JPH0140410B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses a soft magnetic film with easy magnetization perpendicular to the film surface,
The present invention relates to a readout device that reads magnetic recording using the magneto-optical effect, and particularly to a configuration for extending the limit of relative movement speed between a magnetic recording medium and a soft magnetic film surface-perpendicular easily magnetized film. A magnetic recording signal readout device as shown in FIGS. 1 to 3 was first used as a magnetic recording signal readout device that used a soft magnetic film with easy magnetization perpendicular to the surface of the magnetic recording medium and read out magnetic recording signals using the magneto-optical effect. Proposed. That is, in FIG. 1, 1 indicates a magnetic tape,
A normal magnetic tape 1 is used, and the magnetic tape 1 is configured to run, for example, at 19 cm/sec in the direction of the arrow in the same manner as in a normal tape recorder. It is also assumed that a signal frequency-modulated by an audio signal is magnetically recorded on the magnetic tape 1. An easily magnetized film device 2 perpendicular to the soft magnetic film surface is provided at a position in contact with the magnetic tape 1. As shown in FIG. 2, the soft magnetic film easily magnetized perpendicularly to the surface of the soft magnetic film is attached to the surface of the GdGa garnet substrate 2a having a thickness of 0.2 to 0.5 mm on the side facing the magnetic tape 1. Deposit membrane 2b. This soft magnetic film surface perpendicular easily magnetized film 2b has, for example, Y _1.92 Sm _0.1
A 6 μm thick film of YSmCaFeGe garnet such as Ca _0.98 Fe _4.02 Ge _0.98 O ₁₂ is used. In this case GdGa
Both garnet and YSmCaFeGe-based garnet are transparent. Also, this soft magnetic film surface perpendicular easily magnetized film 2
The degree of soft magnetism b needs to be such that leakage magnetic flux of the magnetic recording signal of the magnetic tape 1 does not cause permanent recording in practice. In this example, the coercive force of the domain wall of the easily magnetized film perpendicular to the soft magnetic film surface is 0.3 Oe. Also, the magnetic tape 1 on the GdGa garnet substrate 2a
Anti-reflective coating layer 2c on the opposite side to the facing surface.
A diffusion prevention film 2d made of silicon dioxide with a thickness of 0.2 μm, for example, is deposited on the soft magnetic film surface perpendicular easily magnetized film 2b (lower side in FIG. 2), and on this diffusion prevention film 2d ( For example, the lower side in Figure 2)
A reflective film 2e made of a vapor-deposited aluminum film with a thickness of 0.3 μm is deposited. A protective film 2f of the reflective film 2e is deposited on the reflective film 2e (on the lower side in FIG. 2) in order to increase the abrasion resistance against the magnetic tape.
As this protective film 2f, for example, a layer of silicon dioxide having a thickness of 0.5 μm is used. Incidentally, the diffusion film 2d may be omitted in this soft magnetic film surface perpendicular easily magnetized film device 2. In this case, this soft magnetic film surface perpendicular easily magnetized film device 2
is arranged so that the protective film 2f side faces the magnetic tape 1. When this soft magnetic film surface perpendicular easily magnetized film device 2 is brought into contact with the magnetic tape 1, as shown in FIG. receives modulation corresponding to At this time, magnetic tape 1
Since the magnetic recording signal is frequency modulated, the magnetic domains appearing in the soft magnetic film surface perpendicular easily magnetized film 2b are striped magnetic domains 1c as shown in FIG. 3, and the domain width is frequency modulated. Hereinafter, the group of frequency-modulated striped magnetic domains will be referred to as signal magnetic domains. 1b in Figure 3
is the recording track. Also, since the soft magnetic film surface perpendicular easily magnetized film 2b is soft magnetic, signal magnetic domains corresponding to the magnetic recording signals of the magnetic tape 1 appear every moment as the magnetic tape 1 moves. In this example, the signal magnetic domain of the soft magnetic film surface perpendicular easily magnetized film 2b is read out using the magneto-optical effect. That is, in Fig. 1, 3 is, for example, He-
A light source generating device that generates a light source such as a Ne laser beam is shown. The light generated from the light source generating device 3 is passed through a polarizer 4 to become linearly polarized light, and the light that has passed through a beam splitter 5 is passed through an objective lens 6 to a soft magnetic film. The light is focused on the soft magnetic film 2b of the perpendicular easily magnetized film device 2. In this example, the diameter of the light focusing spot on this light focusing surface is approximately 3 μm. This focused light is reflected by the reflective film 2e of the soft magnetic film surface perpendicular easily magnetized film device 2, enters the objective lens 6 from the opposite direction, is bent at a right angle by the beam splitter 5, and is supplied to the analyzer 7. During this time, the light is perpendicular to the soft magnetic film surface easily magnetized film 2b.
