JP3458012B2 - Magnetic field sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic field sensor utilizing the magneto-optical effect. More specifically, it is a bismuth-substituted iron garnet unit having a compensation temperature grown by the LPE method (liquid phase epitaxial method) as a magneto-optical element. The present invention relates to a magnetic field sensor using a crystal film. This magnetic field sensor is particularly useful when measuring a weak magnetic field of several tens Oe or less.

[0002]

2. Description of the Related Art A magnetic field sensor utilizing the magneto-optical effect is
Since it uses light, it has characteristics such as electrical insulation and non-induction, and has already been put to practical use in the field of electric power and the like. In this type of magnetic field sensor, basically, a first polarizer, a magneto-optical element, and a second polarizer are arranged in this order, measurement light is incident from a light source, and transmitted light is detected by light. It is a configuration to measure with a container. As the magneto-optical element, a magnetic garnet film is usually used because it has a large Faraday rotation coefficient.

The amount of light I detected for the magnetic field H to be measured is expressed by the following equation when the wavelength of light is in the near infrared region where the absorption of the magnetic garnet film can be ignored. I = I ₀ (1 + H × sin (2θ _F × d) / H _S ), where H: external magnetic field I ₀ : amount of light when external magnetic field H is zero θ _F : Faraday rotation coefficient of film d: film thickness H _S : The magnetic field required for film saturation. Therefore, if the change in the light detection light amount when the magnetic field to be measured changes by ΔH is ΔI, the sensitivity A of the sensor is A = ΔI / (ΔH · I ₀ ) = sin (2θ _F × d) / H _S ( / Oe). In the case of a magnetic field sensor for electric power, the measured magnetic field H is generally several tens to several hundreds Oe.

On the other hand, there is a demand for a highly sensitive magnetic field sensor capable of measuring a weak magnetic field of several tens Oe or less. For example, it is possible to detect the inhomogeneity of a minute magnetic field of a permanent magnet, to measure a non-inductive current of a power line of several tens of V for precise experiments, or to detect the rotation speed of a small motor. Therefore, with the aim of increasing the sensitivity A by reducing the saturation magnetization of the film to reduce the magnetic field H _S required for saturation, the bismuth-substituted iron having the composition represented by (GdBi) ₃ (FeAlGa) ₅ O ₁₂ is used. It has been proposed to apply a garnet single crystal film to a magnetic field sensor, and H _S = 200 Oe, and therefore a sensitivity A of 0.12 has been obtained (JP-A-3-24).
9565).

However, the fluctuation rate of this film in the temperature range of -10 to + 60 ° C has a large defect of ± 30 to 40%. Therefore, in order to improve this drawback, the above-mentioned magnetic garnet film is made to pass two types of measurement light having different wavelengths,
A technique for determining the magnetic field strength to be measured from these output differences (Japanese Patent Laid-Open No. 4-143679) or two magnetic garnet films having different film thicknesses are used, and the magnetic field difference to be measured is calculated from the difference in the amount of light transmitted through each film. Technology for determining strength (Japanese Patent Laid-Open No. 4-1
Japanese Patent No. 43680) has been proposed.

[0006]

As described above, in order to improve the sensitivity of the magnetic field sensor, it is necessary to reduce the saturation magnetization of the film to reduce the magnetic field H _S required for saturation. As the LPE film having a small saturation magnetization, the above (Gd
Bi) ₃ (FeAlGa) ₅ O ₁₂ is a typical example because the film having this composition has a compensation temperature T _COMP , which is close to the operating temperature (room temperature). However, in the case of a film composition having a compensation temperature, hysteresis occurs in the magnetic field dependence of magnetization and Faraday rotation angle.

