Phys. Rev. Lett. 118, 057201 (2017)

Thermal gradients along a material may imply the generation of a spin current via the spin Seebeck effect. In magnets, the bulk nature of this effect has been explained based on how thermal gradients affect the characteristic spectrum of magnons. In metal/magnet bilayers, a different mechanism has been put forward, driven by mutual out-of-equilibrium conditions for electron and magnon baths present on the different sides of the interface. However, no experimental report has disentangled the bulk and interfacial mechanisms yet.

Now, Kimling et al. observe the development of a thermally-induced spin accumulation at the interface between metallic Au or Cu and magnetic Y3Fe5O12 on a timescale of a few picoseconds. The researchers use pump–probe magneto-optical Kerr spectroscopy, rather than more common techniques based on the inverse spin Hall effect — which are subject to detect spurious signals and have a limited time resolution. The choice of Au or Cu ensures long spin-relaxation times as well as weak electron–phonon coupling, leading to high temperature values for the electron bath under laser illumination.

The researchers deduce that the short timescale is only compatible with an interfacial mechanism. This is confirmed by the proposed model, able to reproduce the experimental data well for magnetic layers with different thickness values.