Magneto-optical recording

Abstract: An improved magneto-optic recording medium consists of two exchange-coupled RE-TM amorphous layers (a memory layer 56 and a reference layer 52) whose Curie temperatures are almost the same and only one of which is RE-rich without a compensation temperature, the two layers being laminated directly or indirectly with an intermediate layer 54 that allows exchange-coupling to be interposed. Before recording is carried out, the reference layer is magnetised in one direction. Pulses of energy are emitted from the memory layer side so that the temperature of the reference layer remains below its Curie temperature when one of the bit data is recorded but reaches its Curie temperature when the other of the bit data is recorded.

Description: This invention relates to a method, apparatus, and medium for magneto-optical recording. In magneto-optical recording, various methods for direct overwriting have been proposed to improve the data rate. They include a typical example of a light modulation method using a double-layered film, which is disclosed in JA Published Unexamined Patent Application (PUPA) 62-175948. The contents of this application are also reported by Matsumoto et al. in "Direct Overwrite by Light Power Modulation on Magneto-Optical Double-Layered Media," Digest of 53rd Seminar, the Magnetics Society of Japan (1987), p. 87. The method of JA PUPA 62-175948 needs an external field for initialising the reference layer (initialising field) before recording (by the L process or H process), in addition to an external field applied during recording (a bias field). This makes the apparatus complicated. The above method also involves the problem that data recorded in the memory layer are lost owing to the influence of the strong initialising field. Moreover, this method also involves the problem that strict requirements for the Curie temperatures and coercive forces of respective layers result in less flexibility in the selection of materials and necessitate accurate control of the compositions of materials during the preparation of media. Some methods of eliminating the initialising field have been proposed. Among them, T. Fukami and his colleagues' "Novel direct overwriting technology for magneto-optical disks by exchange-coupled RE-TM quadrilayered films," J. Appl. Phys. 67(9), 1 May 1990 uses quadrilayered films as recording media and makes the Curie temperatures, coercive forces, and inter-layer exchange-coupling forces of respective layers different. In this method, however, the number of layers of the medium is increased to four and these layers need to satisfy certain relative requirements with respect to Curie temperatures, exchange-coupling forces, and so on. Therefore, this method not only fails to remove the restrictions on the compositions of materials, but rather increases them. In order to satisfy the requirements, highly accurate control of the composition of each layer is necessary, and hence the production cost of media becomes a problem affecting their practical use. Further, the total thickness of four layers amounts to a value of the order of 2600 angstroms. This results in lower writing efficiency and hence requires higher laser energy. EP-A-0347882 discloses a magnetooptical recording medium capable of performing an overwrite operation upon radiation of a laser beam which is pulse-modulated between high and low levels. The medium is composed of a substrate and two magnetic layers having different Curie temperatures - a recording layer and a reference layer. The reference layer has low coercivity at room temperature and has a compensation temperature near room temperature. EP-A-0347882 teaches that the direction of magnetisation of one layer is influenced by that of the other layer due to a magnetic coupling force, the direction of magnetisation depending on the type of medium. A method of direct overwriting on an exchange-coupled magnetic triple-layer film of a magneto-optical disk has been described by K. Aratani et al in "overwriting on a magneto-optical disk with magnetic triple layers by means of the light intensity modulation method", Proc. SPIE, vol 1078, 265 (1989), p.258-264. The intermediate layer between recording and storage layers reduces interface wall energy - adjusting the strength of exchange coupling. EP-A-0257530 discloses a thermo-magnetic recording method using a thermo-magnetic recording medium having a superposed layer of magnetically coupled first and second magnetic thin films. It is accordingly an object of the invention to provide an improved medium for magneto-optical recording capable of direct overwriting, using a multi-layered film that does not need an initialising field. It is another object of the invention to provide a method and apparatus for recording data on such a medium. There is further provided a method of writing data on such a medium as set out in claim 1. According