Thermoelectric conversion element and method for making the same

Abstract

In order to further improve the spin-current/electric-current conversion efficiency in a spin-current thermoelectric conversion element, a thermoelectric conversion element includes a magnetic material layer having in-plane magnetization; and an electromotive material layer magnetically coupled with the magnetic material layer. The electromotive material layer includes a first conductor with a spin orbit coupling arising, and a second conductor having lower electric conductivity than electric conductivity of the first conductor.

Claims

1. A thermoelectric conversion element, comprising: a magnetic material layer having in-plane magnetization; and an electromotive material layer magnetically coupled with the magnetic material layer and stacked on the magnetic material layer, wherein the magnetic material layer is a magnetic insulator, the electromotive material layer includes a first conductor with a spin orbit coupling arising, and a second conductor having lower electric conductivity than the electric conductivity of the first conductor, the magnetic material layer is magnetically coupled with one of the first conductor and the second conductor, the electromotive material layer has one multi-layered structure in which the first conductor and the second conductor are stacked alternately in a stacking direction of the magnetic material layer and the electromotive material layer, the one multi-layered structure includes more than one layer of at least one of the first conductor and the second conductor, and the stacking direction is perpendicular to the electromotive material layer.

2. The thermoelectric conversion element according to claim 1, wherein the second conductor is formed so as to extend almost parallel to an interface between the magnetic material layer and the electromotive material layer.

3. A method for making a thermoelectric conversion element, comprising: forming a magnetic material layer having in-plane magnetization, the magnetic material layer being a magnetic insulator; and stacking an electromotive material layer on the magnetic material layer at one site, the electromotive material layer magnetically coupled with the magnetic material layer, wherein the forming the electromotive material layer includes stacking, in a stacking direction of the magnetic material layer and the electromotive material layer, a first conductor with a spin orbit coupling arising and a second conductor having lower electric conductivity than the electric conductivity of the first conductor, magnetically coupling one of the first conductor and the second conductor with the magnetic material layer, and forming more than one layer of at least one of the first conductor and the second conductor, and the stacking direction is perpendicular to the electromotive material layer.

4. The thermoelectric conversion element according to claim 1, wherein the magnetic insulator is magnetically coupled with the one multi-layered structure on a single face.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view illustrating a thermoelectric conversion element described in Patent Literature 1.

(2) FIG. 2 is a schematic view illustrating a typical longitudinal type of thermoelectric conversion element.

(3) FIG. 3 is a schematic view illustrating a thermoelectric conversion element in accordance with an exemplary embodiment of the present invention.

(4) FIG. 4 is a schematic view illustrating a configuration example of an electromotive material layer in the thermoelectric conversion element in accordance with the exemplary embodiment of the present invention.

(5) FIG. 5 is a schematic view to explain the operation of the electromotive material layer in the thermoelectric conversion element in accordance with the exemplary embodiment of the present invention.

(6) FIG. 6 is a schematic view illustrating a configuration example of the thermoelectric conversion element in accordance with the exemplary embodiment of the present invention.

(7) FIG. 7A is a graph illustrating output characteristics of a thermoelectric conversion element in accordance with Example 1.

(8) FIG. 7B is a graph illustrating output characteristics of a thermoelectric conversion element in accordance with Example 2.

(9) FIG. 7C is a graph illustrating output characteristics of a thermoelectric conversion element in accordance with Comparative Example 1.

(10) FIG. 8 is a graph illustrating output characteristics of thermoelectric conversion elements in accordance with Example 3, Example 4, and Comparative Example 2.

DESCRIPTION OF EMBODIMENTS

(11) A thermoelectric conversion element and a method for making the same in accordance with an exemplary embodiment of the present invention will be described with reference to accompanying drawings.

1. OVERALL CONFIGURATION

(12) FIG. 3 schematically illustrates a thermoelectric conversion element in accordance with the present exemplary embodiment. The thermoelectric conversion element has a stacking structure including a magnetic material layer 10 and an electromotive material layer 20. Here, the stacking direction of the magnetic material layer 10 and the electromotive material layer 20 is a z direction. The direction perpendicular to the z direction is an in-plane direction. The in-plane direction is defined by an x direction and a y direction which are normal to each other.

