Method and material for magneto-optical recording

Abstract: The recording medium (20) includes alternate thin layers of magnetic rare-earth metal (11) and an electrically conductive material (12), and has "magnetic frozen state" at low temperature. The medium may be magnetised to different ferromagnetic states when heated to different temperatures by a controlled laser beam, under an external magnetic field. The different magnetic states permit of multiple recording.

Description: The present invention relates to a magneto-optical recording medium comprising alternate layers of a magnetic rare-earth metal and an electrically conductive metal, wherein the medium exhibits temperature-dependent ferromagnetism and can be magnetized to a degree dependent on the temperature to which it is heated. The present invention also relates to methods of recording and erasing data on a magneto-optic recording medium. EP-A-0369815 discloses a multi-layer recording medium which exhibits temperature-dependent ferromagnetism. The multi-layer structure of the disclosed medium is arranged to exhibit a "spin-glass" characteristic. Consequently, in order to erase data from the disclosed medium, it must be heated to a temperature greater than that corresponding to peak magnetization. A medium according to the present invention is characterized by a magnetization-temperature characteristic such that a first curve formed by plotting magnetization of the medium against temperature during heating and cooling of the medium under a constant non-zero magnetic field, so as to increase the magnetization of the medium from a first level to a second level, is crossed by a corresponding second curve formed by plotting magnetization against temperature during reduction of the magnetization of the medium from said second level to said first level by heating to a higher temperature and cooling the medium with no magnetic field applied. The present invention provides a magneto-optical recording method, comprising the steps of: applying a magnetic field to a medium, according to the present invention, substantially perpendicular to the surface thereof, and shining a beam of laser light to the medium to heat a region to which said magnetic field is applied, the heating of the medium being controlled on the basis of information to be recorded on the medium. The present invention also provides a method of erasing information recorded on a medium according to the present invention, comprising the steps of: applying no magnetic field to the medium, and shining a beam of laser light on the medium to heat it to a temperature higher than that used for recording the information. The term "magnetic frozen state" refers to a state of the medium in which magnetic spins are kept oriented in random directions. Consequently, as the medium is heated in a magnetic field of an arbitrary intensity, the magnetization increases from its initial magnetization MA at the initial temperature TA to a maximum magnetization at a certain temperature (which will be hereinafter called "peak temperature"), as shown in Fig. 1. 'the magnetization will thereafter be diminished with the call "magnetic freezing", was first found in 1985. (See Physical Review Letters, 56, 259 (1986).) Much study has been made since then on this phenomenon primarily for physical interest. The phenomenon, however,is not fully understood yet: a fundamental problems as to if the phenomenon is attributable to a phase transition as in the case of other magnetic order-disorder cooperative phenomena, still remains unanswered. Nevertheless,the magnetic freezing may be utilized for magneto-optical recording in accordance with the invention, as described in detail below. No use of the magnetic freezing property has been reported in connection with magneto-optical recording. It may be mainly due to the fact that the phenomenon has been taken for granted as being reversible. The inventor, however, found after study of the magnetic freezing phenomenon that it is irreversible under certain conditions. That is, if such a medium as mentioned above is heated under a magnetic field from an initial temperature TA to an arbitrary temperature TB, it retains magnetization MB even after it is cooled to the initial temperature TA and the magnetic field is removed (along the path indicated by arrow A in Fig. 1). This is due to temporary "melting" of the frozen magnetic spins caused by the heating and subsequent reorientation of the spins under a magnetic field to produce ferromagnetism. If the material is heated (along the path B) again without magnetic field, the magnetization reduces approximately to its initial level MA at temperature TB, and back to the very initial magnetization MA if the temperature is reduced thereafter to the initial temperature TA (Path C indicated by arrow C). As briefly discussed above, it is possible to produce a state of an arbitrary degree of magnetization by controlling the heating temperature under magnetic field, and this enables multiple recording. The principle behind this multiple recording will be described in detail later. Also, it should be noted that data writing and data erasing is possible at a temperature arbitrarily selected within a certain temperature range by the use of a laser beam energy. Furthermore, such medium has a further important property that it favors perpendicular magnetization (i.e. magnetization in the direction perpendicular to the surface of the medium), which we call anisotropic perpendicular magnetization property. The invention utilizes these properties of the medium as means for high-density magneto-optical recording. Embodiments of the present invention will now be described, by way of example, with reference to Figures 2 to 12 of the accompanying drawings, in which: Fig. 1 illustrates the principle of the reversible