JP7612382B2 - Magnetic Sensor - Google Patents

Description

The present invention relates to a magnetic sensor.

As a conventional technology, there is a magnetic sensor using a magnetic material classified as a frequency modulation type. Specifically, a magnetic sensor has been proposed in which a transmission line element consisting of a conductor layer, a dielectric layer, and a magnetic material layer is inserted into the feedback circuit of a phase-shift type resonant circuit (see, for example, Patent Document 1). The conductor layer is formed in a spiral planar pattern. The soft magnetic material layer has a substantially rectangular shape consisting of a soft magnetic ribbon or thin film, and is given uniaxial magnetic anisotropy in the longitudinal direction. This magnetic sensor can be both compact and highly sensitive by being driven by a high-frequency current, and the overall configuration, including the peripheral circuits, can be simplified.

Patent No. 3559459

However, this magnetic sensor has a problem in that the sensor portion has a spiral conductor layer and is composed of a multi-layered transmission line element, making the sensor structure complicated.

The present invention aims to provide a magnetic sensor that can improve magnetic sensitivity while further simplifying the configuration.

The magnetic sensor of the present invention comprises:
an inductor including a dielectric substrate, a transmission line made of a conductive material extending on a main surface of the dielectric substrate, and a rectangular soft magnetic thin film adjacent to the transmission line, extending along the transmission line, the long side direction being the hard axis of magnetization and the short side direction being the easy axis of magnetization;
a capacitor that configures an LC resonant circuit together with the inductor;
and a magnet for applying a bias static magnetic field in the longitudinal direction to the soft magnetic thin film.

With this magnetic sensor, the transmission line and soft magnetic thin film are simply formed on the main surface of the dielectric substrate, further simplifying the configuration, while utilizing the ferromagnetic resonance phenomenon of the soft magnetic thin film to improve the magnetic sensitivity to an external AC magnetic field, and therefore the detection accuracy.

1 is an explanatory diagram relating to a configuration of a magnetic sensor according to an embodiment of the present invention; FIG. 2 is a perspective view of a magnetic sensing portion that constitutes the magnetic sensor. FIG. 4 is a top view of a magnetic sensing portion that constitutes the magnetic sensor. FIG. 4 is a schematic cross-sectional view of a magnetic sensing portion that constitutes a magnetic sensor. FIG. 2 is an explanatory diagram of the magnetic properties of a soft magnetic thin film. FIG. 4 is an explanatory diagram of the complex permeability of a soft magnetic thin film. 5A and 5B are diagrams illustrating examples of detection signals by a magnetic sensor.

(composition)
1, a magnetic sensor according to an embodiment of the present invention includes an LC resonant circuit 2. The LC resonant circuit 2 includes a magnetic sensing portion 1 having an inductance L, a first capacitor 21, a second capacitor 22, a frequency adjuster 23, and a signal amplifier 24.

The frequency adjuster 23 is composed of a pair of diodes 231 and 232 facing each other on the cathode side, and a power supply 230. The anode side of one of the diodes 231 is connected to the magnetic sensing unit 1. The anode side of the other diode 232 is connected to ground GND. By adjusting the static voltage Vad applied between the pair of diodes 231 and 232 and ground GND by the power supply 230, the capacitance between the pair of diodes 231 and 232 is adjusted, and the resonant frequency of the LC resonant circuit 2 can be changed. The frequency adjuster 23 may be omitted.

The signal amplifier 24 is composed of a transistor 242 and a power supply 240. The base of the transistor 242 is connected between the magnetic sensing unit 1 and the first capacitor 21, the collector is connected to the power supply 240, and the emitter is connected to the output terminal 26 and is connected to ground GND via a resistor 244.

As shown in FIG. 1, the magnetic sensor further includes a magnet 4 composed of a permanent magnet and/or an electromagnet for applying a bias static magnetic field H to the magnetic sensing portion 1.

As shown in each of Figures 2 to 4, the magnetic sensing unit 1 comprises a dielectric substrate 10, a transmission line 11, and a soft magnetic thin film 12. The transmission line 11 comprises a signal line 13 and a pair of ground lines 141 and 142. To explain the orientation or extension of each component of the magnetic sensing unit 1, a right-handed Cartesian coordinate system is shown in Figures 2 to 4.

The dielectric substrate 10 is made of a dielectric material having a substantially rectangular plate shape. The dielectric substrate 10 may be made of, for example, glass, quartz crystal, sapphire, semiconductor single crystal silicon, germanium, compound semiconductor GaAs, GaN, SiC, ZnSe, CdS, ZnO, InP, or SiGe, or a substrate made of ceramics such as alumina, silicon nitride, aluminum nitride, zirconia, silicon carbide, titania, yttria, or a composite material thereof. The shape of the dielectric substrate 10 may be changed to any shape, such as a disk shape.