(13) The magnetic material layer 10 has magnetization in at least one of in-plane directions. The magnetic material layer 10 is formed of a material with the spin Seebeck effect arising. The material of the magnetic material layer 10 can be ferromagnetic metals or magnetic insulators. The ferromagnetic metals include NiFe, CoFe, CoFeB, and the like. The magnetic insulators include yttrium iron garnet (YIG, Y.sub.3Fe.sub.5O.sub.12), bismuth (Bi)-doped YIG (Bi:YIG), lanthanum (La)-added YIG (LaY.sub.2Fe.sub.5O.sub.12), yttrium gallium iron garnet (Y.sub.3Fe.sub.5-xGa.sub.xO.sub.12), spinel ferrite materials composed of a chemical composition of MFe.sub.2O.sub.4 (M representing a metal element including any one of Ni, Zn, and Co), and the like. It is desirable to use magnetic insulators in the light of suppressing the heat conduction due to electrons.

(14) The electromotive material layer 20 includes a material with the inverse spin Hall effect (spin orbit coupling) arising. The electromotive material layer 20 is formed so as to be magnetically coupled with the magnetic material layer 10. In the present specification, it is referred to as being magnetically coupled to be in a state in which the spin injection phenomenon can arise. The spin injection phenomenon can arise in a case where the magnetic material layer 10 is in immediate contact with the electromotive material layer 20 or in a case where they are so close to each other that the spin angular momentum can be transferred even if they are not in immediate contact with each other. That is to say, even though there is a void between the magnetic material layer 10 and the electromotive material layer 20 or an intermediate layer is inserted between them, it is considered that there is a magnetic coupling if the spin injection phenomenon can arise.

(15) If a temperature gradient in the z direction is applied to such a thermoelectric conversion element, the spin-current is induced at the interface between the magnetic material layer 10 and the electromotive material layer 20. It becomes possible to realize the thermoelectric conversion of generating a thermal electromotive force from a temperature gradient by converting the spin-current into electric electromotive force by the inverse spin Hall effect in the electromotive material layer 20 and taking out the electromotive force as electric power.

2. ELECTROMOTIVE MATERIAL LAYER

(16) The electromotive material layer 20 in the thermoelectric conversion element in accordance with the present exemplary embodiment will be described in detail below. As will become obvious later, according to the present exemplary embodiment, the electromotive material layer 20 is realized which has an excellent spin-current/electric-current conversion efficiency.

2-1. Configuration Example

(17) FIG. 4 is a schematic view illustrating a configuration example of the electromotive material layer 20 in accordance with the exemplary embodiment. In the example illustrated in FIG. 4, the electromotive material layer 20 includes a conductive layer 21 and a weakly conductive layer 22. In more detail, both the conductive layer 21 and the weakly conductive layer 22 have a layered structure parallel to an x-y plane, and the conductive layer 21 and the weakly conductive layer 22 are alternately stacked in the z direction. In other words, the electromotive material layer 20 has a multi-layered structure including the conductive layer 21 and the weakly conductive layer 22.

(18) The conductive layer 21 (a first conductor) is formed out of a material with the inverse spin Hall effect (spin orbit coupling) arising. The conductive layer 21 contains a metal material having large spin orbit coupling, for example. As such a metal material, there are Au, Pt, and Pd with a relatively large spin orbit coupling, transition metals having d orbital or f orbital, or alloy materials containing above-described metals, for example. A similar effect can be obtained by only adding a material such as Fe and Ir to a general metal film material such as Cu in concentrations approximately from 0.5 to 10 mol %. If any one of W, Ta, Mo, Nb, Cr, V, and Ti in the transition metals is used, the voltage can be obtained whose sign is reverse to that using any one of Au, Pt, Pd, and alloys containing them. Alternatively, the material of the conductive layer 21 can be oxides such as ITO (indium tin oxide) or semiconductors.

(19) The weakly conductive layer 22 (a second conductor) has lower electric conductivity than that of the conductive layer 21. The electric conductivity characterizing the weakly conductive layer 22 is concerned with electric conductivity in the direction (the z direction in many cases) parallel to the spin-current injected into the electromotive material layer 20 if the temperature gradient in the z direction is applied to the thermoelectric conversion element. The electric conductivity is relatively low compared with electric conductivity in the direction of electric current arising (in-plane direction) in the conductive layer 21. The electric conductivity described here means electric conductivity which actually appears with the inclusion of the effects of the shape in a state of having made the electromotive material layer 20, the effects of the surface and the interface, and the effects related to external fields such as electric field and magnetic field, temperature, phase transition of a material, and the like.

