Patent Publication Number: US-2018033940-A1

Title: Spin current-electric current conversion structure, thermoelectric conversion element using the same, and method for making the same

Description:
TECHNICAL FIELD 
     The present invention relates to spin current-electric current conversion structures, thermoelectric conversion elements using the structures, and methods for making the thermoelectric conversion elements and, in particular, to a spin current-electric current conversion structure using the spin Hall effect and the inverse spin Hall effect, a thermoelectric conversion element using the structure, and a method for making the thermoelectric conversion element. 
     BACKGROUND ART 
     The expectations for thermoelectric conversion elements are rising as one of thermal management technologies for sustainable society. Heat is the most common energy source that can be recovered in various situations, such as body temperature, solar heat, waste heat produced from engines, and industrial waste heat. This makes it possible to predict that thermoelectric conversion technologies become increasingly important from now on in various uses such as promotion of energy use efficiency, power feeding to ubiquitous terminals and sensors, and thermal flow visualization by heat flow sensing. 
     In these circumstances, a thermoelectric conversion element based on the “spin Seebeck effect”, in which a current of spin angular momentum (spin current) is generated by applying a temperature gradient (temperature difference) to a magnetic material, has been developed in recent years. The thermoelectric conversion element based on the spin Seebeck effect is composed of a double-layered structure including a magnetic material layer having magnetization in a single direction and a conductive electromotive film. When a temperature gradient is applied to the element in a direction perpendicular to the plane of the element (normal direction), a current of spin angular momentum (spin current) is induced in the magnetic material due to the spin Seebeck effect. The spin current is injected into the electromotive film and converted into an electric current due to “the inverse spin Hall effect” in the electromotive film. This enables “the thermoelectric conversion” of generating electricity from a temperature gradient. 
     In order to obtain large electromotive force with such a thermoelectric conversion element, it is important to use a material in which the conversion between the spin current and the electric current is efficiently performed. As the material in which the spin current-electric current conversion is performed, platinum (Pt) with large spin Hall effect has been mainly used conventionally. Specifically, a thermoelectric conversion element can be formed by using single-crystal yttrium iron garnet (Y 3 Fe 5 O 12 : YIG) that is a type of garnet ferrites as magnetic insulator and using platinum (Pt) wire as an electromotive film, for example. It is possible to use gold (Au), iridium (Ir), tantalum (Ta), tungsten (W), and the like for the electromotive film. These are transition metals belonging to the sixth period of the periodic table of the elements and materials falling into a category that is generally called 5d-transition metal. It is known that 5d-transition metal materials have large spin current-electric current conversion efficiency compared to other materials such as 4d-transition metals among metal element materials composed of a single element. 
     Patent Literature 1 discloses an example in which the spin current-electric current conversion is performed by using conductive oxide materials. An electric current-spin current conversion element described in Patent Literature 1 is configured to perform the conversion between electric current and spin current using the spin Hall effect or the inverse spin Hall effect of the 5d-transition metal oxide. It is also described that iridium oxide (IrO 2 ) is a material that exhibits a large spin Hall resistivity and spin Hall angle compared to platinum (Pt). This makes it possible, they say, to obtain the suggestion that iridium oxide (IrO 2 ) has promise as a material for electric current-spin current conversion elements. 
     In addition, Patent Literature 2 discloses related technologies. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] WO 2012/026168 
     [PTL 2] Japanese Unexamined Patent Application Publication No. 2008-070336 
     SUMMARY OF INVENTION 
     Technical Problem 
     As mentioned above, 5d-transition metal materials and oxide materials containing 5d-transition-metal element have been mainly used for the spin current-electric current conversion structure. However, the spin current-electric current conversion structure containing 5d-transition metal element has the problem of low efficiency of the spin current-electric current conversion. Specifically, for example, if a thermoelectric conversion element using a 5d-transition metal material and the spin Seebeck effect is applied to a thermal flow sensor, the sensitivity of the thermal flow sensing is lower than that of an existing thermal flow sensor. Therefore, it is necessary to make the spin current-electric current conversion more efficient in order to achieve higher electromotive force of the thermoelectric conversion element. 
     As mentioned above, there is the problem that the spin current-electric current conversion structure using a material containing 5d-transition metal has low efficiency of spin current-electric current conversion. 
     An object of the present invention is to provide a spin current-electric current conversion structure that solves the above-mentioned problem that a spin current-electric current conversion structure using a material containing 5d-transition metal has low efficiency of spin current-electric current conversion, and provide a thermoelectric conversion element using the structure, and a method for making the thermoelectric conversion element. 
     Solution to Problem 
     A spin current-electric current conversion structure according to an exemplary aspect of the present invention includes a 4d-transition metal oxide structure consisting primarily of an oxide containing a 4d-transition-metal element; a spin current input-output structure configured to allow a spin current to flow into and out in a direction perpendicular to a plane of the 4d-transition metal oxide structure; and an electric current input-output structure configured to allow an electric current to flow into and out, the electric current conducting in an in-plane direction of the 4d-transition metal oxide structure. 