to the invention there is provided a magneto-optical medium as set out in claim 6. There is further provided apparatus for writing data on such a medium as set out in claim 11. Before recording, one of the two layers is magnetised in one direction. Either one of the magnetised RE-TM layers is used as the reference layer, and the other is used as the memory layer. Magnetisation of the reference layer has to be carried out only once, before all data writing processes. In contrast, JA PUPA 62-175948 magnetises the reference layer in a desired writing area every time data writing is carried out, that is, before each emission of a laser pulse. Note, therefore, that the magnetisation of the reference layer performed beforehand in the present invention is not the same as the "initialisation" referred to in JA PUPA 62-175948. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 illustrates the temperature dependence of the coercive force of prior art magneto-optical recording media capable of direct overwriting; Figure 2 is an explanatory diagram showing the principle of a prior art magneto-optical recording method capable of direct overwriting; Figure 3 is a diagram illustrating the temperature dependence of the coercive force of magneto-optical recording media capable of direct overwriting according to the invention; Figure 4 is a diagram illustrating the relationship between the composition and magnetic properties of TbFe films; Figure 5 is a diagram illustrating the temperature dependence of the coercive force of a first example of magneto-optical recording media; Figure 6 is an explanatory view of a magneto-optical recording process using the medium shown in Figure 5; Figure 7 is an explanatory diagram of a magneto-optical recording process using the medium shown in Figure 5; Figure 8 is a diagram illustrating the temperature dependence of the coercive force of a second example of magneto-optical recording media; Figure 9 is an explanatory diagram of a magneto-optical recording process using the medium shown in Figure 8; Figure 10 is an explanatory diagram of a magneto-optical recording process using the medium shown in Figure 8; Figure 11 is a diagram illustrating the temperature dependence of the coercive force of a third example of magneto-optical recording media; Figure 12 is an explanatory diagram of a magneto-optical recording process using the medium shown in Figure 11; Figure 13 is an explanatory diagram of a magneto-optical recording process using the medium shown in Figure 11; Figure 14 is an explanatory diagram of a writing process for domains of variable length; Figure 15 is a schematic diagram illustrating the composition of a magneto-optical recording apparatus according to the invention; Figure 16 is a schematic diagram illustrating the composition of a medium used in an experiment; and Figure 17 is a drawn copy of a photomicrograph of a medium surface after overwriting. A recording medium used in the prior art has a recording layer consisting of two layers, a memory layer and a reference layer, which are exchange-coupled. Overwriting is performed by utilising the difference in temperature dependence of the coercive forces of the two layers. Figure 1 shows the magnetic properties, and Figure 2 shows the overwriting process. As shown in Figure 1, the compositions of the two layers are adjusted so that the coercive force of the reference layer (Hr2) is smaller than that of the memory layer (Hr1) at room temperature (Tamb1), and the Curie temperature of the reference layer (Tc2) is higher than that of the memory layer (Tc1). As shown in Figure 2, one of the characteristics of this method is that an initialising field, as well as a bias field for recording, is applied before data is recorded on the memory layer. The directions of the bias field and the initialising field are anti-parallel. The magnitude of the bias field Hb is set at such a small value as to maintain the magnetisation of the reference layer unreversed in the L process, which will be referred to later. On the other hand, the magnitude of the initialising field Hini is set at a value larger than Hr2 but smaller than Hr1. As a result, only the magnetisation of the reference layer is oriented parallel to Hini (downward in the figure). The data recorded in the memory layer is not affected by Hini. For recording, the H process or L process is performed, depending on the bit data to be recorded. In the L process, a low-power laser beam in the form of pulses is emitted so that the temperature of the memory layer TmL becomes Tc1 < TmL < Tc2. At this time, the magnetisation of the reference layer is not reversed. Therefore, the magnetisation of the memory layer is oriented in a direction determined by the exchange-coupling with the reference layer during the cooling process. The term "exchange-coupling" here means a phenomenon such that the subnetwork magnetisations of RE and TM atoms are