magnetooptical recording. Fig. 2 is a cross section of a multi-layer thin film of a magnetic rare-earth metal and an electrically conductive material. Fig. 3 is a schematic cross section of a Dy/Yb magneto-optical thin film according to the invention. Fig. 4 illustrates a characteristic magnetizationtemperature behavior observed for a thin film of Dy(40Å)/Yb(40Å) (25 periods of Dy-Yb layers), verifying the presence of magnetization freezing. Figs. 5A-C shows some of reversible magneto-optical properties of a thin film of Dy(40Å)/Yb(40Å) (25 periods of Dy-Yb layers) for different temperatures. Fig. 6 shows a chronological change in magnetization of a thin film of Dy(40Å)/Yb(40Å)(25 periods of Dy-Yb layers) under zero magnetic field. Fig. 7 shows the dependence of the magnetic freezing on magnetic field intensity applied for a thin film of Dy(40Å)/Yb(40Å)(25 periods of Dy-Yb layers). Fig. 8 shows the dependence of the magnetization on magnetic field intensity applied for a thin film of Dy(40Å)/Yb(40Å)(25 periods of Dy-Yb layers). Fig. 10 shows the dependence of the magnetic freezing on Yb thickness of a Dy-Yb film (25 periods of Dy-Yb layers with thickness of 40Å). Fig. 11 shows the possible magnetic states in conventional perpendicular magnetization and their Kerr rotational angles. Fig. 12 shows the possible multiple magnetic states in the perpendicular magnetization according to the invention and their Kerr rotational angles. The invention utilizes a multi-layer magnetic medium 20 as shown in Fig. 2, which is obtained by depositing on a base 10 alternate layers of a magnetic rare-earth metal 11 and an electrically conductive material 12. Each layer ranges in thickness from several 10Å to several 100Å. Such multi-layer medium has spin-reorientation characteristic from the "magnetic frozen state" to ferromagnetic state, which arises from the inter-layer magnetic interaction between the rare-earth metal layers with the mediation of conduction electrons. Referring now to Fig. 3, there is shown a multi-layer thin film formed on an silicon substrate. The thin film is formed of alternate layers 2 and 3 of Dy(Dysprosium) and Yb(Ytterbium), respectively by molecular beam deposition technique. Fig.4 shows the magnetization as a function of temperature of such thin film which was once cooled without magnetic field and then heated to an observation temperature under 50 Gauss, the thin film exhibiting the magnetic freezing phenomenon. It has been found that the thin film has the anisotropic perpendicular magnetization. Fig. 5 shows the typical magnetic behaviors observed in the Dy-Yb thin films. The abscissa denotes absolute temperature, while the ordinate perpendicular component of the magnetization. Recording (magnetization) and erasing (demagnetization) were carried out under the following conditions. Light source : Semiconductor laser Wave length : 830 nm Power : 5∼10 mW Beam diameter : 0.5 µm Heating time : 20 ns In this observation, the thin film which was cooled once to a state A without magnetic field is heated to a temperature (e.g. T₁=80K, T₂=100K, and T₃=125K) by irradiating the thin film with laser beam under 50 Gauss, and then cooled to its initial temperature under 50 Gauss. The magnetization of the thin film follows the paths b, c, and d in Figs. 5 A, B, and C, respectively, and reaches the state B, C, and D, respectively. Fig. 6 shows change in magnetization of the thin film with time starting from the state B, C, and D, respectively, after the magnetic field is removed. As seen in the figure, the thin film maintains magnetization even after the removal of the magnetic field. If the thin film is heated by laser beam without magnetic field, the magnetization varies as indicated by arrows e, f, and g of Fig. 5 A, B, and C, respectively, manifesting the property of the thin film that it loses its magnetization at 80K, 100K, and 125K, respectively. The thin film will restore its initial state A through a path indicated by arrows h, i, and j, respectively, as it is cooled to the initial temperature under zero magnetic field. In this manner it is possible to write/erase data into/out of the thin film by heating it with a laser beam to a temperature arbitrarily selected. Furthermore, since variable magnetization may be obtained by choosing the heating temperature, it is possible to attain multiple recording. The dependence of the magnetization of the thin film having magnetic freezing on temperature varies with the intensity of the magnetic field applied as shown in Fig. 7. The abscissa denotes the absolute temperature and the ordinate denotes the magnetization in the direction perpendicular to the surface of the thin film. From this figure, it is seen that magnetization freezing, which is favorable for magnetic recording, is enhanced by strong magnetic field. Fig. 8 shows the effect of an external magnetic field on the magnetization of the thin film which were magnetized under a magnetic field of 2000 Gauss. The ordinate denotes the perpendicular magnetization and the abscissa denotes the intensity of the external magnetic field applied. From this figure, it is clear that the thin film may preserve its magnetization even under the external magnetic field as high as 5000 Gauss in the opposite direction, verifying that such magnetic thin film has extremely strong resistance against external magnetic field. Fig. 9 shows the results of the tests to find an allowable domain of the Dy- and Yb-thicknesses for the magnetic thin film to acquire frozen magnetization. The ordinate is the Yb layer thickness and the abscissa is the Dy layer thickness. Crosses denote samples exhibiting no frozen magnetization. Light circles denote samples exhibiting the frozen magnetization. The results show that the magnetic freezing is independent of the thickness of Dy layer, and that the thickness of Yb