     A thermoelectric conversion element according to an exemplary aspect of the present invention includes a magnetic material layer containing a magnetic material exhibiting spin Seebeck effect; and an electromotive material connected to the magnetic material layer so that a spin current can flow into and out, and configured to generate electromotive force due to inverse spin Hall effect, wherein the electromotive material includes a spin current-electric current conversion structure, which includes a 4d-transition metal oxide structure consisting primarily of an oxide containing a 4d-transition-metal element; a spin current input-output structure configured to allow a spin current to flow into and out in a direction perpendicular to a plane of the 4d-transition metal oxide structure; and an electric current input-output structure configured to allow an electric current to flow into and out, the electric current conducting in an in-plane direction of the 4d-transition metal oxide structure. 
     A memory element according to an exemplary aspect of the present invention includes a magnetic free layer; a barrier layer connected to the magnetic free layer; a magnetic fixed layer configured to form a tunnel junction with the magnetic free layer through the barrier layer; and a conductive layer disposed so that a spin current may arise due to spin Hall effect, and so that the spin current may flow into the magnetic free layer, wherein the conductive layer includes a spin current-electric current conversion structure, which includes a 4d-transition metal oxide structure consisting primarily of an oxide containing a 4d-transition-metal element; a spin current input-output structure configured to allow a spin current to flow into and out in a direction perpendicular to a plane of the 4d-transition metal oxide structure; and an electric current input-output structure configured to allow an electric current to flow into and out, the electric current conducting in an in-plane direction of the 4d-transition metal oxide structure. 
     A method for making a thermoelectric conversion element according to an exemplary aspect of the present invention includes stacking, on a substrate, a magnetic material layer containing a magnetic material exhibiting spin Seebeck effect; stacking, on the magnetic material layer, an electromotive material connected to the magnetic material layer so that a spin current can flow into and out, and configured to generate electromotive force due to inverse spin Hall effect; and forming two electrode sections apart from each other, each of which is electrically connected to the electromotive material, wherein the stacking of the electromotive material includes forming the electromotive material in such a way as to include a the spin current-electric current conversion structure, which includes a 4d-transition metal oxide structure consisting primarily of an oxide containing a 4d-transition-metal element; a spin current input-output structure configured to allow a spin current to flow into and out in a direction perpendicular to a plane of the 4d-transition metal oxide structure; and an electric current input-output structure configured to allow an electric current to flow into and out, the electric current conducting in an in-plane direction of the 4d-transition metal oxide structure. 
     Advantageous Effects of Invention 
     According to the spin current-electric current conversion structure, the thermoelectric conversion element using the structure, and the method for making the thermoelectric conversion element of the present invention, it is possible to make the spin current-electric current conversion more efficient. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating the configuration of a spin current-electric current conversion structure according to a first example embodiment of the present invention. 
         FIG. 2  is a perspective view illustrating the configuration of a thermoelectric conversion element according to a second example embodiment of the present invention. 
         FIG. 3  is a perspective view illustrating the configuration of an evaluation thermoelectric conversion element according to the second example embodiment of the present invention. 
         FIG. 4  is a diagram illustrating the dependence on temperature difference of the thermoelectromotive force of the evaluation thermoelectric conversion element according to the second example embodiment of the present invention. 
         FIG. 5  is a diagram illustrating the dependence on anneal temperature of the thermoelectric coefficient of the evaluation thermoelectric conversion element according to the second example embodiment of the present invention. 
         FIG. 6  is a diagram illustrating the dependence on anneal temperature of the electric resistivity of a ruthenium oxide film included in the thermoelectric conversion element according to the second example embodiment of the present invention. 
         FIG. 7  is a perspective view illustrating the configuration of a memory element according to a third example embodiment of the present invention. 
         FIG. 8  is a diagram illustrating the dependence on temperature difference of the thermoelectromotive force of a thermoelectric conversion element according to example 1 of the present invention. 
         FIG. 9  is a perspective view illustrating the configuration of a thermoelectric conversion element according to example 2 of the present invention. 
         FIG. 10  is a diagram illustrating the dependence on temperature difference of the thermoelectromotive force of the thermoelectric conversion element according to example 2 of the present invention. 
         FIG. 11  is a perspective view illustrating the configuration of a memory element according to example 3 of the present invention. 
     
    
    
     EXAMPLE EMBODIMENT 
     Example embodiments of the present invention will be described below with reference to the drawings. 
     First Example Embodiment 
       FIG. 1  is a perspective view illustrating the configuration of a spin current-electric current conversion structure  100  according to a first example embodiment of the present invention. The spin current-electric current conversion structure  100  includes a 4d-transition metal oxide structure  110 , a spin current input-output structure  120 , and an electric current input-output structure  130 . 
     The 4d-transition metal oxide structure  110  consists primarily of an oxide containing a 4d-transition-metal element. The 4d-transition metal is one of transition metals belonging to the fifth period of the periodic table of the elements, and has the 4d-orbital occupied by electrons in the order of the atomic number from yttrium (Y) of atomic number  39  to silver (Ag) of atomic number  47 . 
     The oxide containing a 4d-transition-metal element that constitutes the 4d-transition metal oxide structure  110  includes at least one of ruthenium oxide, rhodium oxide, and niobium oxide each of which has the rutile crystal structure. 
     The valence of the 4d-transition metal in the oxide containing the 4d-transition-metal element can be determined so that the spin Hall angle of the oxide containing the 4d-transition-metal element may be maximized. The spin Hall angle represents conversion efficiency between spin current and electric current, and is given by the ratio of the spin current to the electric current. 
