Magnetic sensor and heterostructure

Abstract

This magnetic sensor includes: a topological semimetal; a ferromagnetic body which is joined to the topological semimetal; and a measurement device for measuring a magnetic field on the basis of a voltage due to an odd-parity magnetoresistance effect generated in the topological semimetal.

Description

Magnetic Sensors and Heterostructures

[0001] The present invention relates to magnetic sensors and heterostructures.

[0002] Magnetic sensors are used in a variety of situations, such as in airplanes, automobiles, smartphones, etc. For example, Patent Document 1 discloses a magnetic sensor using a Hall element.

[0003] Japanese Patent Application Laid-Open No. 2020-13891

[0004] However, due to the measurement principle of the Hall element, it can only measure magnetic field components perpendicular to the thin film of the element, so a magnetic sensor that can measure magnetic field components in any direction is desired.

[0005] The present invention has been made in view of the above circumstances, and one of its exemplary purposes is to provide a magnetic sensor that can measure magnetic field components in any direction.

[0006] One aspect of the present invention is a magnetic sensor comprising a topological semimetal, a ferromagnetic material bonded to the topological semimetal, and a measuring device that measures a magnetic field based on a voltage due to the odd-function magnetoresistance effect generated in the topological semimetal.

[0007] Another aspect of the present invention is a heterostructure comprising a topological semimetal and a ferromagnet bonded to the topological semimetal, wherein the topological semimetal is α-Sn and the ferromagnet is In. １－ｘ Fe ｘ Except for Sb (x is 0.01 to 0.3).

[0008] Any combination of the above components and any transformation of the present invention into a method, device, system, etc. are also valid aspects of the present invention.

[0009] According to the present invention, it is possible to provide a magnetic sensor that can measure magnetic field components in any direction.

[0010] 5 is a diagram showing the configuration of a magnetic sensor according to an embodiment of the present invention;

[0023] FIG. 5 is a diagram showing the results of observing a sample according to an experimental example with a scanning transmission electron microscope and the results of elemental analysis of the sample;

[0024] FIG. 5 is a diagram showing an arrangement for measuring the resistance of a topological semimetal layer of a sample;

[0025] FIG. 5 is a diagram showing the magnetic field dependence of the resistance and differential resistance of a topological semimetal layer at 300 K;

[0026] FIG. 5 is a diagram showing the magnetic field dependence of the resistance and differential resistance of a topological semimetal layer at 3.5 K;

[0027] FIG. 5 is an enlarged view of region A1 shown in FIG. 5;

[0028] FIG. 5 is a diagram showing the results of measuring the magnetic field dependence of the resistance of an InSb substrate at 3.5 K;

[0029] FIG. 5 is a diagram showing the resistance of a topological semimetal layer when the current is changed from 0.1 μA to 5 mA at a temperature of 2 K and in zero magnetic field;

[0029] FIG. 5 is a diagram showing the resistance of a topological semimetal layer when the current is changed from 0.1 μA to 5 mA at a temperature of 2 K and in a magnetic field of 1 T;

[0029] FIG. 5 is a diagram for explaining the angle of the magnetic field direction;

[0029] FIG. 5 is a diagram showing the magnetic field dependence of the differential resistance when the magnetic field direction is parallel to the current. FIG. 10 is a diagram showing the angular dependence of the differential resistance when the current direction is fixed in the x-axis direction and the magnetic field direction is rotated in the xz plane, yz plane, and xy plane.

[0011] (Background) Magnetic sensors generally convert the magnitude of a magnetic field and the amount of change in the magnetic field into an electrical signal. Magnetic sensors, which make invisible magnetic fields visible by converting them into electrical signals, have been researched for a long time and are now widespread in all aspects of daily life. Magnetic sensors originally began with sensors that used the effect of electromagnetic induction. Physical phenomena such as the galvanomagnetic effect, magnetoresistance effect, and Josephson effect have been applied to magnetic sensors in various ways, and magnetic sensors have been put to practical use.

