Thermo-magnetic recording method

Abstract: The present invention adopts as the basic structure of its thermomagnetic recording medium an arrangement consisting of a first (11) and a second magnetic thin film (12) having perpendicular anisotropy and a third magnetic thin film (13) having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed therebetween, formed into a laminated structure by being magnetically coupled to the adjoining films in turn, modulates and switches, in accordance with information to be recorded, a first heating condition and a second heating condition, with the medium applied with a predetermined external magnetic field H ex in the direction perpendicular to the plane of the film, the first condition being that for raising temperature of the medium to a first temperature T 1 which is virtually above the Curie temperature T C1 of the first magnetic thin film and not causing reversal of the magnetic moment in the second magnetic thin film and the second condition being that for raising temperature of the same to a second temperature T 2 which is virtually above the Curie temperature T C1 and sufficient to cause reversal of the magnetic moment in the second magnetic thin film, to thereby form an information bit (magnetic domains) in the first magnetic thin film, and adapts during the course the medium is cooled from the heated states such that two states established by the different relationships between the directions of magnetization of the first and second magnetic thin films are finally formed whereby the recording of the information is performed, and thus the present invention brings about conditions enabling alteration of recorded information to other information, that is, enabling so-called overwriting. Especially, by virtue of the third magnetic thin film, the present invention achieves simplification of the apparatus for applying the magnetic field.

Description: BACKGROUND OF THE INVENTION The present invention relates to a thermomagnetic recording method as specified in the precharacterizing part of the claim. A similar method is described in EP-A-0 258 978. In the method to record information by thermomagnetic recording an a recording medium, from which information is reproduced by reading information bits (magnetic domains) formed thereon by virtue of magneto-optical interaction, the recording medium having a magnetic thin film formed of a vertically magnetizable film is subjected in advance to initialization, i.e., to a treatment to orient the magnetization in the medium into one direction perpendicular to the plane of the film, and thereafter, magnetic domains having vertical magnetization in the reverse direction to the initial magnetization are formed by heating the medium locally by irradiation of a laser beam or the like, and thereby, the information is recorded thereon as a binarized information bits. In such a thermomagnetic recording method, when altering recorded information, a process must, prior to the alteration, be performed to erase the recorded information (the process corresponding to the above described initialization), so that a certain time is taken to perform the erasing process, and therefore, recording at a high transmission rate cannot be achieved. As countermeasures against that, there have been proposed various real-time recording methods in which overwriting is made possible and thereby the period of time for performing such an independent erasing process can be eliminated. Among such thermomagnetic recording methods executing the overwrite, hopeful ones, for example, are that applies modulated external magnetic field to the medium and that uses two heads, an erasing head as well as a recording head. In the method using modulated external magnetic field, the recording is performed as disclosed, for example, in Japanese Laid-open Patent Publication No. 60-48806 by applying a magnetic field with the polarity corresponding to the state of an input digital signal current to a recording medium, which is provided thereon with an amorphous ferrimagnetic thin film having an axis of easy magnetization perpendicular to the film plane, at its region irradiated by a temperature raising beam. When it is attempted to achieve a high speed recording at a high information transmission rate by the above described external magnetic field modulation method, an electromagnet operating at the rate, for example, on the order of one MHz becomes necessary, and a problem arises that it is difficult to fabricate such an electromagnet, and even if it is fabricated, consumed power and heat generated thereby become huge, and therefore, it cannot be put to practical use. Meanwhile, the two-head method requires an extra head and the two heads must be located apart, and therefore, such a problem occurs that a heavy-load drive system is required and the system becomes uneconomical and unsuitable for mass production. The present applicant earlier proposed thermomagnetic recording methods intended to solve these problems in Japanese Patent Application Nos. 61-194961 and 61-194962 (corresponding to European Patent Application laid open under the number of EP-A-257 530). The thermomagnetic methods proposed in these applications are such that use a thermomagnetic recording medium provided with a first and a second laminated structure of rare earth-transition metal magnetic thin films and switch and modulate, in accordance with information to be recorded, for example, of "0" and "1", a first heating condition to heat the medium to a first temperature T 1 which is virtually above the Curie