2-2. Operation and Effect

(20) Next, with reference to FIG. 5, thermal spin-current/electromotive force conversion in the thermoelectric conversion element in accordance with the present exemplary embodiment will be described. In FIG. 5, the thermoelectric conversion element includes the magnetic material layer 10 having in-plane magnetization M (in x direction) and the electromotive material layer 20 disposed on the magnetic material layer 10. The electromotive material layer 20 has a multi-layered structure including the conductive layer 21 and the weakly conductive layer 22.

(21) The temperature gradient VT in the direction from the magnetic material layer 10 to the electromotive material layer 20 is applied to the thermoelectric conversion element having such a structure. In this case, a thermal spin-current through interaction of spins is generated in the magnetic material layer 10. In addition, a spin injection arises by transferring the spin angular momentum to conduction electrons in the electromotive material layer 20 at the interface between the magnetic material layer 10 and the electromotive material layer 20, and a pure spin-current arises in the electromotive material layer 20. The pure spin-current arises so that an up-spin parallel to the magnetization M of the magnetic material layer 10 may coexist with a down-spin antiparallel to it. The up-spin electrons flow along the temperature gradient and the down-spin electrons flow against the temperature gradient.

(22) When the spin conduction electrons moving as described above pass through the weakly conductive layer 22, the scattering probability of the spin conduction electrons increases. As a result of the scattering (a skew scattering and a side jump), the motion of the spin conduction electrons changes into a motion in a direction perpendicular to both the magnetization M and the temperature gradient, that is, a motion in the lateral direction. As a result, electric current flows in a direction perpendicular to the pure spin-current, that is to say, the inverse spin Hall effect appears. This can be referred to as an extrinsic effect as opposed to an intrinsic effect caused by a crystal structure, a configuration of electron orbitals, and the like.

(23) The spin Hall conductivity arising at this time becomes a very large value reflecting large intra-layer electric conductivity of the conductive layer 21. On the other hand, the electric conductivity in the z direction of the electromotive material layer 20, by which the spin-Hall angle is defined, becomes small due to reflecting the electric conductivity caused by the scattering in the weakly conductive layer 22 and the interlayer. As a result, it becomes possible to obtain a large spin-Hall angle.

(24) As described above, according to the present exemplary embodiment, it is possible to obtain the extrinsic spin Hall effect by providing the weakly conductive layers 22 in the electromotive material layers 20. In addition, a large spin-Hall angle is realized by an unprecedented mechanism by combining a new mechanism of the anisotropy of electric conductivity. As a result, the conversion efficiency of the material used for the spin-current/electric-current conversion increases, and then, the conversion efficiency of the spin-current thermoelectric conversion element also increases largely.

2-3. Generalization

(25) The configuration of the electromotive material layer 20 according to the present exemplary embodiment is not limited to that illustrated in FIG. 4. In general, it is only necessary for the electromotive material layer 20 to include a first conductor with a spin orbit coupling arising and a second conductor having lower electric conductivity than that of the first conductor. This enables to obtain the above-described effect to some extent.

(26) Preferably, the second conductor is formed so as to extend almost parallel to the interface between the magnetic material layer 10 and the electromotive material layer 20, that is, an x-y plane. This enables to obtain a large spin-Hall angle.

(27) It is much preferable for the first conductor and the second conductor to have layered structures parallel to the x-y plane as illustrated in FIGS. 4 and 5. That is to say, it is preferable for the electromotive material layer 20 to have the stacking structure including the conductive layer 21 (the first conductor) and the weakly conductive layer 22 (the second conductor). Here, a multi-layered structure is much preferable in which more than one layer is formed each of which composes at least one of the conductive layer 21 and the weakly conductive layer 22.

(28) The stacking structure enables a flexible device design. For example, it is possible to select particularly a material to increase the spin injection efficiency as the material of the conductive layer 21 to form the interface with the magnetic material layer 10, and select an inexpensive material having high electric conductivity as the material of the other conductive layer 21.

(29) The thickness of each layer in the stacking structure can be optimized so that the device performance may be maximized. The thickness of each layer is not particularly limited. With regard to the minimum value of the thickness of each layer, it is also possible to use a layer with a thickness corresponding to a monoatomic layer. Further, even a layer thinner than the monoatomic layer, that is, a sub-monolayer can be regarded as a film if a two-dimensional potential without discontinuity can be composed due to a spread of the wave function of an element introduced to form a layer.

(30) With regard to the number of stacked layers, the optimum value can be determined, taking into account the spin-current diffusion length, the electric conductivity, and the like in the electromotive material layer 20, so that the thermoelectric conversion output may be maximized.

