Spin Seebeck Effect in insulating SrFeO_3-δ films

SrFeO₃ is a metal with cubic structure and is a helicoidal antiferromagnet (AF) at low temperatures (< 134 K) [1]. Recently, an anisotropic double q spin spiral and isotropic quadruple q spiral hosting a three-dimensional lattice of hedgehog singularities in SrFeO₃ [2] was found suggesting a topologically non-trivial magnetic structure. Upon reduction, SrFeO_3-δ becomes insulating while retaining long wavelength incommensurate AF order. In this work we measured the spin Seebeck effect (SSE)in SrFeO_3-δ films synthesized by molecular beam epitaxy, and detected an SSE signal below 300 K. The measured SSE signal is enhanced below ~ 120 K, near the ordering temperature of incommensurate antiferromagnetism found in SrFeO_3-δ bulk [3]. Control experiment were used to verify that the signal arises from a spin current originating in SrFeO_3-δ and to rule out magnetic proximity effect. By comparing the second derivative of the SSE signal with respect to magnetic field and comparing to a Brillouin function, we find large deviations that suggest strong spin correlations in oxygen reduced SrFeO_3-δ for temperatures below 120 K. Our work suggests SSE an effective way of probing magnetic correlations in non-ferromagnetic insulating films.