temperature T C1 of the first magnetic thin film and not reversing the sub-latice magnetization in the second magnetic thin film and a second heating condition to heat the same to a second temperature T 2 which is above the temperature T C1 and sufficient to reverse the sub-lattice magnetization in the second magnetic thin film, with the medium applied with a required first external magnetic field, so that, in the cooling stage, the direction of sub-lattice magnetization in the first magnetic thin film is brought into agreement with the direction of the sub-lattice magnetization in the second magnetic thin film by virtue of first and second exchange coupling force, whereby recorded bits (magnetic domains), for example, of "0" and "1" are formed in the first magnetic thin film, and the sub-lattice magnetization in the second magnetic thin film is reversed by virtue of a second external magnetic field or by virtue of only the first external magnetic field at room temperature when the composition of the second magnetic thin film has been selected so as to have its compensation temperature between the second temperature T 2 and room temperature, and thereby obtain the conditions to make overwriting possible. Since, throughout the above processes, there is no need of performing a special process (taking a time) for erasing, a high transmission rate can be attained, and thereby, the problems involved in the above described two-head system or the external magnetic field modulation system can be solved. The thermomagnetic recording method according to Japanese Patent Application No. 61-194961 will be described below. The recording of information, for example, of "0" and "1" in this recording method is performed, as shown in FIG. 1 which schematically indicates the above described magnetized states of the first and second magnetic thin films 1 and 2 with small arrows relative to temperature T, by providing, at room temperature T R , a state A with the directions of magnetization in both the magnetic thin films 1 and 2 oriented in one direction and a state B with the same oriented in the reverse directions to each other. And these records are obtained by application of the external magnetic field H ex to the medium and heating the same to the first and second temperatures T 1 and T 2 by laser beam irradiation. For example, a laser beam is first impinged on the position in the state A, with the intensity or time of irradiation of the laser beam modulated in accordance with the recording signal, so that the heating temperature T is brought to the first heating temperature T 1 virtually above the Curie temperature T C1 of the first magnetic thin film 1 and causing no reversal of magnetization in the second magnetic thin film 2 under the influence of the required external magnetic field H ex . By such heating, the first magnetic thin film 1 exhibits a state C where it loses its magnetization but, when the laminated film of the magnetic thin films 1 and 2, after the heating has been finished, is cooled below the temperature T C1 , magnetization is produced in the first magnetic thin film 1. In this case, since it has been previously adapted such that the exchange coupling force with the second magnetic thin film 2 is dominant, the direction of magnetization in the first magnetic thin film 1 is oriented into the same direction as that of the second magnetic thin film 2. Namely, the state A is produced whereby information, for example, of a "0" is recorded. Otherwise, the heating temperature T is brought to the second heating temperature T 2 beyond the above described temperature T 1 and sufficient to reverse the magnetization in the second magnetic thin film 2 with the external magnetic field H ex applied. By performing such heating, a state D in which the first magnetic thin film 1 has lost its magnetization and the second magnetic thin film 2 has reversed its magnetization is brought about. But, when the heating is finished and the laminated film of the magnetic thin films 1 and 2 are cooled below the temperature T C1 , the first magnetic thin film 1 is subjected to the exchange coupling force from the second magnetic thin film 2, whereby a state E, i.e., a magnetized state opposite to the original, initialized state, is produced but by virtue of a subsidiary external magnetic field H sub applied in the vicinity of room temperature T R , the direction of the second magnetic thin film 2 is reversed, and thereby, a magnetized state B with magnetic domain walls 3 formed between both the magnetic thin films 1 and 2, the state B being only different from the magnetized state A in that the magnetization in the first magnetic thin film 1 has been reversed, is brought about, and thus, recording of information, for example, of a "1" is achieved. The recording of information of "0" and "1" is achieved by obtaining the state A and state B as described above. In this case, the light-intensity-modulated overwriting is applicable to both the state A and the state B. More particularly, by having any position of those in the state A and the state B heated to the temperature T 1 or T 2 past the state C, by virtue of selected temperatures T 1 and T 2 as described above, the overwrite of the state A or the state B corresponding to the information "0" or "1" can be achieved no matter whether the original state was the state A or the state B. In the magnetic recording medium of the described structure, the surface between the magnetic thin films 1 and 2 forming the laminated film is under the influence of exchange energy, whereby the magnetic