arXiv subjects

Masahiro Nagao

Publications and source records attributed to Masahiro Nagao.

5 recordsLinked to original sources

arXiv2026

Asymmetric defect distribution generates bulk Dzyaloshinskii-Moriya interaction in Pt/Co/Pt

The Dzyaloshinskii-Moriya interaction (DMI) in magnetic multilayers plays a pivotal role in spintronic applications by enabling the stabilization and manipulation of chiral spin textures, including chiral domain walls and skyrmions. While interfacial DMI in asymmetric multilayers is well established, a substantial DMI persists in symmetric multilayers, where the opposing interface contributions should, in principle, cancel. The microscopic origin of this effect remains unresolved. Here, we show defect-mediated bulk DMI in sputtered nominally symmetric Pt/Co/Pt multilayers. Using Lorentz transmission electron microscopy and aberration-corrected scanning transmission electron microscopy, we systematically elucidate the effect of argon sputter gas pressure on both magnetic and structural properties. Increasing the pressure markedly enhances the net DMI, thereby stabilizing chiral spin textures. Atomic-scale analysis reveals the absence of welldefined interfaces, confirming that the DMI predominantly arises in the bulk rather than at the interfaces. However, macroscopic compositional asymmetries are remarkably weak, and spin-orbit coupling shows no significant enhancement, rendering them insufficient to account for the observed DMI. Crucially, our statistical analysis of dislocations uncovers a strongly biased spatial distribution. These asymmetrically distributed dislocations, accompanied by local atomic displacements, combined with compositional gradients, generate the bulk DMI through the three-site Fert-Levy mechanism. Our results establish defect engineering as an effective strategy for controlling DMI, offering new design principles for spintronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Domain-stored skyrmion structures for a reading error-detectable racetrack memory

Magnetic racetrack memory (RTM) uses a series of either domains or skyrmions as data bits along nanowires. However, it lacks reading error detection capability in nanowires, which requires high electric current density for the deterministic motion of domain walls (DWs) or skyrmions. Here, we propose a method of a reading error-detectable RTM and explore domain-stored skyrmion structures for this RTM. This method uses domains as memory cells and assigns the presence and absence of a skyrmion in a domain to different bits, enabling the electrical signal of any memory cells to output the opposite sign to that of the adjacent memory cells. Using simulations and Lorentz microscopy, we demonstrate that perpendicularly magnetized nanowires with relatively large Dzyaloshinskii-Moriya interaction can achieve domain-stored skyrmion structures. Additionally, our simulations show that when DWs approach to skyrmions during spin-orbit torque-induced motion, angular momentum transfers from DWs to skyrmions, resulting in fast motion of skyrmions.

physics.app-ph↗

arXiv2021

Interaction between skyrmions and antiskyrmions in a coexisting phase of a Heusler material

Coexisting phases of magnetic skyrmions and antiskyrmions have proposed to exhibit a variety of fascinating properties, owing to interactions between them. The recent discovery of the coexisting phase in a Heusler material could offer a platform for skyrmion-antiskyrmion-based spintronics. Here we report Lorentz electron microscopy experiments and micromagnetic simulations in a similar Heusler material, Mn$_{1.3}$Pt$_{1.0}$Pd$_{0.1}$Sn. Around $B_c \sim$ 420$\,$mT, we find a stochastic reversible transformation and a room temperature coexisting phase of elliptical skyrmions and square-shaped antiskyrmions. The closeness of the energy competition is sensitive to the exchange stiffness constants and sample thickness. Furthermore, we reveal isotropic long-range repulsive interaction between the skyrmions and antiskyrmions regardless of their shapes and the skyrmion helicities, in stark contrast to conventional thought of angle- and helicity-dependent short-range pairwise interactions. The observed interaction possibly results from the topological protection against the intrusion of magnetic flux density coming from skyrmions (antiskyrmions) into antiskyrmions (skyrmions). Our results provide new insight into interacting skyrmions and antiskyrmions and a guide for developing skyrmion-antiskyrmion-based spintronics.

cond-mat.mtrl-sci↗

arXiv2021

Observation of domain wall bimerons in chiral magnets

Topological defects embedded in or combined with domain walls have been proposed in various systems, some of which are referred to as domain wall skyrmions or domain wall bimerons. However, the experimental observation of such topological defects remains an ongoing challenge. Here, using Lorentz transmission electron microscopy, we report the experimental discovery of domain wall bimerons in chiral magnet Co-Zn-Mn(110) thin films. By applying a magnetic field, multidomain structures develop, and simultaneously, chained and isolated bimerons arise as the localized state between the domains with the opposite in-plane components of net magnetization. The multidomain formation is attributed to magnetic anisotropy and dipolar interaction, and domain wall bimerons are stabilized by the Dzyaloshinskii-Moriya interaction. In addition, micromagnetic simulations show that domain wall bimerons appear for a wide range of conditions in chiral magnets with cubic magnetic anisotropy. Our results promote further study in various fields of physics.

cond-mat.mtrl-sci↗

arXiv2020

Magnetically induced metal-insulator transition in Pb2CaOsO6

We report on the structural, magnetic, and electronic properties of two new double-perovskites synthesized under high pressure; Pb2CaOsO6 and Pb2ZnOsO6. Upon cooling below 80 K, Pb2CaOsO6 simultaneously undergoes a metal--insulator transition and develops antiferromagnetic order. Pb2ZnOsO6, on the other hand, remains a paramagnetic metal down to 2 K. The key difference between the two compounds lies in their crystal structure. The Os atoms in Pb2ZnOsO6 are arranged on an approximately face-centred cubic lattice with strong antiferromagnetic nearest-neighbor exchange couplings. The geometrical frustration inherent to this lattice prevents magnetic order from forming down to the lowest temperatures. In contrast, the unit cell of Pb2CaOsO6 is heavily distorted up to at least 500 K, including antiferroelectric-like displacements of the Pb and O atoms despite metallic conductivity above 80 K. This distortion relieves the magnetic frustration, facilitating magnetic order which in turn drives the metal--insulator transition. Our results suggest that the phase transition in Pb2CaOsO6 is spin-driven, and could be a rare example of a Slater transition.

cond-mat.str-el↗