Ryo Toyama

Magnetic Weyl semimetals such as Co2MnGa are famous for exhibiting large anomalous Hall effect (AHE) due to intrinsic Berry curvature in their unique electronic band structure. However, because the AHE is also known to originate from extrinsic mechanism such as skew scattering and side-jump, it is important to clarify the role of extrinsic contribution to the AHE. In this study, we investigate the thickness dependence of the extrinsic contribution to the AHE in L21-ordered Co2MnGa single-crystal films from 8 to 90 nm epitaxially grown on MgO(001) substrates. A conventional scaling equation successfully reproduces the AHE of the 90 nm-thick film. The extrinsic contribution ratio reaches around 50%. In contrast, for the lower thicknesses especially below 24 nm, the AHE results are deviated significantly from the conventional scaling. We find that the AHE results are reproduced well using a modified scaling equation including additional extrinsic skew scattering. Our results indicate significant influence of extrinsic contribution on the AHE of the Weyl semimetal Co2MnGa films.

Autonomous high-throughput combinatorial experimentation is a key approach for accelerating materials discovery. However, achieving a fully closed-loop system remains a challenge due to the lack of effective optimization strategies for combinatorial experimentation. Here, we developed a Bayesian optimization method specifically designed for composition-spread films, enabling the selection of promising composition-spread films and identifying which elements should be compositionally graded. Using this approach, we demonstrated an autonomous closed-loop exploration of composition-spread films to enhance the anomalous Hall effect (AHE). Our method optimized the composition of a five-element alloy system consisting of three 3 d ferromagnetic elements of Fe, Co, and Ni and two 5 d heavy elements from Ta, W, or Ir to maximize the AHE. Through our autonomous exploration, we achieved a maximum anomalous Hall resistivity of 10.9 µΩ cm in Fe _44.9 Co _27.9 Ni _12.1 Ta _3.3 Ir _11.7 amorphous thin film on thermally oxidized Si substrates deposited at room temperature.

The development of new materials exhibiting large anomalous Hall effect (AHE) is essential for realizing highly efficient spintronic devices. However, this development has been a time-consuming process due to the combinatorial explosion for multielement systems and limited experimental throughput. In this study, we identify new materials exhibiting large AHE in heavy-metal-substituted Fe-based alloys using a high-throughput materials exploration method that combines deposition of composition-spread films using combinatorial sputtering, photoresist-free facile multiple-device fabrication using laser patterning, simultaneous AHE measurement of multiple devices using a customized multichannel probe, and prediction of candidate materials using machine learning. Based on experimental AHE data on Fe-based binary system alloyed with various single heavy metals, we perform machine learning analysis to predict the Fe-based ternary system containing two heavy metals for larger AHE. We experimentally confirm larger AHE in the predicted Fe–Ir–Pt system. Using scaling analysis, we reveal that the enhancement of AHE originates from the extrinsic contribution.

Magnetic field sensors based on the anomalous Hall effect (AHE) require a magnetic thin film with high anomalous Hall resistivity (ρyxA) and moderate perpendicular uniaxial magnetic anisotropy (Ku) to achieve both a large linear sensitivity and a dynamic range (DR) adaptable for various applications. In this study, we fabricate Fe1−xPtx(0≤x≤1) composition-spread epitaxial thin films and systematically investigate the composition dependence of the AHE, aiming to achieve high sensitivity for the magnetic field sensors. We also explore the underlying physical mechanism of the AHE in the Fe–Pt binary system. Structural analysis reveals the [001]-oriented epitaxial growth, featuring distinct phases of A2-Fe, L12-Fe3Pt, L10-FePt, and L12-FePt3 as increasing x along with their mixed-phases, except for the Pt-rich region. Among the entire Fe1−xPtx, an off-stoichiometric Fe-rich composition Fe0.70Pt0.30 exhibits the highest ρyxA of 4.5 μΩ cm with a linear response to the external magnetic field and a DR of 600 mT, yielding a high sensitivity of 8 μΩ cm/T. The DR can be tuned in a wide range from 600 to 1400 mT by changing the Fe:Pt composition ratio, which is attributed to the variation in Ku and magnetization in the Fe–Pt binary alloy system. The anomalous Hall conductivity (AHC) shows an oscillatory variation with composition x, with the largest AHC of 1261 S/cm observed for Fe0.70Pt0.30. The theoretical calculation of the AHC for A2-Fe and L12-Fe3Pt, including the analysis of the density of states for the off-stoichiometric compositions, suggests that the overall variation in AHC with x can be mostly explained by an intrinsic mechanism and the electron doping effect introduced by Pt addition. Theoretical analysis supports that the highest sensitivity obtained in Fe0.70Pt0.30 originates from the large intrinsic AHC (1759 S/cm) predicted for L12-Fe3Pt.