aligned to those of similar, respectively, even in different layers. Therefore, depending on the compositions of the two layers, the exchange-coupling exerted by one layer during the cooling of the other layer may result in these layers having parallel or anti-parallel directions of magnetisation. Figure 2 shows the case in which the directions of magnetisation of the two layers become parallel as a result of exchange-coupling. In the H process, a high-power laser beam in the form of pulses is emitted, with the result that the temperature of the memory layer TmH becomes Tc2 < TmH. Consequently, during the cooling process, the magnetisation of the reference layer first coincides with the direction of the bias field (upward in the figure). That is, the direction of magnetisation of the reference layer is reversed. When the temperature of the recording layer decreases, the magnetisation of the memory layer is oriented in a direction determined by the exchange-coupling with the reference layer. Since the direction of magnetisation of the reference layer has been reversed from that in the L process, the direction of magnetisation of the memory layer is also reversed from that in the L process. Figure 3 shows two types of temperature dependence of the coercive force of double-layered films used in the present invention. A Curie temperature is a temperature at which the coercive force becomes zero. A compensation temperature is a temperature at which the coercive force diverges. In either case, the following two requirements must be satisfied: (1) The Curie temperatures of the two layers are almost the same; and (2) only one of the layers is RE-rich without a compensation temperature. For example, when a typical magneto-optical recording material, TbFe film, is used for both layers, since Curie temperatures do not substantially vary with the ratio of Tb and Fe, requirement (1) is automatically satisfied. In this respect, considering that the method of JA PUPA 62-175948 controls the combinations and compositions of three to four kinds of elements in order to distinguish the Curie temperatures of two layers, it is easier to prepare media according to the present invention. Requirement (2) can readily be satisfied in the case of a TbFe film by merely setting the composition ratio of Tb at a value above 26% for one of the layers and below 26% for the other layer (see Figure 4). Thus, the invention largely removes restrictions on the materials used for media. In the subsequent description, a medium in the form of a lamination consisting of (1) an RE-rich layer without a compensation temperature and (2) a TM-rich layer is called A-type, whereas a medium in the form of a lamination consisting of (1) an RE-rich layer without a compensation temperature and (2) a RE-rich layer with a compensation temperature is called B-type. In both types, either of the two layers may be used as the memory layer. In the present invention, the layer facing the laser source behaves as the memory layer. Between the memory layer and the reference layer may be interposed an intermediate layer whose thickness does not prevent exchange-coupling between them. Insertion of a layer such as Tb or GdFeCo in order to adjust the strength of exchange-coupling is a known technique. See, for example, K. Aratani et al., "Overwriting on a magneto-optical disk with magnetic triple layers by means of the light intensity modulation method," Proc. SPIE 1078, 265 (1989). The recording method used in the present invention is explained below. Before bit data is written, the magnetisation of the layer that will function as the reference layer is oriented in one direction. As will be understood from the subsequent description, overwriting is possible in whatever state the memory layer is magnetised beforehand. Thus, magnetisation may be oriented in one direction beforehand throughout the entirety of the medium, including the reference layer. Therefore, preliminary magnetisation of the reference layer can be carried out by magnetising it uniformly in a sufficiently strong field before shipment. If the medium is not magnetised before shipment, magnetisation may be carried out by using a magneto-optical recording apparatus referred to later. After magnetisation of the reference layer is complete, bit data is written on the medium in a bias field. It is preferable to use short pulses (of a few nanoseconds) for writing one of the bit data and long pulses (tens of nanoseconds or more) for writing the other of the bit data. As disclosed by T. Ohtsuki et al. in "Direct overwrite by short pulses on double-layered MO media," Conference Digest of Topical Meeting on Optical Data Storage, 172 (1990), when short pulses with a duration of the order of one nanosecond are emitted to the magneto-optical recording medium from the memory layer side, marked temperature gradients are produced in the two layers, and hence only the temperature