Abstract
In this study, we report on the observation of spin current in opposite spin Hall angle materials of polycrystalline bulk-Fe3O4/Co and polycrystalline bulk-Fe3O4/Fe spin Seebeck effect (SSE) devices. In contrast to prior works, a facile and low-cost hot-pressing powder metallurgy process was employed to manufacture the polycrystalline bulk-Fe3O4 samples. The crystal structure, magnetization properties, and electrical resistivity characterizations of the fabricated bulk-Fe3O4, which were performed using x-ray diffraction, vibration sample microscope, and four-point probe, respectively, revealed excellent agreement with those of conventional Fe3O4. By taking advantage of the fact that the SSE signal in our devices is typically contaminated with the anomalous Nernst effect (ANE), we show that the total thermo-voltage obtained from our devices can be enhanced by the significant ANE signals exhibited by the Co and Fe spin detectors. Importantly, the ANE contributions could be filtered out from the main signal by independent measurements of the ANE voltage in SiO2/Co and SiO2/Fe systems, thereby allowing the approximate extraction of the SSE voltage. Our experiments reveal that the polarity of the measured ANE (and pure SSE voltages) are opposite to each other in the bulk-Fe3O4/Co and bulk-Fe3O4/Fe structures, thus proving the opposite spin-hall angles character of these materials. The findings of this work provide a pathway for further exploration of methods through which the thermo-voltage output in future spin-Hall thermopile devices may be improved using materials manufactured via a facile, low-cost, and easily scalable process.
Highlights
Since its discovery by Uchida et al.,1–19 the spin Seebeck effect (SSE) has been garnering significant research interest in the thermoelectric community
The x-ray diffraction (XRD) phase diagram [Fig. 2(a)] of the polycrystalline bulk Fe3O4 matches very well with the phase information corresponding to the details of JCPDS No 01-075-0033 data available for Fe3O4, thereby providing validation for the use of the hot-pressing methodology adopted in this work
The SEM images obtained before and after polishing are shown in Figs. 2(b) and 2(c), where it can be seen that an original roughness of more than 5 μm present in the raw sample is significantly reduced post polishing, as evidenced by the microscopically smooth surface
Summary
Since its discovery by Uchida et al., the spin Seebeck effect (SSE) has been garnering significant research interest in the thermoelectric community. The detected spin signals from SSE devices have been typically low, which, in turn, have led researchers to seek other strategies aimed at increasing the output performance of the SSE devices. Notable of these are the multilayer configuration technique and the spin Hall thermopile technique.. The PMM layer is constructed by a zigzag arrangement of alternating materials having opposite spin Hall angle (θSH) This results in an additive effect on the ISHE and, allows for the detection of an enhanced spin signal. Combining the ANE and SSE in this manner offers a path to greatly enhance the performance of future thermopile structures
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