Determination of the origin of the spin Seebeck effect - bulk vs. interface effects

TL;DR

This paper investigates the origin of the spin Seebeck effect in insulators, demonstrating through thickness dependence measurements that the effect arises from bulk magnonic spin currents rather than interface effects, clarifying a controversial topic.

Contribution

It provides experimental evidence and theoretical analysis showing the bulk origin of the spin Seebeck effect, ruling out interface effects as the primary cause.

Findings

Thickness dependence supports bulk magnonic origin

Interface effects like magnetic proximity are ruled out

Qualitative agreement with thermal magnonic spin current models

Abstract

The observation of the spin Seebeck effect in insulators has meant a breakthrough for spin caloritronics due to the unique ability to generate pure spin currents by thermal excitations in insulating systems without moving charge carriers. Since the recent first observation, the underlying mechanism and the origin of the observed signals have been discussed highly controversially. Here we present a characteristic dependence of the longitudinal spin Seebeck effect amplitude on the thickness of the insulating ferromagnet (YIG). Our measurements show that the observed behavior cannot be explained by any effects originating from the interface, such as magnetic proximity effects in the spin detector (Pt). Comparison to theoretical calculations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite effective magnon propagation length so that the origin of the effect can be traced to genuine bulk magnonic spin currents ruling out parasitic interface effects.