ) is generated by ↓Hb. However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 4 H . 
     When the temperature of the medium is further decreased slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. In this case, since the medium temperature is yet higher than the temperature Tcomp.1, the TM spin is larger than the RE spin (↑ ). As a result, magnetization of  appears in the layer 2 (Condition 5 H ). 
     When the medium temperature is decreased below the temperature Tcomp.1 from the temperature in Condition 5 H , the relationship between the strengths of the TM and RE spins of the layer 1 is reversed (↑ →↑ ). For this reason, magnetization of the layer 1 is reversed to the &#34;A-direction&#34;   (Condition 6 H ). 
     Then, the temperature of the medium decreases from the temperature in Condition 6 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, magnetization of the layer 1 is stably maintained. In this way, bit formation in the &#34;A direction&#34;   is completed. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;A-directed&#34;   bits. 
     The principle of the method of the present invention will be described in detail using a specific medium No. 6 belong to Class 6 (A type, Quadrant II, Type 4) shown in Table 1. 
     The mediun No. 6 satisfies Formula 36: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 28 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 37. The medium No. 6 satisfies Formula 37 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)| 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 40. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 38 and 39. The medium No. 6 satisfies Formulas 38 and 39. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 38 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 39 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 40 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 37 to 39 at the T R  is aligned along the &#34;A direction&#34; (↓ ) by the Hini. which satisfies Formula 40. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 29). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↓. 
     The principle of erasing will be described with reference to FIG. 29. 
     Erasing Mode 
     In Condition 1, the medium temperature is increased to T L  upon irradiation of the nonmodulated leading low-level laser beam. Since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears. In this state, however, since the coersivity H C2  of the layer 2 is sufficiently high, magnetization   of the layer 2 will not be reversed by the bias field ↓Hb. This state is Condition 2 L . 
     In Condition 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the respective spins of the recording layer 1 are influenced by the RE and TM spins (↓ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. As a result, magnetization of ↓ , i.e.,  , appears in the layer 1. This state is Condition 3 L . 
     Condition 3 L  is maintained even if the medium temperature is decreased to the room temperature. 
     As a result, the direction of magnetization of the first layer is aligned in the &#34;non-A direction&#34;  , and the recorded data ia erased. 
     Next, the principle of recording will now be described with reference to FIG. 30. 
     Recording Mode 
     In Condition 1, when the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation further continues and the medium temperature is equal to the temperature T H , since the temperature T H  of the layer 2 is substantially equal to the Curie temperature T C2 , magnetization of the layer 2 also disappears. This state is Condition 3 H . 
     In Condition 3 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization of the layer 2 appears. In this case, magnetization   (↑ ) is generated by ↓Hb. However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 4 H . 
     When the temperature of the medium is further decreased slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. For this reason, magnetization ↑ , i.e.,  , without regard to the bias field ↓Hb, appears in the layer 1. This state is Condition 5 H . 
     Then, the temperature of the medium decreases from the temperature in Condition 5 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, magnetization of the layer 1 is stably maintained. In this way, bit formation in the &#34;A direction&#34;   is completed. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;A-directed&#34; ( ) bits. 
     The principle of the method of the present invention will be described in detail using a specific medium No. 7 belonging to Class 7 (P type, Quadrant III, Type 4) shown in Table 1. 
     The medium No. 7 satisfies Formula 41: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 31 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 42. The medium No. 7 satisfies Formula 42 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2) 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 45. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 43 and 44. The medium No. 7 satisfies Formulas 43 and 44. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 43 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 44 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 45 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 42 to 44 at the T R  is aligned along the &#34;A direction&#34; (↓ ) by the Hini. which satisfies Formula 45. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 32). 
     Condition 1 is held to a point immediately before the recording. In this case, the base field (Hb) is applied in the direction of ↓. 
     The principle of erasing will now be described with reference to FIG. 32. 
     Erasing Mode 
     In Condition 1 immediately before recording, the medium temperature is increased to T L  upon irradiation of the nonmodulated leading low-level laser beam. Since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears. In this state, however, since the coersivity H C2  of the layer 2 is sufficiently high, magnetization   of the layer 2 will not be reversed by the bias field ↓Hb. This state is Condition 2 L . 
     In Condition 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the respective spins of the recording layer 1 are influenced by the RE and TM spins (↓ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. As a result, magnetization of ↓ , i.e.,  , without regard to the bias field ↓Hb, appears in the layer 1. This state is Condition 3 L . 
     Condition 3 L  is maintained even if the medium temperature is decreased to the room temperature. 
     As a result, the direction of magnetization of the first layer is aligned in the &#34;A direction&#34;  , and the recorded data is erased. 
     Next, the principle of recording will now be described with reference to FIG. 33. 
     Recording Mode 
     In Condition 1, when the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation continues and the medium temperature is equal to the temperature T H , since the temperature T H  of the layer 2 is substantially equal to the Curie temperature T C2 , magnetization of the layer 2 also disappears. This state is Condition 3 H . 
     In Condition 3 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization of the layer 2 appears. In this case, magnetization   (↑ ) is generated by ↓Hb. However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 4 H . 
     When the temperature of the medium is further decreased slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 (↑ ) acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. For this reason, magnetization ↑ , i.e.,   appears in the layer 1. This state is Condition 5 H . 
     Then, the temperature of the medium decreases from the temperature in Condition 5 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, magnetization of the layer 1 is stably maintained. In this way, bit formation in the &#34;non-A direction&#34;  is completed. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;non-A-directed&#34; (( ) bits. 
     The principle of the method of the present invention will be described in detail using a specific medium No. 8 belonging to Class 8 (A type, Quadrant IV, Type 2) shown in Table 1. 
     The medium No. 8 satisfies Formula 46: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L ≈Tcomp.2&lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 34 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 47. The medium No. 8 satisfies Formula 47 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)| 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 50. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 48 and 49. The medium No. 8 satisfies Formulas 48 and 49. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 48 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 49 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 50 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 47 to 49 at the T R  is aligned along the &#34;A direction&#34; (↑ ) by the Hini. Which satisfies Formula 50. At this time the recording layer 1 is maintained in the recorded state (Condition 1 in FIG. 35). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↑. 
     The principle of recording will now be described with reference to FIG. 35. 
     Erasing Mode 
     In Condition 1, the medium temperature is increased to T L  upon irradiation of the nonmodulated leading low-lever laser beam. Since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears. In this state, however, since the coersivity H C2  of the layer 2 is sufficiently high, magnetization of the layer 2 will not be reversed by the bias field ↑Hb (Condition 2 L ). 
     In Conditioin 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the respective spins of the recoding layer 1 are influenced by the RE and TM spins (↑ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. As a result, magnetization of ↑ , i.e.,  , without regard to the bias field ↑Hb, appears in the layer 1. This state is Condition 3 L . 
     Condition 3 L  is maintained even if the medium temperature is decreased to the room temperature. 
     As a result, the direction of magnetization of the first layer is aligned in the &#34;non-A direction&#34;  , and the recorded data is erased. 
     Next, the principle of recording will now be described with reference to FIG. 35. 
     Recording Mode 
     In Condition 1, when the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation continues and the medium temperature is slightly higher than the temperature Tcomp.2, the relationship between the strengths of the RE and TM spins is reversed (↑ →↑ ) although the directions thereof (↑ and  ) remain the same. As a result, the direction of the magnetization of the layer 2 is reversed to &#34;non-A direction&#34;  . This state is Condition 3 H . 
     At this temperature, however, since the coersivity H C2  is yet high, magnetization   of the layer 2 will not be reversed by the bias field ↑Hb. Assume that the laser-beam radiation further continues and the medium temperature is increased to T H . Since the temperature T H  is substantially equal to the temperature T C2 , magnetization of the layer 2 also disappears (Condition 4 H ). 
     In Condition 4 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization of the layer 2 appears. In this case, magnetization   (↓ ) is generated by ↑Hb. However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 5 H . 
     When the temperature of the medium is further decreased slightly below the temperature Tcomp.1, the relationship between the strengths of the RE and TM spins is reversed (↓ →↓ ) without reversing the directions thereof (↓ and  ). As a result, the direction of magnetization of the layer 2 is reversed to &#34;non-A direction&#34;  . In this state, since the coersivity H C2  is already sufficiently high, magnetization   of the layer 2 will not be reversed by the bias field ↑Hb. In this case, since the medium temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 6 H . 
     When the medium temperature further decreases slightly below the temperature T C1 , magnetization also appears in the layer 1. At this time, magnetization (↓ ) of the layer 1 influences the layer 1 due to the exchange coupling force to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. For this reason, magnetization ↓ , i.e.,   appears in the layer 1 (Condition 7 H ). 
     Then, the temperature of the medium decreases from the temperature in Condition 7 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, magnetization of the layer 1 is stably maintained. In this way, bit formation in the &#34;A direction&#34;   is completed. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;A-directed&#34; ( ) bits. 
     The principle of the method of the present invention will be descirbed in detail using a specific medium No. 9 belonging to Class 9 (A type, Quadrant IV, Type 4) shown in Table 1. 
     The medium No. 9 satisfies Formula 51: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.H 
    
