Kodchakorn Simalaotao

We study spin magnetic moments near half-metallic Co2FexMn1−xSi (CFMS)/metal interfaces using depth-resolved x-ray magnetic circular dichroism (XMCD) and analyze their robustness against thermal fluctuations. We prepare epitaxial magnetic multilayered films with CFMS/CoFe and CFMS/Ag interfaces and observe the x-ray absorption spectroscopy (XAS) and XMCD spectra for both the interfacial and inner CFMS regions. By analyzing the observed XAS and XMCD spectra, we approximately extract the Mn-related magnetic moments. The results show that the Mn spin magnetic moment at the CFMS/CoFe interface exhibits almost no reduction compared with that in the inner CFMS layer, in contrast to the significant reduction observed at the CFMS/Ag interface. This suggests that the CFMS/CoFe interface provides a larger interfacial exchange stiffness than the CFMS/Ag interface, consistent with theoretical interface exchange stiffness values obtained from first-principles calculations. Furthermore, the large exchange stiffness at the CFMS/CoFe interface leads to the high stability of local spin magnetic moments to thermal fluctuation, resulting in a weaker temperature dependence of the magnetoresistance (MR) ratio in current-perpendicular-to-plane giant MR devices with a CoFe/CFMS/Ag/CFMS/CoFe structure than in those with a CFMS/Ag/CFMS structure.

Compositional disorder plays a critical role in determining transport properties in spintronic heterostructures and ultimately governs their experimentally realizable performance. In this work, we investigate spin-dependent transport in current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) junctions composed of Co2FeGa0.5Ge0.5 (CFGG) electrodes and Cu–Zn spacer alloys, with particular emphasis on the effects of spacer composition and atomic order–disorder. The virtual crystal approximation is employed to treat compositional disorder in the Cu–Zn spacer, thereby extending its application from bulk alloys to spacer alloys embedded in magnetic junctions. Systematic first-principles transport calculations reveal that the majority-spin conductance in the parallel magnetic configuration is maximized at Zn concentrations of approximately 30–50 at. % for both B2-ordered and A2-disordered spacers. This enhancement originates from improved Fermi-surface matching between the CFGG electrodes and the Cu–Zn spacer, leading to an increased interfacial spin-asymmetry coefficient and, consequently, enhanced magnetoresistance performance. In addition, both the conductance and the interfacial exchange stiffness constant exhibit only weak sensitivity to the B2 → A2 transition, indicating that atomic disorder induces only marginal changes in majority-spin transport. These results demonstrate the disorder tolerance of CFGG/Cu–Zn/CFGG junctions with respect to the Fermi-surface matching and the thermal fluctuation, and they provide design guidelines for chemically tunable spacer alloys in CPP-GMR devices.

Novel materials for spintronic devices require not only bulk electronic properties but also electronic structures compatible with magnetic/nonmagnetic interfaces. First-principles calculations have revealed the importance of electron band matching (EBM) at the Fermi energy for transport properties such as giant magnetoresistance (GMR). However, direct evaluation of ballistic conductance in GMR devices using first-principles calculations is computationally expensive and unsuitable for high-throughput screening. Here, we propose a machine learning framework to predict ballistic conductance using image representations of the Fermi surface similarity between magnetic and nonmagnetic materials. The images are decomposed into dictionary atoms, and the resulting feature vectors are subjected to regression analysis, with Gaussian process regression achieving the highest accuracy among the various models. Our results demonstrate that the proposed method effectively captures interfacial features essential for electron transport and provides a rapid means to screen material combinations with large EBM. This framework offers a practical step toward realizing material informatics for spintronics, enabling efficient exploration of heterogeneous material combinations for next-generation GMR devices.

Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices are promising candidates for next-generation hard disk drive (HDD) read heads due to their low resistance-area product (RA) and high-speed operation. This study investigates Cu-based binary alloys (Cu-X) as alternative spacers to conventional Ag spacer in Co2FeGa0.5Ge0.5 (CFGG)-based CPP-GMR devices using first-principles calculations. Majority-spin ballistic conductance, which is essential in the large spin-asymmetry of the interface resistance, was evaluated for CFGG/Cu-X/CFGG(001) trilayers in comparison with CFGG/Ag/CFGG(001) trilayer. The results show that several spacer materials have possibilities to enhance the MR ratio compared to Ag. In particular, we found that B2-CuZn will be the most suitable for CFGG electrode. The large majority-spin conductance of CFGG/CuZn/CFGG(001) is attributed to p-d hybridization between CFGG and Cu s-states near the Γ point, which enhances electron transport efficiency, while the thermal stability of the interface spin-moment at CFGG/CuZn(001) is slightly smaller than that of CFGG/Ag(001). Finally, we proposed that the insertion of Ag layer between the CFGG and CuZn interfaces will enhance not only the spin-asymmetry in the interface resistance but also the thermal stability of the interface spin-moment.

The development of energy-efficient spintronic devices with enhanced magnetoresistance demands materials with low Gilbert damping constant (α) and high half-metallicity. In this study, we report a very low α in sputter-deposited off-stoichiometric Co2FeGa0.5Ge0.5 (CFGG) Heusler alloy thin films and investigate the relation between α and atomic ordering and half-metallicity. CFGG thin films with a composition of Co44.3Fe30.7Ga13.9Ge11.1 were epitaxially deposited on MgO (001) substrates by magnetron sputtering followed by in situ annealing. The density of states calculations revealed that this composition has higher half-metallicity than the stoichiometric composition, owing to Fermi energy tuning. As-deposited and 400 °C-annealed samples exhibited B2-type partially disordered structure, while annealing above 500 °C induced a L21-type ordered structure. The improvement in atomic order resulted in the reduction of α, as demonstrated by ferromagnetic resonance measurement, and enhancement in half-metallicity, as revealed by anisotropic and ordinary magnetoresistance measurements. The 600 °C-annealed samples exhibited an intrinsic α of (3.5 ± 0.3) × 10−4, the low value reported for metallic ferromagnetic materials, demonstrating the potential for its use in energy-efficient spintronic devices.

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.

Half-metallic ferromagnetic Heusler alloys having high spin polarization are promising candidates to realize large magnetoresistance (MR) ratio and high spin-transfer torque (STT) efficiency in next-generation spintronic devices. Since the Heusler alloy properties are sensitive to composition, optimizing the composition is crucial for enhancing device performance. Here, we report the fabrication of high-performance current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices using Co2MnxFe1−xGe (0 ≤ x ≤ 1) Heusler alloy, employing a high-throughput and detailed composition optimization method. The method combined composition-gradient films and local measurements to enable the composition variation from Co2FeGe to Co2MnGe to be efficiently studied on a single library sample with a small composition interval. The CPP-GMR devices fabricated from stacks annealed at 250 °C showed a clear composition dependence of MR with the maximum of MR ratio ∼38% in the Mn-rich region of x = 0.85. By increasing the annealing temperature to 350 °C, the MR ratio increased to ∼45% along with high STT efficiency ∼0.6 in the broad composition range of 0.2 ≤ x ≤ 0.7. The optimal composition for the highest MR changed with annealing temperature because of the stability of the GMR stack being higher in the lower x range. The record high MR for the all-metal CPP-GMR devices, at low annealing temperature of 250 °C was achieved by the detailed composition optimization. These results present the high potential of Co2MnxFe1−xGe and provide a comprehensive guidance on the composition optimization for achieving large MR ratio and high STT efficiency in the CPP-GMR devices.