of the memory layer rises above the Curie temperature, while the reference layer remains at a temperature low enough to maintain the magnetisation. In this case, during the process of cooling the memory layer, a strong exchange-coupling above 1000 (Oe) (1 Oe = 79,6 A/m) is exerted on the memory layer by the reference layer. Therefore, if the magnitude of the bias field is set lower than that of the exchange-coupling, the direction of magnetisation of the memory layer is determined by the exchange-coupling with the reference layer. It is sufficient to apply a field of the order of 500 (Oe) by using, for example, a permanent magnet. In the present method, the process executed while the reference layer is maintained at a low temperature below its Curie temperature is called the L process. In contrast, when long pulses with a duration of tens of nanoseconds or more are emitted, both layers are heated above their Curie temperatures and marked temperature gradients are not induced in the direction of the thickness of the medium. When the heating is finished and the medium has cooled to the Curie temperature, at which magnetisation of the memory layer is determined, magnetisation of each of the two layers is determined according to the direction of the bias field, because the exchange-coupling between the two layers is sufficiently small (if the Curie temperatures of the two layers are absolutely equal, the strength is 0 (Oe)). In the present method, the process that includes a step in which the reference layer is heated to a high temperature near or above its Curie temperature is called the H process. The step in the H process in which the direction of magnetisation of the memory layer is determined is now explained in greater detail. If there is a difference between the Curie temperatures of the two layers, the direction of magnetisation is first determined by the bias field for the layer whose Curie temperature is higher (TcH layer). Therefore, at the instant when the medium has further cooled to the Curie temperature of the other layer (TcL layer), exchange-coupling with the TcH layer as well as the bias field is exerted on the TcL layer. At this time, the larger the difference between the Curie temperatures of the two layers, the greater the strength of the exchange-coupling. In the present invention, however, it is required that the direction of magnetisation of the TcL layer should follow the direction of the bias field. Therefore, the difference between the Curie temperatures of the two layers must be so small that the strength of the exchange-coupling between the two layers does not prevent magnetisation of the memory layer by the bias field. As explained above, it is easy to produce such a double-layered film in which the Curie temperatures are sufficiently close. Presented below is a detailed explanation of how overwriting is carried out when the aforementioned requirements are met. First, with reference to Figures 5 to 7, an overwriting process using an A-type medium is explained. An A-type double-layered film is stable at room temperature in a state in which the directions of magnetisation are oriented anti-parallel to each other by exchange-coupling. It is assumed here that the memory layer 10 is TM-rich and that the reference layer 12 is RE-rich without a compensation temperature. It is also assumed that the magnetisation of the reference layer 12 has been oriented upward beforehand and that the direction of the bias field is upward. When short pulses are emitted, the memory layer 10 is heated to a temperature TmL above its Curie temperature, but the reference layer 12 is heated only to a temperature TrL below its Curie temperature (see Figure 5). As a result, immediately after the heating, only the reference layer 12 in the heated area maintains the magnetisation set before the heating (see (A) in Figure 6). When the memory layer 10 has cooled, the magnetisation is oriented downward by exchange-coupling with the reference layer 12 (see (B) in Figure 6). When long pulses are emitted, both the memory layer 10 and the reference layer 12 are heated to temperatures (TmH and TrH) above their Curie temperatures (see Figure 5). As a result, both layers in the heated area lose their magnetisations set before the heating (see (A) in Figure 7). When the medium has cooled, the magnetisation of both layers is oriented upward by the bias field ((B) in Figure 7). Throughout the L process (Figure 6) and the H process (Figure 7), the magnetisation of the reference layer 12 is maintained upward as in the original state. Even though the memory layer 10 is RE-rich and the reference layer 12 is TM-rich, overwriting is also carried out through the same steps. Next, with reference to Figures 8 to 10, an example of an overwriting process using a B-type medium is explained. The magnetisation direction of a layer with a compensation temperature is changed at the compensation