In recent years, the necessity of clean energy has been brought up as a measure against global warming, and the application of a thermoelectric affect is expected as the clean energy source. For example, the use of Seebeck effect elements has been proposed for the conversion of waste heat from thermal power plants, factories and automobiles to electric power (see Patent Document 1).
However, the efficiency of the current Seebeck effect elements is not sufficiently high, and a further increase in the efficiency of the thermoelectric conversion is required in order to put the Seebeck effect elements into practice as the clean energy source.
The figure of merit Z, which is an indicator of the efficiency in the thermoelectric conversion of a current Seebeck effect element where a dissimilar metal joint made of two types of metals having different Seebeck coefficients is used, can be represented by:Z=S2×(σ/κ)  (1)when S is a Seebeck coefficient, σ is the electric conductivity and κ is the thermal conductivity. In addition, the direction in which the electromotive force V is generated is parallel to the direction of the temperature gradient ∇T.
In this case, the Seebeck coefficient S, the electric conductivity σ and the thermal conductivity κ are all values inherent to the substance, and therefore the figure of merit Z is also a value inherent to the substance, and thus a thermoelectric conversion element having a high figure of merit Z is necessary in order to implement a highly efficient thermoelectric power generation. As a result, it is necessary to develop a new substance in order to increase the figure of merit Z.
Meanwhile, the degree of freedom of the electron charge that is currently used in the field of electronics, such as for semiconductor devices, may be substituted with the degree of freedom of spins that electrons have in addition to their charges, that is to say, the degree of freedom of the spin angular momentum, which is used in spintronics, and this attracts attention as a carrier of the next generation electronic technologies.
By using spintronics where the degree of freedom in the electron charges and the spins is used simultaneously, the aim is to gain performance and characteristics not yet available, and a major part of the spintronics devices is driven by a spin current.
A spin current has little dissipation of energy and is therefore expected to be used for highly efficient energy transfer, and thus it has been urgently demanded to establish methods for generating and detecting a spin current.
Here, spin pumping has been proposed as a method for generating a spin current (see Non-Patent Document 1), and the present inventors have proposed a use of the inverse spin Hall effect (ISHE) as the method for detecting a spin current (see Non-Patent Document 2).
FIG. 12 is a diagram illustrating the inverse spin Hall effect where a pure spin current JS is injected into a sample when a current JC flows in the direction perpendicular to the direction of the pure spin current JS due to the inverse spin Hall effect. There is a potential difference V across the ends of the sample as a result of the inverse spin Hall effect, and therefore this potential difference V can be detected to make it possible to detect whether or not there is a pure spin current JS.
In the thermoelectric conversion using the above-described Seebeck effect, however, the figure of merit Z is large when a substance having a high electric conductivity σ is used, as can be seen from the formula (1). In the case of a metal, however, substances having a high electric conductivity σ also have a high thermal conductivity κ, and therefore a problem arises such that an increase in the figure of merit Z due to an increase in the electric conductivity σ is offset by the effects of the thermal conductivity κ.
Therefore, the present inventors have proposed a spin-Seebeck effect element where the junction between a magnetic body, such as NiFe, and a metal having a large spin-orbit interaction, such as Pt, is used (see Patent Document 2). In this spin-Seebeck effect element, a thermal spin current generated in the magnetic body, such as NiFe, due to the temperature gradient is spin exchanged in the interface with Pt, the pure spin current resulting from the exchange induces the electric current to flow in the direction perpendicular to the direction of the pure spin current, and this electric current is outputted as a voltage across the two ends of the magnetic body.
This was achieved as a result of the findings where there is a difference in the up spin current and the down spin current, which thermally generates a spin current when a temperature gradient is provided to a magnetic body, particularly a ferromagnetic body in a state where an external magnetic field is applied.
The figure of merit Z in this case can be represented by:Z=SS2×(σ1/κF)  (2)where SS is the thermopower of the spin-Seebeck effect element, σ1 is the electric conductivity of the inverse spin Hall member, and κF is the thermal conductivity of the magnetic body. Unlike the conventional figure of merit, the figure of merit in this case can be changed greatly by selecting the materials for the element because the electric conductivity in the numerator and the thermal conductivity in the denominator are carried by different substances.
