Phys. Rev. Mat. 1, 031401 (2017)

The precession of magnetic moments in ferromagnetic materials can be used to inject pure spin currents through the interface with adjacent materials. These precessions can be induced coherently by external magnetic fields under conditions of ferromagnetic resonance or incoherently by thermal gradients — leading to the so-called spin pumping and spin Seebeck effects, respectively. Although these phenomena have been widely studied, a systematic investigation of the influence of the degree of crystallinity of the ferromagnetic material on the injection process is still missing.

Now, Chang et al. report on insulating yttrium iron garnet (YIG) thin films grown by radiofrequency magnetron sputtering on different materials — gadolinium gallium garnet (GGG), silicon and glass. The actual substrate determines the quality of the YIG crystallinity, which is high only in the case of GGG substrates. The injection of spin currents in adjacent platinum layers is then investigated under the effect of microwave magnetic fields or thermal gradients, and the amplitude of the effect is quantified via the detection of an inverse spin Hall voltage across platinum.

The results demonstrate that the polycrystalline morphology of YIG layers strongly limits the spin pumping efficiency. However, the incoherent nature of the thermal excitation leads to a substantial insensitivity of the spin-injection efficiency on the degree of crystallinity of the ferromagnet, with relevant implications for technological applications of the spin Seebeck effect in polycrystalline materials.