(31) Although the conductive layer 21 is in immediate contact with the magnetic material layer 10 in the examples illustrated in FIGS. 4 and 5, the weakly conductive layer 22 may be in immediate contact with the magnetic material layer 20.

(32) It is also possible for a person skilled in the film formation technologies to improve the output by making all kinds of efforts, that is, making a detail of the conductive layer 21 or the weakly conductive layer 22 further multi-layered, introducing a material having non-uniform composition, and continuously depositing only the conductive layer 21 and the weakly conductive layer 22 partly.

(33) The electromotive material layer 20 according to the present exemplary embodiment can be applied not only to the longitudinal type of thermoelectric conversion element but also to a lateral type of thermoelectric conversion element as illustrated in FIG. 1. The same effect can be also obtained by the lateral type of thermoelectric conversion element.

3. A METHOD FOR MAKING

(34) Next, a method for making the thermoelectric conversion element in accordance with the present exemplary embodiment will be described.

(35) Examples of the method for forming the magnetic material layer 10 include a sputtering method, a metal organic decomposition method (MOD method), a sol-gel method, an aerosol deposition method (AD method), a ferrite plating method, a liquid-phase epitaxy method, a solid-phase epitaxy method, a vapor-phase epitaxy method, a dip method, a spray method, a spin coat method, a printing method, and the like. In this case, the magnetic material layer 10 is deposited on some kind of support. Alternatively, it is possible to use as the magnetic material layer 10 a magnetic insulator fiber formed by using a crystal pulling method and the like or a bulk body formed by using a sintering method, a fusion method, and the like.

(36) As a method for forming the conductive layer 21 and the weakly conductive layer 22, similarly, there is a deposition method using any one of a sputtering method, a vapor deposition method, a plating method, a screen printing method, an ink jet method, a spray method, a spin coat method, and the like. It is possible to use a coating and a sintering of nano-colloidal solution (see Japanese Unexamined Patent Application Publication No. 07-188934 and No. 09-20980) and the like.

4. EXAMPLES

(37) With reference to FIG. 6, an example of forming a thermoelectric conversion element by the inventors of the present invention will be described. In this example, a crystalline gadolinium gallium garnet (GGG) wafer 700 m thick was used as a substrate (not illustrated), on which a spin-current thermoelectric conversion element was formed.

(38) Bismuth-substituted yttrium iron garnet (Bi:YIG, BiY.sub.2Fe.sub.5O.sub.12) was used as the material of the magnetic material layer 10. The Bi:YIG film was formed by the metal-organic decomposition method (MOD method). The MOD solution made by Kojundo Chemical Laboratory Co., Ltd was used as a solution. In the solution, metal raw materials made at an appropriate mole fraction (Bi:Y:Fe=1:2:5) are dissolved at a concentration of 3% in acetate ester with a carboxylated state. The solution was applied on the GGG substrate by means of a spin coating (at a rotation speed of 1000 rpm, rotating during 30 s) and dried for five minutes by a hot plate at 150 C. And then, a pre-annealing process was performed for five minutes at 500 C., and finally, a main annealing process was performed for 14 hours at high temperature of 700 C. in an air atmosphere in an electric furnace. This made the crystalline Bi:YIG film about 65 nm thick formed on the GGG substrate.

(39) Subsequently, the electromotive material layer 20 was formed. Concretely, a Pt film 5 nm thick was evaporated by means of a sputtering as the conductive layer 21 to make contact with the magnetic material layer 10. And then, the weakly conductive layer 22 was formed on the conductive layer 21. In addition, a Pt film 5 nm thick was evaporated by means of a sputtering as the conductive layer 21 on the weakly conductive layer 22.

Example 1

(40) As the above-described weakly conductive layer 22, a Ti thin film 1 nm thick was evaporated by means of a sputtering.

Example 2

(41) As the weakly conductive layer 22, a W thin film 1 nm thick was evaporated by means of a sputtering.

Comparative Example 1

(42) As comparative example 1, a sample without the weakly conductive layer 22 was formed. In this case, the electromotive material layer 20 is composed of only a Pt thin film 10 nm thick.

(43) With respect to each of Example 1, Example 2, and Comparative Example 1, cutting out a strip evaluation element 2 mm8 mm in size, the thermoelectric conversion performance was measured. Concretely, spin-Seebeck signals were measured when various temperature differences were applied to each of the evaluation elements in the z direction. FIGS. 7A to 7C illustrate measurement results on Example 1, Example 2, and Comparative Example 1, respectively. In order to see whether or not signals are spin-Seebeck signals, by applying an external magnetic field, it is measured if an output voltage V.sub.ISHE is reversed depending on a reversal of the magnetization direction.