     The graph of FIG. 37 shows this relation. 
     A condition that reverses the direction of magnetization of the reference layer 2 without reversing that of the recording layer 1 by the initial field (Hini.) at the room temperature T R  is represented by Formula 52. The medium No. 9 satisfies Formula 52 at the T R  : 
     
         H.sub.C1 &gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)| 
    
     where 
     H C1  : coersivity of recording layer 1 
     H C2  : coersivity of reference layer 2 
     M S1  : saturation magnetization of layer 1 
     M S2  : saturation magnetization of layer 2 
     t 1  : film thickness of layer 1 
     t 2  : film thickness of layer 2 
     σ w  : interface wall energy. 
     At this time, a condition for the Hini. is represented by Formula 50. If the Hini. is represented by Formula 50. If the Hini. disappears, reversed magnetization of the reference layer 2 is influenced by magnetization of the recording layer 1 due to an exchange coupling force. The condition that can hold the direction of magnetization of the layer 2 is represented by Formulas 53 and 54. The medium No. 9 satisfies Formulas 53 and 54. 
     
         H.sub.C1 &gt;(σ.sub.w /2 M.sub.S1 t.sub.1)              Formula 53 
    
     
         H.sub.C2 &gt;(σ.sub.w /2 M.sub.S2 t.sub.2)              Formula 54 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)&lt;|Hini.|&lt;H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)                                                  Formula 55 
    
     The layer 2&#39;s magnetization of the recording medium which can satisfy Formulas 52 to 54 at the T R  is aligned along the &#34;A direction&#34; (↑ ) by the Hini. which satisfies Formula 55. At this time, the recording layer 1 is maintained in the recorded state (Condition 1 in FIGS. 38 and 39). 
     Condition 1 is held to a point immediately before the recording. In this case, the bias field (Hb) is applied in the direction of ↓. 
     The principle of erasing will be described with reference to FIG. 38. 
     Erasing Mode 
     In Condition 1, the medium temperature is increased to T L  upon irradiation of the nonmodulated leading low-level laser beam. Since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears. In this state, however, since the coersivity H C2  of the layer 2 is sufficiently high, magnetization   of the layer 2 will not be reversed by the bias field ↓Hb. This state is Condition 2 L . 
     In Condition 2 L , when an irradiated portion is separated from the laser beam spot, the medium temperature begins to fall. When the medium temperature is slightly below the temperature T C1 , the recording layer 1 is influenced by the RE and TM spins (↑ ) of the reference layer 2 due to the exchange coupling force. In other words, the exchange coupling force acts to align each of the RE and TM spins (↑ and  ) of the layers 1 and 2. As a result, magnetization of ↑ , i.e.,  , appears in the layer 1. This state is Condition 3 L . 
     Condition 3 L  is maintained even if the medium temperature is decreased to the room temperature. 
     As a result, the direction of magnetization of the first layer is aligned in the &#34;non-A direction&#34;  , and the recorded data is erased. 
     Next, the principle of recording will now be described with reference to FIG. 39. 
     Recording Mode 
     In Condition 1, when the medium temperature is increased to T L  upon irradiation of the high-level of the trailing laser beam, since the temperature T L  is substantially equal to the Curie temperature T C1  of the recording layer 1, magnetization of the layer 1 disappears (Condition 2 H ). 
     When the laser-beam radiation continues and the medium temperature is equal to the temperature T H , the temperature T H  is substantially equal to the temperature T C2 , magnetization of the layer 2 also disappears. This state is Condition 3 H . 
     In Condition 3 H , when an irradiated portion is separated from the laser beam spot, the temperature of the medium begins to decrease. When the temperature of the medium decreases slightly below the temperature T C2 , magnetization of the layer 2 appears. In this case, magnetization   (↓ ) is generated by ↓Hb. However, since the temperature is yet higher than the temperature T C1 , no magnetization appears in the layer 1. This state is Condition 4 H . 
     When the temperature of the medium is further decreased slightly below the temperature T C1 , magnetization appears in the layer 1. At this time, the exchange coupling force from the layer 2 (↓ ) acts to align each of the RE and TM spins (↓ and  ) of the layers 1 and 2. For this reason, magnetization ↓ , i.e.,  , without regard to the bias field ↓Hb, appears in the layer 1. This state is Condition 5 H . 
     Then, the temperature of the medium decreases from the temperature in Condition 5 H  to the room temperature. Since the coersivity H C1  at the room temperature is sufficiently high, magnetization of the layer 1 is stably maintained. 
     As a result, when the trailing writing beam is pulse modulated between &#34;high level&#34; and &#34;basis level&#34; in accordance with data, data can be recorded in the form of the presence/absence or the bit lengths of &#34;A-directed&#34; ( ) bits. 
     According to the present invention as described above, the temperatures T R  and T L  provided by the two beams are elaborately controlled in accordance with data to allow an over write operation. In general, a magnetic thin film constituting a recording layer or a reference layer employs a medium formed into a disk shape. Helical or concentric tracks are formed on the disk-shaped medium. Therefore, when the medium is cut in the radial direction, a plurality of tracks are aligned parallel to each other. In this case, if adjacent tracks are close to each other and are continuous, heat is transferred from a track subjected to erasing or recording to adjacent tracks (the magnetic thin film has a high thermal transfer ratio). Therefore, data on the adjacent tracks may be erroneously erased or rewritten. When tracks are adjacent to each other, a groove for sealing heat is provided between adjacent tracks to thermally insulate the magnetic thin film. In this case, the spot size of the erasing beam is preferably set to be larger than the width of a track. Thus, unnecessary data cannot be left nonerased. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration for explaining a recording principle of a magnetooptical recording method; 
     FIG. 2 is an illustration for explaining a reading principle of the magnetooptical recording method; 
     FIG. 3 is a diagram showing the overall arrangement of a magnetooptical recording apparatus according to Example 10 of the present invention; 
     FIG. 4 is a diagram showing the overall arrangement of a magnetooptical recording apparatus according to Examples 11 and 12 of the present invention; 
     FIG. 5 is a chart showing an output waveform according to an embodiment of the present invention; 
     FIG. 6 is a graph showing the relationship between coersivity and temperature; 
     FIG. 7A is a diagram of a multilayered structure of a recording medium according to the embodiment of the present invention; 
     FIG. 7B is a diagram showing the direction of magnetization of a recording layer and a reference layer; 
     FIG. 8 illustrate changes in the direction of magnetization at high level; 
     FIG. 9 illustrates changes in the direction of magnetization at low level; 
     FIGS. 10A to 10C illustrates flows of the changes in the direction of magnetization shown in FIGS. 8 and 9 for respective P- and A-type media; 
     FIG. 11 is a map wherein the types of media of the present invention as classified into four quadrants; 
     FIGS. 12A to 12D are graphs showing the relationship between the coersivity and the temperature respectively for media of Types I to IV; 
     FIG. 13 is a graph showing the relationship between the coersivity and the temperature for a medium No. 1; 
     FIGS. 14 and 15 are diagrams showing changes in the direction of magnetization in erasing and recording modes of a medium No. 1; 
     FIG. 16 is a graph showing the relationship between the coersivity and the temperature for the medium No. 2; 
     FIGS. 17 and 18 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of a medium No. 2; 
     FIG. 19 is a graph showing the relationship between the coersivity and the temperature for a medium No. 3; 
     FIGS. 20 and 21 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of the medium No. 3; 
     FIG. 22 is a graph showing the relationship between the coersivity and the temperature for a medium No. 4; 
     FIGS. 23 and 24 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of the medium No. 4; 
     FIG. 25 is a graph showing the relationship between the coersivity and the temperature for a medium No. 5; 
     FIGS. 26 and 27 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of the medium No. 5; 
     FIG. 28 is a graph showing the relationship between the coersivity and the temperature for a medium No. 6; 
     FIGS. 29 and 30 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of the medium No. 6; 
     FIG. 31 is a graph showing the relationship between the coersivity and the temperature for a medium No. 7; 
     FIGS. 32 and 33 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of the medium No. 7; 
     FIG. 34 is a graph showing the relationship between the coersivity and the temperature for a medium No. 8; 
     FIGS. 35 and 36 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of the medium No. 8; 
     FIG. 37 is a graph showing the relationship between the coersivity and the temperature for a medium No. 9; and 
     FIGS. 38 and 39 are diagrams showing the changes in the direction of magnetization in erasing and recording modes of the medium No. 9. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described by way of examples. However, the present invention is not limited to the following examples. 
     EXAMPLE 1 
     One of Media No. 1 
     A vacuum vapor deposition apparatus was used in this example. It included two crucibles as evaporation sources shown in Table 2 and two units for heating the respective sources with an electron beam. 
     A 1.2-mm thick, 200-mm diameter glass substrate was placed inside a chamber of the apparatus. The chamber of the apparatus was temporarily evacuated to a vacuum of 1×10 -6  Torr. or less. Thereafter, deposition was performed at a deposition rate of about 3 Å/sec while maintaining the chamber at a vacuum of 1 to 2×10 -6  Torr. then, a 1,000-Å first layer (recording layer) of Gd 14  Gy 12  Fe 74  was formed on the substrate. Subsequently, the evaporation sources were exchanged while maintaining the vacuum state. Then, deposition was performed again to form a 2,000-Å thick second layer (reference layer) of Gd 24  Tb 3  Fe 73 . 
     The first and second layers were perpendicular magnetic layers. 
     In this way, a double-layered recording medium No. 1 belonging to Class 1 (i.e., P-type, Quadrant I, and Type 1) was manufactured. 
     Table 2 below shows the manufacturing conditions and characteristics of the medium No. 1. 
     