domain walls 3 are formed in the first state B. The domain wall energy σ w is expressed as σ w ≒ 2 A 1 K 1 + A 2 K 2 (A 1 and A 2 , K 1 and K 2 are exchange constants and perpendicular magnetic anisotropic constants of the first and second magnetic thin films 1 and 2.) As the conditions required for achieving the overwrite, the condition under which transition from the state B to the state A does not take place at room temperature (- 20°C to 60°C) is given by H C1 > H W1 = σ w /2M S1 h 1 Also, the condition under which transition from the state B to the state E does not take place is given by H C2 > H W2 = σ w /2M S2 h 2 Further, in the state E, in order that the magnetization in the first magnetic thin film 1 is not reversed by the subsidiary external magnetic field H sub , the following condition must be satisfied: H C1 ± H W1 > H sub (4 1 ) where the sign +/- on the left-hand side becomes sign "+" when the first magnetic thin film 1 is a rare earth metal rich film and the second magnetic thin film 2 is a transition metal rich film, whereas it becomes sign "-" when both the first and the second magnetic thin films 1 and 2 are transmission metal rich. Besides, in order that the transition from the state E to the state B takes place, the condition H C2 + H W2 < H sub (4 2 ) must be satisfied. Further, where the heated temperature is in the vicinity of the Curie temperature T C1 of the first magnetic thin film 1, in order that the transition from the state C to the state A takes place, that is, the direction of magnetization in the first magnetic thin film 1 is brought into agreement with the direction of the magnetization in the second magnetic thin film 2, the condition H W1 > H C1 + H ex must be satisfied. Besides, in order that transition from the state B to the state E does not take place, the condition H C2 - H W2 > H ex must be satisfied. In the above expressions, H W1 and H W2 are quantities defined by the expressions (2) and (3), and H C1 and H C2 , M S1 and M S2 , and h 1 and h 2 respectively are coercive forces, saturation magnetizations, and thicknesses of the first and second magnetic thin films. As apparent from these, in order to satisfy the expressions (2) and (3), it is preferred that the domain wall energy σ w at room temperature is as small as possible, but, when assuming that K ≒ 4 X 10 6 erg/cm 3 , A = 2 x 10 -6 erg/cm, we obtain σ w ≒ 3.6 erg/cm 2 . Meanwhile, actual measurements on the hysteresis loop of the two-layer film give σ w = 3 to 6 erg/cm 2 . Now, assuming that σ w = 5 erg/cm 2 and using H c M s ≒ 0.45 x 10 6 erg/cm 2 and H ex = 2 kOe, we obtain h 2 = 1100Å, H C2 = 4kOe, and H W2 ≦ 2kOe as approximate values to satisfy the condition of the expression (6) at room temperature T R , i.e., to satisfy the condition H C2 - H W2 > 2kOe. Thus, a problem is posed that the thickness h 2 of the second magnetic thin film 2 becomes large and the subsidiary external magnetic field H sub becomes large from the expression (4 2 ). OBJECTS OF THE INVENTION It is an object of the present invention to provide a thermomagnetic recording method in which higher Kerr rotation angle is obtained upon playback. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing states of magnetization in a thermomagnetic recording medium used in a thermomagnetic recording method previously applied by the present applicant; FIGs. 2, 4, and 5 are each a sectional view of a thermomagnetic recording medium, wherein Fig. 5 shows an embodiment of the invention; FIGs. 3, 7, and 9 are each a schematic diagram showing states of magnetization in a thermomagnetic recording medium used in a thermomagnetic recording method of the present invention; and FIGs. 6 and 8 are temperature characteristics of magnetic domain wall energy and coercive force energy. DESCRIPTION OF THE PREFERRED EMBODIMENTS A thermomagnetic recording method is adapted such that the above described domain wall energy σ W at room temperature is made small and the temperature characteristic of σ W satisfying the above expression (5) is improved to thereby reduce the film thickness of the second magnetic thin film 2 and lower the subsidiary external magnetic field H sub . More particularly, a thermomagnetic recording medium 10 as shown in FIG. 2 is prepared. The thermomagnetic recording medium 10 is provided thereon with a laminated film 14 made up of first and second magnetic thin films 11 and 12 having perpendicular magnetic anisotropy with a third magnetic thin film 13 having an in-plane magnetic anisotropy or a small amount of perpendicular magnetic anisotropy sandwiched therebetween, these films being magnetically coupled and laminated in turn to the adjoining one. The third magnetic thin film 13 is preferred, even if it has perpendicular magnetic anisotropy, to have sufficiently small perpendicular magnetic anisotropy as against the perpendicular magnetic anisotropy of the first and second magnetic thin films 11 and 12, as low as, for example, 1 x 10 6 erg/cm 3 in perpendicular magnetic anisotropy constant. Recording of information is performed on the recording medium 10 as shown in FIG. 3, in the same way as described with reference to FIG. 1, by heating the laminated film with laser beam irradiation up to the first and second temperatures T 1 and T 2 . More particularly, a first heating condition to raise the temperature to a first temperature T 1 virtually above the Curie temperature T C1 of the first magnetic thin film 11 and causing no reversal of the magnetic moment in the second magnetic thin film 12 and a second heating condition to raise the temperature