Spin caloritronics

The spin Seebeck effect (SSE) [1] is the spin-based analogon to the Seebeck effect. It allows for the efficient generation of pure magnonic spin currents in magnetic insulators due to thermal gradients and thus waste heat can be harvested to obtain useful spin currents. These currents are carried by thermally excited spin waves that propagate through the magnetic material [2], see Fig. 1 (a). Typically, spin currents are detected by exploiting the inverse spin Hall effect [3] in a heavy metal detection layer deposited on top of the magnetic insulator. At the insulator/metal interface a spin accumulation is induced, which in turn gives rise to an electrically detectable voltage signal. Besides ferromagnetic systems, especially the intensively investigated ferromagnetic insulator Yttrium Iron Garnet (YIG), the SSE has been observed as well in antiferromagnets [4,5] and even paramagnets [6].

Using the so-called longitudinal configuration, we observe the SSE by the application of in-plane magnetic fields and out-of-plane temperature gradients that are generated by external Joule heating. In addition to the exploration of new material groups exhibiting the SSE, a more thorough understanding of the fundamental aspects and underlying effects of the yet to be fully understood SSE is required. Temperature dependent SSE experiments performed in compensated ferrimagnets, for instance, allowed for an insight into the coupling between distinct spin wave modes and the conduction electrons of the detection metal [7]. Furthermore a direct correlation between the SSE amplitude and the intrinsic properties of spin waves carrying the spin current as well as the atomic structure of the insulator/metal interface [8].

Finally, the generated spin currents can manipulate the magnetization and for instance displace magnetic domain walls (LINK zur projekt-page DW motion) [9] so that heat is directly used to power magnetic spin-structure based logic or storage applications.

SSE
Figure 1: (a) Longitudinal spin Seebeck effect (LSSE): By an out of plane temperature gradient spin waves are excited in the magnetic insulator that propagate towards the detection layer. (b) In the detection layer a spin accumulation is induced, which in turn is converted into a charge current via the inverse spin Hall effect.

 

[1] K. Uchida et al., Nature 455, 778 (2008).
[2] A. Kehlberger et al., Phys. Rev. Lett. 115, 096602 (2015).
[3] J. Sinova et al., Rev. Mod. Phys. 87, 1213 (2015).
[4] S. Seki et al., Phys. Rev. Lett. 115, 266601 (2015).
[5] S. M. Wu et al., Phys. Rev. Lett. 116, 097204 (2016).
[6] S. M. Wu, J. E. Pearson, and A. Bhattacharya, Phys Rev Lett 114, 186602 (2015).
[7] S. Geprägs et al., Nat. Commun. 7, 10452 (2016).
[8] E.-J. Guo et al., Phys. Rev. X 6, 031012 (2016).
[9] D. Hinzke and U. Nowak, Phys. Rev. Lett. 107, 1 (2011).