                       TABLE 2______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Evaporation Source        GdDy alloy    GdTb alloy(2 Elements) Fe            FeFilm Thickness (Å)        1000          2000Ms (emu/cc)  100           170Hc (Oe)      5600          350Tc (°C.)        150           210Tcomp. (°C.)        110           170σ.sub.w        0.8 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =170° C. and T H  =230° C. (see Example 13), this medium satisfies: 
     
         T.sub.R &lt;Tcomp.1&lt;T.sub.C1 ≈T.sub.L ≈Tcomp.2.&lt;T.sub.C2 ≈T.sub.H                                          Formula 11 
    
     
         H.sub.C1 =5,600 Oe&gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2)=868 Oe       Formula 12 
    
     In Formula 15, since 
     
         H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)=5,100 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=468 Oe 
    
     if an initial field (Hini.) is 600 Oe, the medium No. 1 can satisfy Formula 15. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 1 satisfies: 
     
         H.sub.C1 =5,600 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=400 OeFormula 13 
    
     
         H.sub.C2 =350 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=118 OeFormula 14 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=600 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=600 Oe is applied in the &#34;A direction&#34; (↑), an over-write operation is enabled. Note that since the levels and the directions of the fields Hb and Hini. are equal to each other, the recording apparatus which uses a single applying means for applying both the fields Hb and Hini. can be used. 
     EXAMPLE 2 
     One of Media No. 2 
     A 500-Å thick first layer (recording layer) of Tb 27  Fe 73  and a 2,000-Å thick second layer (reference layer) of Gd 24  Tb 3  Fe 73  were sequentially formed on a substrate in the same manner as in Example 1. Then, a medium No. 2 belonging to Class 2 (i.e., P type, Quadrant I, and Type 2) was manufactured. 
     Table 3 below shows the manufacturing conditions and characteristics of the medium No. 2. 
     
                       TABLE 3______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Evaporation Source        Tb            GdTb alloy(2 Elements) Fe            FeFilm Thickness t(Å)        500           2000Ms (emu/cc)  120           170Hc (Oe)      7000          350Tc (°C.)        135           210Tcomp. (°C.)        None          170σ.sub.w        1.0 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =150° C. and T H  =230° C. (see Example 14), this medium satisfies: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L ≈Tcomp.2&lt;T.sub.C2 ≈T.sub.H                                          Formula 16 
    
     
         H.sub.C1 =7,000 Oe&gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2)=1,330 Oe     Formula 17 
    
     In Formula 20, since 
     
         H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)=6,167 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=497 Oe 
    
     if an initial field (Hini.) is 600 Oe, the medium No. 2 can satisfy Formula 20. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 2 satisfies: 
     
         H.sub.C1 =7,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=833 OeFormula 18 
    
     
         H.sub.C2 =350 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=147 OeFormula 19 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=600 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=600 Oe is applied in the &#34;A direction&#34; (↑), an over-write operation is enabled. Note that since the levels and the directions of the fields Hb and Hini. are equal to each other, the recording apparatus which uses a single applying means for applying both the fields Hb and Hini. can be used. 
     EXAMPLE 3 
     One of Media No. 3 
     A 500-Å thick first layer (recording layer) of Gd 23  Tb 3  Fe 74  and a 1,000-Å thick second layer (reference layer) of Tb 28  Fe 65  Co 7  were sequentially formed on a substrate in the same manner as in Example 1. Then, a medium No. 3 belonging to Class 3 (i.e., P type, Quadrant I, and Type 3) was manufactured. 
     Table 4 below shows the manufacturing conditions and characteristics of the medium No. 3. 
     
