arXiv subjects

Daigo Shimizu

Publications and source records attributed to Daigo Shimizu.

3 recordsLinked to original sources

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↗

arXiv2024

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↗

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↗