[0012] The most sensitive magnetic sensor is the superconducting quantum interference device (SQUID), which uses Josephson junctions made of superconductors. However, SQUIDs require an extremely low-temperature environment and are therefore not used for general-purpose purposes. The most commonly used magnetic sensors are Hall magnetic sensors, which utilize the Hall effect of nonmagnetic semiconductors, and magnetic sensors based on various magnetoresistance effects of ferromagnetic structures (anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunneling magnetoresistance (TMR)). These magnetic sensors utilize the phenomenon in which the resistance of the element material changes with an external magnetic field. For this reason, the sensitivity of a magnetic sensor is evaluated by the rate of change in resistance R relative to the element's resistance R in the absence of a magnetic field (R - R0) / R. Typically, AMR, GMR, and TMR have sensitivities of approximately 5%, 20%, and 100%, respectively.

[0013] Similarly, Hall magnetic sensors are evaluated as the ratio of the Hall resistance R_Hall to the element resistance R0, and their sensitivity is evaluated as S = R_Hall / (R0 x H), where H is the external magnetic field. For example, Asahi Kasei's ultra-sensitive Hall magnetic sensors use high-mobility semiconductor thin films (GaAs, InAs, InSb, etc.), and their sensitivity S is generally around 0.1 to 5.

[0014] Other important indices of magnetic sensors include linearity with respect to the magnetic field and dynamic range. In these indices as well, Hall elements using nonmagnetic semiconductors are superior to magnetoresistive elements using ferromagnetic materials. However, due to the measurement principle of Hall elements, they can only measure magnetic field components perpendicular to the thin film of the element. Furthermore, the sensitivity of Hall elements is limited by the mobility of the material. Improving the sensitivity of magnetic sensors to both the strength and direction of the magnetic field is technically difficult and increases the cost of the element materials.

[0015] (Sn) Sn, along with Si and Ge, is one of the group IV elements. In the α-phase (α-Sn), which has the same diamond-like crystal structure as Si and Ge, it exhibits a variety of unique physical properties, including a zero band gap and a unique band structure in which the conduction band and valence band are inverted due to strong spin-orbit interactions. When strain is applied to an α-Sn thin film, if the in-plane lattice constant is stretched (tensile strain), α-Sn becomes a topological insulator. Conversely, if the in-plane lattice constant is compressed (compressive strain), α-Sn theoretically becomes a topological Dirac semimetal. Furthermore, it is theoretically predicted that when α-Sn is bonded to a ferromagnetic material, the time-reversal symmetry is broken due to the magnetic proximity effect of the ferromagnetic material, resulting in a phase transition to a magnetic Weyl semimetal.

[0016] Topological Dirac semimetals are important materials as parent phases that can transform into various other topological phases. However, there have been very few experimentally confirmed examples, and only Na ３ Bi and Cd ３ As ２ Although α-Sn has a simple single-element structure, it can be transformed into various topological materials through external control. For this reason, α-Sn is a promising material suitable for exploring topological properties and for applications in quantum information devices.

[0017] Recently, the inventors have developed a single-crystal thin film of the topological Dirac semimetal α-Sn, which has the highest quality crystallinity and mobility one to two orders of magnitude higher (up to 30,000 cm) than other research groups in the world. ２ / Vs) [LD Anh et al., Adv. Mater. 33, 2104645 (2021)]. Taking advantage of the advantages of α-Sn's simple crystal structure, good compatibility with major semiconductors, and environmental friendliness, the fabrication process can be simplified and easily controlled. Therefore, the great advantage of ease of industrial application is expected.

[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicate descriptions will be omitted as appropriate. In this specification and drawings, multiple components having substantially the same functional configuration may be distinguished by adding different letters after the same reference numeral.

[0019] In this specification, "a state in which component A is connected to component B" includes not only a case in which component A and component B are directly physically connected to each other, but also a case in which component A and component B are indirectly connected to each other via other components that do not substantially affect the electrical connection state between them or that do not impair the function or effect achieved by their connection.

[0020] Similarly, "a state in which component C is connected (provided) between component A and component B" includes not only a case in which component A and component C, or component B and component C, are directly connected, but also a case in which they are indirectly connected via other components that do not substantially affect the electrical connection state between them or that do not impair the function or effect achieved by their combination.

[0021] 1 is a diagram showing the configuration of a magnetic sensor 1 according to an embodiment of the present invention. The magnetic sensor 1 includes an element 10, a current source 16, and a measuring device 18.