In this case, the direction in which the electromotive force V is generated is perpendicular to the direction of the temperature gradient ∇T because the inverse spin Hall effect is used. The thermopower SS of the spin-Seebeck effect element is proportional to the length in the direction perpendicular to the direction of the temperature gradient ∇T and is thus characterized in that the figure of merit Z can be modulated by adjusting the size of the sample, unlike in the conventional Seebeck effect elements. That is to say, the sample can be formed so as to be long in the direction perpendicular to the temperature gradient ∇T so that the electromotive force V proportional to the length can be gained.
The spin current is not a physical, conserved quantity, and therefore the above-described thermal spin current conversion can be used so that the spin current can be continuously taken out simply by providing a temperature gradient, and accordingly the thermoelectromotive force can also be continuously taken out.
In this spin-Seebeck effect element, however, the thermal spin current generating member is made of a metal having a large thermal conductivity κ, and therefore it is difficult to provide a uniform temperature gradient ∇T when the sample is made large in order to increase the electromotive force V. Accordingly, it is currently difficult to implement a thermoelectric conversion element that is industrially available using a spin-Seebeck effect element entirely made of metal.
Thus, the present inventors have proposed a spin-Seebeck effect element where a magneto-dielectric body having a small thermal conductivity, such as YIG, is used for the thermal spin current generating member instead of metal (see Patent Document 3). Here, a spin-Seebeck effect element having a magneto-dielectric body is described in reference to FIG. 13.
FIG. 13 is a schematic perspective diagram showing a spin-Seebeck effect element using a magneto-dielectric body that is provided with non-magnetic conductors 52 and 53 in strips on a magneto-dielectric layer 51. In this state, an external magnetic field H is applied in the direction of the arrow, and at the same time a uniform temperature gradient ∇T is provided, and thus pure spin currents JS in the opposite symbols respectively flow through the interfaces between the magneto-dielectric body and the non-magnetic conductor located on the high temperature side and on the low temperature side of the element. The pure spin currents JS that have been injected into the normal conductors 52 and 53 are converted to electric currents in the direction perpendicular to the temperature gradient ∇T as a result of the electron relativistic effect so that the thermoelectromotive forces VISHE are generated in the opposite directions in the non-magnetic conductor 52 provided on the high temperature side and the non-magnetic conductor 53 provided on the low temperature side. That is to say, the electromotive force resulting from the inverse spin Hall effect is generated in the direction of the outer product of the injected pure spin current JS and the direction of the polarization of the spins (direction of magnetization M of the magneto-dielectric body).
Any magneto-dielectric body that contains Fe or Co can be used as the magneto-dielectric body 51, but in practice, YIG (yttrium iron garnet) and yttrium gallium iron garnet that are easily available and have small dissipation of the spin angular momentum are used, that is say, a material that can be represented by a general formula: Y3Fe5-xGaxO12 (x<5) is used. In addition, it is desirable to use any of Pt, Au, Pd, Ag, Bi and elements having an f orbital as the non-magnetic conductors 52 and 53 that become inverse spin Hall effect members. These elements have a large spin-orbit interaction, and therefore the thermal spin-wave spin current and the pure spin current can be exchanged at high efficiency in the interfaces between the magneto-dielectric body 51 and the non-magnetic conductor 52 as well as between the magneto-dielectric body 51 and the non-magnetic conductor 53.
FIG. 14 is a schematic diagram illustrating a spin-wave spin current, and as shown in FIG. 14, a spin-wave spin current is provided when spin precesses around the equilibrium position and the change in the phase conveys through the spin system as a wave, and a thermal spin-wave spin current is provided when the change in the phase is caused by heat. A spin-wave spin current is characterized in that it can propagate over a long distance of several mm or several cm or more in contrast to the length of the spin diffusion of a conduction electron-based pure spin current being several nm to several hundreds of nm. This has already been confirmed through various experiments (see Non-Patent Document 3).
In this conversion between a thermal spin-wave spin current and a pure spin current, a thermal spin-wave spin current generated by the temperature gradient in a magneto-dielectric body is exchanged with a spin in a metal electrode so that a pure spin current is generated in the metal electrode, and this pure spin current generates an electric current which generates a thermoelectromotive force VISHE across the two ends of the metal electrode.