to a second temperature T 2 above the Curie temperature T C1 of the first magnetic thin film 11 and sufficient to reverse the magnetic moment in the second magnetic thin film 12 are modulated in accordance with the information signals to be recorded and the heated positions on the medium are cooled down so that the above described state A and state B are obtained there. As described above, the recording of information is achieved by bringing about certain states of magnetization in the first and second magnetic thin films 11 and 12. However, by having the third magnetic thin film 13 sandwiched in-between the two films, the domain wall energy σ W between the first and second magnetic thin films 11 and 12 can be controlled and it is thereby made easier to satisfy the above mentioned expressions (2), (3) and (4 2 ). More particularly, according to the present invention, the states A and B are brought about via the states A - E as shown in FIG. 3, in the same way as described with FIG. 1. That is, the recording of information by the state A in which the first and second magnetic thin films 11 and 12 are magnetized in the same direction and the state B in which they are magnetized in the reverse directions is performed, and at this time, by virtue of existence of the third magnetic thin film 13, the state of formation of the interface domain walls is stabilized, whereby the margin in designing the characteristics of the magnetic thin films are expanded, the domain wall energy is lowered, and the subsidiary external magnetic field required for the transition from the state E to the state B can be decreased. The thermomagnetic recording medium 10 is formed, as shown in FIG. 2, of a light transmitting substrate 15 of a glass plate, an acrylic plate, or the like provided with a laminated film 14 deposited on one surface thereof, via a transparent dielectric film 16 serving as a protecting film or an interference film, by, for example, continuous sputtering of the first magnetic thin film 11, the third magnetic thin film 13, and the second magnetic thin film 12 in turn, the laminated film being covered with a protection film 17 of a nonmagnetic metallic film or a dielectric film. In the thermomagnetic recording medium 10, however, the dielectric film 16 and the protecting film 17 may be omitted. It is further required to overcome the problem of the instability occurring when a reproducing layer, i.e., a magneto-optical reproducing medium having a large Kerr rotation angle, is provided for the above described three-layer basic structure. To meet the aforesaid necessity is used a thermomagnetic recording medium S 1 which as shown in a schematic sectional view of FIG. 4 includes a laminated film consisting of a first magnetic thin film 11 formed of a first component film 11 1 and a second component film 11 2 , each thereof having perpendicular magnetic anisotropy, a second magnetic thin film 12 having perpendicular magnetic anisotropy, and a third magnetic thin film 13 having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed between the first component film 11 1 of the first magnetic thin film 11 and the second magnetic thin film 12, formed into a laminated structure being magnetically coupled to the adjoining films in turn. The Curie temperature T C12 of the second component film 11 2 is set to be higher than the Curie temperature T C11 of the first component film 11 1 of the first magnetic thin film 11. A first heating condition for heating the medium to a temperature T 1 which is in the vicinity of the Curie temperature T C11 , not causing reversal of the magnetic moment in the second magnetic thin film 12, and sufficient to change the magnetic moment in the second component film 11 2 of the first magnetic thin film 11 in compliance with the magnetic moment in the second magnetic thin film 12 and a second heating condition for heating the same to a temperature T 2 which is above the Curie temperature T C12 and sufficient to cause reversal of the magnetic moment in the second magnetic thin film 12 are modulated in accordance with an information signal to be recorded, whereby, while the medium is cooled from the heated states, record magnetization is formed also in the first component film 11 1 in compliance with the magnetization in the second component film 11 2 of the first magnetic thin film 11. Further, to meet the aforesaid requirement is used a thermomagnetic recording medium S 2 which as shown in a schematic sectional view of FIG. 5 includes a laminated film consisting of a first magnetic thin film 11 formed of a first component film 11 1 and a second component film 11 2 , each thereof having perpendicular magnetic anisotropy, a second magnetic thin film 12 having perpendicular magnetic anisotropy, and a third magnetic thin film 13 having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed between the first component film 11 1 of the first magnetic thin film 11 and the second magnetic thin film 12, formed into a laminated structure being magnetically coupled to the adjoining films in turn, further having a magneto-optical reproducing thin film 18 disposed in the front of the first component film 11 1 of the first magnetic thin film 11 magnetically coupled thereto. The Curie temperature T C12 of the second component film 11 2 is set to be higher than the Curie temperature T C11 of the first component film 11 1 of the first magnetic thin film 11, and further, the magneto-optical reproducing film 18 is adapted to satisfy 2M SR h R H CR + 2M S11 h 11 H C11 < σ wa + 2M S12 h 12 H C12 (where M SR , M S11 