JP4171979B2 - Magneto-impedance element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic sensor for detecting a magnetic field and a current sensor using the same, and more particularly to a high-sensitivity magneto-impedance element using the magneto-impedance effect.
[0002]
[Prior art]
In recent years, high performance, downsizing, thinning, and low cost of information equipment and measurement / control equipment have rapidly progressed. With these rapid developments, magnetic sensors and current sensors used in them have become small and low cost. Demand for high sensitivity is increasing.
Known magnetic sensors include Hall elements, magnetoresistive effect elements (MR elements), giant magnetoresistive effect elements (GMR elements), flux sensors, and the like, and current transformers are used as current sensors. Known methods.
[0003]
For example, magnetic heads used in hard disk drives, which are external storage devices for computers, have been improved in performance from conventional bulk type induction type magnetic heads to MR heads. At present, the giant magnetoresistive effect (GMR) is achieved. There is an active research to apply. Further, in a rotary encoder which is a rotation sensor of a motor, the magnetic flux leaking to the outside becomes weak with the miniaturization of the magnet ring, and a highly sensitive magnetic sensor is required instead of the current MR element. Development of electronic devices using current sensors instead of conventional mechanical devices is also progressing for breakers, etc., but it is difficult to reduce the size with conventional current transformer methods, and in terms of sensitivity and detection range, etc. Therefore, high sensitivity and wide range of magnetic sensors are required.
[0004]
In order to satisfy these requirements, a magnetic impedance sensor using the magnetic impedance effect (also referred to as MI effect) of an amorphous wire has been proposed in, for example, Patent Document 1. The magnetic impedance effect means that when an external magnetic field changes in a state where a magnetic material is energized with a high-frequency current, the permeability of the magnetic material changes, and accordingly, the impedance of the magnetic material is several tens to several times that when the magnetic field is zero. It is a phenomenon that changes by 100%. In a sensor using such an effect, a minute external magnetic field change of about several hundred microtesla (μT) can be detected by measuring the voltage across the magnetic material.
The above-mentioned magneto-impedance effect is similarly observed not only in amorphous wires but also in magnetic ribbons and thin films. Especially, thin films can be made small and thin, and have excellent reliability and mass productivity. One of them is proposed, for example, as shown in Patent Document 2.
[0005]
A magneto-impedance element using a thin film is composed of a thin-film magnetic core in which a magnetic permeability is imparted and a high permeability soft magnetic film in which uniaxial anisotropy is induced is processed into a strip shape. The magnetic anisotropy is performed while applying a magnetic field when forming the magnetic film, and is further induced by heat treatment at about 150 to 400 ° C. in a rotating magnetic field or a static magnetic field. The direction of the easy axis of magnetization is generally the short axis (line width) direction of the strip structure. The magneto-impedance element has a characteristic that the impedance is changed by the magnetic field of the longitudinal component. As shown in FIG. 7A, the magneto-impedance characteristics at this time have characteristics in which the impedance peaks depending on whether the magnetic field is positive or negative, and are symmetric depending on whether the magnetic field is positive or negative. Moreover, the change rate shows a very large change of several tens to several hundreds%.
[0006]
FIG. 7A shows characteristics when the easy axis of magnetization is a line width, but magnetic impedance characteristics are exhibited even when magnetic anisotropy is applied in the length direction. As shown in FIG. 7B, the characteristic at that time is a characteristic in which the impedance is greatest when the magnetic field is 0 and decreases as the absolute value of the magnetic field increases. In this case as well, the impedance is symmetric with respect to the positive and negative magnetic fields. The detected magnetic field direction in this case is also a longitudinal component of the thin film magnetic core.
The change in impedance in these magnetic impedance characteristics is caused by a change in magnetic permeability when a high frequency current is applied to the thin film magnetic core. When the impedance is separated into a resistance component and an inductance component, both change due to a change in magnetic permeability, but a resistance component having a large absolute value is dominant in the change. The resistance change due to the permeability change is basically caused by the skin effect that occurs when high-frequency current flows in the magnetic material. To increase the skin effect, the frequency of the high-frequency current is increased, or the thin film magnetic property is increased. A method of increasing the film thickness of the magnetic material that is the core is effective.
