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Hwang, Y.*; Puebla, J.*; 近藤 浩太*; Gonzalez-Ballestero, C.*; 一色 弘成*; S
nchez Mu
oz, C.*; Liao, L.*; Chen, F.*; Luo, W.*; 前川 禎通*; et al.
Physical Review Letters, 132(5), p.056704_1 - 056704_7, 2024/01
被引用回数：19 パーセンタイル：97.55(Physics, Multidisciplinary)Here, we report the observation of strong coupling between magnons and surface acoustic wave (SAW) phonons in a thin CoFeB film constructed in an on-chip SAW resonator by analyzing SAW phonon dispersion anticrossings. We employ a nanostructured SAW resonator design that, in contrast to conventional SAW resonators, allows us to enhance shear-horizontal strain. Crucially, this type of strain couples strongly to magnons. Our device design provides the tunability of the film thickness with a fixed phonon wavelength, which is a departure from the conventional approach in strong magnon-phonon coupling research. We detect a monotonic increase in the coupling strength by expanding the film thickness, which agrees with our theoretical model. Our work offers a significant way to advance fundamental research and the development of devices based on magnon-phonon hybrid quasiparticles.
一色 弘成; 鈴木 和也; 家田 淳一
no journal, ,
Thermoelectric effects, such as the Seebeck effect (SE) and the anomalous Nernst effect (ANE), have been actively studied both for their fundamental properties and for their applications in energy-harvesting devices. Previous studies have mainly focused on measuring the thermoelectric properties of whole materials and devices, discussing their macroscopic thermoelectric performance. However, to investigate the influence of crystal grains and magnetic domains on thermoelectric effects, localized thermoelectric measurements in nanoscale are essential. To address this, we have developed a method to image SE and ANE in nanowires using an atomic force microscope (AFM) with a spatial resolution of approximately 80 nm. In this method, local temperature gradients are created by contacting a standard AFM probe to heated nanowires. Thermoelectric imaging is achieved by mapping the voltages at both ends of the wire. The non-magnetic Seebeck effect and magnetic anomalous Nernst effect can be separated by examining their external magnetic field dependence. So far, we have successfully visualized domains of the cluster magnetic octupole moments (the magnetic order parameter) in the functional antiferromagnet Mn
Sn. In this presentation, in addition to introducing this technique, we report the thermoelectric effects of the high-entropy alloy (HEA) FeCoNiCuPd. The local thermoelectric measurements on the HEA revealed an inhomogeneous distribution of the SE signals, which is attributable to the crystal grains with inhomogeneous compositions within the wires. On the other hand, the ANE signals were found to be nearly uniform across the wires, suggesting the robustness of the anomalous Nernst effect against compositional variations. These results are consistent with the global thermoelectric signals measured for the entire thin films with different compositions (x = 10, 20, 30). While the magnetic properties of HEAs have recently attracted attention, there have been few reports on their thermoelectric properties. In this presentation, we also discuss the impact of elemental substitution on thermoelectric effects, exploring the potential of high-entropy alloys as thermoelectric materials.
一色 弘成; 鈴木 和也; 高梨 弘毅; 今井 正樹; 家田 淳一; 大谷 義近
no journal, ,
The anomalous Nernst effect (ANE) is one of the magneto-thermoelectric effects, where a temperature gradient applied to a ferromagnetic conductor induces an electric field in the direction orthogonal to both the magnetization and the temperature gradient. This effect has attracted considerable attention, as it enables the conversion of heat into electric energy with a simple device structure. To realize higher conversion efficiency, materials with large anomalous Nernst coefficients (SANE [μV/K]) have been explored. Although magnetic topological materials exhibit giant ANE with SANE = 7μV/K, no material has yet been found with sufficient performance to meet the demands of practical heat energy harvesting. Here, we focus on ferromagnetic high-entropy alloys (HEAs) as a new class of materials hosting ANE. These alloys, composed of five or more elements, form stable single-phase solid solutions due to high configurational entropy. While HEAs have been intensively investigated for their mechanical properties, studies on their electronic transport remain limited. In this study, we investigated the anomalous Nernst effect in FeCoNi-based high-entropy alloy thin films. Thin films of FeCoNiCuPd, FeCoNiCuPt, and FeCoNiPdPt with a thickness of 30 nm were deposited on synthetic quartz substrates via sputtering. X-ray analysis confirmed the formation of an fcc structure. First, we changed the Pd composition rate (X) in Fe20Co20Ni20Cu40-xPdx to examine the impact of the heavy element on their ANEs. For the measurement, as illustrated in Fig. 1(a), an in-plane temperature gradient∇xT was applied, and a magnetic field Hz was swept in the out-of-plane direction while the transverse voltage Vy was measured. Figure 1(b)shows the field dependence of Vy for Fe20Co20Ni20Cu40-xPdx with x = 10,20, and 30. In all samples, Vy saturated at approximately 1 T, and the maximum value was nearly constant across compositions, yielding SANE ~ 0.41μV/K. The coefficient can be expressed as SANE =αANEρxx-θAHESSE, where αANE,ρxx,θAHE, and SSE are anomalous Nernst conductivity, longitudinal resistivity, anomalous Hall angle, and Seebeck coefficient, respectively. The first term represents the direct electric field generation from the temperature gradient, and the second term does the contribution from the anomalous Hall effect via the Seebeck effect. Our analysis revealed that, although SANE remained nearly constant, αANE increased by 0.53, 0.61, and 0.74 Am-1K-1 with increasing the Pd ratio x = 10, 20, and 30. This indicates that αANE is enhanced by increasing Pd concentration due to its strong spin-orbit coupling. Subsequently, we examined FeCoNiCuPt and FeCoNiPdPtthin films to further enhance SANE. As shown in Fig. 1(c), the addition of Pt increased SANE to ~0.9μV/K, withαANE reaching 0.96 Am-1K-1. In contrast, the Seebeck coefficient decreased to about two-thirds that of FeCoNiCuPd, limiting the contribution from the term of -θAHESSE to SANE. These results demonstrate that in HEAs,αANE,ρxx,θAHE, and SSE can be tuned by substituting elements and changing the composition rate. Future work will aim to identify the optimal composition to maximize SANE.
鈴木 和也; 一色 弘成; 高梨 弘毅
no journal, ,
5種類以上の元素が無秩序に配列した高エントロピー合金(HEA)は、優れた機械的特性、耐熱性、耐食性、耐照射性を有することから注目されている。本研究では、組成の異なるFeNiCoCuPdハイエントロピー合金薄膜に生じる異方性磁気抵抗効果や異常ホール効果などのスピン依存伝導現象を報告する。