temperature. Therefore, a B-type double-layered film is stable at room temperature in a state in which the directions of magnetisation of the two layers are oriented parallel to each other by exchange-coupling. When the temperature of a layer with a compensation temperature exceeds the compensation temperature, the film becomes stable in a state in which the directions of magnetisation of the two layers are anti-parallel. It is assumed here that the memory layer 20 has a compensation temperature and that the reference layer 22 does not. It is also assumed that the magnetisation of the reference layer 22 has been oriented upward beforehand and that the direction of the bias field is upward. When short pulses are emitted, the memory layer 20 is heated to the temperature TmL above its Curie temperature, but the reference layer 22 is heated only to the temperature TrL below its Curie temperature (see Figure 8). Therefore, immediately after the heating, only the reference layer 22 maintains the magnetisation set before the heating in the heated area (see (A) in Figure 9). When the memory layer 20 is cooled below the Curie temperature, the magnetisation is first oriented downward by the exchange-coupling with the reference layer 22 (see (B) in Figure 9). When the memory layer 20 is further cooled below its compensation temperature, the magnetisation is changed upward, and the medium is stabilised in this state at room temperature. When long pulses are emitted, both the memory layer 20 and the reference layer 22 are heated to temperatures (TmH and TrH) above their Curie temperatures (see Figure 8). As a result, both layers in the heated area lose their magnetisation set before the heating (see (A) in Figure 10). When the medium starts to cool down, the magnetisation of both layers is first oriented upward by the bias field ((B) in Figure 10). When the medium has cooled further and the temperature of the memory layer 20 decreases below its compensation temperature, only the magnetisation of the memory layer 20 is reversed downward ((C) in Figure 10). Throughout the L process (Figure 9) and the H process (Figure 10), the direction of magnetisation of the reference layer 22 remains upward as in the initial state. Next, with reference to Figures 11 to 13, another example of an overwriting process using a B-type medium is explained. It is assumed here that the memory layer 30 does not have a compensation temperature but that the reference layer 32 does. It is also assumed that the magnetisation of the reference layer 32 has been oriented upward beforehand and that the bias field is oriented downward. When short pulses are emitted, the memory layer 30 is heated to a temperature TmL above its Curie temperature, but the reference layer 32 is heated only to a temperature TrL below its Curie temperature (see Figure 11). If the peak temperature TrL is lower than the compensation temperature immediately after the heating, only the reference layer 32 maintains the magnetisation set before the heating in the heated area (see (A) in Figure 12). Therefore, when the memory layer 30 is cooled below its Curie temperature, the magnetisation is oriented upward by exchange-coupling with the reference layer 32 (see (B) in Figure 12). In another case where the peak temperature TrL of the reference layer 32 exceeds the compensation temperature, the direction of magnetisation of the reference layer 32 immediately after the heating is reversed downward ((A') in Figure 12). When the medium is cooled, the magnetisation of the reference layer 32 is again reversed upward. However, regardless of the direction of the magnetisation of the reference layer 32, downward ((B') in Figure 12) or upward ((C') in Figure 12), throughout the process, the final state when the peak temperature TrL exceeds the compensation temperature ((C') in Figure 12) is the same as that of when it does not exceed the compensation temperature ((B) in Figure 12), because exchange-coupling is exerted by the reference layer on the memory layer 30 to orient the magnetisation upward. When long pulses are emitted, both the memory layer 30 and the reference layer 32 are heated to temperatures (TmH and TrH) above their respective Curie temperatures (see Figure 11). Therefore, both layers in the heated area lose their magnetisation set before the heating (see (A) in Figure 13). When the medium starts to cool down, the magnetisation of both layers is first oriented downward by the bias field ((B) in Figure 13). When the medium is cooled further and the temperature of the reference layer 32 decreases below the compensation temperature, only the magnetisation of the reference layer 32 is reversed upward ((C) in Figure 13). In the final state of the L process (Figure 12) and the H process (Figure 13), the magnetisation of the reference layer 32 remains upward as in the initial state. Figure 14 is a schematic diagram of the laser