[0022] The device 10 has a substrate 100, a buffer layer 102, a heterostructure 120, and electrodes 140, 142, 144, and 146. In FIG. 1 , the direction in which the substrate 100, the buffer layer 102, and the heterostructure 120 are stacked is indicated by the z-axis. The direction perpendicular to the z-axis (the in-plane direction of the substrate 100) is indicated by the x-axis, and the direction perpendicular to the x-axis and z-axis is indicated by the y-axis. Hereinafter, the relationship between the x-axis, y-axis, and z-axis is the same as that shown in FIG. 1 .

[0023] The substrate 100 may be made of various semiconductors, such as InSb. In the case of InSb, the substrate 100 may be a (001) oriented substrate. The buffer layer 102 is provided on the substrate 100. The buffer layer 102 may be made of various semiconductors, such as InSb.

[0024] The heterostructure 120 has a ferromagnetic layer 122 and a topological semimetal layer 124 bonded together.

[0025] The ferromagnetic layer 122 is provided on the buffer layer 102. The ferromagnetic layer 122 is made of a ferromagnetic material, for example, InSb, in which part of the In contained in InSb is replaced with Fe. １－ｘ Fe ｘ The value of x is preferably 0.01 to 0.3. This allows the ferromagnetic layer 122 to more reliably impart magnetism to the topological semimetal layer 124 by the proximity effect. Specifically, by setting the value of x to 0.01 or more, the value of In １－ｘ Fe ｘ Sb can reliably provide magnetism, and by setting x to 0.3 or less, In １－ｘ Fe ｘ The crystal structure of Sb can be maintained to be the same as that of InSb.

[0026] The topological semimetal layer 124 is a thin film provided on the ferromagnetic layer 122. The topological semimetal layer 124 is made of a topological semimetal, for example, α-Sn, Na ３ Bi or Cd ３ As ２ Although not shown in FIG. 1 , a layer for protecting the topological semimetal layer 124 from oxidation or the like may be formed on the topological semimetal layer 124.

[0027] When the topological semimetal layer 124 is made of α-Sn, the ferromagnetic layer 122 is made of In １－ｘ Fe ｘ It is preferable that the alloy is made of Sb. １－ｘ Fe ｘ By joining Sb, compressive strain is induced in α-Sn, which makes it a topological Dirac semimetal.

[0028] The topological semimetal layer 124 preferably has a higher electrical conductivity than the other layers (specifically, the substrate 100, the buffer layer 102, and the ferromagnetic layer 122). For example, the topological semimetal layer 124 preferably has a conductivity three orders of magnitude higher than the other layers. This allows the current source 16 to pass a current substantially only through the topological semimetal layer 124.

[0029] The topological semimetal layer 124 exhibits an odd-function magnetoresistance effect due to the magnetic proximity effect from the ferromagnetic layer 122. The odd-function magnetoresistance effect is a phenomenon in which resistance changes when the direction of an external magnetic field is changed from positive to negative, and the difference in resistance (hereinafter also referred to as "differential resistance") is expressed as an odd function with respect to the external magnetic field. Ｂ＿ｏｄｄ is the difference between R(B) and R(-B), that is, R Ｂ＿ｏｄｄ =R(B)-R(-B).

[0030] The electrodes 140, 142, 144, and 146 are provided on the topological semimetal layer 124. The electrodes 140 and 142 are provided on both ends in the x-axis direction of the topological semimetal layer 124. The electrodes 144 and 146 are provided between the electrodes 140 and 142.

[0031] The current source 16 applies a current to the topological semimetal layer 124. The current source 16 according to the present embodiment applies a voltage between the electrodes 140 and 142, thereby applying a current to the topological semimetal layer 124. The current applied by the current source 16 according to the present embodiment flows in the topological semimetal layer 124 in the positive or negative direction of the x-axis.

[0032] The magnitude of the current applied by the current source 16 is preferably 10 μA or less, which more reliably prevents current from flowing through unintended layers other than the topological semimetal layer 124 (specifically, the substrate 100, the buffer layer 102, the ferromagnetic layer 122, etc.).

[0033] The measurement device 18 measures the magnetic field based on the voltage caused by the odd-function magnetoresistance effect generated in the topological semimetal layer 124. In this embodiment, the measurement device 18 measures the magnetic field based on the voltage between the electrodes 144 and 146. The measurement device 18 may include a central processing unit (CPU), a random access memory (RAM), and a read-only memory (ROM) as necessary. The measurement device 18 according to this embodiment includes a resistance calculation unit 180 and a magnetic field calculation unit 182.

