Magneto-optical recording

Abstract: A magneto-optical recording technique is described in which a rare-earth-transition-metal medium may be written to and erased using a single power modulated laser. A bias magnetic field in a direction parallel to the initial magnetisation of the medium is applied to region of the medium at which the laser light is incident. A first bit may be stored by heating the region to a first temperature, below the Curie temperature of the medium, at which the magnetisation of the region becomes parallel to the demagnetising field at the region, and a second bit may be stored by heating the region to a second temperature, higher than the first temperature, at which the magnetisation of the region becomes parallel with the bias field.

Description: This invention relates to magneto-optical (MO) recording, which enables information to be written and erased by the use of a laser beam, and specifically, to an overwritable MO recording method and an apparatus using a single-beam source. MO recording has been put to practical use through a rewritable optical recording approach. MO recording can be classified into two categories, namely the field modulation recording method and the light power modulation recording method. The field modulation recording method permits overwriting to be done easily by the use of a single-beam source, but has the problem that its mechanism becomes complex due to the need for a magnetic head which moves in synchronization with the laser beam. On the other hand, the light power modulation method requires no magnetic head, but has the problem that overwriting cannot be performed easily with a single-beam source. Several studies have been done on ways to overcome the latter problem. The explanation given below is based on two examples. First, JA PUPA (published unexamined patent application) 62,175948 discloses the use of a double-layer film, emission of a high-power laser beam for writing, and emission of a lower-power laser beam for erasing. However, this approach has the problem that the mechanism becomes complex because it requires means for applying a strong initial bias field as well as means for external bias field for writing. In addition, it is complicated to prepare a medium with two layers whose properties are well adjusted. EP-A-316188 shows a biasless magneto-optical system in which a RE-TM medium is heated to a first temperature below the Curie temperature to erase information, and heated to a second temperature around the Curie temperature to write information. EP-A-315415 shows a direct overwrite system in which a RE-TM medium is irradiated with a constant beam of sufficient power level to erase information, and this beam is set at a high level when information is to be written to the medium. JP-A-1 072345 shows a compensation point recording system using an external bias field and a modulated power level laser, in which the medium is either heated to a temperature below the compensation point or heated to a temperature close to the Curie temperature. JP-A-59 113507 describes a system similar to that of EP-A 315415 except that an external bias field is employed. Hang-Ping D. Sheih et al., "Operating Margins for Magneto-Optic Recording Materials with Direct Overwrite Capability", IEEE Transactions on Magnetics, Vol. MAG-23, No. 1, Jan. 1987 discloses the following method: (1) An RE(rare-earth)-rich RE-TM(transition-metal) amorphous film with a compensation temperature not less than 40°C and not exceeding 130°C is used as a recording film. (2) A long pulse of 100 to 400 nsec is used for writing, and a short pulse of 25 to 80 nsec for erasing. The principle of writing and erasing is explained with reference to Figures 6 and 7. Figure 6 shows schematically the properties of an RE-TM film with a compensation temperature higher than room temperature. As the temperature increases, the coercive force (Hc) of the RE-TM film decreases monotonically in the temperature region above the compensation temperature Tcomp, while the saturation magnetization (Ms) first increases monotonically, then decreases monotonically, as shown in the figure. In the temperature region X of Figure 6, the coercive force Hc is low, but the saturation magnetization Ms is still high, so that the demagnetizing field is also high. Therefore, the demagnetizing field may reverse the magnetization of the RE-TM film. It appears that Shieh et al. have ensured that the magnetization of the RE-TM film with property (1) is reversed by the demagnetizing field when the film is heated to temperature region X. The main features of the method disclosed by Shieh et al. are that the demagnetizing field generated by the magnetized RE-TM film itself is utilized, and that no external field is applied. Details of the write and erasure steps are given below. First, it is assumed that the RE-TM film is magnetized initially in an upward direction. The upward magnetization corresponds to storage of "0" and the downward magnetization to storage of "1". In this example, write means reversing the direction of magnetization from upward to downward while erasure means reversing it again from downward to upward. Pulse L, whose duration is 100 to 400 ns, longer than that of the laser pulse L′ used later for erasing, is emitted to the RE-TM film for writing (Figure 7A). As a result, the magnetization of region P irradiated by laser pulse L is reversed by demagnetizing the H′ field which is generated by the surroundings, so that "1" is written (Figure 7B). Next, pulse L′, whose duration is short (25 to 80 ns), is emitted to the center Q of domain P (Figure 7C). As a result, the magnetization of region Q is reversed again by demagnetizing the field H″ generated by