and M S12 ; h R , h 11 , and h 12 ; H CR , H C11 , and H C12 are saturation magnetization, film thickness, and coercive force of the magneto-optical reproducing thin film 18, first, and second component films 11 1 and 11 2 , respectively, and σ wa is domain wall energy between the second component film 11 2 and the second magnetic thin film 12) and having a larger Kerr rotation angle  K than the first component film 11 1 . A first heating condition for heating the medium to a temperature T 1 which is in the vicinity of the Curie temperature T C11 , not causing reversal of the magnetic moment in the second magnetic thin film 12, and sufficient to change the magnetic moment in the second component film 11 2 of the first magnetic thin film 11 in compliance with the magnetic moment in the second magnetic thin film 12 and a second heating condition for heating the same to a temperature T 2 which is above the Curie temperature T C12 and sufficient to cause reversal of the magnetic moment in the second magnetic thin film 12 are modulated in accordance with an information signal to be recorded, whereby, while the medium is cooled from the heated states, record magnetization is formed also in the first component film 11 1 and magneto-optical reproducing thin film 18 in compliance with the magnetization in the second component film 11 2 of the first magnetic thin film 11. In both the above described structures, the Curie temperatures T C11 and T C12 of the first and second component films 11 1 and 11 2 of the first magnetic thin film 11 and the Curie temperatures T C2 and T C3 of the second and third magnetic thin films 12 and 13 are selected to be T C11 < T C12 < T C3 , T C2 . The above described first example of FIG. 4 is characterized in that the first magnetic thin film 11 is formed of the first and second component films 11 1 and 11 2 , and the Curie temperatures T C11 and T C12 of the component films 11 1 and 11 2 are selected to be such that the Curie temperature T C12 of the second component film 11 2 is higher than the other, i.e., T C11 < T C12 . According to this example, the effective coercive force energy of the first magnetic thin film 12 of a two-layer structure is given, as shown, for example, in FIG. 6, by the sum total of the energy of the first and second component films 11 1 and 11 2 having different Curie temperatures, i.e., sum total of E HC11 (= 2M S11 h 11 H C11 ) represented by the curve 204 11 and E HC12 (= 2M S12 h 12 H C12 ) represented by the curve 204 12 , that is, it is represented by the curve 241 which has an inflection point in the vicinity of the Curie temperature T C11 and shows a steeper temperature characteristic on the side lower than T C11 toward room temperature. Meanwhile, E wa has a linear temperature characteristic as shown by the curve 242, and hence, the difference therebetween becomes large at the temperature lower than the temperature T 1 , whereby the recorded information bits, i.e., magnetic domains, can be steadily retained, and further, the temperature T 1 where both the characteristic curves 242 and 241 intersect can be prevented from greatly varying even when some variations are made in E wa , E HC11 , and E HC12 in the manufacturing process of the thermomagnetic recording media. Further, at the high temperature in the vicinity of T 1 , the characteristic of the first magnetic thin film 11 depends only on the characteristic of the second component film 11 2 , so that the effective thickness of the first magnetic thin film 11 is reduced to the small thickness h 12 only of the second component film 11 2 . Further, according to the example described in FIG. 22, the structure used therein is provided with a magneto-optical reproducing thin film 18 having a large Kerr rotation angle  K added to the above described structure and adapted to satisfy the above described expression (11). Hence, in overwriting, the first magnetic thin film 11 is prevented from becoming unstable affected by the direction of the magnetization in the magneto-optical reproducing thin film 18 having a high Curie temperature T C1 and the first component film 11 1 is ensured to form recorded magnetization therein in compliance with the second component film 11 2 in the vicinity of the Curie temperature T C11 of the first component film 11 1 . A thermomagnetic recording medium S 1 used here is provided, as shown in FIG. 4, by depositing, in turn, first and second component films 11 1 and 11 2 constituting a first magnetic thin film 11, a third magnetic thin film 13, and a second magnetic thin film 12, through a dielectric film 16 serving as a protecting film or interference film, over one side of a light transmitting substrate 15 made of a glass plate, acrylic plate, or the like. The first and second component films 11 1 and 11 2 of the first magnetic thin film 11 are rare earth-transition metal thin films made of a material having rather great perpendicular magnetic anisotropy K U , such as TbFeCo. Both the component films 11 1 and 11 2 may be made of either a rare earth rich film or a transition metal rich film but the following conditions must be satisfied. That is, the condition σ wa > 2M S12 h 12 H C12 + 2M S12 h 12 H ex must be satisfied at the temperature right below the Curie temperature T C12 of the second component film 11 2 , and the condition σ wb > 2M S11 h 11 H C11 + 2M S11 h 11 H ex must be satisfied at the temperature right below T C11 (where σ wb is the domain wall energy density on the interface between the first component film 11 1 and the second component film 11 2 , and H ex is the