power and the shape of the domains formed upon execution of overwriting. In order to record data of variable length, a continuous series of short pulses is used for one of the bit data, and long pulses different in duration are used for the other of the bit data. The formed domains are shaped like arrow feathers, in the same way as those formed by the field modulation overwrite method. Therefore, compatibility of data is expected between the light modulation overwrite method and the field modulation overwrite method. Figure 15 shows a schematic view illustrating the composition of a magneto-optical recording apparatus according to the invention. This apparatus includes a means 42, which may be a rotating means, for moving a recording medium 40, a means 44 for generating a bias field, a laser source 46, and a means responsive to the bit data to be recorded for modulating the duration and power of laser pulses. Between the laser source 46 and the recording medium 40 may be interposed a known optical system. The means for generating a bias field may be an electromagnet or a permanent magnet. A permanent magnet is more advantageous from the viewpoint of power consumption and heat generation. If the laser source 46 consists of a semiconductor diode, the means 48 modulates the time width and strength of the current pulses to be supplied to the semiconductor diode, in response to the bit data to be recorded. If the apparatus shown in Figure 15 is used to emit a continuous series of laser beams in a bias field and to heat both layers of the medium 40 above their Curie temperatures, the magnetisation can be oriented in one direction throughout the medium, including the reference layer, before the bit data are recorded. Figure 16 shows the construction of a medium used in an experiment on overwriting. On a glass substrate 50 were deposited, by sputtering, a RE-rich reference layer 52 that is 1200 angstroms thick and made of Tb31Fe69, an intermediate layer 54 that is 6 angstroms thick and made of Tb, a TM-rich memory layer 56 that is 800 angstroms thick and made of Tb20.5Fe79.5, and a protective layer 58 that is 700 angstroms thick and made of SiN. The thicknesses of the respective layers were estimated from their sputtering times. The memory layer 56 and the reference layer 52 of the prepared medium were oriented in one direction beforehand. The diameter of the focused laser spot used in the experiment was 0.47 microns (full-width at half-maximum), and the laser beam was emitted from the side of the protective layer 58 of the medium. The strength of the bias field was 480 (Oe), and the numerical aperture of the objective lens of the optical system was 0.95. The L process (writing) and H process (erasure) of overwriting are as shown in Figures 7 and 8, respectively. In the L process, the duration of the laser pulse was 3.8 ns, and the power was 14.5 mW. In the H process, the duration of the laser pulse was 50 ns, and the power was 4.5 mW. In the foregoing conditions, domains of variable length were written and erased by the writing process (L process) and by the erasing process (H process), after both of which the medium was moved by 0.3 microns. More specifically, domains of variable length were first written (write 1), a part of the domains was erased, and subsequently writing was again performed in the erased area (write 2). Figure 17 is a drawn copy of a photograph of the written domains observed with a polarising microscope from the side of the memory layer 56. It is clear from the figure that overwriting was certainly done in accordance with the H process and the L process. Next, the power range that permits writing was examined by varying the power while maintaining the duration of the laser pulse for the writing process (L process) at 3.8 ns. The successful range was from 10 mW to 37 mW. Thus, in the L process, if short pulses of the order of 1 ns are emitted, the power range permitting the memory layer to be heated while maintaining the reference layer 52 at a low temperature is extended, and the margin is increased, accordingly. In the same conditions, an experiment was carried out to examine the power range that permits writing, by varying the power while maintaining the duration of the laser pulse for the writing process (L process) at 50 ns. As a result, writing was achieved in the range from 1.5 mW to 3.1 mW. The reasons for the success of writing appear to be that a 50 ns pulse can induce rather small temperature gradients along the thickness because of the intermediate layer 54, and that the Curie temperatures of the actually prepared reference layer 52 and memory layer 56 do not strictly coincide. Thus, even when the laser pulse in the L process in the present invention is the same long pulse as that in the H process, it is possible to writ one of the bit data. However, the laser power range that permits writing becomes narrower than for a short pulse.

