JPS61115259A - Photomagnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a film having large coercive force at a low cost by forming a recording film of magnetic garnet having a significant magneto-optical effect and oxidation stability on a heat resistant substrate having a specified coefft. of thermal expansion. CONSTITUTION:A polycrystal film of bismuth substituted garnet represented by the formula is formed on a heat resistant hard substrate of optically isotropic glass, ceramics or the like having 4.0X10<-6>-1.3X10<-5>/ deg.C coefft. of thermal expansion and proof against 600 deg.C. When the coefft. (alpha) of thermal expansion of the substrate is within said range at ordinary temp., the film is hardly cracked or roughened, and a film having large coercive force is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a magneto-optical recording medium in which a polycrystalline garnet film is formed on a hard substrate such as glass, and this is used as a recording layer.

[Conventional technology]

The magneto-optical recording method has the advantage of being rewritable and capable of high-density recording. In magneto-optical recording, when a medium is irradiated with laser light or the like, the temperature of the irradiated area rises due to the thermal effect of the light beam. As a result, the coercive force Hc of the irradiated portion decreases, and when an upward or downward magnetic field is applied to the medium from the outside, the magnetization Ms of this portion is oriented in the direction of the external magnetic field.

If the medium is given perpendicular magnetic anisotropy, the magnetization M
s remains oriented perpendicular to the medium plane. Ms upwards
By setting Ms to "1" and downward Ms to "0", digital recording can be performed by switching the direction of the external magnetic field upward and downward.

On the other hand, for reproduction, magneto-optical effects are used.

That is, when linearly polarized light is irradiated onto a recorded portion, the plane of polarization of the reflected light is rotated by θ0 with respect to the plane of polarization of the incident light.

If the rotation direction of the gold polarization plane is clockwise for upward Ms, it is counterclockwise for downward. Therefore, recording is reproduced by detecting the rotation angle (θ0 or −00) of the polarization plane. At this time, the S/N ratio of the reproduced signal takes a higher value as the rotation angle of the polarization plane becomes larger, and in order to perform high-speed reproduction, it is necessary to use a material with a large 00 value. Become.

[Problem that the invention seeks to solve]

Currently, media materials used for this type of recording method include:
GdCo, TbFe, GdTbFe, Tb(Fe
Amorphous alloy films consisting of rare earth elements such as Co) and transition metals are mainly considered, but these media contain rare earth elements that are susceptible to oxidation, so they have poor oxidation stability, and the magnetic properties of these materials The rotation angle of the deflection surface due to the optical effect is small by about 0.7 degrees ζ at most, which causes difficulties in the film forming method and reproduction method in order to read out a signal with a high S/N.

Magnetic garnet is known as a material with a large magneto-optical effect, but another advantage of this material is that it is extremely stable against oxidation because the material itself is an oxide.

Conventionally, excellent magneto-optical effects have been achieved in single-crystalline garnet films.

However, this becomes a barrier to using the garnet film as a magneto-optical recording medium. That is, since a magneto-optical recording medium is used as a disk with a diameter of at least 3 inches, it is difficult to form a uniform single crystal over the entire surface of the disk. NGO(
NdsGasO+z) single crystal substrates cannot avoid an increase in cost.

[Structure of the invention]

In order to solve the problems of the prior art as described above, the present invention provides a recording film made of magnetic garnet, which has a large magneto-optic effect and is stable against oxidation, compared to a single crystal substrate such as GGG or NGG. An object of the present invention is to provide a magneto-optical recording medium formed on an inexpensive substrate.

In order to achieve the above object, the present invention has a thermal expansion coefficient of 4. OXl0-"/'c~1.3 X 10-
The general formula (Bix R3-X) (A
, Fe5-y)O12 [wherein, R is indolium or a rare earth element, A is an element that can be replaced with Fe, Q<x<
3. Preferably O<x≦2. And 0≦y<5.

] A magneto-optical recording medium is provided, which is made of a polycrystalline film of bismuth-substituted garnet.

It is known that a garnet sputtered film having perpendicular anisotropy can be formed on a substrate made of glass, ceramic, or the like other than nonmagnetic garnet. That is, after sputtering using a sintered target of magnetic garnet, heat treatment is performed to form a polycrystalline garnet film. In this case, the film has no particular crystallographic axis orientation, and therefore its perpendicular anisotropy is due to the interaction of stress and magnetization due to the difference in thermal expansion between the film and the substrate, ie, the inverse magnetostriction effect.

The present inventors have found that by using a substrate with a thermal expansion coefficient appropriately different from that of the magnetic Gurnet7), it has perpendicular anisotropy, and moreover, it has a better property than conventional epitaxial films or polycrystalline films.
It has been found that a bismuth-substituted garnet sputtered film exhibiting a coercive force several times larger can be obtained.

