JPH04339338A - Magneto-optical recording medium - Google Patents

Abstract

PURPOSE:To increase reproduced output and to more surely identify the output by forming the 1st thin magnetic film of the magneto-optical recording medium constituted by magnetical coupling of the 1st and 2nd thin magnetic films to 20 to 70nm thickness. CONSTITUTION:The film thickness (d) of the 1st thin magnetic film 1 of the magneto-optical recording medium 10 is specified to 30nm. The reproduced output of this time yields 50.77dB as C/N in the state E and yields 51.20dB in the state B. An increase by about 0.5dB is thus obtd. The reproduced output yields 49.37dB as C/N in the state E and yields 49.47dB in the state B if the film thickness (d) of the 1st thin magnetic film 1 is specified to 60nm. An increase by about 0.1dB is thus obtd. Namely, the reproduced output in the state B is increased by forming the 1st thin magnetic film to about 20 to 70nm film thickness and, therefore, the Kerr rotating angle and magnitude in the state E are the same and the difference from the state A in which polarities are inverted is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] The present invention relates to a magneto-optical recording medium that utilizes magneto-optical interaction, and particularly to a magneto-optical recording medium consisting of two or more multilayer magnetic thin films. Involved. [0002] A method for recording information on a so-called magneto-optical recording medium in which information bits (magnetic domains) are read by magneto-optical interaction is a method for recording information on a recording medium having a magnetic thin film made of a perpendicularly magnetized film. By performing so-called initialization to align the direction of magnetization in advance in one direction perpendicular to the film surface, and forming a magnetic domain with perpendicular magnetization opposite to this magnetization direction by local heating such as laser beam irradiation, Information is recorded as binary information bits. [0003] In this recording method, prior to rewriting the information, a process of erasing the recorded information corresponding to the above-mentioned initialization, that is, time for erasing is required, and recording at a high transfer rate cannot be realized. . In response to this, various recording methods have been proposed using the so-called overwrite method, which eliminates the need for such independent erasing process time. Among the overwrite recording methods, methods that are considered promising include, for example, an external magnetic field modulation method for the medium, and a two-head method in which an erasing head is provided in addition to a recording head. . [0004] The external magnetic field modulation method is, for example,
As disclosed in Japanese Patent No. 48806, a heating light beam is irradiated onto an amorphous ferrimagnetic thin film recording medium having an axis of easy magnetization perpendicular to the film surface, and the state of the input digital signal current is determined in the irradiated area. Recording is performed by applying a magnetic field with a polarity corresponding to the magnetic field. However, when attempting to perform high-speed recording with a high information transfer rate using the external magnetic field modulation method as described above, an electromagnet that operates at, for example, the MHz order is required, and it is difficult to create such an electromagnet. Even so, there is a problem that power consumption and heat generation are large, making it impractical. Furthermore, the two-head method requires an extra head, and the two heads must be installed apart from each other, which places a large burden on the drive system and is not economical, making it unsuitable for mass production. have. [0007] In order to solve such problems, the present applicant has developed a light (thermal) light (thermal) method that allows for easy rewriting, that is, overwriting, by first switching and controlling the heating temperature of the medium using laser light or the like. For example, the magnetic recording method was
-52354 and JP-A-63-52355. The optical (thermal) magnetic recording method proposed in these applications uses a thermal (optical) magnetic recording medium having a laminated structure of first and second rare earth-transition metal magnetic thin films, and applies a required first external magnetic field. The first magnetic thin film is heated to a first temperature T1 which is approximately equal to or higher than the Curie temperature Tc1 of the first magnetic thin film and at which reversal of the sublattice magnetization of the second magnetic thin film does not occur.
The information to be recorded, for example, "0 ”, “1”, and during the cooling process, the direction of the sublattice magnetization of the first magnetic thin film due to the exchange coupling force of the first and second magnetic thin films is changed to that of the sublattice magnetization of the second magnetic thin film. For example, recording bits (magnetic domains) of "0" and "1" are formed in the first magnetic thin film in the same direction, and a second external magnetic field or the composition of the second magnetic thin film is
By selecting the compensation temperature to exist between room temperature and the second temperature T2, overwriting is possible by inverting the sublattice magnetization of the second magnetic thin film only by the first external magnetic field at room temperature. This is to ensure that a state is obtained. [0008] In this case, a special process (time) for erasing is not required, a high transfer rate can be achieved, and the problems associated with the above-mentioned two-head system or external magnetic field modulation system can be solved. . To explain the thermal (optical) magnetic recording method according to the above-mentioned Japanese Patent Application Laid-Open No. 63-52534, in this recording method, the above-mentioned first and second