[0034] The resistance calculation unit 180 calculates the resistance value of the topological semimetal layer 124 based on the voltage due to the odd-function magnetoresistance effect in response to the current flowing through the topological semimetal layer 124. Specifically, the resistance calculation unit 180 calculates the resistance value of the topological semimetal layer 124 based on the voltage in the direction (x-axis direction) of the current flowing through the topological semimetal layer 124. The resistance calculation unit 180 according to this embodiment calculates the resistance value of the topological semimetal layer 124 based on the voltage between the electrodes 144 and 146.

[0035] The resistance calculation unit 180 can calculate the resistance value of the topological semimetal layer 124 when the magnetic field is oriented in a first direction, and the resistance value of the topological semimetal layer 124 when the magnetic field is oriented in a second direction opposite to the first direction. For example, the resistance calculation unit 180 can calculate the resistance value of the topological semimetal layer 124 when the magnetic field is oriented in the positive direction of the x-axis, and the resistance value of the topological semimetal layer 124 when the magnetic field is reversed so that the magnetic field is oriented in the negative direction of the x-axis. Alternatively, two elements 10 may be arranged in opposite directions with respect to the magnetic field, and the resistance value of the topological semimetal layer 124 may be calculated for each element 10.

[0036] The magnetic field calculation unit 182 calculates a magnetic field component in a direction parallel to the current flowing through the topological semimetal layer 124 based on the resistance value calculated by the resistance calculation unit 180. The magnetic field calculation unit 182 can calculate the magnetic field component in a direction parallel to the current based on the difference (differential resistance) between the resistance value calculated by the resistance calculation unit 180 when the magnetic field is oriented in a first direction and the resistance value calculated by the resistance calculation unit 180 when the magnetic field is oriented in a second direction.

[0037] The topological semimetal layer 124 according to this embodiment exhibits an odd-function magnetoresistance effect, and therefore its resistance value changes when the direction of the magnetic field is reversed. As shown in the experimental example described below, the difference in resistance value corresponds to the magnetic field component in a direction parallel to the current. Therefore, the magnetic field calculation unit 182 can measure the magnetic field component in a direction parallel to the current based on the differential resistance.

[0038] Furthermore, whether the differential resistance is positive or negative is determined depending on whether the magnetic field component is positive or negative. Therefore, the magnetic field calculation unit 182 can determine whether the magnetic field component is positive or negative based on whether the differential resistance is positive or negative.

[0039] Although the present embodiment has been described with reference to an example in which a current flows in the x-axis direction, the present invention is not limited to this example. The magnetic sensor 1 may be configured to allow a current to flow three-dimensionally through the topological semimetal layer 124. For example, in the element 10, multiple electrodes may be arranged to allow a current to flow three-dimensionally. The current source 16 can use these electrodes to change the direction of the current flowing through the topological semimetal layer 124 three-dimensionally. This allows a single element to measure a magnetic field three-dimensionally, and specifically, it becomes possible to measure magnetic field components in any direction, including the y-axis direction and the z-axis direction, in addition to the x-axis direction.

[0040] The configuration of the magnetic sensor 1 according to this embodiment has been described above. With the magnetic sensor 1 according to this embodiment, the measurement device 18 measures a magnetic field based on a voltage due to the odd-function magnetoresistance effect generated in the topological semimetal layer 124. The odd-function magnetoresistance effect (specifically, the differential resistance caused by this effect) has a magnitude corresponding to the magnetic field component parallel to the direction of the current flowing in the topological semimetal layer 124. Therefore, with the magnetic sensor 1 according to this embodiment, it is possible to measure a magnetic field component in any direction by adjusting the direction of the current flowing in the topological semimetal layer 124.

[0041] In the case of a magnetic sensor using Hall elements, in order to measure magnetic field components three-dimensionally, a Hall element for measuring the magnetic field component is required for each of three orthogonal axes. In contrast, the magnetic sensor 1 according to this embodiment can measure magnetic field components in any direction depending on the current direction, making it possible to measure magnetic field components three-dimensionally using a single element 10.

[0042] (Experimental Example) Hereinafter, an experimental example will be described in which an element including a heterostructure of a topological semimetal and a ferromagnet was fabricated and its odd-function magnetoresistance effect was observed.

