Decoupling anomalous Nernst and longitudinal spin Seebeck effects in Fe3O4/Pt thin films around the Verwey transition

Abstract

Magnetite (Fe3O4) is a unique half-metallic magnetic material for longitudinal spin Seebeck effect (LSSE) investigations because of its characteristic Verwey transition (TV ≈ 125 K). Since Fe3O4 is metallic above TV, the measured LSSE signal is contaminated by the anomalous Nernst effect (ANE) arising from the Fe3O4 layer. Earlier study on the LSSE in Fe3O4/Pt [1] did not disentangle the LSSE and ANE signals as a function of temperature above and below TV. To address this, we have performed a comprehensive study of the temperature dependence of ANE and LSSE in Fe3O4 thin films (80 nm) grown on Si and Al2O3 substrates. We find that while both effects are present in Fe3O4/Pt at T > TV, the ANE signal is much smaller compared to the intrinsic LSSE signal. Below TV, the total signal across Fe3O4/Pt is completely dominated by the LSSE. Through a comprehensive analysis of temperature dependent LSSE and ANE, we decouple their contributions that allow us, for the first time, to observe an anomalous temperature dependence of the intrinsic LSSE signal in Fe3O4/Pt around TV. A comparative study of the intrinsic LSSE in Si/Fe3O4/Pt and Al2O3/Fe3O4/Pt points to the important role of the ordering and density of spins on the Fe3O4 surface on the spin transport and spin-charge conversion efficiency [2,3]. The LSSE voltage measured at 300 K in Si/Fe3O4/Pt is about two times greater than that of Al2O3/Fe3O4/Pt. This finding is fully supported by the magnetic force microscopy study that reveals the higher density of magnetic domains on the surface of the Fe3O4 film grown on Si compared to the Fe3O4 film grown on Al2O3. Our study not only provides deeper insights into the LSSE and its temperature evolution in magnetic oxides like Fe3O4 but also an effective way for improving the spin-charge conversion efficiency in magnetic films through surface magnetism engineering.