Universal scaling of the temperature dependence of longitudinal spin Seebeck effect in compensated ferrimagnets

Abstract

The longitudinal spin Seebeck effect (LSSE) is an efficient method for generating incoherent magnon excitations by applying a vertical temperature gradient across a magnetic material. To avoid the contamination of the thermally generated “pure” spin current by the electronic degrees of freedom, magnetic insulators are exclusively preferred for LSSE. Although the ferrimagnetic insulator Y3Fe5O12 (YIG) is known as the hallmark system for LSSE because of its ultra-low Gilbert damping,[1] other members of the insulating iron garnet family, including the compensated ferrimagnet Gd3Fe5O12 (GdIG), have recently attracted considerable interest due to their unique compensated spin configurations.[2] In particular, the fascinating observation of the change in sign of the LSSE signal in a GdIG/Pt bilayer around the magnetic compensation temperature due to reorientation of sublattice magnetizations has motived us to address to an emerging fundamental question: “Is there a general trend for the temperature dependence of LSSE in compensated ferrimagnets?” Therefore, we have performed a comprehensive investigation of LSSE in GGG/GdIG(t)/Pt(5nm) bilayers with 5 different thicknesses ranging between 31 and 272 nm. We find that all the GdIG films possess an in-plane magnetic easy axis, and the compensation temperature decreases from 270 to 220 K with decreasing film-thickness from 272 to 31 nm. Using a proposed rescaling method, we have demonstrated, for the first time, a “universal scaling” behavior for the temperature dependence of LSSE signal for our GdIG films with different compensation temperatures, which, we believe, holds true for other compensated ferrimagnetic systems as well. In addition, we have performed the LSSE study on a 31 nm GdIG film grown on lattice-mismatched GSGG substrate that exhibits an out-of-plane magnetic easy axis and observed a clear distinction in the magnetic field dependent LSSE signal, relative to the 31 nm GGG/GdIG film showing an in-plane magnetic easy axis. This highlights a strong correlation between the LSSE signal and the magnetic anisotropy in compensated ferrimagnets. Our findings will pave the way to explore LSSE in other novel compensated ferrimagnets.