                       TABLE 4______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Evaporation Source        GdTb alloy    Tb(2 Elements) Fe            FeCo alloyFilm Thickness t(Å)        500           1000Ms (emu/cc)   90            180Hc (Oe)      8000          3000Tc (°C.)        150            200Tcomp. (°C.)        120           Noneσ.sub.w        1.0 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =170° C. and T H  =220° C. (see Example 15), this medium satisfies: 
     
         T.sub.R &lt;Tcomp.1&lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.HFormula 21 
    
     
         H.sub.C1 =8,000 Oe&gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2)=4,389 Oe     Formula 22 
    
     In Formula 25, since 
     
         H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)=6,889 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=3,278 Oe 
    
     if an initial field (Hini.) is 4,000 Oe, the medium No. 3 can satisfy Formula 25. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 3 satisfies: 
     
         H.sub.C1 =8,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=1,111 OeFormula 23 
    
     
         H.sub.C2 =3,000 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=278 OeFormula 24 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=4,000 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=300 Oe is applied in the &#34;non-A direction&#34; (↓), an over-write operation is enabled. 
     EXAMPLE 4 
     One of Media No. 4 
     A 1,000-Å thick first layer (recording layer) of Tb 13  Dy 13  Fe 74  and a 1,000-Å thick second layer (reference layer) of Gd 14  Dy 14  Fe 72  were sequentially formed on a substrate in the same manner as in Example 1. Then, a medium No. 4 belonging to Class 4 (i.e., P type, Quadrant I, and Type 4) was manufactured. 
     Table 5 below shows the manufacturing conditions and characteristics of the medium No. 4. 
     
                       TABLE 5______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Evaporation Source        TbDy alloy    GdDy alloy(2 Elements) Fe            FeFilm Thickness t(Å)        1000          1000Ms (emu/cc)   80            160Hc (Oe)      8000          3000Tc (°C.)         100           140Tcomp. (°C.)        None          Noneσ.sub.w        0.8 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =120° C. and T H  =160° C. (see Example 16), this medium satisfies: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.HFormula 26 
    
     
         H.sub.C1 =8,000 Oe&gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2)=3,750 Oe     Formula 27 
    
     In Formula 30, since 
     
         H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)=7,500 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=3,250 Oe 
    
     if an initial field (Hini.) is 4,000 Oe, the medium No. 4 can satisfy Formula 30. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 4 satisfies: 
     
         H.sub.C1 =8,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=500 OeFormula 28 
    
     
         H.sub.C2 =3,000 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=250 OeFormula 29 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=4,000 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=300 Oe is applied in the &#34;non-A direction&#34; (↓), an over-write operation is enabled. 
     EXAMPLE 5 
     One of Media No. 5 
     A 500-Å thick first layer (recording layer) of Gd 13  Dy 13  Fe 74  and a 600-Å thick second layer (reference layer) of Tb 18  Fe 74  Co 8  were sequentially formed on a substrate in the same manner as in Example 1. Then, a medium No. 5 belonging to Class 5 (i.e., A type, Quadrant II, and Type 3) was manufactured. 
     Table 6 below shows the manufacturing conditions and characteristics of the medium No. 5. 
     
                       TABLE 6______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Evaporation Source        GdDy alloy    Tb(2 Elements) Fe            FeCo alloyFilm Thickness t(Å)        500           600Ms (emu/cc)   90           150Hc (Oe)      6000          3000Tc (°C.)        145           200Tcomp. (°C.)        120           Noneσ.sub.w        0.8 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =165° C. and T H  =210° C. (see Example 17), this medium satisfies: 
     
         T.sub.R &lt;Tcomp.1&lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.HFormula 31 
    
     
         H.sub.C1 =6,000 Oe&gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)|=3,445 OeFormula 32 
    
     In Formula 35, since 
     
         H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)=6,889 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=3,444 Oe 
    
     if an initial field (Hini.) is 4,000 Oe, the medium No. 5 can satisfy Formula 35. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 5 satisfies: 
     
         H.sub.C1 =6,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=889 OeFormula 33 
    
     
         H.sub.C2 =3,000 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=444 OeFormula 34 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=4,000 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=300 Oe is applied in the &#34;non-A direction&#34; (↓), an over-write operation is enabled. 
     EXAMPLE 6 
     One of Media No. 6 
     A 3-element RF magnetron sputtering apparatus was used and included three targets Tb, Fe, and FeCo alloy shown in Table 7. Targets Tb and Fe were first used, and Tb and FeCo alloy were then used. A 1.2-mm thick, 200-mm diameter glass substrate was placed inside a chamber of the apparatus. 
     The chamber of the apparatus was temporarily evacuated to a vacuum of 7×10 -7  Torr. or less, and 5×10 -3  Torr. of Ar gas was introduced therein. Then, sputtering was performed at a deposition rate of about 2 Å/sec. Thereby, a 500-Å thick first layer (recording layer) of Tb 27  Fe 73  was formed on the substrate. Subsequently, the targets were replaced while maintaining the vacuum state. Thereafter, sputtering was performed again to form a 1,000-Å thick second layer of Tb 18  Fe 74  Co 8  on the first layer. Note that the first and second layers were perpendicular magnetic films. 
     In this way, a double-layered magnetooptical recording medium No. 6 belonging to Class 6 (i.e., A type, Quadrant II, and Type 4) was manufactured. 
     Table 7 shows the manufacturing conditions and characteristics of the medium No. 6. 
     
