arXiv subjects

Shinji Isogami

Publications and source records attributed to Shinji Isogami.

8 recordsLinked to original sources

arXiv2026

Nitrospinics as a platform from orbital-torque memory to artificial intelligence

The exploration of energy-efficient and functional spintronics has attracted considerable attention. Orbital transport has opened new pathways for current-induced torque generation beyond the conventional spin transport based on the spin Hall effect. In addition, artificial intelligence computing has been demonstrated using spintronic devices. Further progress of these devices can be anticipated through the development of unique and functional materials beyond the existing heavy metals and topological systems with strong spin-orbit coupling. The nitride materials exhibit unique chemical, magnetic, and structural versatility, including antiferromagnetism, high thermal stability, and compatibility with diverse device architectures. Here, we propose Nitrospinics as a conceptual and functional framework that exploits nitride materials for applications ranging from orbital-torque-based spintronic devices to artificial intelligence hardware. Using Cr2N, a two-dimensional nitride MXene with an atomic layered structure, as a prototype system, we discuss how nitrogen contributes to the structural stability, the orbital torque generation, and the interfacial orbital and spin conversion. We further outline key challenges and opportunities toward establishing nitride-based materials for next-generation computing technologies.

cond-mat.mtrl-sci↗

arXiv2026

Efficient magnetization switching driven by orbital torque originating from light 3d-transition-metal nitrides

The orbital Hall effect (OHE) in light transition metals offers a promising route to generate orbital torques for efficient magnetization control, providing an alternative to conventional spin Hall effect approaches that rely on heavy metals. We demonstrate perpendicular magnetization switching in [Co/Pt]3 multilayers driven by the OHE in a light 3d transition metal nitride, VN, with 111-texture of face-center cubic structure. Second harmonic Hall measurement reveals a large torque efficiency of -0.41 in the VN(7.5 nm)/[Co(0.35nm)/Pt(0.3 nm)]3, which significantly surpasses that in the control samples with Co, Py, and CoFeB ferromagnets, suggesting strong conversion of orbital current originating from VN to spin current by [Co/Pt]3 ferromagnet. Full switching by in-plane current is achieved with an in-plane magnetic field, while partial field-free switching occurs without it. The critical current density for the switching is found to be comparable to that of the W-based spin-orbit torque device. First-principles calculations confirm a large orbital Hall conductivity in VN, with a small spin Hall conductivity around the Fermi energy. Our results highlight the potential in the combination of light 3d transition metal nitrides and Co/Pt ferromagnetic multilayer with 111-texture to maximize the magnetization switching efficiency of orbitronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Induced magnetic moment at two-dimensional MXene/ferromagnetic interface evaluated by angle-dependent hard X-ray photoemission spectroscopy

Emergent ferromagnetism on the surface of recent two-dimensional (2D) MXene is investigated by X-ray magnetic circular dichroism (XMCD) and angle-dependent hard X-ray photoemission spectroscopy (HAXPES). Focusing on the Cr2N as one of the 2D-MXenes, the bilayers of Cr2N/Co and Cr2N/Pt are prepared by magnetron sputtering technique. XMCD reveals the induced magnetic moment of Cr in the Cr2N/Co interface, while it is not observed in the Cr2N/Pt interface at room temperature. To distinguish the possible origins of either the interlayer magnetic exchange coupling or the charge transfer at the interfaces, the additional controlled Cr2N/Cu bilayer, whose work function of Cu is consistent with Co, is prepared. HAXPES spectra for the Cr 2p core level near the interface of Cr2N/Cu is consistent with that of Cr2N/Co, indicating that the induced magnetic moment of Cr observed by XMCD for the Cr2N/Co can be attributed to the interlayer magnetic exchange coupling, rather than the charge transfer, which is a specific characteristics emerged at the interface with 2D-MXene.

cond-mat.mtrl-sci↗

arXiv2025

Berry curvature-induced intrinsic spin Hall effect in light-element-based CrN system for magnetization switching

The current-induced spin-orbit torque-based devices for magnetization switching are commonly relied on the 4d and 5d heavy metals owing to their strong spin-orbit coupling (SOC) to produce large spin current via spin Hall effect (SHE). Here we present the sizable SHE in CrN, a light element-based system and demonstrate the current-induced magnetization switching in the adjacent ferromagnetic layer [Co(0.35nm)/Pt(0.3nm)]3, which exhibits perpendicular magnetic anisotropy. We found the switching current density of 2.6 MA/cm2. The first principles calculation gives the spin Hall conductivity (SHC) to be 120 (hcross/e) S/cm due to intrinsic Berry curvature arising from SOC induced band splitting near Fermi-energy. The theoretically calculated intrinsic SHC is close to the experimental SHC extracted from second harmonic Hall measurement. We estimated spin Hall angle to be 0.09, demonstrating efficient charge-to-spin conversion in CrN system.

cond-mat.mtrl-sci↗

arXiv2025

Unconventional Spin-orbit Torques by Two-dimensional Multilayered MXenes for Future Nonvolatile Magnetic Memories

