JP7501677B2 - Magnetic Sensor - Google Patents

Description

The present invention relates to a magnetic sensor.

Micro- and nano-electromechanical systems, known as MEMS (Micro Electro Mechanical Systems) and NEMS (Nano Electro Mechanical Systems), have resonant structures with low vibration loss (high Q factor), and have been used as highly sensitive sensors of various physical quantities such as weight and charge. In particular, applied research into magnetic sensors has a long history spanning two or three decades, and sensors capable of detecting a single electron spin and several hundred nuclear spins have been reported at the basic research level (Non-Patent Document 1, Non-Patent Document 2).

In these studies, a tiny ferromagnetic piece attached to the tip of a cantilever is subjected to a magnetic correlation force by the magnetic field created by the spin magnetic moment in the sample, and highly sensitive sensing is achieved by monitoring the resulting change in the resonant frequency of the cantilever. However, in order to sense the tiny magnetic field emitted from the sample, this technology requires sophisticated spatial control technology to place the cantilever very close to the sample, expensive equipment to enable this spatial control, and special measurement technology to read out the extremely small magnetic correlation force, making it impractical in terms of device robustness and simplicity of measurement method.

One of the most versatile and convenient magnetic sensors using NEMS/MEMS is the surface acoustic wave (SAW) type that uses the magnetostrictive effect (Non-Patent Document 3). SAW is actually used as a general high-frequency signal filter in many devices, including wireless communication terminals. This sensor has a very simple structure with a thin ferromagnetic film evaporated onto its surface. In addition, the magnetization in the ferromagnetic material that changes when exposed to the magnetic field to be detected can be piezoelectrically detected as a change in vibration intensity via the magnetoelastic effect, typified by magnetostriction.

D. Rugar et al., "Single spin detection by magnetic resonance force microscopy", NATURE, vol. 430, pp. 329-332, 2004. C. L. Degen et al., "Nanoscale magnetic resonance imaging", Proceedings of the National Academy of Sciences of the United States of America, vol. 106, no. 5, pp. 1313-1317, 2009. M. Kadota et al., "Magnetic Sensor Based on Surface Acoustic Wave Resonators", Japanese Journal of Applied Physics, vol. 50, 07HD07, 2011.

However, because magnetoelastic effects such as magnetostriction are usually small, in existing NEMS/MEMS sensors, the size of the magnetic thin film that constitutes the magnetic detection part and the SAW is large, reaching the order of millimeters on one side, which limits the sensing sensitivity, including spatial resolution.

Conventional magnetic sensors detect magnetism through changes in the intensity of the SAW's propagating wave. However, because the medium on the vibration side of the magnetoelastic effect is the propagating wave, this coupling effect is small. For this reason, in conventional magnetic sensors, in order to ensure a sufficient S/N ratio and electrically detect the signal change, it was necessary to increase the size of the ferromagnetic thin film that is the magnetic detection section and the piezoelectric detection section (interdigital transducer, IDT). As a result, conventional technologies using SAWs led to larger sensor device structures and reduced sensing capabilities such as low sensitivity and resolution.

On the other hand, attempts are being made to overcome these challenges by using micromechanical oscillators such as cantilevers. This technology has achieved ultra-high resolution and resolution at the basic research level, such as highly sensitive sensing, such as the detection of single electron spins and minority nuclear spins, as well as magnetic resonance imaging (MRI) observation of microstructures such as polymers. However, technology that uses micromechanical oscillators requires expensive equipment and advanced operating techniques for operation and measurement, making it inconvenient from the standpoint of application and practical use.

As described above, conventional technology had the problem that it was not easy to perform magnetic detection with high sensitivity and resolution using small, inexpensive equipment without requiring advanced operating techniques.

The present invention has been made to solve the above problems, and aims to enable magnetic detection with high sensitivity and resolution using a small, inexpensive device without requiring advanced operating techniques.

The magnetic sensor according to the present invention comprises a plate-shaped substrate made of a piezoelectric material, a magnetic layer made of a magnetic material formed on the substrate, a phononic crystal structure formed on the substrate surrounding the magnetic layer, a first electrode formed on the substrate at one location sandwiching the phononic crystal structure for generating an elastic wave on the surface of the substrate, a second electrode formed on the substrate at the other location sandwiching the phononic crystal structure for detecting the elastic wave, and a magnetic field application unit for applying a magnetic field to the magnetic layer, and the elastic waves confined in the region inside the phononic crystal structure where the magnetic layer is formed resonate, and when resonance occurs inside the phononic crystal structure, the magnetization of the magnetic layer resonates in the magnetic field applied by the magnetic field application unit.

