The spin Seebeck effect refers to the generation of spin voltage as a result of a temperature gradient in ferromagnetic or ferrimagnetic materials. When a conductor is attached to a magnet under a temperature gradient, the thermally generated spin voltage in the magnet injects a spin current into the conductor, which in turn produces electric voltage owing to the spin-orbit interaction. The spin Seebeck effect is of increasing importance in spintronics, since it enables direct generation of a spin current from heat and appears in a variety of magnets ranging from metals and semiconductors to insulators. Recent studies on the spin Seebeck effect have been conducted mainly in paramagnetic metal/ferrimagnetic insulator junction systems in the longitudinal configuration in which a spin current flowing parallel to the temperature gradient is measured. This 'longitudinal spin Seebeck effect' (LSSE) has been observed in various sample systems and exclusively established by separating the spin-current contribution from extrinsic artefacts, such as conventional thermoelectric and magnetic proximity effects. The LSSE in insulators also provides a novel and versatile pathway to thermoelectric generation in combination of the inverse spin-Hall effects. In this paper, we review basic experiments on the LSSE and discuss its potential thermoelectric applications with several demonstrations.

Energy harvesting technologies, which generate electricity from environmental energy, have been attracting great interest because of their potential to power ubiquitously deployed sensor networks and mobile electronics. Of these technologies, thermoelectric (TE) conversion is a particularly promising candidate, because it can directly generate electricity from the thermal energy that is available in various places. Here we show a novel TE concept based on the spin Seebeck effect, called 'spin-thermoelectric (STE) coating', which is characterized by a simple film structure, convenient scaling capability, and easy fabrication. The STE coating, with a 60-nm-thick bismuth-substituted yttrium iron garnet (Bi:YIG) film, is applied by means of a highly efficient process on a non-magnetic substrate. Notably, spin-current-driven TE conversion is successfully demonstrated under a temperature gradient perpendicular to such an ultrathin STE-coating layer (amounting to only 0.01% of the total sample thickness). We also show that the STE coating is applicable even on glass surfaces with amorphous structures. Such a versatile implementation of the TE function may pave the way for novel applications making full use of omnipresent heat.

The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in magnetic materials including insulators. The SSE is applicable to thermoelectric generation because the thermally generated spin current can be converted into a charge current via spin-orbit interaction in conductive materials adjacent to the magnets. The insulator-based SSE device exhibits unconventional characteristics potentially useful for thermoelectric applications, such as simple structure, device-design flexibility, and convenient scaling capability. In this article, we review recent studies on the SSE from the viewpoint of thermoelectric applications. Firstly, we introduce the thermoelectric generation process and measurement configuration of the SSE, followed by showing fundamental characteristics of the SSE device. Secondly, a theory of the thermoelectric conversion efficiency of the SSE device is presented, which clarifies the difference between the SSE and conventional thermoelectric effects and the efficiency limit of the SSE device. Finally, we show preliminary demonstrations of the SSE in various device structures for future thermoelectric applications and discuss prospects of the SSE-based thermoelectric technologies.

We present temporal evolution of the spin Seebeck effect in a YIG|Pt bilayer system. Our findings reveal that this effect is a sub-microseconds fast phenomenon governed by the temperature gradient and the thermal magnons diffusion in the magnetic materials. A comparison of experimental results with the thermal-driven magnon-diffusion model shows that the temporal behavior of this effect depends on the time development of the temperature gradient in the vicinity of the YIG|Pt interface. The effective thermal-magnon diffusion length for YIG|Pt systems is estimated to be around 700 nm.The spin Seebeck effect (SSE) [2-9] is one of the most fascinating phenomena in the contemporary era of spincaloritronics [10]. Analogous to the classical Seebeck effect, the SSE is a phenomenon where a spin current is generated in spin-polarized materials like metals [2], semiconductors [4,5], and insulators [6-9] on the application of a thermal gradient. Generally, the generated spin current is measured by the inverse spin Hall effect (ISHE) [11] in a normal metal like Pt, placed in contact with the spin-polarized material. Currently, this phenomenon has attracted much attention due to its potential applications, for example, recent progresses show that based on this effect thin-film structures can be fabricated to generate electricity from waste-heat sources [12]. Further advancements in industrial applications like temperature sensors, temperature gradient sensors, and thermal spincurrent generators require an in-depth understanding of this effect.Although there have been numerous experimental and theoretical studies about this effect, the underlying physics is yet not well understood. The most accepted theory predicts that the SSE is driven by the difference in the local temperatures of magnon-, phonon-, and electron baths [13,14] of the system. However, no clear evidences of such differences have been observed experimentally [15]. So, the origin of this effect is still under discussion. Some experimental studies show that the interface proximity effect in the YIG|Pt system could exhibit similar behavior as observed for the SSE [16]. However, very recent measurements claim no such proximity effects [17]. Moreover, the question whether the SSE is an interface or bulk effect, is still open [18,19].To shed light on this controversial physics, we developed an entirely new experimental approach where we studied the temporal evolution of the SSE in YIG|Pt bilayer structures. The observations were realized in the longitudinal configuration of the SSE [7,8]. In the longitudinal spin Seebeck effect (LSSE), a thermal gradient is created perpendicular to the film plane, and the spin current generated by thermal excitations of magnetiza-