MXenes have attracted considerable attention in recent years owing to their two-dimensional (2D) layered structures with various functionalities similar to those of graphene and transition metal dichalcogenides. To open a new application field for MXenes in the realm of electronic devices, such as ultrahigh-integrated magnetic memory, we have developed a spin-orbit torque (SOT) bilayer structure comprising bare MXene of Cr2N: substrate//Cr2N/[Co/Pt]3/MgO using the magnetron sputtering technique. We demonstrated field-free current-induced magnetization switching (CIMS) in the bilayer structure, regardless of the charge current directions with respect to the mirror symmetry lines of Cr2N crystal. This is a specific characteristic for the 2D MXene-based SOT-devices, originating from an unconventional out-of-plane SOT. As the SOT efficiency increases with increasing the Cr2N thickness, the first-principles calculations predict an intrinsic orbital-Hall conductivity with the dominant out-of-plane component, comparing to the spin-Hall conductivity in the Cr2N. X-ray magnetic circular dichroism reveals the out-of-plane uncompensated magnetic moment of Cr in the Cr2N layer at the interface, induced by contact with the Co in the [Co/Pt]3 ferromagnetic layer. Therefore, the intrinsic bulk orbital Hall effect in MXene and the interfacial contribution such as spin-filtering-like effect owing to uncompensated magnetic moment of Cr are considered as possible major mechanisms for the unconventional out-of-plane SOT in the device, rather than a crystal symmetry and/or an interlayer exchange coupling.

cond-mat.mes-hall↗

arXiv2023

Ultrafast spin-to-charge conversions of antiferromagnetic (111)-oriented $\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$

Antiferromagnetic $\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$ combines outstanding spin-transport properties with magnons in the terahertz (THz) frequency range. However, the THz radiation emitted by ultrafast spin-to-charge conversion via the inverse spin Hall effect remains unexplored. In this study, we measured the THz emission and transmission of a permalloy/(111)-oriented $\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$ multilayer by THz time-domain spectroscopy. The spin Hall angle was determined to be approximately constant at 0.035 within a frequency range of 0.3-2.2 THz, in comparison with the THz spectroscopy of a permalloy/Pt multilayer. Our results not only demonstrate the potential of $\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$ as a spintronic THz emitter but also provide insights into the THz spin transport properties of $\mathrm{L1_2}$-$\mathrm{Mn_3Ir}$.

cond-mat.mtrl-sci↗

arXiv2020

Anomalous Hall and Nernst effects in ferrimagnetic Mn$_4$N films: possible interpretation and prospect for enhancement

Ferrimagnetic Mn$_4$N is a promising material for heat flux sensors based on the anomalous Nernst effect (ANE) because of its sizable uniaxial magnetic anisotropy ($K_{\rm u}$) and low saturation magnetization ($M_{\rm s}$). We experimentally and theoretically investigated the ANE and anomalous Hall effect in sputter-deposited Mn$_4$N films. It was revealed that the observed negative anomalous Hall conductivity ($\sigma_{xy}$) could be explained by two different coexisting magnetic structures, that is, a dominant magnetic structure with high $K_{\rm u}$ contaminated by another structure with negligible $K_{\rm u}$ owing to an imperfect degree of order of nitrogen. The observed transverse thermoelectric power ($S_{\rm ANE}$) of $+0.5\, \mu{\rm V/K}$ at $300\, {\rm K}$ gave a transverse thermoelectric coefficient ($\alpha_{xy}$) of $+0.34\, {\rm A/(m \cdot K)}$, which was smaller than the value predicted from first-principles calculation. The interpretation for $\alpha_{xy}$ based on the first-principles calculations led us to conclude that the realization of single magnetic structure with high $K_{\rm u}$ and optimal adjustment of the Fermi level are promising approaches to enhance $S_{\rm ANE}$ in Mn$_4$N through the sign reversal of $\sigma_{xy}$ and the enlargement of $\alpha_{xy}$ up to a theoretical value of $1.77\, {\rm A/(m \cdot K)}$.

cond-mat.mtrl-sci↗

arXiv2019

Contributions of magnetic structure and nitrogen to perpendicular magnetocrystalline anisotropy in antiperovskite $\epsilon$-Mn$_4$N

To study how nitrogen contributes to perpendicular magnetocrystalline anisotropy (PMA) in the ferrimagnetic antiperovskite Mn$_4$N, we examined both the fabrication of epitaxial Mn$_4$N films with various nitrogen contents and first-principles density-functional calculations. Saturation magnetization ($M_{\rm s}$) peaks of 110 mT and uniaxial PMA energy densities ($K_{\rm u}$) of 0.1 MJ/m$^3$ were obtained for a N$_2$ gas flow ratio ($Q$) of $\sim 10 \%$ during sputtering deposition, suggesting nearly single-phase crystalline $\epsilon$-Mn$_4$N. Segregation of $\alpha$-Mn and nitrogen-deficient Mn$_4$N grains was observed for $Q \approx 6\%$, which was responsible for a decrease in the $M_{\rm s}$ and $K_{\rm u}$. The first-principles calculations revealed that the magnetic structure of Mn$_4$N showing PMA was "type-B" having a collinear structure, whose magnetic moments couple parallel within the c-plane and alternating along the c-direction. In addition, the $K_{\rm u}$ calculated using Mn$_{32}$N$_x$ supercells showed a strong dependence on nitrogen deficiency, in qualitative agreement with the experimental results. The second-order perturbation analysis of $K_{\rm u}$ with respect to the spin-orbit interaction revealed that not only spin-conserving but also spin-flip processes contribute significantly to the PMA in Mn$_4$N. We also found that both contributions decreased with increasing nitrogen deficiency, resulting in the reduction of $K_{\rm u}$. It was noted that the decrease in the spin-flip contribution occurred at the Mn atoms in face-centered sites. This is one of the specific PMA characteristics we found for antiperovskite-type Mn$_4$N.

cond-mat.mtrl-sci↗