Claims: A method of writing data on a magneto-optical medium (40) consisting of two exchange-coupled RE-TM amorphous layers (10, 12) having similar Curie temperatures and only one of which is RE-rich without a compensation temperature, said two layers being laminated directly or indirectly with an intermediate layer (54) that allows exchange-coupling interposed therebetween, the method comprising the steps of: (a) magnetising one of said two layers in one direction beforehand; (b) moving said medium relative to an energy source in a bias field with said one layer farther from the energy source than the other layer; (c) applying to said medium when recording one of the bit data a pulse of energy such that the temperature of said one layer remains below its Curie temperature while that of said other layer becomes near or above its Curie temperature; and (d) applying to said medium when recording the other of the bit data a pulse of energy such that the temperatures of said two layers become near or above their Curie temperatures, wherein any difference between the Curie temperatures of the two layers is sufficiently small that, on cooling of said medium after applying a pulse of energy and recording the other of the bit data, exchange-coupling between the two layers does not prevent magnetisation of said other layer by the bias field. A method according to claim 1, wherein said two layers of the medium are a TM-rich layer and a RE-rich layer without a compensation temperature. A method according to claim 1, wherein said two layers of the medium are a RE-rich layer with a compensation temperature and a RE-rich layer without a compensation temperature. A method according to claim 1 wherein the source (46) of said energy pulses is a laser. A method according to any one of claims 1 to 4, wherein in said step of recording said one of the bit data, said pulse of energy is of such a short duration as to induce marked temperature gradients in said two layers, whilst in said step of recording the other of the bit data, said pulse of energy is of such a long duration as not to induce marked temperature gradients in said two layers. A magneto-optical medium for use in a method according to claim 1, the medium consisting of two exchange-coupled RE-TM amorphous layers only one of which is RE-rich without a compensation temperature, characterised in that said two layers are laminated directly or indirectly with an intermediate layer that allows exchange-coupling interposed therebetween and in that said two layers have similar Curie temperatures, any difference between the Curie temperatures of the two layers being sufficiently small that, on cooling of said medium after applying a pulse of energy and recording the other of the bit data in a method of writing data according to claim 1, exchange-coupling between the two layers would not prevent magnetisation of said other layer by the bias field. A medium according to claim 6, wherein said two layers are a TM-rich layer and a RE-rich layer without a compensation temperature. A medium according to claim 6, wherein said two layers are a RE-rich layer with a compensation temperature and a RE-rich layer without a compensation temperature. A medium according to any one of claims 6 to 8, wherein both of said two layers are comprised of TbFe. A medium according to any one of claims 6 to 9, wherein one of said two layers is premagnetised in one direction. A magneto-optical apparatus for use in a method according to any one of claims 1 to 5, comprising: (a) an energy source (46) capable of emitting a pulse of energy; (b) means (44) for generating a bias field; (c) means (42) for moving said medium (40) relative to said energy source in said bias field with said one layer farther from the energy source than the other layer; and (d) means responsive to bit data signals for switching said pulse of energy emitted from said energy source between a first pulse such that the temperature of said one layer remains below its Curie temperature while that of the other layer becomes near or above its Curie temperature, and a second pulse such that the temperatures of said two layers become near or above their Curie temperatures. A magneto-optical apparatus according to claim 11, including means for magnetising one of said two layers in one direction beforehand.

Inventors: OHTSUKI TOMOKO, OHTSUKI, TOMOKO
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1992
Main classification: G11B11
IPC codes: G11B11