Spin Seebeck effect
The recently emerged field of spin caloritronics involves the study of the interaction of spins with heat currents. One of the well-established phenomena of the spin caloritronics is a (longitudinal) spin Seebeck effect (SSE), which was discovered in 2010 by Uchida et.al.
 preceded by the
discovery of a transverse SSE in 2008  The SSE refers to the conversion of a heat current into a spin current in a magnetically ordered material, typically ferro- or ferrimagnet (FM). The SSE can be used in a thermoelectric generation because the thermally generated spin current can be converted
to an electrical current by means of the inverse spin Hall effect (ISHE)  in the attached conductive nonmagnetic metal (NM) with a large spin Hall angle [4,5]. A necessary condition for the detection of SSE by ISHE is that the directions of the spin current (or heat current), spin polarization of the spin current, and electrical current have mutually perpendicular components. In analogy with the Seebeck effect, the spin Seebeck coefficient can be defined as a ratio of an electric field generated by ISHE and applied thermal gradient SSSE
The conversion efficiency from a temperature gradient into the SSE voltage is mainly determined by the following three factors :
- efficiency of the conversion of heat to spin current JS in the magnetic materials, depending mainly on the spin polarization σ and magnon propagation length in the magnet;
- efficiency of the spin momentum transfer through the magnetic material/metal thin layer interface, characterized by the spin mixing conductance;
- efficiency of the conversion of the spin to charge current in metallic thin layer, characterized by the spin Hall angle θSH.
As regards the spin current conversion into electrical current by ISHE, non-magnetic materials with metallic conductivity (NM) exhibiting strong spin-orbit coupling and hence large spin Hall angle are required. The transition 4d
metals, including their electrically conducting oxides and alloys, are typical examples. Platinum with positive spin Hall angle (the same polarity of magnetic field and spin Seebeck) and Wolfram with negative θSH
(opposite polarity of magnetic field and spin Seebeck) are among the most efficient representatives.
The magnetic material for spin current pumping are usually ferro- or ferri-magnetic materials (FM) with TC above room temperature, typical are alloys of transition metals or oxides like garnets, ferrites or perovskites. For the fundamental study of SSE it is more convenient to use the insulating rather than conducting magnetic material, in order to avoid parasitic signals such as a planar or anomalous Nernst effect (ANE). On the other hand, the contribution of Nernst effects to resulting voltage is beneficial for applications.
- K. Uchida, H. Adachi, T. Ota, H. Nakayama, S. Maekawa, and E. Saitoh,
Observation of longitudinal spin-Seebeck effect in magnetic insulators,
Appl. Phys. Lett. 97, 172505 (2010).
- K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S. Maekawa, and E. Saitoh,
Observation of the spin Seebeck effect,
Nature 455, 778 (2008).
- E. Saitoh, M. Ueda, H. Miyajima, and G. Tatara,
Conversion of spin current into charge current at room temperature: Inverse spin-Hall effect,
Appl. Phys. Lett. 88, 182509 (2006).
- A. Hoffmann,
Spin Hall effects in metals,
IEEE Trans. Magn. 49, 5172 (2013).
- J. Sinova, S. O. Valenzuela, J. Wunderlich, C. H. Back, and T. Jungwirth,
Spin Hall effects,
Rev. Mod. Phys. 87, 1213 (2015).
- K. Uchida, H. Adachi, T. Kikkawa, A. Kirihara, M. Ishida, S. Yorozu, S. Maekawa, and E. Saitoh,
Thermoelectric Generation Based on Spin Seebeck Effects,
Proceedings of the IEEE 104, 1946 (2016).
Spin Seebeck effect in core-shell magnetic nanocomposites
The aim of the project is the preparation of nanocomposites capable to generate electrical energy by means of transverse thermoelectric effects, in particular the recently experimentally confirmed spin Seebeck effect (SSE). This effect consists of spin current generation by temperature gradient applied across a thin magnet and the conversion of it to electrical current by means of the inverse spin Hall effect in the attached metallic layer. Proposed device will have nanocomposite character formed by ferrimagnetic cores (FM) coated with a thin layer of non-magnetic metal (NM). Due to alternation of FM and NM materials at nanoscale and connection of NM layers in parallel, the nanocomposites will solve two obstructions which limit the use of SSE for effective thermoelectric conversion: the high resistance of the thin NM layer and the small usable volume of FM caused by the short propagation of the spin current. The starting FM will be magnetite Fe3
which has presently the best SSE performance and our team has long-standing experience with its preparation in the form of nanoparticles
J. Hirschner, M. Maryško, J. Hejtmánek, R. Uhrecký, M. Soroka J. Buršík, A. Anadón, M. H. Aguirre, K. Knížek
Spin Seebeck effect in Y-type hexagonal ferrite thin films, Phys. Rev. B 96, 064428 (2017).
K. Knížek, M. Pashchenko, P. Levinský, O. Kaman, J. Houdková, P. Jiříček, J. Hejtmánek, M. Soroka, J. Buršík
Spin Seebeck effect in epsilon-Fe2O3 thin layer with high coercive field, J. Appl. Phys. 124, 213904 (2018).