Spin Seebeck effect in iron oxide thin films: Effects of phase transition, phase coexistence, and surface magnetism

TL;DR

This study investigates how phase transitions, coexistence, and surface magnetism influence the spin Seebeck effect in iron oxide thin films, revealing the dominant role of interfacial magnetism on LSSE signals in different substrates.

Contribution

It provides a comprehensive analysis of temperature-dependent LSSE in biphase iron oxide films, highlighting the impact of phase transitions and surface magnetism on spin to charge conversion efficiency.

Findings

LSSE signal is dominated by LSSE over ANE below Verwey transition.

Surface magnetic moments significantly affect LSSE magnitude.

Si/BPIO/Pt exhibits nearly twice the LSSE signal compared to Al2O3/BPIO/Pt.

Abstract

Understanding impacts of phase transition, phase coexistence, and surface magnetism on the longitudinal spin Seebeck effect (LSSE) in a magnetic system is essential to manipulate the spin to charge current conversion efficiency for spincaloritronic applications. We aim to elucidate these effects by performing a comprehensive study of the temperature dependence of LSSE in biphase iron oxide (BPIO = alpha-Fe2O3 + Fe3O4) thin films grown on Si (100) and Al2O3 (111) substrates. A combination of temperature-dependent anomalous Nernst effect (ANE) and electrical resistivity measurements show that the contribution of ANE from the BPIO layer is negligible compared to the intrinsic LSSE in the Si/BPIO/Pt heterostructure even at room temperature. Below the Verwey transition of the Fe3O4 phase, the total signal across BPIO/Pt is dominated by the LSSE. Noticeable changes in the intrinsic LSSE signal for both Si/BPIO/Pt and Al2O3/BPIO/Pt heterostructures around the Verwey transition of the Fe3O4 phase and the antiferromagnetic (AFM) Morin transition of the alpha-Fe2O3 phase are observed. The LSSE signal for Si/BPIO/Pt is found to be almost two times greater than that for Al2O3/BPIO/Pt, an opposite trend is observed for the saturation magnetization though. Magnetic force microscopy reveals the higher density of surface magnetic moments of the Si/BPIO film compared to the Al2O3/BPIO film, which underscores a dominant role of interfacial magnetism on the LSSE signal and thereby explains the larger LSSE for Si/BPIO/Pt.