Such hysteresis causes a large error in measurement by the magnetic field sensor. In the case of detecting static magnetic fields, for example inhomogeneities of the microscopic magnetic field of permanent magnets, the hysteresis has a decisive influence on the measurement error. When the transmission current is measured by detecting an AC magnetic field, for example, a magnetic field generated around an electric power transmission line, if there is a hysteresis, a second harmonic wave or a third harmonic wave of the AC frequency appears. That is, in addition to the fundamental wave of 50Hz, 100Hz, 15
High-accuracy cannot be obtained because the 0Hz harmonic component is mixed and the output stability deteriorates. If the basic frequency is constant, it is possible to deal with it to some extent by selecting the element, but when measuring the magnetic field of indefinite frequency such as measuring the motor rotation speed by detecting the change of the magnetic field. In addition, since hysteresis induces a harmonic component, it brings about a great decrease in accuracy.

Therefore, when the film has hysteresis, a countermeasure may be taken such as providing an electric circuit in which hysteresis characteristics are stored in advance and correcting the detected value each time. However, since the hysteresis itself is not necessarily highly reproducible, high-accuracy measurement cannot be expected even if it is corrected by an electric circuit. In addition, even chips made of the same film have different hysteresis, so it is necessary to select relatively good ones, which causes a cost increase.

Further, as described above, the structure using two kinds of measuring light beams having different wavelengths and the structure using two magnetic garnet films having different film thicknesses are effective in improving the sensitivity, but both of them are provided by the apparatus. It becomes complicated, large-sized, and costly.

After all, unless the area surrounded by the hysteresis loop becomes almost zero, the basic problem cannot be solved. Further, the problems of the above-mentioned film are that the extinction ratio of the unsaturated magnetic field is extremely lowered and that the width of the magnetic domain is large with respect to the light beam diameter, which also cause the measurement error. .

An object of the present invention is to use a magneto-optical element composed of a bismuth-substituted iron garnet LPE film having a compensation temperature, and there is almost no hysteresis in the magnetic field dependence of the magnetization and the Faraday rotation angle, so that an electrical correction circuit is unnecessary. It is also to provide a magnetic field sensor that has a high detection sensitivity even with a simple configuration and can reduce the measurement error.

[0012]

The inventors of the present invention heat-treat a bismuth-substituted iron garnet single crystal film grown by the LPE method and having a compensating temperature to obtain hysteresis etc. concerning the magnetic field dependence of the Faraday rotation angle before and after the heat treatment. Studied the nature of. As a result, when the heat treatment is performed under a certain specific condition, the Faraday rotation angle, the saturation magnetization, and the magnetic field required for the saturation are not changed, and the area S surrounded by the hysteresis loop is greatly reduced.
Further, it has been found that the area S becomes almost zero under the optimum conditions. At the same time, they also found that this heat treatment significantly reduced the width of the magnetic domain. The present invention has been completed based on the knowledge of such facts.

Basically, the present invention is as shown in FIG.
The first polarizer 10, the film-shaped magneto-optical element 12, and the second
Are arranged in this order, and the measurement light from the light source 16 is incident perpendicularly to the film surface of the magneto-optical element 12 through the first polarizer 10 and the transmitted light is transmitted to the second polarizer. 1
4 is a magnetic field sensor configured to be measured by the photodetector 18 through 4. In this magnetic field sensor, the magnetization of the magneto-optical element is not saturated
It measures the magnetic field in various areas. Here, a feature of the present invention is that the magneto-optical element 12 is a bismuth-substituted iron garnet single crystal film grown by a liquid phase epitaxial method and having a compensation temperature at a temperature of 1110 ° C. or higher and 1190 ° C. or lower for 9 hours. was heat-treated at top conditions within,
The point is to use a film having an area S surrounded by hysteresis of 0.5 Kdeg / Oe or less .

The film composition of the magneto-optical element is (GdB
i) ₃ (FeAlGa) ₅ O ₁₂ is preferred. As the heat treatment condition, a top condition of a temperature of 1120 ° C. or more and 1180 ° C. or less and within 7 hours is preferable, and particularly, a top condition of holding at 1150 ° C. for 2 hours to 7 hours is optimal.