The magnetization states in each of the magnetic thin films 1 and 2 are schematically shown by the arrows in each of the thin films 1 and 2 shown, and the directions of magnetization of both the magnetic thin films 1 and 2 are the same at room temperature TR. For example, information such as "0" and "1" is recorded in two ways: state A and state B, which are opposite to each other. These recordings are performed by applying a recording magnetic field, that is, a first external magnetic field Hex, and by heating at first and second heating temperatures T1 and T2 by laser beam irradiation. First, a laser beam is irradiated onto a region in state A, and the intensity or irradiation time of this laser beam is modulated and controlled in accordance with a recording signal to increase the heating temperature T.
The second magnetic thin film 2 is heated to a first heating temperature T1 which is approximately equal to or higher than the Curie temperature Tc1 of the magnetic thin film 1 and at which magnetization reversal does not occur in the second magnetic thin film 2 by a required recording magnetic field (external magnetic field) Hex. When such heating is performed, the first magnetic thin film 1 exhibits a state C in which it loses magnetization, but when this heating is completed, the magnetic thin films 1 and 2
When the temperature of the laminated film drops below Tc1, magnetization occurs in the first magnetic thin film 1. At this time, the exchange coupling force with the second magnetic thin film 2 is made to be dominant, so that the direction of magnetization of the first magnetic thin film 1 is the same as that of the second magnetic thin film 2. be done. In other words, state A is generated and, for example, information of "0" is recorded. On the other hand, the heating temperature T is heated to a second heating temperature T2 which is higher than the above-mentioned temperature T1 and at which the magnetization of the second magnetic thin film 2 can be reversed by the recording magnetic field (external magnetic field) Hex. When such heating is performed, the first magnetic thin film 1 loses its magnetization, and on the other hand, a state D occurs in which the magnetization of the second magnetic thin film 2 is reversed by the recording magnetic field Hex, but when this heating ends, the magnetic thin film When the laminated films 1 and 2 drop to a temperature Tc1, the exchange coupling force of the second magnetic thin film 2 causes the first magnetic thin film 1 to enter state E, that is, a state in which the direction of magnetization is opposite to the original initial state. It is formed. At this time, an initializing magnetic field, that is, a second external magnetic field, so to speak, an external auxiliary magnetic field Hsub is applied to the second magnetic thin film 2, so that only the second magnetic thin film 2, whose coercive force is selected to be relatively low near room temperature TR, is By reversing the direction of magnetization, the bimagnetic thin film 1
A magnetic state B is created in which a domain wall MW is generated between the first magnetic thin film 1 and the magnetic wall MW, that is, a state B is created in which only the direction of magnetization of the first magnetic thin film 1 is reversed from the magnetic state A, and information of, for example, "1" is recorded. In this way, state A and state B provide information "
0" and "1" are recorded, and the direction of magnetization by this first magnetic thin film 1 is read out and detected by Kerr rotation of laser beam irradiation. In this case, in either of these states A and B, It is also possible to overwrite light intensity modulation on this. That is, from either state A or state B, the temperature T1
and T2, by going through the process of state C, the information "0" and “1” causes state A and state B
can be overwritten. By the way, in the magneto-optical recording medium having the above-described structure, exchange energy acts on the interface between the laminated films of the first and second magnetic thin films 1 and 2, and therefore the first state B In this case, a domain wall MW is generated, and this domain wall energy σw is given by the following equation 1. [Equation 1] σw ≒ 2 ((A1 K1 ) 1/2 + (A2 K2
)1/2)(A1 and A2, K1 and K2
(are the exchange constant and perpendicular magnetic anisotropy constant of the first and second magnetic thin films 1 and 2, respectively) The conditions necessary for overwriting are expressed by the following equations. First, the conditions for preventing transition from state B to state A at room temperature (-20° C. to 60° C.) are the following equation 2. [Equation 2] Hc1 > Hw1 = σw /2Ms1h1
Furthermore, in order to prevent transition from state B to state E, it is necessary to satisfy the following condition 3. [Equation 3] Hc2 > Hw2 = σw /2Ms2h2
Furthermore, in state E, in order to prevent the first magnetic thin film 1 from being reversed by the external auxiliary magnetic field Hsub, it is necessary to satisfy the following equation 4. [Equation 4]Hc1 ±Hw1 >Hsub Here, +- on the left side indicates that the first magnetic thin film 1 is a rare earth metal-dominated film, and the second magnetic thin film 2 is a transition metal-dominated film. If the first and second magnetic thin films 1 and 2 are both transition metal-dominated films, the value is "-". On the other hand, in order to cause a transition from state E to state B, it is necessary to satisfy the following equation 5. [Equation 5] Hsub > Hc2 + Hw2 = Hc2 +σw /
2Ms2 h2 Further, when the heating temperature is near the Curie temperature Tc1 of the first magnetic thin film 1,
In order to transition from state C to state A, that is, to align the magnetization direction of the first magnetic thin film 1 with the magnetization direction of the second magnetic thin film 2, the following condition 6 must be satisfied. It is. [Equation 6] Hw1 > Hc1 +Hex [0028] Furthermore, in order to prevent a transition from state C to state E, it is necessary that the following condition 7 is satisfied. [Equation 7] Hc2 −Hw2 >Hex Furthermore, when transitioning from state E to state B, the magnetization of the first magnetic thin film 1 is affected by the external auxiliary magnetic field Hsub