[0043] First, a 125 nm thick InSb buffer layer was formed on a (001) oriented InSb substrate. Next, a 10 nm thick (In,Fe)Sb ferromagnetic layer with 13.8% Fe substitution for In was grown as a single crystal on the buffer layer using molecular beam epitaxy (MBE). Next, a 3 nm thick α-Sn topological semimetal layer was grown as a single crystal on the ferromagnetic layer using MBE to prepare a device sample. The sample area was 1 × 5 mm. ２ It was decided.

[0044] Figure 2 shows the results of observing the sample of this experimental example using a scanning transmission electron microscope (STEM) and the results of elemental analysis of the sample. As shown in Figure 2, a heterostructure of (In,Fe)Sb and α-Sn was formed on InSb. The surface of the α-Sn was oxidized, and the thickness of the α-Sn was approximately 20 ml.

[0045] The resistance of the fabricated sample in a magnetic field was measured. Figure 3 shows the layout for measuring the resistance of the topological semimetal layer 224 of the sample 20. As shown in Figure 3, the topological semimetal layer 224 is composed of a substrate 200, a buffer layer 202, and a heterostructure 220 stacked in this order from bottom to top. In the heterostructure 220, a ferromagnetic layer 222 and a topological semimetal layer 224 are stacked in this order from bottom to top. In Figure 3, the z-axis indicates the direction in which the substrate 200, the buffer layer 202, and the heterostructure 220 are stacked. In Figure 3, the in-plane direction of the topological semimetal layer 224 is indicated by the x-axis, and the direction perpendicular to the x-axis and z-axis is indicated by the y-axis.

[0046] In this experimental example, the resistance of the topological semimetal layer 224 was measured using a four-terminal method. Electrodes 240, 242, 244, and 246 were formed on the topological semimetal layer 224. A current source 26 was connected to the electrodes 240 and 242 formed at both ends of the topological semimetal layer 224 in the x-axis direction, and a voltmeter 28 was connected to the electrodes 244 and 246 formed between the electrodes 240 and 242. A magnetic field parallel to the direction of the current (1 mA DC) (x-axis direction) was applied to the topological semimetal layer 224 while changing the magnitude of the magnetic field in the order 1 T → -1 T → 1 T. The resistance of the topological semimetal layer 224 was measured at temperatures of 300 K and 3.5 K.

[0047] Using the measured resistance, the differential resistance R Ｂ＿ｏｄｄ (B) was calculated. The differential resistance R Ｂ＿ｏｄｄ (B) represents the difference between the resistance R(B) of the topological semimetal layer 224 in a magnetic field B and the resistance R(-B) of the topological semimetal layer 224 in a magnetic field in the opposite direction, and R Ｂ＿ｏｄｄ (B) = R(B) - R(-B).

[0048] FIG. 4 shows the resistance R and differential resistance R of the topological semimetal layer 224 at 300 K. Ｂ＿ｏｄｄ 5 shows the magnetic field dependence of the resistance R and the differential resistance R of the topological semimetal layer 224 at 3.5 K. Ｂ＿ｏｄｄ4 and 5, the horizontal axis represents the magnetic field B (kOe) applied to the topological semimetal layer 224, the left vertical axis represents the resistance R (mΩ) of the topological semimetal layer 224, and the right vertical axis represents the differential resistance R Ｂ＿ｏｄｄ Shows.

[0049] 4 and 5, the resistance R in a positive magnetic field is smaller than the resistance R in a negative magnetic field of the same magnitude. Therefore, the differential resistance R in a positive magnetic field is Ｂ＿ｏｄｄ becomes negative, and R in the negative magnetic field Ｂ＿ｏｄｄ is positive. Thus, the differential resistance R Ｂ＿ｏｄｄ is reversed in sign for a positive magnetic field and a negative magnetic field, and it can be said that the topological semimetal layer 224 exhibits an odd-function magnetoresistance effect at both temperatures of 3.5K and 300K.

