Abstract
The spin-Seebeck effect (SSE) is an advective transport process traditionally studied in bilayers composed of a ferromagnet (FM) and a non-magnetic metal (NM) with strong spin-orbit coupling. In a temperature gradient, the flux of magnons in the FM transfers spin-angular momentum to electrons in the NM, which by the inverse spin-Hall effect generates an SSE voltage. In contrast, the Nernst effect is a bulk transport phenomenon in homogeneous NMs or FMs. These effects share the same geometry, and we show here that they can be added to each other in a new combination of FM/NM composites where synthesis via in-field annealing results in the FM material (MnBi) forming aligned needles inside an NM matrix with strong spin-orbit coupling (SOC) (Bi). Through examination of the materials’ microstructural, magnetic, and transport properties, we searched for signs of enhanced transverse thermopower facilitated by an SSE contribution from MnBi adding to the Nernst effect in Bi. Our results indicate that these two signals are additive in samples with lower MnBi concentrations, suggesting a new way forward in the study of SSE composite materials.
Highlights
The spin-Seebeck effect (SSE) is the most recent addition to the family of spin-related thermal effects [1]
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We observed that the length-to-width aspect ratio of the MnBi particles was optimized for the sample containing 10 at.% MnBi, in which the MnBi formed needlelike shapes within the matrix that corresponded with maximized magnetic anisotropy at higher temperatures
Summary
The spin-Seebeck effect (SSE) is the most recent addition to the family of spin-related thermal effects [1]. Boona et al [10] have shown that it is possible to combine the ANE and SSE in composite materials where the NM/FM interfaces are not planar, but randomly distributed throughout the bulk of the material, such as in coatings of spherical Ni particles with Pt nanoparticles This type of composite approach has multiple advantages over thin-film heterostructures for energy conversion applications, such as the use of scalable manufacturing techniques to prepare bulk quantities of material, leading to potentially more efficient thermoelectric/spin-caloritronic energy conversion overall. The peritectic decomposition temperature at 446 ◦C, the HT phase melts into liquid Bi with Mn in solution and solid Mn. While there are several previous studies of MnBi-Bi composites with compositions at or near the eutectic, the location of the liquidus line indicates that, under equilibrium conditions below ~355 ◦C, Mn-Bi alloys with Mn content up to ~10 at.% are expected to contain phase-pure MnBi particles precipitated from solution. In conjunction with microstructural characterization, we measured the anisotropic resistivity, Seebeck, and Nernst effects of a range of MnBi concentrations to search for evidence of a contribution from the SSE to the materials’ transport properties
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