the surroundings. Furthermore, the re-reversed domain expands (Figure 7D) and the magnetization of the whole of domain P is finally re-reversed. Thus erasure is completed (Figure 7E). This method has the merit that no external field needs to be applied, but involves the following problems: (a) Short pulses have to be emitted inside the written domains. Therefore, the medium has to be turned one extra time in order to locate the written domains before the erasure step. This is not overwriting in the strict sense of the word. (b) Written domains become large because they are formed with long pulses. Therefore, the recording density cannot be increased. Viewed from one aspect the invention provides a method for overwritable magneto-optical recording on an amorphous rare-earth-transition-metal (RE-TM) recording medium using a modulatable power level laser, the method comprising the steps of: moving said medium relative to said laser, applying a constant external bias magnetic field to said medium in a direction parallel to the initial magnetisation of the medium, emitting a pulsed laser beam of a first power level sufficient to heat said recording medium to within a range of temperatures under the Curie temperature at which the saturation magnetization of the medium is higher than the external bias field and at which the coercive force is sufficiently reduced so that the direction of magnetization of a heated region becomes aligned with the direction of an internal demagnetizing field at said region, in order to write a first bit of information, emitting a pulsed beam of a second higher power level sufficient to heat said recording medium to a temperature close to or higher than the Curie temperature at which the internal demagnetising field becomes lower in strength than the external bias field so that the direction of magnetization of the heated region becomes aligned with the bias magnetic field in order to write a second bit of information. The inventor has recognised that the magnetization reversal by the demagnetizing field is a phenomenon that occurs naturally in RE-TM films with high saturation magnetization Ms even when the coercive force becomes sufficiently small at the time of heating, and that a combination of such films and the novel recording approach makes it possible to overwrite without causing the above problems. In preferred embodiments the recording medium has a Curie temperature not less than [about] 300°C. In other words, it has a sufficiently high saturation magnetization Ms, around 200°C. Another property is that the coercive force Hc drops to a level that allows the magnetization to be reversed, because of the demagnetizing field. It is sufficient if the coercive force is approximately 39789 A/m (500 Oe) or less at 200°C. If those properties are satisfied, the composition of the recording medium can be either on the TM-rich side, where the sublattice magnetization of transition-metals is predominant, or on the RE-rich side, where the sublattice magnetization of the rare-earth metals is predominant. TM-rich RE-TM single-layer films are known. S.Takayama et al. disclose some examples of them in "Magnetic and magneto-optical properties of Tb-Fe-Co amorphous films", J.Appl. Phys. 61 (7), 1 April 1987. However, overwriting with such films by the light power modulation recording technique has never been achieved. The proposed invention makes it possible to overwrite using a TM-rich RE-TM single-layer film. The invention provides for MO recording that allows real overwrite by light power modulation with a combination of a storage medium consisting of a single-layer RE-TM film and a single-laser source. Viewed from a further aspect the invention provides a data recording system comprising an amorphous rare-earth-transition-metal (RE-TM) recording medium, and an overwritable magneto-optical data recording apparatus comprising: a modulatable power level laser for heating a region of said medium as said medium is moved relative to said laser, means for generating a bias magnetic field at said region in a direction parallel to the initial magnetisation of said medium, and means for modulating the laser power between first and second power levels, characterised in that: said laser power modulating means (18) further comprises means for emitting a pulsed laser beam of said first power level which is sufficient to heat said region to within a range of temperatures, below the Curie temperature of said medium at which the saturation magnetization of the medium is higher than the external bias field and at which the coercive force is sufficiently reduced so that the direction of magnetization of a heated region becomes aligned with the direction of an internal demagnetizing field at said region in order to write a first bit of information, and means for emitting a pulsed laser beam of said second laser power level which is sufficient to heat said region to a temperature close to or higher than the Curie temperature at which the internal demagnetising field becomes lower in strength than the external bias field so that the direction of magnetization of the heated region becomes aligned with the bias magnetic field, in order to write a second bit of information. