Magnon diffusion lengths in bulk and thin film Fe3O4 for spin Seebeck applications

The magnon diffusion length (MDL) is understood to play a major role in the thickness dependence of spin Seebeck effect (SSE) voltages in Fe3O4/Pt thin films. Here we extract the MDL in an Fe3O4 single crystal using inelastic neutron scattering (INS) and in Fe3O4/Pt thin films using accurate heat flux SSE and static magnetization measurements. The INS MDLs were 34 ± 8 nm at 300 K and 27 ± 6 nm at 50 K. The SSE MDLs decreased with temperature (19 ± 2 nm at 300 K and 13 ± 4 nm at 50 K), but were markedly smaller. Whilst the bulk MDL is expected to be an upper limit of the MDL in thin films, we show that the film magnetization must be considered to obtain MDLs from SSE measurements. This study highlights the importance of disentangling the role of various effects in SSE measurements which is crucial in increasing the efficiencies of thermomagnetic energy harvesting devices

Enhancement of spin Seebeck effect in Fe3O4/Pt thin films with α-Fe nanodroplets

In this study, we demonstrate an enhancement of the measured spin Seebeck coefficient in Fe3O4/Pt bilayer films due to an increase in Fe nanodroplets formed by pulsed laser deposition. Four bilayer films were deposited at the same time from a highly textured target, resulting in a general increase in droplet formation that was confirmed to be Fe rich by scanning electron microscope and transmission electron microscope-dispersive x-ray spectroscopy. Of these four films, there were two distinct groupings with differing density of α-Fe droplets, where the bilayer with higher droplet density exhibited a 64% increase in the measured spin Seebeck coefficient from 38 to 63 nV m/W

Measurement of the heat flux normalized spin Seebeck coefficient of thin films as a function of temperature

The spin Seebeck effect (SSE) has generated interest in the thermoelectric and magnetic communities for potential high efficiency energy harvesting applications and spintronic communities as a source of pure spin current. Understanding the underlying mechanisms requires characterization of potential materials across a range of temperatures; however, for thin films, the default measurement of an applied temperature gradient (across the sample) has been shown to be compromised by the presence of thermal resistances. Here, we demonstrate a method to perform low temperature SSE measurements where, instead of monitoring the temperature gradient, the heat flux passing through the sample is measured using two calibrated heat flux sensors. This has the advantage of measuring the heat loss through the sample as well as providing a reliable method to normalize the SSE response of thin film samples. We demonstrate this method with an SiO2/Fe3O4/Pt sample where a semiconducting–insulating transition occurs at the Verwey transition, TV, of Fe3O4 and quantify the thermomagnetic response above and below TV