The above heat treatment conditions were derived from the results of various experiments described in the section of Examples. The reason for setting such a range is as follows. If the temperature is lower than 1110 ° C, the effect of reducing the hysteresis cannot be seen. If it exceeds 1190 ° C, the Faraday rotation angle decreases. It is considered that this is because bismuth is scattered from the crystal. 9 more
When the time is exceeded, the magnetic field H _S required for saturation increases. It is considered that this is because the perpendicular magnetic anisotropy was lowered and the film became closer to the in-plane magnetized film.

[0016]

When the bismuth-substituted iron garnet single crystal film having the compensation temperature grown by the LPE method is appropriately heat-treated,
The Faraday rotation angle, the saturation magnetization, and the magnetic field required for saturation do not change, and the area S surrounded by the hysteresis related to the magnetic field dependence of the Faraday rotation angle is greatly reduced, and the area S becomes almost zero under the optimum conditions. At the same time, the extinction ratio of the unsaturated magnetic field is improved and the magnetic domain width is reduced.

[0017]

[Example] (GdBi) ₃ (F) grown by the LPE method
A single crystal film of eAlGa) ₅ O ₁₂ was processed into 2 × 7 × 0.36 mm, and the 2 × 7 mm surface was made a mirror surface. The substrate was scraped off. The specific composition of the single crystal film is Gd _2.02 Bi _0.98 Fe
_4.43 Al _0.44 Ga _0.13 O ₁₂ , which has a compensation temperature at -5 ° C. The heat treatment was performed by placing the element on a platinum plate and placing it in an electric furnace in an air atmosphere. The rate of temperature increase and the rate of temperature decrease were constant at 120 ° C./hour, and the maximum temperature (top temperature) and the holding time were variously changed. Crystal evaluation is
The Faraday rotation measurement was performed by using light having a wavelength of 1.31 μm and changing the magnetic field with an electromagnet. VS for measuring saturation magnetization
The magnetic domains were measured with a metallurgical microscope equipped with an infrared camera. The measurement results are shown in Table 1. Also,
The heat treatment conditions of the present invention determined by these experimental results are shown in FIG.

[0018]

[Table 1]

In Table 1, when the hysteresis is asymmetric (asymmetry is significant when not heat-treated), the coercive force Hc is a value obtained by dividing 2Hc by 2, and S is the area surrounded by the hysteresis related to the magnetic field dependence of the Faraday rotation angle. And the unit was kdeg / Oe. The Faraday rotation of the untreated film and the measurement results of the magnetic field dependence of the extinction ratio are shown in FIGS. When untreated, there was a large variation in the characteristics of each element, and two typical types were shown. Some have a small coercive force Hc, but the hysteresis area S is large. Further, the extinction ratio drops to 15 dB or less in the unsaturated region. 1150
FIG. 3 shows the measurement results of the magnetic field dependence of the Faraday rotation of the magneto-optical element (product of the present invention) heat-treated at 3 ° C. for 3 hours. The area S surrounded by the hysteresis is 0.1 kdeg / Oe, and the extinction ratio is 30 dB or more.

FIG. 6 shows the magnetic domains of the untreated film, and FIG. 5 shows the magnetic domains of the film after heat treatment at 1150 ° C. for 3 hours. It can be seen that the untreated film has a magnetic domain width of about 0.11 mm, whereas the heat-treated film has a very narrow width of 0.04 mm.

In FIG. 2, the shaded region (including the boundary line) is the range of the top conditions of the heat treatment in the present invention (Sample Nos. 6 to 11 and 13 in Table 1). Since the heat treatment goes through a process of raising the temperature, maintaining the top condition, and lowering the temperature in the furnace, the top condition is that the holding time is zero (that is, the process of raising the temperature to a predetermined temperature and then immediately lowering the temperature). Including. When the heat treatment temperature is lower than 1110 ° C. (Sample Nos. 1 to 5), the coercive force Hc becomes large and the area S surrounded by the hysteresis is not reduced. When the temperature exceeds 1190 ° C (Sample Nos. 14 and 15), the Faraday rotation angle decreases, so that the film thickness must be increased and the surface becomes rough. It is considered that this is because bismuth scatters from the inside of the crystal, and the composition also shifts. When the heat treatment time becomes longer than 9 hours (Sample No. 12), the magnetic field H _S required for saturation increases. It is considered that this is because the perpendicular magnetic anisotropy decreases and approaches the in-plane magnetized film. As a result of these various experiments, a heat treatment at a temperature of 1150 ° C. for 7 hours was optimum. In the case where the heat treatment was not performed, there was a large variation in the characteristics among the elements, but when the heat treatment was performed, there was almost no variation in the characteristics.