(44) A spin-Seebeck constant S was estimated roughly from the value of the measured output voltage V.sub.ISHE and the temperature difference applied to an entire sample. As can be seen in FIGS. 7A to 7C, a larger spin-Seebeck constant S is obtained in each of Example 1 and Example 2 as compared with that in Comparative Example 1. Since the value of internal resistance R in Example 1 and Example 2 is almost the same as that in Comparative Example 1, one could argue that the larger conversion efficiency exceeding that in Comparative Example 1 has been obtained in each of Example 1 and Example 2.

Example 3

(45) As the conductive layer 21, a Cu film 5 nm thick was evaporated by means of a sputtering instead of the above-described Pt film. As the weakly conductive layer 22, a Pt thin film 1 nm thick was evaporated by means of a sputtering.

Example 4

(46) As the conductive layer 21, a Cu film 5 nm thick was evaporated by means of a sputtering instead of the above-described Pt film. As the weakly conductive layer 22, a W thin film 1 nm thick was evaporated by means of a sputtering.

Comparative Example 2

(47) As comparative example 2, a sample without the weakly conductive layer 22 was formed. In this case, the electromotive material layer 20 is composed of only a Cu thin film 10 nm thick.

(48) With respect to each of Example 3, Example 4, and Comparative Example 2, the thermoelectric conversion performance was measured as is the case with the above. FIG. 8 illustrates measurement results concerning each of Example 3, Example 4, and Comparative Example 2.

(49) Cu is a material having a small spin-Hall angle. Therefore, a spin-Seebeck constant S with a very small positive number is observed in Comparative Example 2.

(50) In Example 3, as a result of selecting Pt, as the weakly conductive layer, which has electric conductivity smaller than that of Cu and a spin-Hall angle larger than that of Cu, a spin-Seebeck constant S about 10 times as large as that of Comparative Example 2 can be obtained.

(51) In Example 4, as a result of selecting W, as the weakly conductive layer, which has electric conductivity smaller than that of Cu and a spin-Hall angle with the sign opposite to that of Pt and Cu, the spin-Seebeck constant S about three times as large as that of Comparative Example 2 with the reversed sign can be obtained.

(52) As described above, it is possible to control the spin-current/electric-current conversion function of the entire electromotive material layer 20 by combining materials having various magnitudes or different signs of the spin-Hall angle.

(53) The exemplary embodiment of the present invention has been described above with reference to the accompanying drawings. However, the present invention is not limited to the foregoing embodiment, but various changes may be made therein by those of ordinary skill in the art without departing from the spirit and scope of the present invention.

(54) The whole or part of the exemplary embodiments disclosed above can be described as, but not limited to, the following supplementary notes.

(55) (Supplementary Note 1)

(56) A thermoelectric conversion element, comprising: a magnetic material layer having in-plane magnetization; and an electromotive material layer magnetically coupled with the magnetic material layer, wherein the electromotive material layer includes a first conductor with a spin orbit coupling arising, and a second conductor having lower electric conductivity than electric conductivity of the first conductor.

(57) (Supplementary Note 2)

(58) The thermoelectric conversion element according to Supplementary note 1, wherein the second conductor is formed so as to extend almost parallel to an interface between the magnetic material layer and the electromotive material layer.

(59) (Supplementary Note 3)

(60) The thermoelectric conversion element according to Supplementary note 1 or 2, wherein the first conductor and the second conductor have layered structures.

(61) (Supplementary Note 4)

(62) The thermoelectric conversion element according to the Supplementary note 3, wherein more than one layer is formed each of which composes at least one of the first conductor and the second conductor.

(63) (Supplementary Note 5)

(64) A method for making a thermoelectric conversion element, comprising: a step for forming a magnetic material layer having in-plane magnetization; and a step for forming an electromotive material layer magnetically coupled with the magnetic material layer, wherein the step for forming the electromotive material layer includes a step for forming a first conductor with a spin orbit coupling arising, and a step for forming a second conductor having lower electric conductivity than electric conductivity of the first conductor.

(65) This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-011338, filed on Jan. 24, 2013, and the disclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

(66) 10 Magnetic material layer

(67) 20 Electromotive material layer

(68) 21 Conductive layer

(69) 22 Weakly conductive layer