     The spin current input-output structure  120  allows a spin current  10  to flow into and out in a direction perpendicular to the plane of the 4d-transition metal oxide structure  110 . For example, the interface between the 4d-transition metal oxide structure  110  and a magnetic material can be used as the spin current input-output structure  120 . 
     The electric current input-output structure  130  allows an electric current  20  to flow into and out, and the electric current  20  conducts in an in-plane direction of the 4d-transition metal oxide structure  110 . For example, two terminals or electrode sections can be used as the electric current input-output structure  130 , and the two terminals or electrode sections are electrically connected to the 4d-transition metal oxide structure  110  respectively and disposed apart from each other. 
     According to the spin current-electric current conversion structure  100  of the present example embodiment, the above-described configuration enables the spin current-electric current conversion to become more efficient. 
     The spin current-electric current conversion structure  100  of the present example embodiment is configured to include the 4d-transition metal oxide structure  110  that consists primarily of the oxide containing the 4d-transition-metal element. This makes it possible to achieve the effect of high stability and good corrosion-resistance of the material. As described above, the spin current-electric current conversion structure  100  of the present example embodiment enables spintronics devices such as thermoelectric conversion elements to improve in performance. 
     Second Example Embodiment 
     Next, a second example embodiment of the present invention will be described.  FIG. 2  is a perspective view illustrating the configuration of a thermoelectric conversion element  200  according to the second example embodiment of the present invention. The thermoelectric conversion element  200  according to the present example embodiment is a thermoelectric conversion element using the spin current-electric current conversion structure according to the first example embodiment as an electromotive material. 
     The thermoelectric conversion element  200  includes a magnetic material layer  210  and a conductive 4d-transition metal oxide layer  220  that serves as the electromotive material. The magnetic material layer  210  contains a magnetic material exhibiting the spin Seebeck effect. The conductive 4d-transition metal oxide layer  220  is connected to the magnetic material layer  210  so that a spin current can flow into and out, and generates electromotive force (electric current  20 ) due to the inverse spin Hall effect. The conductive 4d-transition metal oxide layer  220  is configured to include the spin current-electric current conversion structure according to the first example embodiment. 
     The thermoelectric conversion element  200  can be configured to further include a substrate  230  on which the magnetic material layer  210  is mounted, and two electrode sections that are electrically connected to the conductive 4d-transition metal oxide layer  220  and disposed apart from each other. The interface between the conductive 4d-transition metal oxide layer  220  and the magnetic material layer  210  configures the spin current input-output structure  120 , and the electrode sections configure the electric current input-output structure  130 . As illustrated in  FIG. 2 , the electrode sections may be composed of pad sections  241 A and  241 B and terminal sections  242 A and  242 B. 
     The magnetic material layer  210  is made of a magnetic material that exhibits the spin Seebeck effect, and generates a spin current  10  (Js) from a temperature gradient, nabla T (temperature difference ΔT), in a direction perpendicular to the plane of the layer (normal direction) due to the spin Seebeck effect. The direction of the spin current Js is parallel or antiparallel to that of the temperature gradient, nabla T. In the example illustrated in  FIG. 2 , the temperature gradient, nabla T, is applied in the minus z direction, and the spin current Js along the plus z or minus z direction is generated. 
     Materials such as yttrium iron garnet (Y 3 Fe 5 O 12 : YIG), YIG doped with bismuth (Bi) (Bi: YIG, BiY 2 Fe 5 O 12 ), or Ni—Zn ferrite ((Ni, Zn) x Fe 3-x O 4 ) can be used for the magnetic material layer  210 . The smaller the thermal conductivity of the magnetic material layer  210  is, the larger the thermoelectric conversion efficiency becomes. Consequently, it is preferable to use, as the magnetic material layer  210 , a magnetic insulator through which the electric current does not flow, that is, electrons do not transport heat. 
     The conductive 4d-transition metal oxide layer  220  converts the spin current  10  arising and inflowing due to the spin Seebeck effect in the magnetic material layer  210  into the electromotive force (electric current  20 ) due to the inverse spin Hall effect. In other words, the conductive 4d-transition metal oxide layer  220  generates the electromotive force from the spin current Js due to the inverse spin Hall effect, which causes the electric current  20  to flow. Thus the conductive 4d-transition metal oxide layer  220  functions as the spin current-electric current conversion structure. 
     The direction of the electromotive force (electric field E) generated above is given by the cross product of the direction of the magnetization M in the magnetic material layer  210  and the direction of the temperature gradient, nabla T. That is to say, there is a relation of E˜M×nabla T. The thermoelectric conversion element  200  of the present example embodiment is configured so that the direction of the electromotive force may be an in-plane direction of the conductive 4d-transition metal oxide layer  220  that serves as the electromotive material. In the example illustrated in  FIG. 2 , the direction of the magnetization M of the magnetic material layer  210  is the plus y direction, the direction of the temperature gradient, nabla T, is the minus z direction, and the direction of the electromotive force is the minus x direction. 