[0050] Typically, in a material that maintains time-reversal symmetry, the magnetoresistance effect remains unchanged even when the magnetic field is reversed, and the resistance must be an even function of the magnetic field (Onsager reciprocity). On the other hand, if there is magnetic interaction within the material and time-reversal symmetry is broken, reversing the magnetic field will result in a slight change in resistance. This change in resistance is the odd-function magnetoresistance effect. The odd-function component of the resistance with respect to the magnetic field is often small, and it is rare to observe the odd-function magnetoresistance effect with only DC current, as in this experimental example. Furthermore, when AC measurements (lock-in (f = 5261 Hz), ω, and 2ω) were performed on the topological semimetal layer 224 of this experimental example, the odd-function magnetoresistance effect was observed, similar to the DC measurements.

[0051] The sensitivity of the magnetic sensor is β = {R Ｂ＿ｏｄｄ (1T) / R(0T)}×100%, the sensitivity of the sample 20 was β=110% at 300 K and β=250% at 3.5 K. Furthermore, when the resistance of the topological semimetal layer 224 was measured at 2 K, the sensitivity was β=2100%.

[0052] FIG. 6 is an enlarged view of region A1 shown in FIG. 5. In FIG. 6, open circles indicate the resistance value when the magnetic field is changed in the decreasing direction, and solid circles indicate the resistance value when the magnetic field is changed in the increasing direction. As shown in FIG. 6, the resistance value shifts to the left (in the direction indicated by the solid arrow from zero magnetic field) when the magnetic field is decreased, and to the right (in the direction indicated by the dashed arrow from zero magnetic field) when the magnetic field is increased, indicating hysteresis. Because the conductivity of α-Sn is about three orders of magnitude higher than that of (In,Fe)Sb, the resistance R shown in FIG. 6 is primarily the resistance of α-Sn. Therefore, the hysteresis shown in FIG. 6 suggests a proximity effect from the ferromagnetic layer 222 in α-Sn. Based on this hysteresis, the ferromagnetic coercivity Hc is estimated to be 200 Oe.

[0053] Figure 7 shows the results of measuring the magnetic field dependence of the resistance of an InSb substrate at 3.5 K. As shown in Figure 7, the resistance of the InSb substrate was the same regardless of the direction of the magnetic field, and no odd-function magnetoresistance effect was observed. The sensitivity β was 2%. A heterostructure of α-Sn and InSb was also fabricated, and the magnetic field dependence of the resistance in this heterostructure was measured. Although the odd-function magnetoresistance effect was observed, the differential resistance value was very small.

[0054] Figure 8 shows the resistance R of the topological semimetal layer 224 when the current is changed from 0.1 μA to 5 mA at a temperature of 2 K in zero magnetic field. In Figure 8, the horizontal axis represents the current (μA), and the vertical axis represents the resistance R (Ω) of the topological semimetal layer 224. As shown in Figure 8, the resistance R rapidly decreases when the current exceeds 400 μA. This is thought to be because, when the current is 400 μA or less, the current flows mainly through the α-Sn, whereas when the current exceeds 400 μA, the current also flows through the buffer layer and the InSb substrate, causing the resistance R to decrease.

[0055] Figure 9 shows the resistance R of the topological semimetal layer 224 when the current is changed from 0.1 μA to 5 mA at a temperature of 2 K and in a magnetic field of 1 T. In Figure 9, the resistance R when the magnetic field direction is perpendicular to the surface of the topological semimetal layer 224 (0 deg) is shown by a circle, the resistance R when the magnetic field direction is antiparallel to the current (90 deg) is shown by a square, and the resistance R when the magnetic field direction is parallel to the current (270 deg) is shown by a triangle.

[0056] As shown in Figure 9, regardless of the direction of the magnetic field applied, the resistance R remains constant when the current is 10 μA or less, and decreases rapidly when the current exceeds 10 μA. This rapid decrease in resistance R is thought to be caused by current also flowing through InSb. Therefore, in order to prevent current from flowing through InSb, it is preferable that the current magnitude be 10 μA.

[0057] FIG. 10 shows the differential resistance R of the topological semimetal layer 224 at 2K. Ｂ＿ｏｄｄ 11 is a diagram illustrating the angle φ of the magnetic field direction.

[0058] FIG. 10 shows the differential resistance R at 2 K for magnetic fields of 0.2 T, 0.4 T, 0.6 T, 0.8 T, and 1 T. Ｂ＿ｏｄｄ 10, the horizontal axis represents the angle φ (deg.) of the magnetic field direction, and the vertical axis represents the differential resistance R Ｂ＿ｏｄｄ The magnitude of the current is 10 μA.

