Enhancement of the Anomalous Nernst Effect in Polycrystalline Co2MnGa/AlN Multilayers

Abstract

I. INTRODUCTIONThermoelectric generators (TEG) that convert waste heat to electricity is regarded as alternative and environment-friendly technology for harvesting and recovering heat [1]. Although the conventional thermoelectric devices based on the Seebeck effect (SE) are promising, SE-based TEGs suffer from the high fabrication cost due to their complex structure and the usage of toxic, rare, and expensive elements. Furthermore, the one-dimensional relationship between the heat and carrier transports defined in SE makes it very challenging to achieve a high energy conversion efficiency and be only limited to a flat heat source surface [1, 2]. Recently its counterpart, the anomalous Nernst effect (ANE) in ferromagnetic materials, has gained increasing interests with its potential merits to further improve the figure-of-merit, ZT, simplify a thermopile structure with low-cost materials, and be applicable for a non-flat surface [2, 3]. However, to date, the reported thermoelectric conversion efficiency of the ANE is too small compared with the SE, which hampers its application as TEG. Thus, it is quite meaningful to explore valid approaches to achieve larger ANE output.In this study, we focus on the multilayer structure with full Heusler alloy Co2MnGa which is a famous ferromagnet exhibiting the large ANE thanks to its characteristic electronic band structure [3, 4]. In addition to the ANE originating from the bulk property of Co2MnGa, we expect the enhancement of ANE in the multilayer structures as reported previously [5, 6]. Here, polycrystalline Co2MnGa/AlN multilayer films were prepared on the amorphous substrates, the layer thickness dependence of ANE was investigated. We found that the large anomalous Nernst thermopower (SANE) of 4.9 ± 0.4 µV K-1 can be obtained even for the polycrystalline Co2MnGa.II. EXPERIMENTAL METHODThe film stacking structure is Si/SiO2/AlN(20)/[Co2MnGa(t)/AlN(5)]25/t (t = 2.5, 5.0, 12.5, and 25.0, thickness in nm), which was fabricated by DC magnetron sputtering at room temperature. Then all the films were post-annealed at 500 oC for 3 hours. For the ANE measurement, the thin films were first patterned into Hall-cross shapes through the use of photolithography and Ar ion milling. After milling, the on-chip thermometers made of the 100-nm-thick sputtered Pt were prepared by the lift-off process. The in-plane temperature gradient was generated by an on-chip heater, and the temperature on the cold and hot sides of the sample was monitored by two Pt on-chip thermometers. An external magnetic field (± 2 T) was applied perpendicular to the sample plane.III. RESULT AND DISCUSSIONThe structural characterization was carried out using the x-ray diffraction. For all samples, the Co2MnGa layers showed the (110)-oriented textures on the SiO2 substrate. The 25-nm-thick Co2MnGa film (single layer) exhibited the weak Co2MnGa 220 peak. On the other hand, [Co2MnGa (12.5)/AlN (5)]2 multilayer film (multilayer) with the Co2MnGa layers sandwiched by dielectric AlN layers shows a much stronger (110) texture. Moreover, the satellite peaks were observed around 2θ = 44.6° of the multilayer sample, indicating shape and abrupt Co2MnGa/AlN interfaces. Furthermore, the largest SANE of 4.9 ± 0.1 µV K-1 was achieved for the multilayer film while the value of SANE was 3.8 ± 0.4 µV K-1 for the single layer film. Although the present SANE of 4.9 ± 0.1 µV K-1 is smaller than the record-high value of SANE which is ~ 8.0 μV K−1 for single crystal Co2MnGa bulk [7], it is still promising one order of magnitude larger than those for other ferromagnets with similar magnetizations [8]. The results indicate that the large ANE output can be obtained even for the polycrystalline Co2MnGa which quite meaningful for practical application. This fact also implies that multilayers and/or superlattices with low dimension and sharp interfaces are promising platform/candidates to achieve large ANE. The present results provide new insight into the correlation between microstructure and ANE.We consider that the size and interface effect are the primary origins for the enhancement of ANE in the proposed Co2MnGa/AlN multilayer films. There are three possible scenarios for explaining the enhancement of ANE in the multilayer samples. First, the distinct thermal expansion coefficient, elastic modulus, and lattice mismatch between metals (Here Co2MnGa) and ceramics (here AlN) would introduce tremendous interface stress during the annealing, i.e. heating and cooling processes. The interface stress in the Co2MnGa layer may be a driving force to promote the chemical ordering of Co2MnGa. Second, the lattice distortion in Co2MnGa due to the interface stress will modify its electronic band structures. Third, the composition variation in the Co2MnGa layers may occur via the interdiffusion through the interface. The detailed structural analyses will be addressed in the presentation.This work successfully demonstrated a way to achieve giant ANE thermopower for the polycrystalline Co2MnGa layer on the amorphous substrate by exploiting the dielectric AlN layers. This is an important finding for practical applications. This work was supported by KAKENHI (18H05246) and CSRN. **