As described above, according to the present invention, a phononic crystal structure is provided surrounding a magnetic layer formed on a plate-shaped substrate made of piezoelectric material, and a first electrode is provided on one side of the phononic crystal structure and a second electrode is provided on the other side, making it possible to perform magnetic detection with high sensitivity and resolution using a small, inexpensive device without requiring advanced operating techniques.

FIG. 1 is a diagram showing the configuration of a magnetic sensor according to a first embodiment of the present invention. FIG. 2A is a cross-sectional view showing a partial configuration of the magnetic sensor according to the first embodiment of the present invention. FIG. 2B is a cross-sectional view showing a partial configuration of the magnetic sensor according to the first embodiment of the present invention. FIG. 3A is a plan view showing a partial configuration of the magnetic sensor according to the first embodiment of the present invention. FIG. 3B is a plan view showing a partial configuration of the magnetic sensor according to the first embodiment of the present invention. FIG. 4 is a plan view showing the configuration of the comb-shaped electrodes. FIG. 5 is a characteristic diagram showing the relationship between the frequency of the vibration efficiently excited from the first electrode 104 and the vibration of a surface acoustic wave (SAW). FIG. 6 is an explanatory diagram for explaining the principle of the present invention. FIG. 7A is a diagram showing the configuration of a magnetic sensor according to a second embodiment of the present invention. FIG. 7B is a cross-sectional view showing a partial configuration of another magnetic sensor according to the second embodiment of the present invention. FIG. 8 is a photograph of the magnetic sensor that was actually fabricated.

The following describes a magnetic sensor relating to an embodiment of the present invention.

[First embodiment]
First, a magnetic sensor according to a first embodiment of the present invention will be described with reference to Fig. 1. This magnetic sensor includes a magnetic layer 102 formed on a base 101, a phononic crystal structure 103 formed on the base 101 so as to surround the periphery of the magnetic layer 102, a first electrode 104, and a second electrode 105.

This magnetic sensor also includes a magnetic field application unit 106 that applies a magnetic field to the magnetic layer 102. In FIG. 1, the substrate 101 is shown as a plane. The magnetic field application unit 106 can be disposed, for example, above the magnetic layer 102 on the surface of the substrate 101. The magnetic field application unit 106 can be disposed, for example, below the magnetic layer 102 on the back side of the substrate 101.

The substrate 101 is made of a piezoelectric material, for example, lithium niobate crystal ( _LiNbO3 ), zinc oxide (ZnO), aluminum nitride (AlN), gallium arsenide (GaAs), etc. The magnetic layer 102 is made of a magnetic material, for example, nickel, iron, etc.

The first electrode 104 is formed on one of the portions of the substrate 101 that sandwich the phononic crystal structure 103, and generates a surface acoustic wave on the surface of the substrate 101. The second electrode 105 is formed on the other portion of the substrate 101 that sandwiches the phononic crystal structure 103, and detects the surface acoustic wave. The first electrode 104 and the second electrode 105 can be composed of comb electrodes.

The phononic crystal structure 103 is composed of a plurality of lattice elements 107, which are periodically arranged in a lattice pattern at intervals of n times (n is a natural number) half the wavelength of the above-mentioned surface acoustic wave. In other words, the phononic crystal structure 103 has a lattice constant of n times half the wavelength of the surface acoustic wave. The magnetic layer 102 is formed in the portion of the phononic crystal structure 103 where there are no lattice elements 107 (in the defective portion).

Here, if the periodic interval of the comb electrodes constituting the first electrode 104 or the second electrode 105 is p as shown in Figure 4, and the speed of the surface acoustic waves of the base 101 is v, then the frequency of vibration efficiently excited from the first electrode 104 (the center frequency of the filter constituted by the base 101, the first electrode 104, and the second electrode 105) is expressed as _f0 = v/p. The periodic interval p of the comb electrodes is the wavelength of the surface acoustic waves. Furthermore, if the number of periods of the comb electrodes is N, then the excitation frequency range (3 dB band) is Δf = _f0 /N (Figure 5).