                       TABLE 7______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Target       Tb            Tb(2 Elements) Fe            FeCo alloyFilm Thickness t(Å)        500           1000Ms (emu/cc)  120            150Hc (Oe)      7000          3000Tc (°C.)        135            200Tcomp. (°C.)        None          Noneσ.sub.w        1.5 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =155° C. and T H  =220° C. (see Example 18), this medium satisfies: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.HFormula 36 
    
     
         H.sub.C1 =7,000 Oe&gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)|=3,750 OeFormula 37 
    
     In Formula 40, since 
     
         H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)=8,250 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=3,500 Oe 
    
     if an initial field (Hini.) is 4,000 Oe, the medium No. 6 can satisfy Formula 40. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 6 satisfies: 
     
         H.sub.C1 =7,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=1,250 OeFormula 38 
    
     
         H.sub.C2 =3,000 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=500 OeFormula 39 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=4,000 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=300 Oe is applied in the &#34;non-A direction&#34; (↓), an over-write operation is enabled. 
     EXAMPLE 7 
     One of Media No. 7 
     A 1,000-Å thick first layer (recording layer) of Tb 21  Fe 79  and a 1,000-Å thick second layer (reference layer) of Tb 18  Fe 74  Co 8  were sequentially formed on a substrate in t he same manner as in Example 6. Then, a medium No. 7 belonging to Class 7 (i.e., P type, Quadrant III, and Type 4) was manufactured. 
     Table 8 shows the manufacturing conditions and characteristics of the medium No. 7. 
     
                       TABLE 8______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Target       Tb            Tb(2 Elements) Fe            FeCo alloyFilm Thickness t(Å)        1000          1000Ms (emu/cc)   70            150Hc (Oe)      7000          3000Tc (°C.)         135           200Tcomp. (°C.)        None          Noneσ.sub.w        1.5 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =155° C. and T H  =220° C. (see Example 19), this medium satisfies: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.HFormula 41 
    
     
         H.sub.C1 =7,000 Oe&gt;H.sub.C2 +(σ.sub.w /2 M.sub.S1 t.sub.1)+(σ.sub.w /2 M.sub.S2 t.sub.2)=4,571 Oe     Formula 42 
    
     In Formula 45, since 
     
         H.sub.C1 -(σ.sub.w /2 M.sub.S1 t.sub.1)=5,929 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=3,500 Oe 
    
     if an initial field (Hini.) is 4,000 Oe, the medium No. 7 can satisfy Formula 45. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 7 satisfies: 
     
         H.sub.C1 =7,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=1,071 OeFormula 43 
    
     
         H.sub.C2 =3,000 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=500 OeFormula 44 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=4,000 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=300 Oe is applied in the &#34;non-A direction&#34; (↓), an over-write operation is enabled. 
     EXAMPLE 8 
     One of Meida 8 
     A 500-Å thick first layer (recording layer) of Tb 21  Fe 79  and a 2,000-Å thick second layer (reference layer) of Gd 24  Tb 3  Fe 73  were sequentially formed on a substrate in the same manner as in Example 6. Then, a medium No. 8 belonging to Class 8 (i.e., A type, Quadrant IV, and Type 2) was manufactured. 
     Table 9 shows the manufacturing conditions and characteristics of the medium No. 8. 
     
                       TABLE 9______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Target       Tb            GdTb alloy(2 Elements) Fe            FeFilm Thickness t(Å)        500           2000Ms (emu/cc)   70           170Hc (Oe)      7000          350Tc (°C.)        135           210Tcomp. (°C.)        None          170σ.sub.w        1.5 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =155° C. and T H  =230° C. (see Example 20), this medium satisfies: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L ≈Tcomp.2&lt;T.sub.C2 ≈T.sub.H                                          Formula 46 
    
     
         H.sub.C1 =7,000 Oe&gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)|=2,273 OeFormula 47 
    
     In Formula 50, since 
     
         H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)=9,143 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=570 Oe 
    
     if an initial field (Hini.) is 800 Oe, the medium No. 8 can satisfy Formula 50. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 8 satisfies: 
     
         H.sub.C1 =7,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=2,143 OeFormula 48 
    
     
         H.sub.C2 =350 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=220 OeFormula 49 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=800 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=800 Oe is applied in the &#34;A direction&#34; (↑), an over-write operation is enabled. Note that since the levels and the directions of the fields Hb and Hini. are equal to each other, the recording apparatus which uses a single applying means for applying both the fields Hb and Hini. can be used. 
     EXAMPLE 9 
     One of Media 9 
     A 1,000-Å thick first layer (recording layer) of Gd 4  Tb 19  Fe 77  and a 500-Å thick second layer (reference layer) of Tb 29  Fe 61  Co 10  were sequentially formed on a substrate in the same manner as in Example 1. Then, a medium No. 9 belonging to Class 9 (i.e., A type, Quadrant IV, and Type 4) was manufactured. 
     Table 10 shows the manufacturing conditions and characteristics of the medium No. 9. 
     