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 schematically illustrates the write and erasure steps of the described embodiment; Figure 2 is a graph showing the coercive force dependence of Tb₁₉Fe₄₅Co₂₉Cr₇ on the temperature: Figure 3 is a graph showing one example of modulating the power level of laser beams; Figure 4 shows bits on the storage medium written in accordance with the described embodiment; Figure 5 is a block diagram showing an MO recording apparatus in accordance with the described embodiment; Figure 6 is a graph showing the coercive force and saturation magnetization dependence of RE-TM film with temperature; and Figure 7 shows a series of steps of the MO recording technique in the prior art using an RE-TM single-layer film. The whole of an RE-TM film is magnetized in the "0" direction before the recording steps. The recording method is to heat the film under a bias field to an appropriate temperature under the Curie temperature Tc, where the coercive force Hc drops sufficiently but the saturation magnetization Ms is still high enough for writing. For erasure, it is heated to a temperature near or above the Curie temperature Tc. Therefore, if a single-laser beam source is used, its power level is set highest for erasure, low for writing, and much lower for reading when magnetization reversal is not required. Figure 1 illustrates the recording method in accordance with the invention by contrasting it with the prior method disclosed by Shieh et al., which was mentioned with reference to Figure 7. First, the writing step is performed by emitting laser beam L with the first power level (Figure 1A). Since the bias field Hex is weak, the magnetization of the heated spot is reversed by the sufficiently strong demagnetizing field H′ generated by the spot itself and the surroundings (Figure 1B). Next, the erasure step is performed by emitting of laser beam L with a second power level higher than the first power level (Figure 1C). Because the film is heated to a temperature near or higher than the Curie temperature, the bias field Hex, although weak, reverses the magnetization again (Figure 1D). If a laser beam with the first power level is emitted as shown in Figure 1B, where "1" has been written, the magnetization direction of the irradiated domain remains "1", because it becomes parallel to the direction of the demagnetizing field H′ which is still high. If the laser beam with the second power level is emitted as shown in Figure 1D, where "0" has been written, the magnetization direction of the irradiated domain remains "0", because the magnetization of the surrroundings drops and the demagnetizing field Hex works. The main features of the invention can be understood from the above description. However, the details of the theory of operation of the invention are still incompletely understood, and therefore the exaplanation with reference to Figure 1 represents the current understanding of the observed behaviour and is intended to give guidance only. In accordance with the invention, the recording density can be improved, because laser beam spots for writing or erasing one bit of information can be made small. Erasure is possible with the aid of the bias field, even if the erasure beam doesn't overlap the written spot exactly. As a result, it becomes unnecessary to locate written domains before the erasure beam is emitted, which makes real overwrite feasible. Details of an experimental example are given below. A TM-rich RE-TM film whose composition was Tb₁₉Fe₄₅Co₂₉Cr₇ was formed on a five-inch glass substrate by the DC magnetron sputtering method. Figure 2 shows the dependence of the coercive force Hc and the saturation magnetization Ms on the temperature. As seen in the figure, the Curie temperature Tc of the film exceeded 300°C and the coercive force at temperatures over 200°C was less than approximately 39789 A/m (500 Oe). A semiconductor laser beam with a wavelength of 822 nm was directed onto a recording medium moving at a linear velocity of 9m/sec. As shown in Figure 3, when the laser spot was over the region where the information was to be recorded, the power was modulated between the write level, higher than the read level, and a much higher erasure level. The power level was set at 1.5 mW for reading, 6 mW for writing, and 8 mW for erasing. Further, a bias field of approximately 7958 A/m (100 Oe) was applied at the position hit by the laser beam spot. Figure 4 shows the appearance of the written bits formed in this way. It was found that bits of arbitary size can be written and erased in accordance with the invention, as can be seen from the figure. Table 1 shows compositions on the TM-rich side, which were found to be overwritable in the same situation as above. Table 2 shows compositions on the RE-rich side, which were also found to be overwritable in the same situation. The values of the coercive force in the tables were measured at room temperature. Here, Nb, Cr, and Ta are transition-metal elements that were added to increase the corrosion resistance. They can be replaced with other elements that can achieve the same purpose, such as Ti and Pt. Figure 5 is a block diagram that gives an overview of the MO recording apparatus. This apparatus has a rotation means (12) as one example of a means of moving the recording medium, a means of applying a bias field (14), a laser beam source (16), and a modulator (18) of pulses to the first level, suitable for writing, or to the second level, suitable for erasing. Of course a known optical system can exist between a laser beam source (16) and a recording medium (12). An electromagnet or, preferably, a permanent magnet can be used as a means of applying a bias field (14). A laser beam source (16), for instance, can be used for initial magnetization of the recording medium by irradiating it with the second power level continuously. In at least the preferred embodiment the invention provides for MO recording which allows real overwrite by light power modulation with a combination of a storage medium consisting of a single-layer RE-TM film and a single-laser source, and for overwritable MO recording by light power modulation with a simple mechanism and with high recording density.