In the case of a polycrystalline garnet sputtered film, its perpendicular anisotropy is caused by the difference in thermal expansion coefficients between the film and the substrate, so from the viewpoint of obtaining a large perpendicular anisotropy, the larger the difference in the thermal expansion coefficients, the better. However, if the difference in thermal expansion coefficients is too large, cranking and film roughness will occur. Therefore, it is necessary to make the difference in thermal expansion coefficients as large as possible within a range that does not cause cranking or film roughness. The thermal expansion coefficient of magnetic garnet is, for example, YIG at room temperature 1. Oxio-S,"c. Then, a film was formed by sputtering, and then heated at 600°C.
When a garnet polycrystalline film is obtained by the above heat treatment, the thermal expansion coefficient α of the substrate is 4. Oxio-'/'c<α
- It was found that within the range of <1.3 Xl0-'/°C, there is little cranking and film roughness (and a film with high coercive force can be obtained).

However, in this case, since a temperature of at least 600° C. or higher is required to crystallize bismuth-substituted garnet, the substrate needs to have heat resistance of 600° C. or higher. In addition, in order to be used as a substrate for magneto-optical recording media, optically isotropic hard substrates are basically required. It needs to be a substrate.

The coefficient of thermal expansion is 4. OX 10-”/l~1.3X
10-'/'C, heat-resistant to a temperature of 600 DEG C., and optically isotropic hard substrates such as glass and ceramic are easily commercially available.

For example, glasses include barium borosilicate glass and aluminosilicate glass, and ceramics include alumina, forsterite, zirconia, and alumina.

Some titanium carbide and the like satisfy the above conditions.

It is known that the magnetostriction constant λS of a polycrystalline magnetic garnet film differs in sign and magnitude depending on the type of rare earth element. For those with λs <O, Dy, Ho
, T+++, Er+Y+ Yb+Gd, and Eu+Tb, Ss for those with λs > O.
There are each garnet. Therefore, in order to obtain perpendicular anisotropy by the inverse magnetostriction effect, in the case of λs<0, a substrate with a thermal expansion coefficient smaller than that of the garnet film and λ
In the case of S>0, it is necessary to use a substrate having a larger coefficient of thermal expansion than the garnet film.

A 20 mm square barium borosilicate glass substrate (Corning Glass Co., Ltd. product r#7059J) was coated with Bi.

Y r , s G −F −40lt sintered garnet was used as a target by RF sputtering to a thickness of 1
A polycrystalline garnet film was obtained on the substrate by creating a μm film and heat-treating it in air at 650° C. for 3 hours. The thermal expansion coefficient of this barium borosilicate glass is 4', 6 x 10-''/''c at room temperature.

The polycrystalline garnet film thus obtained had almost no cracks.

FIG. 1 shows the Faraday hysteresis loop of this polycrystalline garnet film. The squareness ratio is 1, indicating that it is a perpendicular magnetization film. Further, the coercive force is IKOe. This value is usually 200 to 3000 for conventional epitaxial garnet films on GGG and other single crystal substrates.
e, so it is more than three times larger than that.

Bi, 2Eu
, a Gao, b Fe*, a O+z
A film with a thickness of 1 μm was created by RF sputtering using the sintered garnet as a target, and this was heat-treated in air at 650° C. for 3 hours to obtain a polycrystalline garnet film on the substrate.

The polycrystalline garnet film thus obtained was in good condition with almost no cracks.

FIG. 2 shows the Faraday hysteresis loop of this polycrystalline garnet film. The squareness ratio is 1, indicating that it is a perpendicular magnetization film. Furthermore, the coercive force is 1.2 KOe, which is four times larger than that of the conventional one.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the glass is cheaper than the single crystal garnet-based substrate.

A magneto-optical recording medium is provided in which a polycrystalline film of bismuth-substituted garnet, which has a large magneto-optical effect and is resistant to oxidation, is formed on a hard substrate such as ceramic. It is easier to create a homogeneous film over a larger area than in the case of a film;
It also has a large coercive force.

[Brief explanation of the drawing]

FIG. 1 is a graph diagram showing a Faraday hysteresis loop of a polycrystalline garnet film according to an example of the present invention, and FIG. 2 is a graph diagram showing a Faraday hysteresis loop of a polycrystalline garnet film according to another example.

Claims (1)

[Claims] 1. Thermal expansion coefficient is 4.0 x 10^-^6/°C ~ 1 at room temperature
．． The general formula (Bi_xR_3_-_x) (A_yF
e_5_-_y) O_1_2 [wherein, R is yttrium or a rare earth element, A is an element that can be replaced with Fe, 0<x
<3.0≦y<5. ] A magneto-optical recording medium made of a polycrystalline film of bismuth-substituted garnet. 2. The magneto-optical recording medium according to claim 1, wherein the substrate is made of glass or ceramic. 3. The general formula (Bi_xR_3_-_x) (A_yF
e_5_-_y) 0<x≦2.0 in O_1_2,
The magneto-optical recording medium according to claim 1 or 2, wherein 0≦y<5.