In order to avoid inversion, it is necessary that the following condition 8 be satisfied. [Formula 8] Hsub <Hc1 −Hw1 =Hc1 −σw
/(2Ms1 h1 ) In each of the above equations, Hw1 and Hw2 are the effective magnetic fields due to the exchange coupling force defined by Equations 2 and 3 above, and Hc1 and H
c2, Ms1 and Ms2, h1 and h2 are the coercive force, saturation magnetization and thickness of the first and second magnetic thin films, respectively. As is clear from these, at room temperature, the domain wall energy σ can satisfy the above equations 2 and 3.
Although it is desirable that w be small, it actually shows a fairly high value, and from Equations 5 and 7 above, as the thickness h2 of the second magnetic thin film 2 increases, the external auxiliary magnetic field Hsub
becomes large. On the other hand, in the magneto-optical recording method described above with reference to FIG. In order to reduce the thickness h2 of the magnetic thin film 2 and the external auxiliary magnetic field Hsub, the present applicant previously proposed a thermomagnetic recording method in JP-A-2-24801 and JP-A-2-121103. . In the recording method proposed in the above-mentioned Japanese Patent Application Laid-Open No. 2-24801, a magneto-optical recording medium 1 shown in FIG.
0 is prepared. This magneto-optical recording medium 10 is
First and second magnetic thin films 1 and 2 having perpendicular magnetic anisotropy are sequentially magnetically coupled via a third magnetic thin film 3 having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy. It has laminated films. Information is recorded by heating the recording medium 10 to the first and second temperatures T1 and T2, as shown in FIG. 4, in the same way as explained in FIG. 3. I do. That is, the first temperature T1 is approximately equal to or higher than the Curie temperature Tc1 of the first magnetic thin film 1 and at which no reversal of the magnetic moment of the second magnetic thin film 2 occurs.
and a second heating state in which the first magnetic thin film 1 is heated to a second temperature T2 which is higher than the Curie temperature Tc1 of the first magnetic thin film 1 and which is sufficient to reverse the magnetic moment of the second magnetic thin film 2. The states A and B described above are obtained by modulating the state according to the information signal to be recorded and cooling from each heating state. In this method as well, information is recorded depending on the magnetization state of the first magnetic thin film 1.
In this method, the first and second magnetic thin films 1 and 2
are magnetically coupled via the third magnetic thin film 3 having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy, thereby reducing the domain wall energy between the first and second magnetic thin films 1 and 2. σw is reduced, and as a result, the above equation 5
This makes it easier to satisfy the above conditions, which makes it possible to reduce the external auxiliary magnetic field Hsub for transitioning from state E to B, that is, for initializing the second magnetic thin film 2, and to reduce the thickness of the entire laminated film. It is possible to reduce the Based on the operating principle of light intensity modulation overwriting as described above, the magnetization state in the laminated film becomes state E in FIG. 3 or 4 after irradiation with high-temperature laser light, but the next overwriting For operation, it is necessary to reverse the magnetization of only the second magnetic thin film 2 using an external auxiliary magnetic field Hsub to achieve state B. Therefore, reproduction after high-level laser light irradiation is performed for state B. ,
A state in which the direction of the magnetic moment is opposite between the first magnetic thin film 1 and the second magnetic thin film 2 will be observed. In such a thermomagnetic recording medium, in order to perform laser irradiation from the first magnetic thin film 1 side,
Depending on the thickness of the first magnetic thin film 1, even the second magnetic thin film 2 may be observed through the first magnetic thin film 1. Therefore, it was necessary to select the thickness of the first magnetic thin film 1 so that the above-mentioned state E and state B can be reliably distinguished. SUMMARY OF THE INVENTION The present invention aims at optimizing the film thickness of the first magnetic thin film in the above-described thermomagnetic recording medium to improve reproduction signal output. Means for Solving the Problems FIG. 1 shows an enlarged schematic cross-sectional view of an example of the magneto-optical recording medium of the present invention. The present invention uses a magneto-optical recording medium 10 in which at least first and second magnetic thin films 1 and 2 are magnetically coupled and stacked. 2
The temperature T at which no reversal of the magnetic moment of the magnetic thin film 2 occurs
1 and a second heating state where the second magnetic thin film 2 is heated to a temperature T2 which is equal to or higher than the temperature Tc1 and which is sufficient to reverse the magnetic moment of the second magnetic thin film 2. In the magneto-optical recording method in which recording magnetization is formed in the magneto-optical recording medium 10 by modulating the temperature according to the heating state and cooling from each heating state, the thickness d of the first magnetic thin film 1 is set to 20 nm to 70 nm. [Operation] Also in the magneto-optical recording medium according to the present invention,
As explained in FIGS. 3 and 4, binary recording of, for example, "1" and "0" is performed by forming state A and state B. As mentioned above, in the magneto-optical recording medium of the present invention, the thickness d of the first magnetic thin film 1 is 20 nm.
By selecting a film thickness in this range, it is possible to increase the Kerr rotation angle of the reproducing laser beam in state B in FIGS. 3 and 4, and to It was possible to increase the difference in the Kerr rotation angle from state B, ie, for example, the reproduction output of "1" and "0". FIG. 2 shows the first magnetic thin film 1 of the magneto-optical recording medium.