The signal line 13 of the transmission line 11 is formed in a substantially rectangular or strip-like shape extending in the x-direction on the main surface of the dielectric substrate 10. The pair of ground lines 141 and 142 of the transmission line 11 are formed in a substantially rectangular or strip-like shape extending in the x-direction so as to sandwich the signal line 13 in the y-direction. The signal line 13 and the ground lines 141 and 142 are made of a conductive material, and may be Al, Cu, Au, Pt, Ag, Ti, or a laminated thin film of these materials.

The soft magnetic thin film 12 is formed in a substantially rectangular shape with the x direction as the longitudinal direction and the y direction as the transverse direction in the vicinity of the signal line 13 of the transmission line 11. When a high-frequency current I(ω _h ) in the x direction flows through the signal line 13, an AC magnetic field H(ω _h ) rotating in the y-z plane is generated around the signal line 13 as shown in FIG. 4. The strength of the AC magnetic field H(ω _{h ) decreases rapidly as it moves away from the signal line 13 according to Biot-Savart's law. In addition, the AC magnetic field H(ω h} ) is strongly concentrated in the gap between the signal line 13 and the ground lines 141 and 142 due to the superposition of the magnetic field distribution formed by the forward high-frequency current I(ω _h ) flowing through the signal line 13 and the reverse-direction current -I(ω _{h )} flowing through the ground lines 142 and 142. Therefore, by arranging the soft magnetic thin film 12 with high magnetic permeability near the signal line 13, a change in inductance that is sensitive to the external AC magnetic field H(ωs) can be obtained. By producing a change in inductance, a highly sensitive magnetic sensor is realized.

In this embodiment, the transmission line 11 and the soft magnetic thin film are laminated in this order on the main surface of the dielectric substrate 10 so that the soft magnetic thin film 12 partially overlaps the signal line 13 at the longitudinal center when viewed from the z direction. A layer made of a dielectric may be formed between the soft magnetic thin film 12 and the dielectric substrate 10 and/or the transmission line 11. The dielectric is preferably a low-loss dielectric. The soft magnetic thin film 12 does not necessarily overlap the signal line 13 when viewed from the z direction. Another dielectric substrate may be disposed approximately parallel to and close to the dielectric substrate 10, and the soft magnetic thin film 12 may be formed on the main surface of the other dielectric substrate with a gap therebetween so as to face the signal line 13 formed on the dielectric substrate 10.

The short side length b of the soft magnetic thin film 12 is greater than the width of the signal line 13. It is desirable that the short side length b is approximately equal to the width b0 of the signal line 13 plus the gap b1 between the signal line 13 and the ground line 141 and the gap b2 between the signal line 13 and the ground line 142. It is desirable that the aspect ratio (long side length a)/(short side length b) of the soft magnetic thin film 12 is in the range of 10 to 20. This is because, when the aspect ratio a/b of the soft magnetic thin film 12 is less than 10, the magnetic sensitivity decreases due to the stronger influence of the demagnetizing field in the long side length a direction, which is the detection axis direction of the AC magnetic field H(ωs), and, when the aspect ratio a/b exceeds 20, the magnetic sensitivity decreases due to the increased transmission loss of the line length and the increased magnetic resistance of the soft magnetic thin film 12.

The soft magnetic thin film 11 has a hard axis along its length and an easy axis along its width. The soft magnetic thin film 12 has magnetic properties along its hard axis, which is its length, that are represented by a magnetization curve as shown in FIG. 5 by a solid line. On the other hand, the soft magnetic thin film 12 has magnetic properties along its easy axis, which is its width, that are represented by a magnetization curve as shown in FIG. 5 by a dotted line.

The soft magnetic thin film 12 may be a Co-Fe-Si-B or Co-Nb-Zr amorphous thin film, a Fe-Si or Fe-Zr-N microcrystalline thin film, or a multilayer thin film formed by stacking these thin films with a thin insulating layer such as SiO sandwiched between them, a Ni-Fe or Fe-Si-Al crystalline thin film, or even a bulk material such as an Fe-Si alloy, an Fe-Co-Ni alloy, or a Sendust alloy, or a thin film made of Co-Fe-Ni-Si-B, Fe-Co-Ni-Zr, Fe-Ni-B, Co-Fe-Zr, or Co-Zr amorphous alloy ribbon, or a thin film made of soft magnetic ferrite.

The signal line 13 of the transmission line 11 and the pair of ground lines 141, 142 are arranged so that the resonant frequency of the LC resonant circuit 2 resulting from the parasitic capacitance between the signal line 13 of the transmission line 11 and each of the pair of ground lines 141, 142 is shifted to the high frequency side (for example, 4 GHz band) compared with the resonant frequency ω ₀ of the LC resonant circuit 2 resulting from the inductor formed by the dielectric substrate 10, the transmission line 11, and the soft magnetic thin film 12, and the first capacitor 21 and the second capacitor 22. This is because the parasitic capacitance between the signal line 13 and the pair of ground lines 141, 142 is easily affected by the surrounding environment, and this is avoided in order to maintain stability of operation.