layer must not exceed 200Å for the magnetic freezing to occur. Fig. 10 shows the temperature dependence of the magnetization of Dy-Yb thin films having different Yb thicknesses. Fig. 10 verifies that magnetic freezing is enhanced as the thickness of the Yb layers is decreased. Thus, thin films having thin Yb layers are advantageous for magnetic recording. Since the stronger magnetic freezing appears in those films having thinner Yb layers, the magnetic freezing may be attributed to the Dy inter-layer magnetic interactions mediated by the conduction electrons. The disappearance of the frozen magnetic state in the thin films having thick Yb layers supports this view, since the disappearance would be attributed to the destruction of the inter-layer magnetic interactions by the thick Yb layers. It is thus anticipated that thin layers of electrically conductive material must exist in between the Dy layers in order to provide the thin film with frozen magnetization. This would be the case for any multi-layer magnetic thin films composed of other magnetic rare-earth metals and electrically conductive materials mentioned previously. Referring now to Figs. 11 and 12, a method for multiple recording is described. A conventional perpendicular magnetic recording medium has only two magnetic states "a" and "b" at a given temperature as shown in Fig, 11A, with "a" being the state obtained under a perpendicular magnetic field and "b" obtained under opposite perpendicular magnetic field. The states "a" and "b" may represent binary states "0" and "1", or vice versa, respectively, as shown in Fig. 11B. These magnetic states may be read through observation of either the rotational angles a and b associated with them in Kerr effect or Faraday effect. In contrast, the magnetic thin film of this invention may have more than two magnetic states, depending on the recording temperatures. In Fig. 12A are shown two different recording temperatures T₁ and T₂ for inducing four magnetic states "a′", "b′", "c′", and "d′", which correspond, respectively, to four different Kerr rotational angles shown in Fig. 12B, and may represent distinct quadri-states "00", "10","01", and "11". Therefore, the magnetic medium shown in the figure may provide doubly multiple recording of information as compared with conventional one. The Kerr rotational angle  a′ for "a′" state was measured to be 2 degrees under an exemplary experiment, and the maximum angular difference between "a′" state and "d′" state amounted to 4 degree, which is far greater than the value of 1 degree for conventional magnetic media, so that the magnetic medium of this invention may provide sufficient S/N ratio. It is not difficult to extend the number of the states to 8 even with current measurement techniques which are widely used. Such 8 states may provide triplet information. It should be noted that the magneto-optical recording medium according to the invention is not limited to the Dy-Yb multi-layer thin film described above, but may be extended to include multi-layer thin films of any magnetic rare-earth thin films interlaced with conductive metal layers, since the magnetic freezing, which is favorable for magneto-optical recording, is attributed to the magnetic inter-layer interaction between rare-earth metal layers. The multi-layered magnetic thin film according to the invention may be manufactured economically using rare-earth metals such as Dy, without using expensive metals like Tb (Terbium). Such magnetic thin film having anisotropic perpendicular magnetization property along with the magnetic freezing property, allows multiple and high density magneto-optical recording using a controlled beam of laser light if one chooses suitable heating temperatures.

Claims: A magneto-optical recording medium (20) comprising alternate layers of a magnetic rare-earth metal (11) and an electrically conductive metal (12), wherein the medium exhibits temperature-dependent ferromagnetism and can be magnetized to a degree dependent on the temperature to which it is heated, characterized by a magnetization-temperature characteristic such that a first curve (A) formed by plotting magnetization of the medium (20) against temperature during heating and cooling of the medium (20) under a constant non-zero magnetic field, so as to increase the magnetization of the medium (20) from a first level (M A ) to a second level (M B ), is crossed by a corresponding second curve (B,C) formed by plotting magnetization against temperature during reduction of the magnetization of the medium (20) from said second level (M B ) to said first level (M A ) by heating to a higher temperature and cooling the medium (20) with no magnetic field applied. A magneto-optical recording method, comprising the steps of: applying a magnetic field to a medium, according to claim 1, substantially perpendicular to the surface thereof, and shining a beam of laser light to the medium to heat a region to which said magnetic field is applied, the heating of the medium being controlled on the basis of information to be recorded on the medium. A method according to claim 2, wherein the power of the laser light is controlled to heat the medium to one of a plurality of predetermined temperatures in dependence on said information according to a multi-level code. A method of erasing information recorded on a medium according to claim 1, comprising the steps of: applying no magnetic field to the medium, and shining a beam of laser light on the medium to heat it to a temperature higher than that used for recording the information.

Inventors: MAEDA ATSUSHI, MAEDA, ATSUSHI, MAEDA, ATSUSHI, SANYO DENKI TAKEZONO SHATAKU-26
Applicants: JAPAN RES DEV CORP, SANYO ELECTRIC CO, RESEARCH DEVELOPMENT CORPORATION OF JAPAN, SANYO ELECTRIC CO., LTD.
Application year: 1991
Main classification: G11B11
IPC codes: G11B11