     The oxide materials containing the 4d-transition-metal element such as a ruthenium oxide (RuO x ) conductive film, rhodium oxide (RhO x ), and niobium oxide (NbO x ) can be used as the conductive 4d-transition metal oxide layer  220 . It is preferable for the thickness of the conductive 4d-transition metal oxide layer  220  in the direction perpendicular to the plane to be nearly equal to the spin diffusion length of the 4d-transition metal oxide contained in the layer, and the thickness is preferably not less than 2 nanometers (nm) and not more than 30 nanometers (nm). 
     The pad sections  241 A and  241 B are disposed at both ends of, electrically connected to, the conductive 4d-transition metal oxide layer  220 . This makes it possible to extract the electromotive force efficiently from the thin-film conductive 4d-transition metal oxide layer  220  to the outside. It is preferable to use metal materials having small resistivity as the pad sections  241 A and  241 B, and materials such as gold (Au), platinum (Pt), tantalum (Ta), and copper (Cu) can be used, for example. It is preferable for the film thickness of the pad sections  241 A and  241 B to be thicker than that of the conductive 4d-transition metal oxide layer  220 , and to be not less than 30 nanometers (nm). 
     The electromotive force is extracted to the outside through the terminal sections  242 A and  242 B that are connected to the pad sections  241 A and  241 B, respectively. If the thermoelectric conversion element  200  is used as a thermal flow sensor, for example, the amount of heat flowing through the thermoelectric conversion element  200  can be evaluated by measuring the open voltage between the two terminal sections  242 A and  242 B by a voltmeter  250 . 
     The electrode section may have a configuration in which the terminal sections  242 A and  242 B are formed directly on the conductive 4d-transition metal oxide layer  220  without the pad sections  241 A and  241 B. 
     Next, a method for making the thermoelectric conversion element  200  according to the present example embodiment will be described. 
     In the method for making the thermoelectric conversion element  200  of the present example embodiment, first, the magnetic material layer  210  containing a magnetic material exhibiting the spin Seebeck effect is stacked on the substrate  230 . On the magnetic material layer  210 , the conductive 4d-transition metal oxide layer  220  is stacked that serves as an electromotive material that is connected to the magnetic material layer  210  so that a spin current can flow into and out, and generates electromotive force due to the inverse spin Hall effect. Finally, two electrode sections each of which is electrically connected to the conductive 4d-transition metal oxide layer  220  are formed apart from each other, which completes the thermoelectric conversion element  200 . In stacking the conductive 4d-transition metal oxide layer  220 , the conductive 4d-transition metal oxide layer  220  is formed in such a way as to include the spin current-electric current conversion structure according to the first example embodiment. 
     In order to form the magnetic material layer  210 , any one of the methods can be used that include a sputtering method, a metal organic decomposition (MOD) method, a pulsed laser deposition (PLD) method, a sol-gel method, an aerosol deposition (AD) method, a ferrite plating method, and a liquid phase epitaxy (LPE) method. 
     In order to form the conductive 4d-transition metal oxide layer  220 , methods can be used that include a reactive sputtering method in the presence of oxygen and the metal organic decomposition (MOD) method. In order to form the pad sections  241 A and  241 B, methods can be used that include the sputtering method, a vacuum deposition method, an electron beam deposition method, and a plating method. 
     According to the above-mentioned thermoelectric conversion element  200  and the method for making the thermoelectric conversion element, it is possible to make the spin current-electric current conversion more efficient. The effect of the thermoelectric conversion element  200  according to the present example embodiment will be described in more detail below. 
     In order to verify the above-mentioned effect, an evaluation thermoelectric conversion element  201  illustrated in  FIG. 3  was prepared and evaluated. As illustrated in the figure, a ruthenium oxide (RuO x ) conductive film was used as the conductive 4d-transition metal oxide layer  220 . 
     The evaluation thermoelectric conversion element  201  was made as follows. First, an yttrium iron garnet (Y 3 Fe 5 O 12 : YIG) magnetic film 120 nanometers (nm) thick was formed on a gadolinium gallium garnet (Gd 3 Ga 5 O 12 : GGG) substrate approximately 0.5 millimeters (mm) thick. A ruthenium oxide (RuO x ) conductive film 10 nanometers (nm) thick was formed on the above-described magnetic film. The metal organic decomposition (MOD) method included in coating-based deposition methods was used in order to form the YIG magnetic film. Specifically, the YIG magnetic film was formed by applying a solution of an organic metal (MOD solution) containing yttrium (Y) and iron (Fe) by spin coat technology at a rotational speed of approximately 1,000 rpm (revolution per minute), and then annealing it at approximately 700° C. 
     The ruthenium oxide (RuO x ) conductive film was formed by using the reactive sputtering method. The reactive sputtering was performed under conditions that a ruthenium (Ru) target was used at room temperature at approximately 0.5 Pa pressure (argon Ar flow rate of 2.9 sccm, and oxygen O 2  flow rate of 7.5 sccm). Although the stoichiometric stable composition of the ruthenium oxide is expressed in RuO 2 , an oxygen defect or an excess of oxygen arises depending on fabrication conditions such as heat treatment. Accordingly, post-annealing for approximately one hour was performed under atmospheric conditions or nitrogen (N 2 ) flow conditions in different temperature conditions (anneal temperature T an ) after the sputtering. Then the evaluation was performed on a plurality of evaluation thermoelectric conversion elements  201  that differ in oxidation state. 
