Observation of the spin Seebeck effect in a paramagnetic insulator VO₂

The spin Seebeck effect (SSE) involves the generation of spin current in the presence of a temperature gradient across the material.  Here we present a systematic study of the transverse SSE in the paramagnetic insulating phase of VO₂ at low temperatures. In the nonlocal geometry, the spin current is thermally generated by an AC current flowing in one Pt wire and the SSE is detected by another Pt wire using a lock-in amplifier at the second harmonic via the inverse spin Hall effect. The pronounced SSE at 3 K, decreases sharply as the temperature increases to 10K, and suddenly changes sign at 15 K, before vanishing above about 40 K. The SSE shows the expected angle dependence with the in-plane field; however, when the field is out-of-plane, the signal is accompanied by a Nernst effect contribution. We will also discuss the dependence of the observed SSE on heater-detector distance.