A wavelength of 1.31 μm under optimum heat treatment conditions
When the film thickness is m and the film thickness is 191 μm, the Faraday rotation angle θ _F = -1180 deg / cm, the magnetic field required for saturation H _S = 84 Oe sin (2θ _F × d) = 1, and A = 0.012 is obtained as the sensitivity constant A. Was given.

[0023]

As described above, the present invention is a magnetic field sensor that uses a magneto-optical element heat-treated under appropriate conditions on an LPE film of bismuth-substituted iron garnet single crystal having a compensation temperature, and therefore retains its basic features. At the same time, the magnetic field and coercive force required for saturation can be reduced, and at the same time, the area S surrounded by hysteresis can be significantly reduced (can be made to be near zero). As a result, an electrical correction circuit is not required, and the measurement error is greatly reduced even with the simplest configuration, so that the device can be downsized. Further, the heat treatment eliminates variations among chips, which can greatly contribute to cost reduction. Further, the extinction ratio in the unsaturated magnetic field is improved and the magnetic domain width is reduced, so that the measurement error caused by it is also reduced.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a basic configuration of a magnetic field sensor according to the present invention.

FIG. 2 is an explanatory diagram showing a range of heat treatment conditions applied to the magneto-optical element used in the present invention.

FIG. 3 is an explanatory diagram showing the magnetic field dependence of the Faraday rotation angle and the extinction ratio of the magneto-optical element used in the present invention.

FIG. 4 is an explanatory diagram showing an example of magnetic field dependence of a Faraday rotation angle and an extinction ratio of a conventional magneto-optical element.

FIG. 5 is an explanatory diagram showing magnetic domains of the magneto-optical element used in the present invention.

FIG. 6 is an explanatory diagram showing a magnetic domain of a conventional magneto-optical element.

[Explanation of symbols]

10 First polarizer 12 Magneto-optical element 14 Second polarizer 16 light sources 18 Photodetector

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsuguo Tokumasu 5-36-11 Shimbashi, Minato-ku, Tokyo Within Fuji Denki Kagaku Co., Ltd. (72) Hiroshi Rikukawa 5-36-11 Shinbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd. (56) Reference JP-A-3-249565 (JP, A) JP-A-6-77081 (JP, A) Patent 2729464 (JP, B2) (58) Fields investigated (Int .Cl. ⁷ , DB name) G01R 15/24 G01R 33/032 G02F 1/09 505

Claims (3)

(57) [Claims] 1. A film of a magneto-optical element, in which a first polarizer, a film-shaped magneto-optical element, and a second polarizer are arranged in this order, and measurement light from a light source is passed through the first polarizer. When the incident light is perpendicular to the plane and the transmitted light is measured by the photodetector through the second polarizer , the magnetization of the magneto-optical element is not
In a magnetic field sensor for measuring a magnetic field in a saturated region, a bismuth-substituted iron garnet single crystal film grown by a liquid phase epitaxial method and having a compensation temperature is used as the magneto-optical element at a temperature of 1110 ° C. or higher and 1190 ° C. or lower. , and it was heat-treated at top conditions within 6 hours, of the hysteresis
A magnetic field sensor using a film having an enclosing area S of 0.5 Kdeg / Oe or less . 2. The composition of the magneto-optical element is (GdBi) ₃
The magnetic field sensor according to claim 1, which is (FeAlGa) ₅ O ₁₂ . 3. The top condition of heat treatment is 1150 ° C. and 2
The magnetic field sensor according to claim 2, wherein the time is from 7 hours to 7 hours.