                       TABLE 10______________________________________      Recording Layer 1                  Reference Layer 2______________________________________Evaporation Source        GdTb alloy    Tb(2 Elements) Fe            FeCo alloyFilm Thickness t(Å)        1000          500Ms (emu/cc)   30           200Hc (Oe)      7000          3000Tc (°C.)         150          220Tcomp. (°C.)        None          Noneσ.sub.w        1.0 erg/cm.sup.2(Room Temperature)______________________________________ 
    
     If T L  =170° C. and T H  =220° C. (see Example 21), this medium satisfies: 
     
         T.sub.R &lt;T.sub.C1 ≈T.sub.L &lt;T.sub.C2 ≈T.sub.HFormula 51 
    
     
         H.sub.C1 =7,000 Oe&gt;H.sub.C2 +|(σ.sub.w /2 M.sub.S1 t.sub.1)-(σ.sub.w /2 M.sub.S2 t.sub.2)|=4,167 OeFormula 52 
    
     In Formula 55, since 
     
         H.sub.C1 +(σ.sub.w /2 M.sub.S1 t.sub.1)=8,667 Oe 
    
     
         H.sub.C2 +(σ.sub.w /2 M.sub.S2 t.sub.2)=3,500 Oe 
    
     if an initial field (Hini.) is 4,000 Oe, the medium No. 9 can satisfy Formula 55. Thus, the direction of magnetization of the reference layer 2 is reversed at the room temperature due to the initial field (Hini.) without reversing that of the recording layer 1. 
     In addition, since the medium No. 9 satisfies: 
     
