Christopher Cox

In this article, we show the measurement of the magneto-thermoelectric effect (spin Seebeck and anomalous Nernst effects) of bulk composite pellets prepared from nanopowders of Fe3O4 and Pt. We compare structural, magnetic, and thermoelectric data as the Pt% is increased against a control series of pellets, where sintering temperature and time are varied and a 5 nm layer of Pt is sputtered onto the surface. We also compare the measured thermoelectric efficiency and figure of merit of the composite pellets to our control and a benchmark thin film, where we find an order-of-magnitude increase. This suggests that composite pellets could be a route forward in scaling up the spin Seebeck effect from thin film to bulk.

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.

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, T_V, of Fe₃O₄ and quantify the thermomagnetic response above and below T_V.

Separation of the anomalous Nernst and spin Seebeck voltages in bilayer devices is often problematic when both layers are metallic, and the anomalous Nernst effect (ANE) becomes non-negligible. Co₂MnSi, a strong candidate for the spin generator in spin Seebeck devices, is a predicted half-metal with 100% spin polarisation at the Fermi energy, however, typically B2 or L2₁ order is needed to achieve this. We demonstrate the optimisation of thin film growth of Co₂MnSi on glass, where choice of deposition and annealing temperature can promote various ordered states. The contribution from the ANE is then investigated to inform future measurements of the spin Seebeck. A maximum ANE coefficient of 0.662 µV K⁻¹ is found for an A2 disordered polycrystalline Co₂MnSi film. This value is comparable to ordered Heusler thin films deposited onto single crystal substrates but obtained at a far lower fabrication temperature and material cost.

We study the properties of polycrystalline bulk yttrium iron garnet (YIG) pellets prepared by the solid-state method, where the choice of the sintering temperature can lead to mixed phases of yttrium iron perovskite (YIP) and YIG or single phase YIG. Magnetometry shows multiple switching regimes in the mixed-phase pellets where the saturation magnetization is dominated by the proportion of YIG present. Ferromagnetic resonance was used to corroborate the saturation magnetization from magnetometry and to extract the spin wave damping α. The lowest damping was observed for the YIG pellet, which resulted in a spin Seebeck effect (SSE) coefficient that was approximately 55% of single crystal YIG. This demonstrates that macroscale crystallization does not play a major role in the SSE and paves the way for utilising polycrystalline samples for thermomagnetic applications.

The spin Seebeck effect (SSE) has been widely studied as a potential mechanism for energy harvesting. However, the efficiency of such devices, utilizing the spin thermoelectric effect in thin film form, has not yet reached a sufficient value to make them economically viable. It is therefore imperative that advances are made to investigate means by which the thermoelectric signal can be enhanced. Multilayers of Co₂MnSi and Pt are fabricated and characterized in an attempt to observe enhanced voltages. We report that bilayers of ferromagnetic conductor/normal metal (FM/NM) exhibit a Longitudinal SSE response and that repetitive stacking of such bilayers results in an increased thermoelectric voltage that is highly dependent upon the quality of CMS/Pt and Pt/CMS interfaces.