     For comparison of the performance of the evaluation thermoelectric conversion element  201 , a comparative thermoelectric conversion element was also made that used, as a conductive film, platinum (Pt) generally used as a conductive film for spin Seebeck elements or iridium oxide (IrO x ) included in the 5d-transition metal oxide. The comparative thermoelectric conversion element using platinum (Pt) was made by the same production method as the above-described production method. In other word, on a GGG (Gd 3 Ga 5 O 12 ) substrate was formed a YIG (Y 3 Fe 5 O 12 ) magnetic film 120 nanometers (nm) thick, on which a platinum (Pt) film 10 nanometers (nm) thick was formed by using the sputtering method, by which the comparative thermoelectric conversion element was made. Regarding the comparative thermoelectric conversion element using iridium oxide (IrO x ), the YIG (Y 3 Fe 5 O 12 ) magnetic film was formed in a similar way, and then an iridium oxide (IrO x ) film 10 nanometers (nm) thick was formed by using the reactive sputtering using an iridium (Ir) target. The condition of the reactive sputtering was the same as that for forming the above-mentioned ruthenium oxide (RuO x ) conductive film. Then post-annealing was performed at approximately 400° C. in the atmosphere. 
     The wafer produced in the above-mentioned processes was cut in the shape of a sample approximately two-by-eight millimeters (mm), and its thermoelectric properties were evaluated with the temperature gradient (temperature difference ΔT) illustrated in  FIG. 3  applied. If the temperature difference ΔT is applied in the thickness direction (perpendicular to the plane) of the element including the substrate as seen above, the electromotive force V is generated in an in-plane direction perpendicular to both the direction of the magnetization M of the magnetic film and that of the temperature gradient. In this instance, the direction (sign) and magnitude of the electromotive force are determined by the spin Hall angle that is a parameter inherent in the conductive material. 
       FIG. 4  illustrates the dependence of the thermoelectromotive force V of the evaluation thermoelectric conversion element  201  on the temperature difference ΔT. The evaluation thermoelectric conversion element  201  had the above-mentioned RuO x /YIG/GGG structure, on which the annealing treatment was performed under the conditions of the temperature T an =600° C. and nitrogen (N 2 ) flow. The diagram also illustrates the evaluation results of the comparative thermoelectric conversion element having the IrO x /YIG/GGG structure and annealed in the atmosphere at the temperature T an =400° C. and the comparative thermoelectric conversion element having the Pt/YIG/GGG structure. 
     As can be seen from  FIG. 4 , the dependence on temperature difference of the thermoelectromotive force of the evaluation thermoelectric conversion element  201  having the ruthenium oxide (RuO x ) conductive film is opposite in sign to that of the comparative thermoelectric conversion element using platinum (Pt) or iridium oxide (IrO x ). The absolute value of the thermoelectric coefficient of the evaluation thermoelectric conversion element  201  was 2.2μ, V/K, which was approximately three times as large as that of the comparative thermoelectric conversion element using platinum (Pt) and approximately 40 times as large as that of the comparative thermoelectric conversion element using iridium oxide (IrO x ). 
       FIG. 5  illustrates the dependence on post-anneal temperature T an  of the thermoelectric coefficient V/ΔT of the evaluation thermoelectric conversion element  201  having the RuO x /YIG/GGG structure. As can be seen from the diagram, the thermoelectric performance significantly depends on the anneal temperature T an . That is to say, the evaluation thermoelectric conversion element treated at the anneal temperature T an  of 300° C. has thermoelectromotive force smaller than that of the element without annealing treatment. However, it can be seen that the thermoelectromotive force is reversed in sign in the evaluation thermoelectric conversion element with the annealing treatment at 400° C., and the thermoelectromotive force increases raising the anneal temperature further. 
     It can be obtained as new insight from the result that the efficiency in spin current-electric current conversion also varies depending on the oxidation state of the 4d-transition metal oxide, that is, the valence of the 4d-transition metal ion. This enables the spin current-electric current property to be optimized by controlling the valence of the metal ion by a process such as annealing treatment. In other words, the method for making the thermoelectric conversion element can be configured to include a process for performing thermal treatment so that a valence of the 4d-transition metal in the 4d-transition metal oxide containing a 4d-transition-metal element may have a value by which to maximize the spin current-electric current conversion efficiency, that is, the spin Hall angle of the oxide containing a 4d-transition-metal element. It is preferable to perform the annealing treatment in the approximately 500° C. to 650° C. range for the above-mentioned evaluation thermoelectric conversion element  201  including the ruthenium oxide (RuO x ) conductive film. 
       FIG. 6  illustrates the dependence of the electric resistivity of ruthenium oxide (RuO x ) on the anneal temperature T an . The four-terminal measurement method was used for the measurement of the electric resistivity. It can be seen from the diagram that the electric conduction property of ruthenium oxide (RuO x ) also varies depending on the anneal temperature. The electric resistivity reaches, at the anneal temperature T an  of 400° C., a minimum of 6.2×10 −5  Ωcm, which is close to the values that has been reported in the literature. On the other hand, the electric resistivity increases when the anneal temperature is further raised; consequently, when the annealing treatment is performed at 600° C., the electric resistivity is equal to 1.06×10 −3  Ωcm, which is an order of magnitude or more greater than that with annealing at 400° C. It was found that the conductivity disappeared when the annealing was performed at 700° C. or higher. 