The solid line B shows the case in the above-mentioned state B, and the broken line E shows the case in the above-mentioned state E for comparison. In this case, as shown in FIG. 1, first, third, and second magnetic thin films 1, 3, and 2 are laminated on a light-transmissive substrate 5 via a dielectric film 6, and a protective film 7 is further laminated thereon. However, the Kerr rotation angle measured through the substrate 5 and the dielectric film 6 is calculated. As can be seen from FIG. 2, the Kerr rotation angle measured through the first magnetic thin film in state E is almost constant regardless of the film thickness beyond a certain thickness; The Kerr rotation angle measured through the magnetic thin film 1 changes depending on the film thickness of the first magnetic thin film 1, and is 20
It becomes large at ~70 nm and shows a maximum value at about 40 nm. Since the direction of magnetization in state A mentioned above is opposite to the direction of magnetization in state E, the Kerr rotation angle in state A has the same magnitude as state E, but only the polarity is reversed. It can be considered as Therefore, by selecting the film thickness of the first magnetic thin film from 20 nm to 70 nm, the Kerr rotation angle in state B shown by solid line B in FIG. 2 is made to exceed the Kerr rotation angle in state E shown by broken line E. According to the invented magneto-optical recording medium, the signal “1” level and “
The difference in the 0" level becomes large, and the magneto-optical reproduction signal can be further increased. [Example] Each example of the magneto-optical recording medium of the present invention will be explained in detail below with reference to FIG. In this example, first and second magnetic thin films 1 and 2 having perpendicular magnetic anisotropy are successively interposed via a third magnetic thin film 3 having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy. A case is shown in which a laminated film 4 is magnetically coupled and laminated. As shown in FIG. A transparent dielectric film 6 consisting of Tb (Fe95
The first magnetic thin film 1 is made of Gd (Fe9
The third magnetic thin film 3 is made of GdTb (5Co5) or the like;
A disk-shaped magneto-optical recording medium 10 was constructed by sequentially laminating a second magnetic thin film 2 made of Fe70Co30 or the like and a protective film 7 made of a transparent dielectric film such as silicon nitride by sputtering or the like. Here, the thickness of the dielectric film 6 is 80 nm, the thickness of the third magnetic thin film 3 is 20 nm, and the thickness of the third magnetic thin film 3 is 20 nm.
The thickness of the magnetic thin film 2 is 70 nm, and the thickness of the protective film 7 is 50 nm.
It was set as nm. This magneto-optical recording medium 10 is rotated at 2400 rpm.
The reproduction output was measured at a radius of 40 mm from the center of the substrate 5. In this case, the recording external magnetic field Hex is 500
[Oe], external auxiliary magnetic field Hsub is 4.0k [Oe]
, the power of the high-level laser beam was fixed at 16.0 mW, the power of the low-level laser beam was fixed at 6.0 mW, and the frequency was 5.0 MHz.
The initializing magnetic field, that is, the external auxiliary magnetic field Hsub
Changes in the reproduced signal after recording was performed in a state E without applying an external auxiliary magnetic field and in the reproduced signal after a state B was achieved by applying an external auxiliary magnetic field were measured. Example 1 In the magneto-optical recording medium 10 described above, the first magnetic thin film 1
The film thickness d was 30 nm. At this time, the reproduced output in state E obtained a C/N of 50.77 dB, and in state B, 51.20 dB was obtained, indicating an increase of about 0.5 dB. Example 2 In the magneto-optical recording medium 10 described above, the first magnetic thin film 1
The film thickness d was set to 60 nm. At this time, the reproduction output in state E obtained a C/N of 49.37 dB, and in state B, 49.47 dB was obtained, an increase of about 0.1 dB. As can be seen from these, by setting the thickness of the first magnetic thin film 1 to about 20 to 70 nm, the reproduction output in state B can be increased, and therefore, in state E
It is possible to make a large difference from state A in which the magnitude is the same as the Kerr rotation angle in , but only the polarity is reversed. In the conventional magneto-optical recording medium, no limit is given to the thickness of the first magnetic thin film 1, and the condition for transitioning from state C to state A in FIG. However, as described above, in the magneto-optical recording medium of the present invention, , by focusing on the film thickness dependence of the Kerr rotation angle explained in FIG.
The thickness selection of the magnetic thin film 1 can be optimized,
This made it possible to increase the reproduction output. According to the above-mentioned FIG. 2, the film thickness d of the first magnetic thin film 1 should be selected from 20 nm to 70 nm, but in reality, in order to ensure overwriting, that is, rewriting. In addition, it is desirable to set the film thickness d to about 20 nm or more and about 55 nm or less, for example, taking into consideration the power and the like. Furthermore, the present invention is not limited to the above-mentioned embodiments, and can be applied to various other material configurations, such as a two-layer media system in which the first and second magnetic thin films 1 and 2 are in direct contact with each other. It can be applied to magneto-optical recording media according to the invention. As described above, according to the magneto-optical recording medium of the present invention, it is possible to increase the reproduction output in state B shown in FIGS. 3 and 4 in light intensity modulation overwriting. Therefore, state A and state B can be distinguished more reliably, and reliability can be improved.