[0007]
As described above, the magneto-impedance element is characterized in that the impedance greatly changes with respect to the magnetic field. However, by applying a bias magnetic field to the element and operating it at a point where the change in impedance is large with respect to the magnetic field, The sensor responds with high sensitivity to the magnetic field. A point A shown in FIG. 7A indicates the bias point (bias magnetic field). To apply this bias magnetic field, a coil (bias coil) is formed around the element, and a current is applied to the coil. It is necessary to generate a magnetic field by doing so. A coil is also required for a method of applying a negative feedback magnetic field for the purpose of expanding the magnetic field detection range. When an amorphous wire is used, a structure in which a Cu wire or the like is wound directly around the wire to form a coil is used, but the coil is formed on the same substrate as the magneto-impedance element formed on the thin film. (For example, refer to Patent Document 3).
[0008]
As described above, the sensitivity of the magneto-impedance element can be controlled by the bias coil and the measurement range can be expanded by the negative feedback coil. However, in the case of the magneto-impedance element formed by a thin film, the bias coil and the negative feedback coil are also available. If they are not formed on the same substrate, the advantages of small size and light weight cannot be utilized. However, in that case, the process is such that a magnetic film is formed on the lower coil via an insulating film, an upper coil is further formed via an insulating film, and the upper coil and the lower coil are electrically connected. Since it becomes complicated, it is difficult to control the characteristic variation between elements, and it becomes unstable in terms of reliability, which is disadvantageous in mass productivity.
[0009]
On the other hand, in the characteristic curve as shown in FIG. 7A, the magnetic field at which the magnetic impedance has a peak corresponds to an anisotropic magnetic field. Therefore, by controlling the magnetic anisotropy of the magnetic film, a predetermined value can be obtained. The sensitivity in the external magnetic field can be changed. That is, in order to increase the sensitivity in the weak magnetic field region, the anisotropic magnetic field may be reduced and the peak position of the magnetic impedance characteristic may be shifted inward. Conversely, in order to increase the sensitivity in the strong magnetic field region, the anisotropic magnetic field is increased and the peak position of the magnetoimpedance characteristic is shifted outward. Furthermore, when it is desired to extend the measurement range, magnetic anisotropy can be imparted in the length direction of the strip-shaped magnetic film to realize the “Λ” -shaped magneto-impedance characteristic as shown in FIG. 7B. . For example, Patent Document 4 discloses a technique for changing such magnetic anisotropy.
[0010]
[Patent Document 1]
JP-A-06-281712 (page 2-4, FIG. 1)
[Patent Document 2]
Japanese Patent Application Laid-Open No. 08-075835 (page 4, FIG. 1)
[Patent Document 3]
JP 11-109006 A (page 3-4, FIG. 1)
[Patent Document 4]
JP 2002-189067 A (page 2-3, FIG. 1)
[0011]
[Problems to be solved by the invention]
However, since the structure disclosed in Patent Document 4 is not configured to directly transmit the strain of the PZT piezoelectric element to the sensor, there is a problem that the efficiency is not good.
Accordingly, an object of the present invention is to enable the magnetic anisotropy to be efficiently controlled by allowing the strain of the PZT piezoelectric element to be directly transmitted to the sensor.
[0012]
[Means for Solving the Problems]
In order to solve the above problems, the invention of claim 1 is directed to applying a high frequency current to a magnetic thin film processed into a non-magnetic substrate in a highly magnetic permeability strip shape or a zigzag shape, and the impedance of the magnetic film by an external magnetic field. Is an element using the magneto-impedance effect, in which a piezoelectric thin film is formed on an insulator or semiconductor substrate, a magnetic thin film is formed thereon, and another piezoelectric thin film is formed thereon. It is characterized by that. In this way, the magnetic film has a bimorph structure sandwiched with a piezoelectric thin film, so that a voltage can be applied to the piezoelectric thin film in parallel to expand and contract, and stress can be applied evenly to the upper and lower surfaces of the magnetic film. The problem of warping of the magnetic film during application of stress can be solved.
[0013]
The invention of claim 2 is characterized in that, in the invention of claim 1, the magnetic material is an amorphous magnetic material. Co-based and Fe-based amorphous magnetic materials are known, but since there is no magnetocrystalline anisotropy, magnetic anisotropy due to stress is dominant. For this reason, it is easy to control the magnetic anisotropy by the inverse effect of magnetostriction.