2A, the lattice elements 107 can be recesses formed in the surface of the substrate 101. The lattice elements 107 can also be holes formed in the surface of the substrate 101. For example, the recesses and holes can be formed by microfabrication using well-known lithography and etching techniques.

Also, the lattice elements 107 can be convex portions formed on the surface of the substrate 101, as shown in FIG. 2B. For example, the convex portions can be formed by microfabrication using well-known lithography and etching techniques. Also, for example, the convex portions made of metal can be formed by well-known lithography, metal deposition, and lift-off techniques.

The lattice element 107 can be composed of two or more types of elastic bodies having different elastic properties (mass density and Young's modulus) from the base 101. The lattice element 107, which is a recess or hole, can also be filled with a different material. This different material can have a different elastic coefficient from the base 101. The shape of the lattice element in plan view is not limited to a circle, and can also be a star shape in plan view. As shown in Figures 3A and 3B, the phononic crystal structure 103a can be composed of a periodic structure of lattice elements 107a that are rectangular in plan view. As shown in Figure 3A, the phononic crystal structure 103a can be disposed between the magnetic layer 102 and the first electrode 104, and between the magnetic layer 102 and the second electrode 105. As shown in Figure 3B, the phononic crystal structure 103a can also be disposed outside the first electrode 104 and the second electrode 105 in plan view, with the magnetic layer 102 positioned at the center.

Here, the surface acoustic waves confined in the region where the magnetic layer 102 is formed inside the phononic crystal structure 103 are assumed to resonate. In addition, when resonance occurs inside the phononic crystal structure 103, the magnetization of the magnetic layer 102 is assumed to resonate with the magnetic field applied by the magnetic field application unit 106.

The magnetic sensor described above is composed of a composite structure of two resonators: magnetization and mechanical. The interaction between these two resonators allows the change in magnetization to be observed through the movement of the machine, and the change in the machine to be observed through the movement of the magnetization. Mechanical resonance can be measured by simply measuring the vibration intensity of a specific frequency (resonant frequency), by measuring the phase of resonance, or by tracking the resonant frequency using a feedback circuit, and these measurement methods can be used to measure (detect) micro-magnetism.

The magnetic sensor according to the first embodiment described above is a highly sensitive and highly resolved mechanical magnetic sensor that utilizes an elastic periodic structure, such as a phononic crystal. By utilizing magnetoelastic coupling, such as the magnetostrictive effect (Reference 1) or the magnetic rotation coupling effect (Reference 2), the magnetization of the magnetic detection unit provided on the substrate 101 is resonated by vibration [ferromagnetic resonance (FMR) or spin wave resonance (SWR)]. When this resonance is used, even if there is a slight change in the external magnetic field, the resonant frequency changes and the magnetization strength also changes significantly. Then, mechanical properties such as vibration strength and distortion change through the magnetoelastic effect, so that the change in the magnetic field can be known mechanically.

The key to this invention is the introduction of an artificial elastic periodic structure, such as a phononic crystal, into this vibration structure. Vibration loss can be reduced and spatial localization (reduced mode volume) can be achieved through the band gap effect. In the former, the reduced vibration loss effectively lengthens the interaction time between magnetization and vibration, enhancing the magnetoelastic effect and improving sensitivity. In the latter, the band gap allows the vibration to be tightly confined to a microscopic space, reducing the vibration mode volume to the wavelength scale, which is expected to result in improved magnetic resolution.

The following is a more detailed explanation. When an AC voltage with a frequency f ₀ is applied to the first electrode 104, it is possible to piezoelectrically induce vibrations of that frequency. A part of the excited vibration propagates on the surface of the substrate 101, and the magnetic layer 102 on this surface is also shaken by the propagating surface acoustic wave. In the case of magnetostriction, the magnetic layer 102 is distorted by vibration, and the free energy of magnetization is modulated, and as a result, an effective magnetic field vibrating at a frequency f ₀ is applied to the magnetization of the magnetic layer 102. At the same time, if a static magnetic field μ ₀ H _ex is applied from the outside to the magnetic layer 102 by the magnetic field application unit 106, and the value of the applied magnetic field is adjusted so that the resonance frequency of ferromagnetic resonance (FMR) or the like becomes f ₀ , a magnetic resonance phenomenon such as FMR can be induced using the vibration.