The signal undergoes Faraday optical rotation to the left or right depending on the magnetization direction of the domain, and therefore the light obtained on the output side of the analyzer 7 undergoes width modulation in accordance with this signal magnetic domain. This optical modulation signal is supplied to a photoelectric conversion element 8 such as a photodiode, and the photoelectric conversion element 8
An electrical signal corresponding to this optical modulation signal is obtained at the output terminal 9 of the
Supplied to FM playback device and played. According to the above-mentioned apparatus, the magnetic tape 1 receives signals recorded frequency-modulated magnetically on the magnetic tape 1, for example, at 19 cm/
I was able to play it while running at sec.
What should be noted in this reproducing device is (a) The diameter of the condensing spot on the easily magnetized film 2b perpendicular to the soft magnetic film surface is approximately 3 μm.
This is sufficient, and the track width is approximately 1/300 of the track width of conventional audio magnetic recording (approximately 1 mm), making it possible to improve the recording density by 300 times. (b) Since the soft magnetic film surface perpendicular easily magnetized film 2b is used, the direction of the deflected beam and the magnetization direction are parallel, and the Faraday rotation can be utilized most effectively, and the Faraday rotation angle can be doubled by the reflective film 2e. Therefore, the S/N of the reproduced signal can be improved. (c) When using magnetic bubbles for signal reproduction in a reproduction device using conventional magnetic bubble materials, a bias magnetic field to hold the bubbles, a transfer mechanism to move the bubbles, and a hard magnetic film surface are required. In a device using a perpendicular magnetization film, magnetic signals are stored in the perpendicular magnetization film on the surface of this hard magnetic film, so when magnetic signals are sent moment by moment by a magnetic tape, a mechanism is used to demagnetize the stored magnetic signals. However, this device does not require these additional mechanisms and is extremely simple, making it more suitable for practical use. In the above-described embodiment, a frequency-modulated signal was magnetically recorded on the magnetic tape 1, but the same effect as described above can be obtained by magnetically recording a pulse-code modulated, pulse-width modulated, etc. signal instead. Using such a device, we measured the high-speed response limit at each wavelength by changing the recording wavelength on the magnetic tape and changing the running speed of the magnetic tape.
The results shown in Table 1 were obtained.

[Table] Therefore, although this device can be used as an audio tape recorder, it cannot be used to reproduce video signals unless multiple tracks are used simultaneously.
For example, if the material of the soft magnetic film surface perpendicularly easily magnetized film (hereinafter referred to as magnetic garnet film) is selected, the shortest recording wavelength that can be transferred and reproduced can be about 4 μm, but when transmitting a signal with a band of 5 MHz such as a video signal, the carrier wavelength can be reduced to 10 μm.
In order to reproduce this as an FM, at least a critical speed of 50 m/sec is required, making it impossible to reproduce with conventional equipment. The present invention discovers the causes that determine the above-mentioned speed limit, and takes countermeasures against them to realize a reading device that can also be used for reproducing video signals. The domain walls that occur in conventional magnetic garnet films are twisted domain walls due to the magnetic field generated from the magnetic poles on the surface of the magnetic domains. Therefore, if we examine the azimuth angle φ of the magnetization in the thickness direction in the domain wall that occurs when no signal magnetic field is applied from the outside, the curve N in FIG. 4 will be obtained. When an external magnetic field with a signal wavelength of 30 μm is applied, the pattern is transferred to the garnet film, and due to the influence of the in-plane component of the external magnetic field, the azimuth of magnetization changes as shown by H and M in Figure 4. φ
changes. M indicates the time when the maximum signal is given, and H indicates the time when the intermediate value is given. When there is an in-plane magnetic field, the magnetization azimuth angle curve stands, and the difference in the magnetization azimuth angle between the top surface and the bottom surface is small. Become. Next, when the structure of the domain wall was determined by calculation when signal magnetic fields of the same wavelength were moving at a constant speed, it was found that the structure was as shown in A and B in FIG. 5. In other words, 0
In the range of ~9.5m/sec, the structure is as shown in Figure 5A,
The speed of movement of the domain wall relative to the movement of the magnetic field is the same, and no phase shift occurs. However, 10m/sec~