external magnetic field, i.e., the external recording magnetic field). A thermomagnetic recording medium S 2 is provided with the above described structure of the thermomagnetic recording medium S 1 and additionally a magneto-optical reproducing thin film 18 as shown in FIG. 5. More particularly, also in the thermomagnetic recording medium S 2 , a light transmitting substrate 15 made of a glass plate, acrylic plate, or the like is used as shown in FIG. 5, and a magneto-optical reproducing thin film 18, first and second component films 11 1 and 11 2 constituting a first magnetic thin film 11, a third magnetic thin film 13, and a second magnetic thin film 12, are deposited, in turn, through a dielectric film 16 serving as a protecting film or interference film, over one side of the substrate. The depositing of the films 16, 11 1 , 11 2 , 13, and 12, or the films 16, 18, 11 1 , 11 2 , 13, and 12 of the thermomagnetic recording media S 1 and S 2 are each achieved by making laminating sputtering in a successive or simultaneous manner through the use, for example, of a magnetron type sputtering apparatus performing, for example, multiple-source sputtering, namely, sputtering from multiple-source targets. The third magnetic thin film 13 of each of the thermomagnetic recording media S 1 and S 2 is desired to have in-plane magnetic anisotropy or lower perpendicular magnetic anisotropy than that of the first and second magnetic thin films 11 and 12, as low as, for example, 1 X 10 6 erg/cm 3 at room temperature and, in addition, be made of a rare earth rich metallic film having the temperature characteristic of its effective magnetic anisotropy constant K being convex upward or linear and the saturation magnetization M S at room temperature being 0 to 450 emu/cm 3 . In the media S 1 and S 2 , the second magnetic thin film 12 can be formed of GdTbFeCo having great perpendicular magnetic anisotropy. The thermomagnetic recording medium S 1 will first be described mentioning an embodiment of it. A thermomagnetic recording medium S 1 of the structure as shown in FIG. 4 including the magnetic thin films 11 1 , 11 2 , 13, and 12 having the compositions, magnetic characteristics, and film thicknesses as shown in Table 1 below was prepared. Thin Film Compo-Sition Magnetization (emu/cc) Coercive Force (kOe) Curie Temp. (°C) Film Thickness (Å) (11 1 ) TbFeCo 30 15 170 250 (11 2 ) TbFeCo 20 23 210 250 (13) GdFeCo 400 - 235 150 (12) GdTbFeCo 180 3.2 350 580 The manner of operations when thermomagnetic recording is made with the above described thermomagnetic recording medium S 1 will be described with reference to the drawing of FIG. 7 showing magnetized states. In FIG. 7, the directions of the spin of the transition metal Fe in the films 11 1 , 11 2 , 13, and 12 are indicated by arrows. In this case, the directions of the external magnetic field H ex and the subsidiary external magnetic field H sub differ with the composition of the second magnetic thin film 12, but the illustrated case is where a transition metal rich film is used for it. The first temperature T 1 is selected, for example, to be right below the Curie temperature T C11 of the first component film 11 1 , and the second temperature T 2 is selected to be above the Curie temperature T C12 of the second component film 11 2 . Also in this case, in the same manner as described with reference to FIG. 1 and FIG. 3, information is recorded by the states A and B, that is, by the state A wherein the first and the second magnetic thin films 11 and 12 are magnetized oriented in the same direction and the state B wherein the same are magnetized oriented in the reverse directions. In this case, once the medium has been heated at the first temperature T 1 (the erasing temperature T 1 in FIG. 6) by irradiation, for example, of a laser beam, the direction of the spin in the second component film 11 2 in the process of the medium cooled from that temperature is brought to the state C wherein it is in agreement with that of the second magnetic thin film 12, no matter whether the previous state was A or B, according to the above described expression (9) and the intersection of the curves 241 and 242 in FIG. 23, and in the process cooled down to right below the Curie temperature T C11 of the first component film 11 1 , the direction of the spin in the first component film 111 is brought into agreement with that of the second component film 11 2 by the arrangement made so that expression (9) is satisfied. Thus, no matter whether the state is A or B, overwriting of the state A is achieved by bringing the medium to the first temperature T 1 . By heating the medium to the second temperature T 2 above the Curie temperature T C2 of the second magnetic thin film 12, namely, above the first and second Curie temperatures T C11 and T C12 , similarly by irradiation, for example, of a laser beam, the direction of the spin in the second magnetic thin film 12 is reversed by virtue of the external magnetic field (recording field) H ex , and in the subsequent cooling stage, the state E is brought about wherein the directions of the spin in the first and second component films 11 1 and 11 2 of the first magnetic thin film 11 are in agreement with the direction of the spin in the second magnetic thin film 12 according to the conditional expressions (8) and (9). And in the state cooled down to room temperature, the state E is changed by virtue of the subsidiary external magnetic field H sub to the state B wherein the direction of the spin in the second magnetic thin film 12 is reversed. In order that this