This phenomenon consumes vibration energy and causes resonance, which in turn reduces the vibration strength of the surface acoustic wave as a reaction. Furthermore, resonance causes a significant change in magnetization dynamics, which in turn affects and changes various characteristics of the surface acoustic wave (SAW), such as the sound speed, resonance frequency, and Q value, via magnetostriction. By reading (measuring) these changes in various characteristics, it is possible to measure (detect) changes in the FMR of the magnetic detection unit from multiple angles. This is the basic operating principle of a magnetic sensor (see Figure 6).

[Embodiment 2]
Next, a magnetic sensor according to a second embodiment of the present invention will be described with reference to Figs. 7A and 7B. This magnetic sensor includes a magnetic layer 102 formed on a substrate 101, a phononic crystal structure 103 formed on the substrate 101 so as to surround the magnetic layer 102, a first electrode 104, and a second electrode 105. This magnetic sensor also includes a magnetic field application unit 106 that applies a magnetic field to the magnetic layer 102. Note that the magnetic field application unit 106 is omitted in Fig. 7B. This magnetic sensor also includes a magnetic field application unit 106 that applies a magnetic field to the magnetic layer 102. These configurations are the same as those of the first embodiment described above.

In the second embodiment, the base 101 in the region in which the magnetic layer 102 and the phononic crystal structure 103 are formed has a cavity 108 therein, and the region in which the elastic wave propagates is a thin-film portion 121 (thin-film structure). The thin-film portion 121 is spaced apart from the main body 122 of the base 101.

In the second embodiment, the lattice elements 107 can be holes formed on the surface of the substrate 101 as shown in FIG. 7A. The lattice elements 107 can be convex portions formed on the surface of the thin film portion 121 of the substrate 101 as shown in FIG. 7B. As in the first embodiment, the lattice elements 107 formed as convex portions can be made of two or more types of elastic bodies having different elastic properties (mass density and Young's modulus) from the substrate 101 (thin film portion 121). The lattice elements 107 formed as holes can also be filled with a different substance. This different substance can have a different elastic coefficient from the substrate 101 (thin film portion 121). The shape of the lattice elements in plan view is not limited to a circle, and can also be a star shape in plan view.

A more detailed description will be given of the magnetic sensor according to the second embodiment, which has a so-called thin-film structure. In view of the above-mentioned basic operating principle of the magnetic sensor according to the present invention, the sensitivity and resolution of sensing are determined by how strong the magnetic elastic coupling is and how small the area where the magnetic elastic coupling occurs can be. For example, on the outside of two comb electrodes spaced apart on the surface of the substrate, an elastic periodic structure is formed parallel to the propagation direction of the surface acoustic wave and spaced apart from each other, with the same spacing as the wavelength of the surface acoustic wave. In this configuration, if the surface acoustic wave leaking to the two comb electrodes satisfies the Bragg condition, the incident wave and the reflected wave from the elastic periodic structure cause destructive interference. As a result, the propagation of the surface acoustic wave is prohibited (band gap), and the periodic structure on the substrate functions as a so-called reflector. This causes the surface acoustic wave to resonate, and the vibration loss is reduced.

However, in a normal surface acoustic wave resonator, if you try to strongly confine the surface acoustic wave in order to suppress the vibration mode volume, the proportion of the reflected wave that couples with the bulk mode in the substrate increases, which actually leads to an increase in vibration loss, i.e., a decrease in the Q value. For this reason, there is a trade-off between suppressing vibration loss (high Q value) and reducing the vibration mode volume.

To solve this problem, a phononic crystal resonator (Reference 3) is effective, in which the vibrating part (region) is made into a thinner film and separated from the main body of the supporting substrate. If attempting to strongly confine vibrations increases vibration loss, then a structure in which the vibrating part is made thinner and separated can be used. For phononic crystal structures too, creating a periodic arrangement of through holes can enhance the confinement effect, achieving both a high Q value and a low mode volume at the same time. Of course, this can also be achieved by creating periodic grooves or protrusions, just like with surface acoustic wave devices.

According to Reference 3, the Q value of the resonator is 2,000 on average, and the mode volume is the cube of λ _saw (up to μm ³ order of magnitude). Therefore, if the thin film structure of the second embodiment is used, the sensitivity is expected to improve by nearly two orders of magnitude, and the resolution is expected to improve by nearly three orders of magnitude, from mm ³ to μm ^3. Figure 8 shows a photograph of the magnetic sensor that was actually fabricated. Figure 8 (a) shows a point defect phononic crystal on which a nickel thin film is formed. Figure 8 (b) shows a coupling structure of a phononic crystal resonator and a waveguide, and Figure 8 (c) shows a cross section of a phononic crystal with a thin film structure.