In the range of 26.5 m/sec, the structure becomes as shown in Figure 5B, and a so-called horizontal brochure line occurs on the domain wall, causing a decrease in the moving speed of the domain wall, a phase lag, etc., and it becomes impossible to follow higher speeds. Divided. This value is when the signal magnetic field is at its maximum, but the calculated value when the magnetic field is half that is 33.5 m/sec, and the actual measured value of the above-mentioned critical speed almost matches the average of both. Therefore, it is assumed that the critical speed in the conventional device is due to the occurrence of horizontal Bloch lines. By the way, as shown in FIG. 5, when the speed reaches such a level that a horizontal bloch line occurs, the change in φ becomes large and the curve has many curved parts. Therefore, I came up with the idea that if there was a way to set this up, it would be possible to prevent the occurrence of horizontal brochure lines and, in turn, increase the critical speed. In other words, if a magnetic garnet thin film with orthorhombically symmetric magnetic anisotropy is used instead of the conventional magnetic garnet film, it will have in-plane anisotropy in addition to uniaxial anisotropy perpendicular to the film surface. We realized that if the direction of the internal anisotropy is set perpendicular to the domain wall, the magnetization at the midpoint in the thickness direction within the domain wall can be made perpendicular to the domain wall surface. Therefore, as a magnetic garnet film, Eu _2.09 La _0.75
A thin film consisting of Ca _0.16 Ge _0.16 Al _0.55 Ga _0.12 Fe _4.17 O ₁₂ is
It is grown and deposited on the (110) oriented surface of the substrate 2a similar to the above device, and the direction of in-plane anisotropy is made parallel to the running direction of the magnetic tape using the same device as shown in FIG. I chose it and conducted an experiment. This thin film
Ku/2πMs ² (Ku is the uniaxial anisotropy constant, Ms is the saturation magnetization) is 7.4, and Δ/2πMs ² (Δ is the in-plane anisotropy constant) is 4.0, so the signal is similar to the conventional garnet film. A signal magnetic domain was generated by the magnetic field, which could be converted into an electrical signal by the magneto-optical effect. Moreover, since there is in-plane anisotropy,
This seems to prevent the occurrence of horizontal bloat lines, and the actual measured values of the critical speed are shown in Table 2 below, indicating that the intended purpose of reproducing video signals can be achieved.

[Table] By using a magnetic garnet thin film with orthorhombically symmetrical magnetic anisotropy and selecting the direction of in-plane magnetic anisotropy parallel to the running direction of the magnetic recording medium, it is possible to In comparison, it was found that the maximum speed of the domain wall could be increased by 2 to 10 times. According to various experiments and calculations, materials that can be used for this purpose should be selected with a small value of Ku/2πMs ² in order to widen the transferable wavelength range and increase the mobility of the domain wall. To increase the critical speed by standing perpendicular to the membrane surface as described above, Ku/
2πMs ² >Δ/2πMs ² 1 is required. Note that this inequality does not take into account the in-plane magnetic field of the signal magnetic field, but the in-plane magnetic field that is actually required should be set to have a magnetization of 8 Ms or more. Therefore, the magnitude of the in-plane magnetic anisotropy Δ is the sum of the magnetic field due to the signal magnetic field.
If it exceeds 8 Ms, there is no practical problem, and therefore even a material with a value of Δ slightly smaller than the Δ that satisfies the above-mentioned inequality can be used depending on the strength of the signal magnetic field. In addition to the above-mentioned materials, a magnetic garnet thin film containing Bi having orthorhombically symmetrical magnetic anisotropy can also be used. In the above embodiments, the present invention was applied to a magnetic tape, but it is of course possible to apply the present invention to other magnetic recording media such as a magnetic sheet. Furthermore, it goes without saying that the present invention is not limited to the above-described present invention, and that various other configurations can be taken without departing from the gist of the present invention.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an example of a magnetic recording signal readout device, FIG. 2 is a sectional view showing an example of a soft magnetic film surface perpendicular easy magnetization film device, and FIG. 3 is a diagram for explaining FIG. 1. FIGS. 4 and 5 are diagrams showing the domain wall structure in a magnetic garnet thin film, respectively. 1 is a magnetic tape, 1b ₁ , 1b ₂ , 1b ₃ . . . are signal tracks, and 2 is a soft magnetic film device with easy magnetization perpendicular to the surface.

Claims (1)

[Claims] 1. A device for disposing soft magnetic easily magnetized films perpendicular to the magnetic recording medium, moving them relative to each other, and reading out signal magnetic domains formed in the soft magnetic easily magnetized films perpendicular to the surface by using the magneto-optical effect. In this method, a magnetized film that also has in-plane magnetic anisotropy is used as the soft magnetic easily perpendicularly magnetized film, and the direction of the in-plane magnetic anisotropy is set between the magnetic recording medium and the soft magnetic easily perpendicularly magnetized film. A magnetic recording signal reading device characterized in that the magnetic recording signal is selected so as to be parallel to the direction of relative movement with respect to the magnetic recording signal.