Designing efficient spin Seebeck-based thermoelectric devices via simultaneous optimization of bulk and interface properties

Abstract

The spin Seebeck effect (SSE) refers to a direct energy conversion of heat to electricity that exploits magnon current in a ferromagnet (FM)/normal metal (NM) heterostructure. Since SSE is a transverse thermoelectric effect, it can be employed to realize transverse thermoelectric devices, which could potentially overcome the inherent limitations of conventional Seebeck effect-based devices. Yet, the widespread use of SSE is currently hindered by its low conversion efficiency. The thermoelectric efficiency of SSE can be improved through an optimization of bulk FM as well as FM/NM interface. A bulk FM can enhance the thermoelectric efficiency by introducing a 'phonon-glass magnon-crystal' concept, wherein scattering centers selectively scatter the heat-carrying phonons while not affecting the magnons that contribute to SSE. A high-quality FM/NM interface can boost the spin pumping across the interface as well as the electrical transport in NM. Here, by adopting polycrystalline nickel-ferrite (NFO)/Pt as a platform, we demonstrate the successful optimization of bulk FM together with FM/NM interface via a simple heat treatment scheme. The phase separation of NFO via such heat treatment results in a distinctive hierarchical microstructure of nano-sized NFO embedded in micro-sized NiO precipitates; this structure selectively scatters phonons while barely affecting magnons, leading to reduced thermal conductivity without variation of spin Seebeck coefficient. The simultaneous formation of a clean and smooth NFO/Pt interface provides efficient spin pumping at the interface and high electrical conductivity of Pt. As a result, the energy conversion efficiency of SSE in NFO/Pt system can be significantly improved. The proposed facile approach can be applied to design highly efficient oxide-based SSE devices.