     It is clear from the above-described results that a great spin current-electric current conversion effect can be obtained according to the thermoelectric conversion element  200  of the present example embodiment using ruthenium oxide (RuO x ), a conductive 4d-transition metal oxide, as an electromotive film. As a result, a thermoelectric conversion output voltage can be obtained that is larger than that of the thermoelectric conversion element using a conductive 5d-transition metal oxide or platinum (Pt) of noble metal as the electromotive film. 
     For single metal element, more efficient spin current-electric current conversion can be achieved as the element gets heavy with atomic weight larger because the spin orbit interaction is enhanced. In other words, 5d-transition metal generally has greater spin current-electric current conversion effect than 4d-transition metal has. In contrast, if transition metal oxides are used, the thermoelectric conversion element  200  according to the present example embodiment using 4d-transition metal oxide has greater spin current-electric current conversion effect than the comparative thermoelectric conversion element using 5d-transition metal has. This result is contrary to the above-mentioned empirical rule for single metal element that has previously been known. Therefore, it is clear that the configuration of the thermoelectric conversion element  200  according to the present example embodiment cannot be easily conceived from the configurations of these publicly known thermoelectric conversion elements. 
     As mentioned above, according to the thermoelectric conversion element  200  and the method for making the element of the present example embodiment, it is possible to make the spin current-electric current conversion more efficient. This makes it possible to obtain a large output voltage (electromotive force); therefore, it becomes possible to achieve high sensitivity for thermal flow sensing and the like. 
     In addition, according to the thermoelectric conversion element  200  and the method for making the element of the present example embodiment, it is possible to configure a thermoelectric conversion element relatively inexpensively. In contrast, conventional thermoelectric conversion elements using 5d-transition metal have the problem that material costs are high. That is to say, the 5d-transition metal such as platinum (Pt), gold (Au), and iridium (Ir) is noble metal; consequently, the material cost is high. As a result, there has been the problem that it is difficult to apply materials for spin current-electric current conversion containing 5d-transition metal to large area elements. 
     In addition, the thermoelectric conversion element  200  and the method for making the element of the present example embodiment produce the effect of high stability and high corrosion resistance of the material because the oxide is used as the conductive film (electromotive material). That is to say, the stability of the material is extremely important because thermoelectric conversion element is often used under harsh circumstances such as high-temperature and humidity. Metal materials generally have challenges such as easily-oxidizable and corrosion-prone properties at high temperature. In particular, 5d-transition metal materials such as tantalum (Ta) and tungsten (W) are easily oxidized at high temperature, and are problematic in terms of reliability depending on its application. In contrast, it is possible to avoid the above-described problems because the oxide materials are less likely to corrode and highly stable. 
     Third Example Embodiment 
     Next, a third example embodiment of the present invention will be described.  FIG. 7  is a perspective view illustrating the configuration of a memory element  300  according to the third example embodiment of the present invention. The memory element  300  of the present example embodiment is a memory element using the spin current-electric current conversion structure according to the first example embodiment as a conductive layer. 
     The memory element  300  includes a magnetic free layer  310 , a barrier layer  320  connected to the magnetic free layer  310 , a magnetic fixed layer  330  to form a tunnel junction with the magnetic free layer  310  through the barrier layer  320 , and a conductive 4d-transition metal oxide film  340  as a conductive layer. The conductive 4d-transition metal oxide film  340  is disposed so that a spin current may arise due to the spin Hall effect, and the spin current  10  may flow into the magnetic free layer  310 . The conductive 4d-transition metal oxide film  340  is configured to include the spin current-electric current conversion structure according to the first example embodiment. These configurations can be disposed on a substrate  350 . 
     The memory element  300  is capable of performing the operation of writing information by an electric current and the operation of reading out information by resistance detection. The writing operation is performed by passing a writing electric current  30  between both terminals of writing electrode terminals  371 A and  371 B that are electrically connected to both ends of the conductive 4d-transition metal oxide film  340 . In the reading-out operation, the stored information can be detected by measuring a resistance in the stacked direction of the magnetic free layer  310 , the barrier layer  320 , and the magnetic fixed layer  330 . In order to detect the resistance, the memory element can be configured to include a reading-out electrode  360  electrically connected to the magnetic fixed layer  330  and a reading-out electrode terminal  372  electrically connected to the reading-out electrode  360 . 
     The magnetic free layer  310  and the magnetic fixed layer  330  have in-plane magnetization in the x direction of each layer, and form a tunnel junction through the barrier layer  320 . The magnetic fixed layer  330  has sufficiently large coercivity, and has a fixed magnetization MA with the magnetization always fixed in the plus x direction. In contrast, the magnetization direction of the magnetic free layer  310  is defined as any one of plus x and minus x direction and is inverted by external drive that is called spin torque. In other words, the magnetic free layer  310  has a variable magnetization MB. The magnetization direction of the magnetic free layer  310  becomes parallel or antiparallel to that of the magnetic fixed layer  330  depending on the magnetization direction of the magnetic free layer  310 ; consequently, the resistance of the tunnel junction changes. The resistance change corresponds to the information ‘0’ or ‘1’ for the memory element  300 . 
