Insulating Spintronics

One of the leading loses in electronics comes in the form of Joule heating from the movement of the electrons. Magnetic insulators allow for the production and investigation of pure magnonic spin currents without the associated Joule heating. Magnonic spin currents can be produced in several ways including the spin Seebeck effect [1] and via the spin Hall effect (SHE) [2] in a neighbouring heavy metal layer such as platinum (Pt), tungsten (W) or Tantalum (Ta).

The SSE 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 [3], see Fig. 1 (a). Typically, spin currents are detected by exploiting the inverse SHE [2] 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].

We observe the local 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].

By utilising the SHE, a transverse spin current can be produced in response to a charge current flowing through a heavy metal layer with a large spin-orbit coupling such as Pt [2]. This spin current can then excite spin waves in a neighbouring magnetic material which can propagate. At some non-local location, the inverse SHE can be utilised to detect this magnon current as an electrical one allowing for the study of the spin transport through a magnetic insulator [9].


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] K. Ganzhorn et al, AIP Advances, 7, 085102 (2017).