In invention also invention of claim 1 of claim 3, wherein the magnetic film is made of a giant magnetostrictive soft magnetic material. Laves phase intermetallic compound REFe ₂ (RE = Tb, Sm, Dy) is known as a giant magnetostrictive material. Among them, TbFe ₂ and DyFe ₂ alloyed (Tb, Dy) Fe ₂ is giant magnetostrictive. Although it is a material having a soft magnetic material, it exhibits a magnetostriction 100 to 1000 times that of a normal magnetic material. Therefore, by using these materials, it is possible to realize a magnetic detection element in which the magnetic anisotropy changes sensitively due to stress caused by expansion and contraction of the piezoelectric body.
[0014]
The invention of claim 4 is characterized in that, in the invention of claim 1, a ferroelectric thin film is used as the piezoelectric thin film . It is known that a ferroelectric thin film such as Pb (Zr, Ti) O ₃ , so-called PZT, is epitaxially grown on a substrate having a close crystal structure, and on a Si substrate or the like through an appropriate buffer layer. Can also be epitaxially grown. These ferroelectric thin films are expected to show a higher piezoelectricity than conventional ZnO piezoelectric thin films, and by applying stress to the magnetic film at a lower voltage, it is possible to easily control the sensitivity characteristics. .
Further, the invention is also the invention of claim 1 of claim 5, characterized by using a piezoelectric high content child film as the piezoelectric thin film. Forming a magnetic film on a highly workable piezoelectric polymer film and bonding it to an IC substrate, for example, to integrate it with an IC, but also control sensitivity characteristics by changing magnetic anisotropy It becomes possible. By using a material having a low Young's modulus as the adhesive, it is possible to make it difficult to transmit the distortion of the piezoelectric body to the substrate.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a block diagram showing the principle of the present invention .
As shown in the figure, an amorphous magnetic film 1 showing a magnetoimpedance effect is formed on a PZT piezoelectric substrate 2. Here, the magnetic film 1 has a folded shape, which is formed by, for example, photolithography by applying photosensitive polyimide on an amorphous magnetic film having a thickness of 4 μm formed on the entire surface of the PZT substrate 2 by magnetron sputtering. It was developed into a folded pattern by a technique and processed by ion beam etching using it as a mask. The Au electrodes 3a and 3b are formed on the side surface of the piezoelectric substrate, and a voltage is applied through the Au electrodes 3a and 3b to distort the substrate. As a result, stress is applied to the magnetic film 1 and the magnetic anisotropy changes, so that the peak position of the magnetic impedance characteristic shifts.
[0016]
FIG. 2 is a graph showing the result of controlling the peak position of the magnetic impedance characteristic by the above method. The control of magnetic anisotropy will be specifically described below.
For example, as a magnetic film, a CoHfTaPd amorphous magnetic film formed by a magnetron sputtering method is used. After the film formation, a static magnetic field treatment is performed at 300 ° C. and 48000 A / m for 1 h, and one strip is 0.2 mm (w) × 1 mm (l ) × 4 μm (t), a magnetic impedance characteristic having an anisotropic magnetic field of about 800 A / m in the width (w) direction of the strip can be obtained (see FIG. 2A). The size of the PZT substrate was 0.1 mm (w) × 1 mm (l) × 0.5 mm (t).
[0017]
Next, the strain δw / w when a voltage of 100 V is applied in the width direction of the PZT substrate is determined using the piezoelectric strain constant d ₃₃ = 285 × 10 ⁻¹² [m / V],
δw / w = d ₃₃ × V / w = 285 × 10 ⁻¹² [m / V]
× 100 [V] / (0.1 × 10 ⁻³ [m])
= 2.85 × 10 ⁻⁴
It becomes. Assuming that the magnetic film on the PZT substrate is also distorted as much as this, the Young's modulus Y of the CoHfTaPd amorphous magnetic film is about 88 GPa, so the stress σ in the width direction applied to the magnetic film is
σ = Y × δw / w = 88 × 10 ⁹ [N / m ² ] × 2.85 × 10 ⁻⁴
= 2.5 × 10 ⁷ [N / m ² ]
It becomes.
[0018]
Since the saturation magnetostriction λs of the CoHfTaPd amorphous magnetic film is about −2 × 10 ⁻⁶ , the change amount ΔK of the magnetic anisotropy energy due to this stress is
ΔK = (3/2) × λs × σ = (3/2) × (−2 × 10 ⁻⁶ ) × 2.5
× 10 ⁷ = −75 [J / m ³ ]
It becomes. On the other hand, the magnetic anisotropy energy K ^{0 in} the width direction before the magnetic film is distorted is that the anisotropic magnetic field Hk ⁰ is 800 A / m and the saturation magnetization Ms is about 1 [T].