(Action and Effect)
In the magnetic sensor having this configuration, the magnet 4 applies a bias static magnetic field + _Hs or _-Hs, which is approximately equal in magnitude to the anisotropic magnetic field Hk of the soft magnetic thin film 12, to the soft magnetic thin film 12 in its longitudinal direction, so that the magnetization M is saturated in the hard axis direction of the soft magnetic thin film 12 (see ± _Ms in FIG. 5). When the LC resonant circuit 2 resonates in this state at a resonant angular frequency _ωr , an AC current I( _ωr ) flows in the longitudinal direction of the transmission line 11, and an AC magnetic field H( _ωr ) is formed in the soft magnetic thin film 12, vibrating in its lateral direction, i.e., in the direction of the easy axis of magnetization (see FIGS. 3 and 4). As a result, the electron spins aligned in the hard axis direction of the soft magnetic thin film 12 precess, inducing ferromagnetic resonance.

FIG. 6 shows the real part μ' and imaginary part μ" of the complex permeability μ in the hard axis direction of the soft magnetic thin film 12. The amount of change in the real part μ' and imaginary part μ" of the complex permeability μ is maximized in the vicinity of the angular frequency _ωc at which ferromagnetic resonance occurs. Accordingly, the inductance formed by the transmission line 11 and the soft magnetic thin film 12 can change significantly. At this time, the LC resonant circuit 2 oscillates at a resonant angular frequency _ωr corresponding to the inductance of the magnetic sensing unit 1 and the capacitance of the first capacitor 21 and the second capacitor 22, and the oscillation signal is detected from the output terminal 26 of the LC resonant circuit 2. The oscillation signal is transformed into the frequency domain f by FFT, and a peak P0 corresponding to the resonant frequency _ωr of the LC resonant circuit 2 is detected as shown in FIG. 7.

Here, when an external AC magnetic field H(ωs) that vibrates in the longitudinal direction of the soft magnetic thin film 12 exists, the speed or frequency of precession of some of the electron spins aligned in the direction of the hard magnetization axis of the soft magnetic thin film 12 changes, and accordingly, a modulation component occurs in the inductance formed by the transmission line 11 and the soft magnetic thin film 12. As a result, a modulation component also occurs in the oscillation signal of the LC resonant circuit, and the oscillation signal is converted into the frequency domain by FFT, so that peaks _{P+ and P- corresponding to the modulation component are detected on the high-frequency side and the low-frequency side of the peak P0 corresponding to the resonant frequency f r} =ω _r /2π of the LC resonant circuit, as shown in FIG. 7. Then, the strength |H(ωs)| of the external AC magnetic field H(ωs) is estimated from the heights of the peaks P+ and P-. The frequency f=ω/2π of the external AC magnetic field H(ω) is estimated from the frequency deviation of the peaks P+ and P- from the peak P0.

1: magnetic sensing part, 2: LC resonant circuit, 10: dielectric substrate, 11: transmission line, 12: soft magnetic thin film, 13: signal line, 141, 142: ground lines, C1, C2: capacitors.

Claims (4)

an inductor including a dielectric substrate, a transmission line made of a conductive material extending on a main surface of the dielectric substrate, and a rectangular soft magnetic thin film adjacent to the transmission line, extending along the transmission line, the long side direction being the hard axis of magnetization and the short side direction being the easy axis of magnetization;
a capacitor that configures an LC resonant circuit together with the inductor;
a magnet for applying a bias static magnetic field in a longitudinal direction to the soft magnetic thin film;
A magnetic sensor characterized in that a ratio of a length of the main surface of the soft magnetic thin film in a front-rear direction to a length of the main surface in a left-right direction is in a range of 10-20. The transmission line includes a signal line and a pair of ground lines extending parallel to the signal line and sandwiching the signal line therebetween.
2. The magnetic sensor according to claim 1 . the transmission line includes a signal line and a pair of ground lines extending parallel to the signal line and sandwiching the signal line therebetween;
the short side length of the soft magnetic thin film is greater than the short side length of the signal line;
2. The magnetic sensor according to claim 1. an inductor including a dielectric substrate, a transmission line made of a conductive material extending on a main surface of the dielectric substrate, and a rectangular soft magnetic thin film adjacent to the transmission line, extending along the transmission line, the long side direction being the hard axis of magnetization and the short side direction being the easy axis of magnetization;
a capacitor that configures an LC resonant circuit together with the inductor;
a magnet for applying a bias static magnetic field in a longitudinal direction to the soft magnetic thin film;
the transmission line includes a signal line and a pair of ground lines extending parallel to the signal line and sandwiching the signal line therebetween;
A magnetic sensor characterized in that the signal line and the pair of ground lines are arranged so that the resonant frequency of the LC resonant circuit resulting from the capacitance between the signal line and each of the pair of ground lines is shifted to a higher frequency side compared to the resonant frequency of the LC resonant circuit resulting from the inductor and the capacitor.

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