[0059] 11 , when the angle φ of the magnetic field direction is 0°, the magnetic field direction is perpendicular to the topological semimetal layer 224. The magnetic field direction was rotated by one full turn from the 0° direction to a direction tilting in the plane of the topological semimetal layer 224. Specifically, the magnetic field direction was rotated so that when the angle φ was 90°, the magnetic field direction was antiparallel to the current direction; when the angle φ was 180°, the magnetic field direction was perpendicular to the topological semimetal layer 224 and opposite to the 0° direction; and when the angle φ was 270°, the magnetic field direction was rotated so that the magnetic field direction was parallel to the current direction.

[0060] Returning to FIG. 10, the measurement results will be described. As shown in FIG. 10, the odd-function magnetoresistance effect was observed regardless of the magnitude of the magnetic field. The graph interpolated in FIG. 10 (the vertical axis is the differential resistance R Ｂ＿ｏｄｄ , the horizontal axis represents the angle φ from 0 to 360°. As shown in Fig. 1, when the magnitude of the magnetic field is 0.2 T, the resistance R Ｂ＿ｏｄｄ is roughly proportional to a sine function, and the resistance R Ｂ＿ｏｄｄ corresponds to the magnetic field component parallel to the current.

[0061] When the magnetic field B is set to 1 T, the voltage V applied to the topological semimetal layer 224 is ｉｎ , V Ｈ is the difference between the measured voltage V(1T) and the measured voltage V(-1T), and the sensitivity coefficient V Ｈ / BV ｉｎ was calculated to be 23 (mV / mT / V). The sensitivity coefficient of Asahi Kasei's ultra-high sensitivity magnetic sensor (InSb 250Ω element + magnetic flux amplification) was 7.4 (mV / mT / V), and it can be seen that in this experimental example, a sample 20 with extremely high sensitivity was produced.

[0062] FIG. 12 shows the differential resistance R when the magnetic field direction is parallel to the current (B / / I, φ=270 deg). Ｂ＿ｏｄｄ As shown in FIG. Ｂ＿ｏｄｄ is proportional to the square of the magnetic field B. Therefore, the differential resistance R Ｂ＿ｏｄｄ By measuring the magnetic field component parallel to the current direction, it is possible to measure the magnetic field component parallel to the current direction.

[0063] The large odd-function magnetoresistance observed in this experiment is very unusual, and its physical origin is not yet fully understood. However, it is expected to be caused by chiral anomalies that arise when an external magnetic field and current are parallel in an α-Sn / ferromagnet heterostructure undergoing a phase transition to a topological Weyl semimetal.

[0064] In the α-Sn heterostructure, the time-reversal symmetry and the spatial inversion symmetry are broken by the magnetization of (In,Fe)Sb. The band structure calculations revealed that the band structure changes due to the magnetic proximity effect of (In,Fe)Sb on α-Sn in the heterostructure. Specifically, one of the two Dirac cones disappears.

[0065] It is possible that such a change in the band structure of α-Sn is responsible for the large odd-function magnetoresistance effect observed in the sample of this example. Therefore, it is speculated that the odd-function magnetoresistance effect can be realized not only with the magnetic semiconductor (In,Fe)Sb, but also with any ferromagnetic layer that can impart a magnetic proximity effect to α-Sn.

[0066] In this experimental example, a giant odd-function magnetoresistance effect was observed in α-Sn grown on a ferromagnetic semiconductor (In,Fe)Sn, in which the resistance changes linearly in proportion to the external magnetic field parallel to the current flowing through the thin film. This giant odd-function magnetoresistance effect offers the following advantages as a magnetic sensor: (1) Because the topological semimetal layer is composed of the single-element material Sn, material control is easy. Sn is non-toxic and has high controllability with semiconductor technology, making it advantageous for future device applications. (2) The sensitivity of current Hall magnetic sensors is limited by the upper limit of the material's mobility. For this reason, further improvements in sensitivity will be difficult and costly. On the other hand, the magnetic sensor proposed in this disclosure exhibits a resistance change rate of 1 T at room temperature and 2100% at a low temperature of 2 K compared to the resistance R in no magnetic field, achieving performance comparable to the most ultra-sensitive magnetic sensors currently available. (3) When the magnetic field direction is reversed, the differential resistance R Ｂ＿ｏｄｄ Since the sign of is reversed, the differential resistance R Ｂ＿ｏｄｄ The sign of the magnetic field can be determined based on the sign of the differential resistance R. Ｂ＿ｏｄｄis proportional to the magnetic field component parallel to the direction of the current flowing through the thin film. Therefore, by changing the current direction in a single magnetic sensor, it is possible to measure any three-dimensional magnetic field component. (5) The odd-function magnetoresistance effect is almost linearly proportional to the magnetic field up to magnetic fields higher than 1 T, and has a wide dynamic range. The realization of an ultra-sensitive magnetic sensor with the above features enables functionality that conventional magnetic sensors could not achieve.