K ⁰ = (1/2) × Hk ⁰ × Ms = (1/2) × 800 [A / m] × 1 [T]
= 400 [J / m ³ ]
[0019]
Therefore, the magnetic anisotropy energy K in the width direction before the distorted magnetic film is distorted is
K = K ⁰ + ΔK = 400 [J / m ³ ] −75 [J / m ³ ] = 325 [J / m ³ ]
The anisotropic magnetic field Hk due to this is
Hk = 2 × K / Ms = 2 × 325 [J / m ³ ] / 1 [T] = 650 [A / m]
As a result, the peak position of the magnetic impedance characteristic shifts inward by about 150 A / m (see FIG. 2B). Thereby, the sensitivity in a low magnetic field region is improved. For example, in the case of an element having an impedance variation ΔZ of 100Ω, a sensitivity improvement of 25% or more is expected.
[0020]
When the voltage applied in the width direction of the PZT substrate is further increased, for example, to 550 V, the change amount ΔK of the magnetic anisotropy energy is
ΔK = −412.5 [J / m ³ ]
Thus, the magnetic anisotropy energy in the width direction that the magnetic film had before being distorted is canceled, and conversely, it has magnetic anisotropy in the length direction. At this time, the magnetic impedance characteristic of the magnetic film becomes a “Λ-shaped” curve having a peak in a zero magnetic field (see FIG. 2C), and the magnetic field can be detected in a wider range than the “V-shaped”.
[0021]
In this embodiment, a Co-based amorphous magnetic film is used as the magnetic film, but an Fe-based amorphous magnetic film may be used. The Fe system has a positive magnetostriction constant on the order of 10 ⁻⁴ , so that the magnetic anisotropy can be greatly changed at a lower voltage than the Co system. Further, by using a giant magnetostrictive soft magnetic material such as (Tb, Dy) Fe2, the magnetic anisotropy can be changed more sensitively.
Regarding the piezoelectric substrate, PZT is used as a material in this embodiment, but other piezoelectric materials may be used. In addition, although the electrodes are formed on the side surfaces in the width direction of the substrate, the electrodes may be formed in the length direction to control the strain, and the electrodes are formed on all four side surfaces so as not to be in electrical contact with each other. It is also possible to control the magnetic anisotropy of the magnetic film by strain in the width direction and the length direction.
[0022]
FIG. 3 shows a modification of FIG .
This is an example in which the magnetic film 1 is formed on the piezoelectric polymer film 2a and processed into a folded shape. Metal electrodes 3a and 3b are formed at both ends of the polymer film 2a, and a voltage is applied to the metal electrodes 3a and 3b to cause the polymer film 2a to contract, thereby applying stress to the magnetic film 1 to control magnetic anisotropy. Since the polymer film 2a can be bent freely, there is a feature that it can be used in a shape according to the application. For example, when detecting a circular magnetic field generated by a current flowing through an electric wire, the magnetic field can be efficiently received when used in such a manner that it is wound around the electric wire.
[0023]
FIG. 4 shows another modification of FIG .
In this example, a PZT thin film 4 is formed on an Al ₂ O ₃ substrate 2b by a magnetron sputtering method, and then a magnetic film 1 processed into a folded shape is formed thereon. Voltage application to the PZT thin film 4 is performed by the electrodes 3a and 3b. The electrodes 3a and 3b are formed so as to cover not only the upper surface but also the entire side surface so that the PZT thin film 4 expands and contracts uniformly in the thickness direction. Yes. The electrodes may be formed on the side surfaces in the length direction, or may be formed on all four side surfaces so as not to be in electrical contact with each other. Thereby, the laminated structure of the magnetic film / piezoelectric body can be very miniaturized. Here, Al ₂ O ₃ is used as the substrate, but an insulating substrate such as glass or SrTiO ₃ can also be used, and it can also be formed directly on the Si substrate or via an appropriate buffer layer. Therefore, the advantage of downsizing can be utilized by forming the IC substrate on-chip. Further, although the PZT thin film is used as the piezoelectric thin film, other ferroelectric films, ZnO thin films, and AlN thin films can also be used. Furthermore, a similar configuration can be realized by a simpler method by forming a magnetic film on the piezoelectric polymer film and attaching it to the substrate with an adhesive.
[0024]
FIG. 5 shows an embodiment of the present invention .