     The conductive 4d-transition metal oxide film  340  is disposed below the magnetic free layer  310  in order to write and rewrite the information into the memory element  300 . The operation of writing and rewriting the information can be performed by passing a writing electric current between the writing electrode terminal  371 A and the writing electrode terminal  371 B that are electrically connected to both ends of the conductive 4d-transition metal oxide film  340 . 
     Specifically, when the writing electric current  30  is passed in the minus y direction in  FIG. 7  through the conductive 4d-transition metal oxide film  340 , a part of the writing electric current  30  is converted, due to the spin Hall effect, into a spin current in the z direction, that is, a flow in the z direction of the spin angular momentum in the x direction. The conductive 4d-transition metal oxide film  340 , therefore, functions as the spin current-electric current conversion structure. This spin current  10  is injected into the magnetic free layer  310  and inverts the magnetization of the magnetic free layer  310  by applying the spin torque to the magnetic free layer  310 . This makes it possible to rewrite the information. 
     Oxide materials containing 4d-transition-metal element such as ruthenium oxide (RuO x ) conductive film, rhodium oxide (RhO x ), and niobium oxide (NbO x ) are used as the conductive 4d-transition metal oxide film  340 . It is preferable for the film thickness to be nearly equal to the spin diffusion length of the 4d-transition metal oxide material to be used, and the thickness is preferably not less than 3 nanometers (nm) and not more than 30 nanometers (nm). 
     Ferromagnetic materials such as CoFeB, cobalt (Co), and iron (Fe) can be used as the magnetic free layer  310  and the magnetic fixed layer  330 . Insulating materials such as magnesium oxide (MgO) and aluminum oxide (Al 2 O 3 ) can be used as the barrier layer  320 . Each film thickness of the magnetic free layer  310  and the magnetic fixed layer  330  preferably ranges from approximately 1 nanometer (nm) to approximately 20 nanometers (nm), and the film thickness of the barrier layer  320  preferably ranges from approximately 0.3 nanometers (nm) to approximately 3 nanometers (nm). Materials such as tantalum (Ta) and gold (Au) can be used as the reading-out electrode  360 . 
     Next, a method for making the memory element  300  according to the present example embodiment will be described. 
     In the method for making the memory element  300  according to the present example embodiment, first, the conductive 4d-transition metal oxide film  340  is formed by using a reactive sputtering method in the presence of oxygen or a metal organic decomposition (MOD) method. The region on the conductive 4d-transition metal oxide film  340  where to form a magnetic tunnel junction is patterned with resist by using a method such as a photolithography method and an electron beam lithography method. Then the magnetic free layer  310 , the barrier layer  320 , the magnetic fixed layer  330 , and the reading-out electrode  360  are formed, respectively. A method such as a sputtering method can be used for the formation of these films. Finally, the resist is removed by using a lift-off process; consequently, a pillar-like magnetic tunnel junction is formed. This completes the memory element  300 . 
     Example 1 
     An example of the method for making the thermoelectric conversion element according to the second example embodiment of the present invention will be described below. 
     The thermoelectric conversion element  200  according to the second example embodiment is configured to include the conductive 4d-transition metal oxide layer  220  of a conductive oxide film that serves as an electromotive material. For the formation of the conductive oxide film, a non-vacuum process by a coating method can be used. This makes it possible to perform all steps in the process for making the thermoelectric conversion element together with the formation of a magnetic insulator film such as YIG by using a coating-based formation method. In the present example, the description will be made for a method for making the thermoelectric conversion element by such an all-coating-based process and for the characteristics of the thermoelectric conversion element obtained by the method. 
     Both the magnetic insulator film (YIG) and the conductive film (RuO x ) were formed using a metal organic decomposition (MOD) method that is a coating-based film-formation method. In accordance with the MOD method, an oxide film can be formed by applying an organometallic solution containing a metal ion such as yttrium (Y), iron (Fe), and ruthenium (Ru) using a spin-coat method and by annealing the applied solution. 
     In the present example, a YIG film 120 nanometers (nm) thick was formed under the condition that a rotational speed of spin-coating was 1,000 rpm, and an anneal temperature was 700° C. Then a RuO x  film approximately 40 nanometers (nm) thick was formed on the YIG film by using a coating method under the condition that a rotational speed of spin-coating was 4,000 rpm, and an anneal temperature in nitrogen (N 2 ) atmosphere was 600° C. The thermoelectric conversion element made by the above-described processes was cut into a specimen with an area of approximately 8×2 mm 2 . The resistance of the RuO x  film in this specimen was 6.7 kΩ. 
       FIG. 8  illustrates the dependence on the temperature difference ΔT of the electromotive force V of the thermoelectric conversion element according to the present example. The evaluation was performed by a method similar to that in the second example embodiment. As illustrated in the diagram, clear signals of thermoelectromotive force proportional to the temperature difference ΔT were observed. This validates for the first time the fact that a thermoelectric conversion element (spin thermoelectric element) was able to be fabricated by the all-coating-based process. The sign of the electromotive force is the same as that of the thermoelectric conversion element  200  including the RuO x  film formed by the sputtering method and post-annealing described in the second example embodiment (see  FIG. 4 ), but it is opposite to that of the element using platinum (Pt). 