The magnetic sensor according to the above-mentioned embodiment can be used not only in the atmosphere but also in measuring the micromagnetic field of a specimen in a solution, for example. In this case, the grooves and holes that become the lattice elements 107 constituting the phononic crystal structure 103 are filled with liquid. The substrate 101, which is the medium of the magnetic sensor, is solid, and the mass density, bulk modulus, or Young's modulus and Poisson's ratio are significantly different between solids and liquids. Therefore, even in liquid, the confinement of vibration due to the band gap effect of the phononic crystal structure 103 works. The mass density of water (H ₂ O) is 997.2 kg/m ³ , and the bulk modulus of water is 0 to 2.3 GPa. In contrast, the mass density of silicon (Si) is 2330 kg/m ³ , and the bulk modulus of silicon is 102 GPa.

As described above, according to the present invention, a phononic crystal structure is provided surrounding the periphery of a magnetic layer formed on a plate-shaped substrate made of a piezoelectric material, and a first electrode is provided on one side of the phononic crystal structure, and a second electrode is provided on the other side. This makes it possible to perform magnetic detection with high sensitivity and resolution using a small, inexpensive device without requiring advanced operating techniques. According to the present invention, a mechanical resonator with a high Q value can achieve high magnetic sensing (detection) sensitivity. Furthermore, according to the present invention, a low mode volume can achieve high-resolution sensing.

[Reference 1] L. Dreher et al., "Surface acoustic wave driven ferromagnetic resonance in nickel thin films: Theory and experiment", Physical Review B, vol. 86, no. 13, 134415, 2012.
[Reference 2] D. Kobayashi et al., "Spin Current Generation Using a Surface Acoustic Wave Generated via Spin-Rotation Coupling", Physical Review Letters, vol. 119, no. 7, 077202, 2017.
[Reference 3] D. Hatanaka and H. Yamaguchi, "Real-Space Characterization of Cavity-Coupled Waveguide Systems in Hypersonic Phononic Crystals", Physical Review Applied, vol. 13, no. 2, 024005, 2020.

101...substrate, 102...magnetic layer, 103...phononic crystal structure, 104...first electrode, 105...second electrode, 106...magnetic field application section, 107...lattice element.

Claims (7)

A plate-shaped base body made of a piezoelectric material;
a magnetic layer made of a magnetic material formed on the substrate;
a phononic crystal structure formed on the substrate so as to surround the magnetic layer;
a first electrode that generates an acoustic wave on a surface of the base and is formed on one of the portions of the base that sandwich the phononic crystal structure;
a second electrode for detecting the elastic wave, the second electrode being formed on the substrate at the other location between which the phononic crystal structure is sandwiched;
a magnetic field application unit that applies a magnetic field to the magnetic layer,
The elastic wave confined in the region inside the phononic crystal structure where the magnetic layer is formed resonates,
A magnetic sensor comprising: a magnetic field applied by the magnetic field application unit, the magnetic layer being magnetized in resonance with the magnetic field applied by the magnetic field application unit while the resonance is occurring inside the phononic crystal structure. 2. The magnetic sensor according to claim 1,
A magnetic sensor characterized in that the phononic crystal structure is composed of a plurality of lattice elements, and the plurality of lattice elements are arranged periodically in a lattice pattern at intervals of n times (n is a natural number) half the wavelength of the elastic wave. 3. The magnetic sensor according to claim 2,
The magnetic sensor according to claim 1, wherein the grating elements are convex portions formed on the surface of the base. 3. The magnetic sensor according to claim 2,
The magnetic sensor according to claim 1, wherein the grating elements are recesses or holes formed in the surface of the substrate. 5. The magnetic sensor according to claim 4,
A magnetic sensor comprising a different material filled in the recess or the hole, the different material having a different elastic modulus from that of the substrate. The magnetic sensor according to any one of claims 1 to 5,
The magnetic sensor according to claim 1, wherein the first electrode and the second electrode are configured as comb electrodes. The magnetic sensor according to any one of claims 1 to 6,
A magnetic sensor characterized in that the base in the region in which the magnetic layer and the phononic crystal structure are formed has a cavity therein and the region in which the elastic wave propagates is made thin film-like.

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