transition takes place, the subsidiary external magnetic field H sub is selected to satisfy the following condition. H sub > H C2 + σ wa / 2M S2 h 2 . In the present embodiment, referring to FIG. 23 showing temperature characteristics of the coercive force energy in the first magnetic thin film 11 and the domain wall energy between the first and the second magnetic thin films, the domain wall energy is kept sufficiently small in the vicinity of room temperature as indicated by the curve 242, and therefore, the subsidiary external magnetic field H sub in expression (10) can be made sufficiently small. Further, in order to stabilize the magnetized state of the first and second component films 11 1 and 11 2 of the first magnetic thin film 11 in the state B, the following condition must be satisfied. M S11 h 11 H C11 + M S12 h 12 H C12 M S11 h 11 + M S12 h 12 > H sub + σ wa 2(M S11 h 11 + M S12 h 12 ) A thermomagnetic recording medium of a threelayer structure of the structure shown in FIG. 4 but the first magnetic thin film therein is formed of a single film was used, as a reference example Compositions, magnetic characteristics, and film thicknesses of the constituent films in this case are shown in Table 2 below. Thin Film Compo-Sition Magnetization (emu/cc) Coercive Force (kOe) Curie Temp. (°C) Film Thickness (Å) (11) TbFeCo 30 15 170 500 (13) GdFeCo 400 - 235 125 (12) GdTbFeCo 180 3.2 350 580 Temperature characteristics of the domain wall energy E wa (= σ wa ) between the first and second magnetic thin films 11 and 12 and the coercive force energy of the first magnetic thin film 11 in this case are shown by the curves 251 and 252, respectively, in FIG. 8. Variations of the erasing temperature T 1 for the above embodiment and the reference example will be considered referring to FIG. 6 and FIG. 8. Variations of the domain wall energy E wa with change in the film thickness h 3 of the third magnetic thin film 13 or the like are such, at the temperature 20°C, that E wa ≒ 2 erg/cm 2 when the film thickness h 3 = 125 Å and that E wa ≒ 1.6 erg/cm 2 when the film thickness h 3 = 150 Å. It is found in the case of FIG. 8 for the reference example 2 that σ wa = 2 erg/cm 2 and the temperature at which E HCl = E wa is obtained is around 130 °C, while in the case of FIG. 6 for the above embodiment, σ wa = 1.6 erg/cm 2 and the temperature T 1 ≒ 165°C. Since 0.2 erg/cm 2 or so of error for σw a generally occurs in the manufacture, it is now assumed that it has become σ wa = 1.8 erg/cm 2 (at 20°C). Then, from FIG. 8, T 1 becomes T 1 ≒ 148°C for the three-layer film of the reference example 2, while it becomes T 1 ≒ 162°C for the embodiment 6 of FIG. 6. Hence, while 18°C of variation in the temperature T 1 is produced in the three-layer film against a change of 0.2 erg/cm 2 in σ wa , that for the four-layer film of the above embodiment is kept to a variation as small as 3°C. Thus, by forming the first magnetic thin film of two layers having different Curie points, it becomes possible, at the time of mass production, to reduce the variations in the temperature T 1 against changes in E wa or E HC . With the above described structure, it is preferred that T C11 - T C12 is 10 - 70°C. This is because, if it is less than 10°C, the effect as described in FIG. 23 is not obtained so much, and, if it exceeds 70°C, i.e., if T C12 becomes too high, it becomes necessary to raise the second temperature T 2 and hence to have large recording power. Thermomagnetic recording media S 2 of the structure as shown in FIG. 5 including the magnetic thin films 18, 11 1 , 11 2 , 13, and 12 having the compositions, magnetic characteristics, and film thicknesses as shown in Table 3 below were prepared. Thin Film Compo-Sition Magnetization (emu/cc) Coercive Force (kOe) Curie Temp. (°C) Film Thickness (Å) (18) GdFeCo 30 0.4 400 h R (11 1 ) TbFeCo 30 15 170 250 (11 2 ) TbFeCo 20 23 210 250 (13) GdFeCo 400 - 235 150 (12) GdTbFeCo 180 3.2 350 580 A thermomagnetic recording medium S 2A was prepared by setting the thickness h R of the magneto-optical reproducing thin film 18 to 75 Å, and a thermomagnetic recording medium S 2B was prepared by setting the thickness hR of the magneto-optical reproducing thin film 18 to 150 Å. These media S 2A and S 2B were structured so as to satisfy the above described expression (7). The manner of operations for thermomagnetic recording using these thermomagnetic recording media S 2A and S 2B is shown in FIG. 9. Referring to FIG. 9, the directions of the spin in the transition metal Fe are shown by arrows drawn in each of the films 18, 11 1 , 11 2 , 13, and 12. Also in this case, the first temperature T 1 was selected to be the Curie temperature T C11 of the first component film 11 1 and the second temperature T 2 was selected to be above the Curie temperature T C12 of the second component film 11 2 . Also in this case, the same as described in FIG. 1 or FIG. 3, recording of information is made by the states A and B, namely, by the state A wherein the first and the second magnetic thin films 11 and 12 are magnetized in the same direction and the state B wherein they are magnetized in the reverse directions. In either of the states, the reproducing thin film 18 is magnetized in the same direction as the first magnetic thin film 11. In this case, if the medium is irradiated, for example, by a laser beam and heated up, for example, to the temperature T 1 right below the Curie temperature T C11 of the first component film 11 1 , the direction of the second component film 11 2 is brought into agreement with that of the second magnetic