Low temperature spin Seebeck effect in non-magnetic vanadium dioxide

TL;DR

This study reports a surprising paramagnetic spin Seebeck effect in insulating VO2 at low temperatures, with a magnitude comparable to magnetic materials, challenging previous assumptions about its magnetic state.

Contribution

It demonstrates the presence of a paramagnetic SSE response in VO2, a non-magnetic insulator, and provides a quantitative analysis of its magnitude and temperature dependence.

Findings

Paramagnetic SSE response observed below 50 K.

Spin Seebeck coefficient comparable to magnetic insulators.

Inconsistent with triplet spin transport models.

Abstract

The spin Seebeck effect (SSE) is sensitive to thermally driven magnetic excitations in magnetic insulators. Vanadium dioxide in its insulating low temperature phase is expected to lack magnetic degrees of freedom, as vanadium atoms are thought to form singlets upon dimerization of the vanadium chains. Instead, we find a paramagnetic SSE response in VO2 films that grows as the temperature decreases below 50 K. The field and temperature dependent SSE voltage is qualitatively consistent with a general model of paramagnetic SSE response and inconsistent with triplet spin transport. Quantitative estimates find a spin Seebeck coefficient comparable in magnitude to that observed in strongly magnetic materials. The microscopic nature of the magnetic excitations in VO2 requires further examination.