     The magnitude of the output voltage is smaller than that of the thermoelectric conversion element  200  including the RuO x  film formed by the sputtering method according to the second example embodiment (see  FIG. 4 ). This is because the RuO x  film used in the present example is thick, approximately 40 nanometers (nm) thick. As a result, it is preferable for the film thickness of the RuO x  film to be equal to or less than 30 nanometers (nm). 
     Example 2 
     Another example of the thermoelectric conversion element and the method for making the element according to the second example embodiment of the present invention will be described below. 
     In the present example, a thermoelectric conversion element is described that uses rhodium oxide (RhO x ) instead of ruthenium oxide (RuO x ) as the 4d-transition metal oxide material. 
       FIG. 9  is a perspective view of a thermoelectric conversion element  202  according to the present example. A GGG substrate that is made of the same material as that of the substrate used for the evaluation thermoelectric conversion element  201  according to the second example embodiment was used for a substrate, and ytterbium (Yb)-doped YIG (YbY 2 Fe 5 O 12 ) 60 nanometers (nm) thick was used as a magnetic material layer. The Yb-doped YIG film was formed by a process similar to the process using the metal organic decomposition method (MOD method) in the second example embodiment. 
     The rhodium oxide (RhO x ) film was formed using the reactive sputtering method as is the case with the second example embodiment. The reactive sputtering was performed under conditions that a rhodium (Rh) target was used at room temperature at 0.5 Pa pressure (Ar flow rate of 2.9 sccm, and O 2  flow rate of 7.5 sccm). 
     Although the stoichiometric stable composition of rhodium oxide (RhO x ) is expressed in RhO 2 , an oxygen defect or an excess of oxygen arises depending on fabrication conditions such as heat treatment. Accordingly, post-annealing for one hour was performed under atmospheric conditions or nitrogen (N 2 ) flow conditions in different temperature conditions (anneal temperature T an ) after the sputtering, as is the case in RuO x  according to the second example embodiment. Thus a plurality of samples were prepared that differed in oxidation state of a RhO x  film. 
       FIG. 10  illustrates the dependence on the temperature difference ΔT of the thermoelectromotive force V of the thermoelectric conversion element having a RhO x /Yb:YIG/GGG structure. This element is subjected to anneal treatment under the conditions of the temperature T an  equal to 600° C. and nitrogen (N 2 ) flow. 
     The thermoelectromotive force larger than that of the element using platinum (Pt) was achieved, as is the case with the evaluation thermoelectric conversion element  201  having the RuO x /YIG/GGG structure according to the second example embodiment. The sign of the electromotive force is opposite to that of the RuO x  film after annealing used in the second example embodiment and Example 1, and it is the same as that of the element using platinum (Pt). 
     As described in the above-mentioned second example embodiment and example, a high-performance spin current-electric current conversion structure can be achieved by using the conductive 4d-transition metal oxide having the rutile crystal structure such as ruthenium oxide (RuO x ) and rhodium oxide (RhO x ). 
     Example 3 
     An example of the memory element and the method for making the memory element according to the third example embodiment of the present invention will be described below.  FIG. 11  illustrates the configuration of a memory element  301  according to the present example. 
     A thermal silicon oxide (SiO 2 /Si) substrate that was oxidized to a depth of 100 nanometers (nm) from the surface was used as a substrate. A conductive 4d-transition metal oxide film composed of RuO x  10 nanometers (nm) thick was formed on the substrate by using the reactive sputtering and post-annealing. 
     Then the region where to form the magnetic tunnel junction was patterned with resist by using an electron beam lithography method. Subsequently, Co 20 Fe 60 B 20  2 nanometers (nm) thick serving as a magnetic free layer, MgO 1 nanometer (nm) thick serving as a barrier layer, and Co 20 Fe 60 B 20  4 nanometers (nm) thick serving as a magnetic fixed layer were formed. In addition, tantalum (Ta) 10 nanometers (nm) thick serving as a reading-out electrode was formed sequentially by using the sputtering method. Finally, the resist was removed by lift-off process; consequently, a pillar-like magnetic tunnel junction was formed. The pillar had an elliptical shape, and was formed so that the major axis may become equal to 150 nanometers (nm) and minor axis may become equal to 100 nanometers (nm). 
     The memory element using a conductive 4d-transition metal oxide as a spin current-electric current conversion structure was fabricated by the above-described steps. 
     As described above, the present invention has been described by using the above example embodiments as typical examples. However, the present invention is not limited to the above example embodiments. In other words, various aspects of the present invention that are conceivable to those skilled in the art can be applied within the scope of the present invention. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2015-036159, filed on Feb. 26, 2015, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
         
           
               100  spin current-electric current conversion structure 
               110  4d-transition metal oxide structure 
               120  spin current input-output structure 
               130  electric current input-output structure 
               200 ,  202  thermoelectric conversion element 
               201  evaluation thermoelectric conversion element 
               210  magnetic material layer 
               220  conductive 4d-transition metal oxide layer 
               230  substrate 
               241 A,  241 B pad section 
               242 A,  242 B terminal 
               250  voltmeter 
               300 ,  301  memory element 
               310  magnetic free layer 
               320  barrier layer 
               330  magnetic fixed layer 
               340  conductive 4d-transition metal oxide film 
               350  substrate 
               360  reading-out electrode 
               371 A,  371 B electrode terminal 
               372  reading-out electrode terminal 
               10  spin current 
               20  electric current 
               30  writing current