Longitudinal spin Seebeck effect in thulium iron garnet (TmIG) thin films*

The longitudinal spin Seebeck effect (LSSE) induces incoherent magnon excitations with the application of a temperature gradient across the thickness of a magnetic material. To avoid the mixing of the thermally generated pure spin current with the electronic degrees of freedom, magnetic insulators are preferred for LSSE. Although the ferrimagnetic insulator Y₃Fe₅O₁₂ is known as the benchmark system for LSSE, other members of the insulating iron garnet family, e.g. the ferrimagnet Tm₃Fe₅O₁₂ (TmIG) are of interest and have received less attention from the point of view of spin-caloritronics. We have performed a systematic investigation of LSSE in the TmIG/Pt bilayer for two different TmIG thicknesses: 236 and 89 nm. We found that the temperature dependence of the LSSE signal shows a sudden decrease below 200 K for both the films, which coincides with the sudden drop in the temperature dependence of saturation magnetization (M_S). This anomalous drop in both M_S and LSSE voltage can be interpreted in terms of spin reorientation caused by increase in the effective magnetic anisotropy as revealed independently in radio frequency transverse susceptibility experiments. Our findings will help in understanding the influence of spin reorientation on the LSSE in other insulating garnets.