Claims: A method for overwritable magneto-optical recording on an amorphous rare-earth-transition-metal (RE-TM) recording medium using a modulatable power level laser, the method comprising the steps of: moving said medium relative to said laser, applying a constant external bias magnetic field to said medium in a direction parallel to the initial magnetisation of the medium, emitting a pulsed laser beam of a first power level sufficient to heat said recording medium to within a range of temperatures under the Curie temperature at which the saturation magnetization of the medium is higher than the external bias field and at which the coercive force is sufficiently reduced so that the direction of magnetization of a heated region becomes aligned with the direction of an internal demagnetizing field at said region, in order to write a first bit of information, emitting a pulsed beam of a second higher power level sufficient to heat said recording medium to a temperature close to or higher than the Curie temperature at which the internal demagnetising field becomes lower in strength than the external bias field so that the direction of magnetization of the heated region becomes aligned with the bias magnetic field in order to write a second bit of information. A method as claimed in claim 1, wherein the Curie temperature equals or exceeds 300°C. A method as claimed in claims 1 and 2, wherein said medium is an RE-TM film whose coercive force drops to substantially 3979 A/m (50 Oe) and below when heated to 200°C and over. A method as claimed in any of claims 1 to 3, wherein said RE-TM film is TM-rich. A method as claimed in any of claims 1 to 3, wherein said RE-TM film is RE-rich. A method as claimed in any of claims 1 to 5, wherein said medium is a single-layer film. A method as claimed in any of claims 1 to 7, wherein said laser emission is performed with a single-laser power source. A data recording system comprising: an amorphous rare-earth-transition-metal (RE-TM) recording medium (10) and an overwritable magneto-optical data recording apparatus comprising: a modulatable power level laser (16) for heating a region of said medium as said medium is moved relative to said laser, means (14) for generating a bias magnetic field at said region in a direction parallel to the initial magnetisation of said medium, and means (18) for modulating the laser power between first and second power levels, characterised in that: said laser power modulating means (18) further comprises means for emitting a pulsed laser beam of said first power level which is sufficient to heat said region to within a range of temperatures, below the Curie temperature of said medium at which the saturation magnetization of the medium is higher than the external bias field and at which the coercive force is sufficiently reduced so that the direction of magnetization of a heated region becomes aligned with the direction of an internal demagnetizing field at said region, in order to write a first bit of information and means for emitting a pulsed laser beam of said second laser power level sufficient to heat said region to a temperature close to or higher than the Curie temperature at which the internal demagnetising field becomes lower in strength than the external bias field so that the direction of magnetization of the heated region becomes aligned with the bias magnetic field, in order to write a second bit of information. A data recording system as claimed in claim 8, wherein the Curie temperature of said medium equals or exceeds 300°C. A data recording system as claimed in claims 8 or 9, wherein said medium is an RE-TM film whose coercive force drops to about 3979 A/M (50 Oe) and below when heated to 200°C and over. A data recording system as claimed in claims 8 to 10, wherein said RE-TM film is TM rich. A data recording system as claimed in claims 8 to 10, wherein said RE-TM film is RE rich. A data recording system as claimed in claims 8 to 12, wherein said medium is a single-layer film. A data recording system as claimed in claims 8 to 13, wherein said laser comprises a single-laser power source.

Inventors: TAKAYAMA SHINJI
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: G11B11
IPC codes: G11B11