thin film 12 according to the characteristics shown in FIG. 6. Since, at this time, the Curie temperature T CR of the magneto-optical reproducing thin film 18 is high, either a state C A or a state C B is brought about depending on whether the previous state was the state A or the state B. However, as the medium is cooled toward room temperature T R , even if there has been produced the state C B , it is ensured to be changed to the state A during the cooling stage because conditions satisfying expression (7) have been provided, or more particularly, the sum total of the coercive force energy of the magneto-optical reproducing thin film 18 and the coercive force energy of the first component film 11 1 has been selected to be smaller than the sum total of the domain wall energy σ wa between the second component film 11 2 and the second magnetic thin film 12 and the coercive force energy of the second component film 11 2 . Thus, by subjecting the medium to the first temperature T 1 , recording of information in the state A is achieved. Similarly, by heating the medium with irradiation, for example, of a laser beam up to the second temperature T 2 above the Curie temperature T C2 of the second magnetic thin film 12, i.e., above the first and second Curie temperatures T C11 and T C12 , the direction of the spin in the second magnetic thin film 12 is reversed under the influence of the external magnetic field (recording field) H ex , and in the cooling stage of the medium, the state E is brought about wherein the directions of both the first and the second component films 11 1 and 11 2 of the first magnetic thin film 11 are in agreement with the direction of the second magnetic thin film 12 according to the above described expressions (8) and (9). When the medium is cooled down to room temperature, the state E is changed by the influence of the subsidiary external magnetic field H sub to the state B wherein the spin of the second magnetic thin film 12 is reversed. To effect this, the subsidiary external magnetic field H sub has been selected to satisfy the above described expression (10), and further, the subsidiary external magnetic field H sub can be made sufficiently small the same as described in above first embodiment. An arrangement of the second magnetic thin film 12 formed of a two-layer structure may be applied to the medium S 2 having a magneto-optical reproducing thin film 18 as described in FIG. 5, and in such ways, various modifications of the embodiment other than those described above can be made.

Claims: A thermomagnetic recording method using a thermomagnetic recording medium including a laminated film having a first magnetic thin film (11) formed of a first component film (11 1 ) and a second component film (11 2 ), each thereof having perpendicular magnetic anisotropy, a second magnetic thin film (12) having perpendicular magnetic anisotropy, and a third magnetic thin film (13) having inplane magnetic anisotropy or small perpendicular magnetic anisotropy interposed between said second component film (11 2 ) of said first magnetic thin film (11) and said second magnetic thin film (12) magnetically coupled to the adjoining films in turn, the Curie temperature T C12 of said second component film (11 2 ) of said first magnetic thin film (11) being higher than the Curie temperature T C11 of said first component film (11 1 ) thereof, said method comprising the step of: modulating in accordance with an information signal to be recorded a first heating condition for heating the medium to a temperature T 1 which is in the vicinity of said Curie temperature T C11 , not causing reversal of the magnetic moment in said second magnetic thin film (12), and sufficient to change the magnetic moment in said second component film (11 2 ) of said first magnetic thin film (11) in compliance with the magnetic moment in said second magnetic thin film (12) and a second heating condition for heating the same to a temperature T 2 which is above said Curie temperature T C12 and sufficient to cause reversal of the magnetic moment in said second magnetic thin film (12); characterized in that said thermomagnetic recording medium further includes a magneto-optical reproducing thin film (18) disposed in the front of said first component film (11 1 ) of said first magnetic thin film (11) magnetically coupled thereto, said magneto-opitcal reproducing film (18) satisfying 2M SR h R H CR + 2M S11 h 11 H C11 < σ wa + 2M S12 h 12 H C12 where M SR , M S11 and M S12 ; h R , h 11 , and h 12 ; H CR , H C11 , and H C12 are saturation magnetization, film thickness, and coercive force of said magneto-optical reproducing thin film (18), first, and second component films (11 1 , 11 2 ), respectively, and σ wa is domain wall energy between said second component film (11 2 ) and said second magnetic thin film (12), and having a larger Kerr rotation angle than said first component film, said method further comprising the step of: cooling the medium from the heated states so that record magnetization is formed, in compliance with the magnetization in said second component film (11 2 ) of said first magnetic thin film (11), also in said first component film (11 1 ) thereof and said magneto-optical reproducing thin film (18).

Inventors: ARATANI KATSUHISA, KANEKO MASAHIKO, MUTO YOSHIHIRO, ARATANI, KATSUHISA, KANEKO, MASAHIKO, MUTO, YOSHIHIRO, Aratani, Katsuhisa, c/o Sony Corp., Kaneko, Masahiko, c/o Sony Corp., Muto, Yoshihiro, c/o Sony Corp.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1989
Main classification: G11B11
IPC codes: G11B11