[Brief explanation of drawings]

FIG. 1 is a schematic enlarged cross-sectional view of an example of a thermomagnetic recording medium of the present invention.

FIG. 2 is a diagram showing the relationship between the film thickness of the first magnetic thin film of the thermomagnetic recording medium and the Kerr rotation angle.

FIG. 3 is a schematic diagram showing the magnetization state of an example of a thermomagnetic recording medium.

FIG. 4 is a schematic diagram showing the magnetization state of another example of a thermomagnetic recording medium.

[Explanation of symbols]

1 First magnetic thin film 2 Second magnetic thin film 3 Third magnetic thin film 4 Laminated film 5. Light-transparent substrate 6 Dielectric film 7 Protective film

Claims (1)

[Claims] 1. Using a magneto-optical recording medium in which at least first and second magnetic thin films are magnetically coupled and stacked, the first magnetic thin film has a temperature approximately equal to or higher than the Curie temperature Tc1, and the second A first heating state in which the magnetic thin film is heated to a temperature T1 at which reversal of the magnetic moment does not occur, and a second heating state in which the magnetic thin film is heated to a temperature T2 which is equal to or higher than the temperature Tc1 and is sufficient to reverse the magnetic moment of the second magnetic thin film. In the magneto-optical recording method, the magneto-optical recording medium forms recorded magnetization on the magneto-optical recording medium by modulating the heating state of the magneto-optical recording medium according to the information signal to be recorded, and cooling from each of the heating states. Thin film thickness is 20 nm to 70 nm
A magneto-optical recording medium characterized in that it is made in nm.