[0067] FIG. 13 shows the differential resistance R when the current direction is fixed in the x-axis direction and the magnetic field direction is rotated in the xz plane, yz plane, and xy plane. Ｂ＿ｏｄｄ 13 is a graph showing the angle dependence of the resistance R. In FIG. 13, the horizontal axis represents the angle φ (0° to 360°) by which the magnetic field direction is rotated, and the vertical axis represents the differential resistance R. Ｂ＿ｏｄｄ When the magnetic field direction is rotated in the xz plane and the xy plane, when φ = 0°, the magnetic field direction is parallel to the current direction, i.e., the magnetic field direction is in the x-axis direction. When the magnetic field direction is rotated in the yz plane, when φ = 0°, the magnetic field direction is perpendicular to the plane (z-axis direction). The measurement conditions are a measurement temperature of 2 K, a magnetic field strength of 0.2 T, and a current magnitude of 10 μA.

[0068] The theoretical model of the tilted Weyl cone is described in the following reference: (Reference) A. Kundu, et al., New J. Phys. 22, 083081 (2020). The present inventors have confirmed that the measurement results shown in Figure 13 can be explained by the tilted Weyl cone model described in this reference.

[0069] The present invention has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the components and treatment processes, and that such modifications are also within the scope of the present invention.

[0070] The present invention can be used in a magnetic sensor.

[0071] 1 magnetic sensor, 10 element, 16, 26 current source, 18 measuring device, 20 sample, 28 voltmeter, 100, 200 substrate, 102, 202 buffer layer, 120, 220 heterostructure, 122, 222 ferromagnetic layer, 124, 224 topological semimetal layer, 140, 142, 144, 146, 240, 242, 244, 246 electrode, 180 resistance calculation unit, 182 magnetic field calculation unit.

Claims

1. A magnetic sensor comprising: a topological semimetal; a ferromagnetic material joined to the topological semimetal; and a measuring device that measures a magnetic field based on a voltage due to the odd-function magnetoresistance effect generated in the topological semimetal.

2. The magnetic sensor according to claim 1, wherein the topological semimetal is α-Sn.

3. The ferromagnetic material is In １－ｘ Fe ｘ 3. The magnetic sensor according to claim 2, wherein the magnetic material is Sb, where x is 0.01 to 0.

3.

4. A magnetic sensor as described in claim 1, further comprising a current source that applies a current to the topological semimetal, wherein the measuring device has a resistance calculation unit that calculates a resistance value of the topological semimetal based on a voltage due to the odd function magnetoresistance effect in response to the current flowing through the topological semimetal, and a magnetic field calculation unit that calculates a magnetic field component in a direction parallel to the current flowing through the topological semimetal based on the resistance value, wherein the magnetic field calculation unit calculates the magnetic field component based on the difference between the resistance value when the magnetic field direction is in a first direction and the resistance value when the magnetic field direction is in a second direction opposite to the first direction.

5. The magnetic sensor according to claim 4, wherein the current source changes a direction of a current flowing through the topological semimetal in three dimensions.

6. The magnetic sensor according to claim 4, wherein the magnetic field calculation section determines whether the magnetic field component is positive or negative based on whether the difference is positive or negative.

7. The magnetic sensor according to any one of claims 4 to 6, wherein the current is 10 μA or less.

8. A heterostructure comprising a topological semimetal and a ferromagnet bonded to the topological semimetal, wherein the topological semimetal is α-Sn and the ferromagnet is In. １－ｘ Fe ｘ Sb (x is 0.01 to 0.3).