This is because the PZT thin film 4 is formed on the substrate 2c, the magnetic film 1 is further formed into a zigzag shape, the gap of the processed magnetic film 1 is filled with the insulator 5, and the magnetic film 1 is again formed thereon. A PZT thin film 4a is formed. Then, the electrodes 3a and 3b are formed so as to cover the side surfaces of the PZT thin films 4 and 4a, and a voltage is applied to the electrodes 3a and 3b to distort the PZT thin films 4 and 4a. To control the peak position. The position of the electrode may be formed on the side surface in the length direction, or may be formed on all four side surfaces. The substrate 2c may be an Al ₂ O ₃ substrate as in FIG. 4, or another nonmagnetic plate.
By adopting such a bimorph type structure, equal stress can be applied to the upper surface and lower surface of the magnetic film 1, so that the uniformity of stress distribution in the thickness direction can be increased. As a result, the problem that the film warps due to the difference in elongation between the upper and lower surfaces of the magnetic film 1 can also be solved. Although the PZT thin film is used as the piezoelectric thin film, other ferroelectric thin films, ZnO thin films, AlN thin films, piezoelectric polymer films, and the like can also be used.
[0025]
FIG. 6 shows another principle configuration of the present invention .
In this example, the magnetic film 1 is formed on the circumference of a tube-shaped PZT piezoelectric body 2d and processed into a folded shape, and electrodes 3a and 3b are formed on the upper and lower surfaces of the PZT tube 2d, respectively. Then, a voltage is applied to the PZT piezoelectric body 2d through the electrodes 3a and 3b to expand and contract in the tube height direction, thereby applying stress to the magnetic film 1 to control the peak position of the magnetic impedance characteristic. The magnetic film 1 is formed by magnetron sputtering while rotating the PZT tube 2d, and the patterning process of the photosensitive polyimide is also performed while rotating. The processing may be performed by ion beam etching while rotating the PZT tube 2d, but wet etching is simpler. The magnetic detection element of this embodiment is particularly suitable for use in detecting a magnetic field generated by a current flowing through an electric wire. That is, by passing the electric wire 6 through the hole of the PZT tube 2d as shown, it is possible to efficiently receive the circular magnetic field generated around the electric wire.
[0026]
【The invention's effect】
According to the present invention, it is possible to realize a magnetic detection element using the magneto-impedance effect capable of adjusting sensitivity and switching the measurement range without requiring a complicated process for forming a bias coil and a negative feedback coil. Moreover, there is also an advantage that a magneto-impedance element having an optimum form can be provided according to the use by appropriately selecting the shape of the piezoelectric body.
[Brief description of the drawings]
1 is a diagram illustrating the principle of the present invention; FIG. 2 is a characteristic diagram illustrating the magneto-impedance characteristics of FIG . 1; FIG . 3 is a configuration diagram illustrating a modification of FIG . 1; FIG. 5 is a block diagram showing an embodiment of the present invention. FIG. 6 is a block diagram showing another principle configuration of the present invention. FIG. 7 is a characteristic diagram showing a general example of magnetic impedance characteristics. Explanation】
1 ... magnetic film, 2 ... PZT substrate, 2a ... piezoelectric polymer film, 2b ... Al ₂ O ₃ substrate, 2c ... substrate, 2d ... tubular piezoelectric, 3a, 3b ... electrode, 4, 4a ... PZT thin film, 5 ... insulator, 6 ... electric wire.

Claims (5)

Magnetic impedance effect, in which high magnetic permeability strip-like magnetic thin films or alternating magnetic thin film patterns are formed on a non-magnetic substrate to form a magnetic thin film pattern, and the high frequency current is applied to this to change the impedance of the magnetic material. An element that uses
A piezoelectric thin film is formed on an insulator or a semiconductor substrate, a magnetic thin film is formed thereon, another piezoelectric thin film is formed thereon, and the magnetic thin film is sandwiched between the piezoelectric thin films. A magneto-impedance element, wherein a voltage is applied to a thin film to expand and contract, and stress is applied to the magnetic thin film to change magnetic anisotropy. The magneto-impedance element according to claim 1, wherein the magnetic thin film is made of an amorphous magnetic material . Magneto-impedance element according to claim 1, wherein the magnetic thin film is characterized Rukoto such a super magnetostrictive soft magnetic material including TbFe _2, DyFe _2. The magneto-impedance element according to claim 1, wherein a ferroelectric thin film is used as the piezoelectric thin film . 2. The magneto-impedance element according to claim 1, wherein a piezoelectric polymer film is used as the piezoelectric thin film and is bonded to a substrate .

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