         H.sub.C1 =7,000 Oe&gt;(σ.sub.w /2 M.sub.S1 t.sub.1)=1,667 OeFormula 53 
    
     
         H.sub.C2 =3,000 Oe&gt;(σ.sub.w /2 M.sub.S2 t.sub.2)=500 OeFormula 54 
    
     if the initial field (Hini.) is removed, magnetization of the layers 1 and 2 can be maintained. 
     Therefore, when the initial field (Hini.)=4,000 Oe is applied in the &#34;A direction&#34; (↑) and the bias field Hb=300 Oe is applied in the &#34;non-A direction&#34; (↓), and over-write operation is enabled. 
     EXAMPLE 10 
     Magnetooptical Recording Apparatus 
     This apparatus is for recording only, and FIG. 3 shows the overall arrangement thereof. 
     The apparatus basically comprises: 
     (a) a rotating means 21 as a means for moving a recording medium 20; 
     (b) an initial field (Hini.) applying means 22; 
     (c) a light source 23 L  for generating a leading low-level erasing laser beam; 
     (d) a light source 23 H  for generating a trailing high-level writing laser beam; 
     (e) a means 24 for pulse modulating, based on binary data to be recorded, a beam intensity of the writing laser beam between (1) high level that sets the medium at a temperature T H  suitable for forming one of a bit having upward magnetization and a bit having downward magnetization, and (2) basis level equal to or lower than the low level; and 
     (f) a bias field (Hb) applying means 25; and 
     (g) a control means 26 for setting the erasing beam at &#34;low level&#34;. 
     The spot diameter of the erasing beam on the medium irradiation surface is 1.3μ, and that of the writing beam is also 1.3μ. The centers of these beams are separated by 3μ. The erasing beam is continuously generated without being modulated in the recording mode. 
     The bias field (Hb) applying means 25 normally comprises an electromagnet or, preferably, a permanent magnet. Alternatively, a stray field from a portion other than recording tracks of the recording medium can be used as the bias field Hb. In this case, the applying means 25 indicates a region of perpendicular magnetic films (first and second layers) of the recording medium 20, which produces the stray field. 
     In this example, a permanent magnet which produced a bias field Hb=300 Oe and had a direction of magnetization along the &#34;non-A direction&#34; (↓) was used as the applying means 25. The permanent magnet 25 had a rod-like shape having a length corresponding to the radius of the disk-shaped medium 20 and was fixed in the apparatus. The permanent magnet 25 was not moved together with a recording head (pickup) including the light source 23 L  and 23 H . This made the pickup light, and allowed high-speed access. 
     An electromagnet or, preferably, a permanent magnet was used as the initial field (Hini.) applying means 22. In this example, a permanent magnet which produced an initial field (Hini.)=4,000 Oe and had a direction of magnetization along the &#34;A direction&#34; (↑) was used. The permanent magnet 22 had a rod-like shape having a length corresponding to the radius of the disk-shaped medium 20 and was fixed in the apparatus. 
     Note that a reproduction apparatus can be added to the recording apparatus of this example as a recording/reproduction apparatus. 
     EXAMPLE 11 
     Magnetooptical Recording Apparatus 
     This apparatus is for recording only, and FIG. 4 shows the overall arrangement thereof. 
     The apparatus basically comprises: 
     (a) a rotating means 21 as a means for moving a recording medium 20; 
     (b) a light source 23 L  for generating a leading low-level erasing laser beam; 
     (d) a light source 23 H  for generating a trailing high-level writing laser beam; 
     (e) a means 24 for pulse modulating, based on binary data to be recorded, a beam intensity of the writing laser beam to obtain (1) high level that sets the medium at a temperature T H  suitable for forming one of a bit having upward magnetization and a bit having downward magnetization, and to obtain (2) basis level equal to or lower than the low level; and 
     (b, f) a bias field (Hb) applying means 25 which is commonly used as an initial field (Hini.) applying means 22; and 
     (g) a control means 26 for settng the erasing beam at &#34;low level&#34;. 
     The spot diameter of the erasing beam on the medium irradiation surface is 1.5μ, and that of the writing beam is also 1.3μ. The centers of these beams are separated by 3μ. 
     When the directions of the bias field (Hb) and the initial field (Hini.) coincide with each other, the applying means 25 can also be used as the applying means 22. More specifically, if the applying means 25 is arranged at a recording position (a spot region irradiated with a beam) at which a magnetic field is to be concentrated, it is impossible to concentrate the magnetic field at a point. In other words, a leaking magnetic field is inevitably applied around the recording point. Therefore, if the leaking magnetic field is utilized, the initial field (Hini.) can be applied before recording. For this reason, in the apparatus of this example, the means 25 served as the means 22. 
     The means 25 and 22 normally comprise electromagnets or, preferably, permanent magnets. In this example, permanent magnets which produced a bias field Hb (Hini.)=600 Oe and had a direction of magnetization along the &#34;A direction&#34; (↑) were used as means 22 and 25. The permanent magnets 22 and 25 had a rod-like shape having a length corresponding to the radius of the disk-shaped recording medium 20. The magnets 22 and 25 were fixed to the apparatus of this example, and were not moved together with a pickup including the light source 23 L  and 23 H . This made the pickup light, and allowed high-speed access. 
     EXAMPLE 12 
     Magnetooptical Recording Apparatus 
     This apparatus is for recording only, and FIG. 4 shows the overall arrangement thereof. 
     This apparatus is basically the same as that in Example 11, except for a magnetic field intensity of a bias field (Hb) applying means 25 which also serves as an applying means 22. 
     In this example, permanent magnets which produced a bias field Hb (Hini.)=600 Oe and had a direction of magnetization along the &#34;A direction&#34; (↑) were used as means 22 and 25. The permanent magnets 22 and 25 had a rod-like shape having a length corresponding to the radius of the disk-shaped recording medium 20. The magnets 22 and 25 were fixed to the apparatus of this example, and were not moved together with a pickup including the light source 23 L  and 23 H . 
     EXAMPLE 13 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 11 (see FIG. 4). First, the recording medium 20 of Example 1 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 8.8 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 9.3 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. Inn this example, the data to be recorded was a signal having a frequency of 1 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 1 MHz. As a result, a 1-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 51 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 5 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 48 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 1-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =230° C. at high level and T L  =170° C. at low level. 
     EXAMPLE 14 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 11 (see FIG. 4). First, the recording medium 20 of Example 2 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 7.6 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 9.3 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 1 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 1 MHz. As a result, a 1-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 52 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 5 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 49 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 1-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =230° C. at high level and T L  =150° C. at low level. 
     EXAMPLE 15 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 10 (see FIG. 3). First, the recording medium 20 of Example 3 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 6.6 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 8.9 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 5 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 5 MHz. As a result, a 5-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 51 dB, and it was confirmed therefrom that the signal was actually recorded. 
     A signel of a frequency of 2 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 54 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 5-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =220° C. at high level and T L  =170° C. at low level. 
     EXAMPLE 16 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 10 (see FIG. 3). First, the recording medium 20 of Example 4 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 4.3 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 6.1 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 5 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 5 MHz. As a result, a 5-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 47 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 2 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a /CN ratio of 50 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 5-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =160° C. at high level and T L  =120° at low level. 
     EXAMPLE 17 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 10 (see FIG. 3). First, the recording medium 20 of Example 5 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 6.4 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 8.4 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 5 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 5 MHz. As a result, a 5-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 48 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 4 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 49 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 5-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =210° C. at high level and T L  =165° C. at low level. 
     EXAMPLE 18 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 10 (see FIG. 3). First, the recording medium 20 of Example 6 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 5.9 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 8.1 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 5 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 5 MHz. As a result, a 5-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 49 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 3 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 51 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 5-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =220° C. at high level and T L  =155° C. at low level. 
     EXAMPLE 19 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 10 (see FIG. 3). First, the recording medium 20 of Example 7 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 5.9 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 8.9 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 5 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 5 MHz. As a result, a 5-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproductuion apparatus, a C/N ratio was 49 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 2 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 52 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 5-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =220° C. at high level and T L  =155° C. at low level. 
     EXAMPLE 20 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 12 (see FIG. 4). First, the recording medium 20 of Example 8 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing means were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 7.8 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 9.3 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 1 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 1 MHz. As a result, a 1-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 52 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 2 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 51 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 1-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =230° C. at high level and T L  =155° C. at low level. 
     EXAMPLE 21 
     Magnetooptical Recording 
     A magnetooptical recording operation was performed using the recording apparatus of Example 10 (see FIG. 3). First, the recording medium 20 of Example 9 was rotated at a constant speed of 8.5 m/sec by the rotating means 21. The two leading and trailing beams were radiated on the medium 20. The leading erasing beam was adjusted by the means 26 to obtain an output of 6.6 mW (on disk) at low level. The trailing writing beam was adjusted by the means 24 to have an output of 8.9 mW (on disk) at high level and to have an output of 0.5 mW (on disk) at basis level. 
     The writing beam was pulse modulated by means 24 in accordance with data to be input. In this example, the data to be recorded was a signal having a frequency of 5 MHz. Therefore, the laser beam was radiated onto the medium 20 while being modulated at the frequency of 5 MHz. As a result, a 5-MHz signal was recorded. When this signal was reproduced by another magnetooptical reproduction apparatus, a C/N ratio was 51 dB and it was confirmed therefrom that the signal was actually recorded. 
     A signal of a frequency of 6 MHz was newly recorded on an already recorded region of the medium 20. 
     When the signal was similarly reproduced, it could be reproduced at a C/N ratio of 49 dB. In this case, a bit error rate was 10 -5  to 10 -6 . In addition, the 5-MHz signal (preceding data) was not reproduced at all. 
     As a result, it was found that an over-write operation was enabled. 
     Note that under these conditions, the temperature of the medium reached T H  =220° C. at high level and T L  =170° C. at low level.