Kosuke Karube

Chiral magnets host topologically protected spin textures whose nonequilibrium dynamics are crucial in phase transitions and domain evolution, yet ultrafast defect-mediated processes remain poorly understood. Here, we investigate a photothermally induced helical-to-paramagnetic phase transition in Co9Zn9Mn2 using pump–probe Lorentz transmission electron microscopy (LTEM). Following the suppression of the magnetic stripe contrast induced by femtosecond laser pulses, we observe a directional recovery process of magnetic order driven by the anisotropic thermal diffusion, toward the thick region that effectively acts as a heat sink. Remarkably, around a magnetic edge dislocation, the magnetic contrast recovery exhibits a pronounced delay accompanied by a transient blurring of LTEM contrast. These findings suggest that the recovery dynamics around the magnetic edge dislocation proceed through multiple relaxation paths that are selected stochastically. Our results indicate a possible enhancement of stochasticity around topological defects during the recovery dynamics of magnetic phase transitions.

Quantitative visualization of three-dimensional magnetic textures is essential in understanding emergent topological spin textures. However, existing transmission electron microscopy approaches face trade-offs between spatial resolution, sample robustness, experimental complexity and the ability to reconstruct full vector fields. Here, we present a general, experimentally practical workflow for 3D reconstruction of vector fields based on dual-axis-tilt integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) tomography. The method combines low magnification iDPC imaging, dual-axis-tilt acquisition, scalar field reconstruction using Simultaneous Iterative Reconstruction Technique (SIRT) and computational calculations to extract the magnetic induction fields. Here, we demonstrate the method on room temperature magnet Fe _1.9 Ni _0.9 Pd _0.2 P (FNPP), revealing 3D strip domains with two types of domain walls. Our approach provides a versatile and accessible solution for resolving 3D magnetic induction in thin-film and bulk-derived specimens, bridging the gap between phase-contrast STEM and full-vector tomography in the investigations of magnetic materials.

Emergent topological spin textures, such as nanometric skyrmions and antiskyrmions, not only exhibit a wealth of novel physical phenomena but also represent promising candidates for next-generation spintronic devices due to their topological stability and low-current-driven dynamics. Investigating their responses to electric currents is essential for uncovering unique electromagnetic properties and advancing their integration into electronic technologies. While considerable progress has been made through extensive research over the past decade, key challenges still persist. In this study, we demonstrate the novel, current-driven oppositely rotating dynamics of skyrmion and antiskyrmion assemblies in terms of Lorentz transmission electron microscopy. Their senses of rotation are strongly dictated by their inherent topological charges and remain largely unaffected by current direction, showing consistent behavior observed across various sample geometries. Further experimental analyses have revealed a reduction in angular velocity as (anti)skyrmions move away from the sample edge. The theoretical analyses, combined with micromagnetic simulations, highlight the critical roles of boundary-induced confining potentials and gyrotropic forces in steering the rotational dynamics of (anti)skyrmions. These findings offer new insights into current-driven (anti)skyrmion dynamics and open promising avenues for advancing topological concepts in confined spintronic systems.

Topological magnetic semimetals with kagomé lattices have attracted significant attention due to their nontrivial electronic band structures and pronounced electromagnetic responses. The search for kagomé‐lattice topological semimetals exhibiting magnetic ordering above room temperature is essential for advancing their potential in device applications. In this work, we report direct observations of topological magnetic textures and anomalous Hall effects driven by topological nodal lines in MnRhP, a room‐temperature ferromagnet with a distorted kagomé lattice. Using single‐crystal magnetization measurements and powder neutron diffraction, we reveal a weak uniaxial magnetic anisotropy. Lorentz transmission electron microscopy observations confirm the presence of stable magnetic skyrmions above room temperature. Moreover, both the ordinary and anomalous Hall effects are significantly enhanced upon cooling, with a large anomalous Hall conductivity (AHC) observed at low temperatures. First‐principles calculations indicate significant contributions to electronic states near the Fermi level from both in‐plane and out‐of‐plane d orbitals of Mn and Rh, resulting in the low magnetic anisotropy energy. The calculated Berry curvature reproduces the experimentally observed large AHC, providing direct evidence for an intrinsic mechanism linked to the topological nodal lines. These findings establish MnRhP as a promising kagomé‐lattice magnet for investigating topological magnetic textures and anomalous transport phenomena at room temperature.

Chiral magnets under broken time-reversal symmetry can give rise to rectification of moving electrons, called nonreciprocal transport. Several mechanisms, such as the spin fluctuation–induced chiral scattering and asymmetry in the electronic band dispersion with and without the relativistic spin-orbit interaction, have been proposed, but clear identification and theoretical description of these different contributions are desired for full understanding of nonreciprocal transport phenomena. Here, we investigate a chiral magnet Co ₈ Zn ₉ Mn ₃ and find the nonreciprocal transport phenomena consisting of different contributions with distinct field and temperature dependence across the magnetic phase diagram over a wide temperature range including above room temperature. We successfully separate the nonreciprocal resistivity into different components and identify their mechanisms as spin fluctuation–induced chiral scattering and band asymmetry in a single material with the help of theoretical calculations.

The Hall effect is one of the most fundamental but elusive phenomena in condensed matter physics due to the rich variety of underlying mechanisms. Here we report an exceptionally large Hall effect in a frustrated magnet GdCu₂ with high conductivity. The Hall conductivity at the base temperature is as high as the order of 10⁴–10⁵ Ω⁻¹cm⁻¹ and shows abrupt sign changes under magnetic fields. Remarkably, the giant Hall effect is rapidly suppressed as the longitudinal conductivity is lowered upon increasing temperature or introducing tiny amount of quenched disorder. Our systematic transport measurements combined with neutron scattering measurements, ab initio band calculations and spin model calculations indicate that the unusual Hall effect can be understood in terms of spin-splitting induced emergence/disappearance of Fermi pockets as well as skew scattering from spin-chiral cluster fluctuations in a field-polarized state. The present study demonstrates complex interplay among magnetization, spin-dependent electronic structure, and spin fluctuations in producing the giant Hall effect in highly conductive frustrated magnets with a distorted triangular lattice.

Chiral cubic Co‐Zn‐Mn magnets exhibit diverse topological spin textures, including room‐temperature skyrmion phases and robust far‐from‐equilibrium metastable states. Despite recent advances in understanding metastable skyrmions, the interplay between compositional disorder and varying magnetic anisotropy on the stability and decay of metastable textures, particularly near room temperature, remains incompletely understood. In this work, the equilibrium and metastable skyrmion formation in Co ₈ Zn ₉ Mn ₃ is examined, revealing transformations between distinct metastable spin textures induced by temperature and magnetic field. At room temperature, the decay dynamics of metastable skyrmions exhibits a strong dependence on magnetic anisotropy, showcasing a route towards tailoring relaxation behavior. Furthermore, a nascent double‐ q spin texture, characterized by two coexisting magnetic modulation vectors q , is identified as a minority phase alongside the conventional triple‐ q hexagonal skyrmion lattice. This double‐ q texture can be quenched as a metastable state, suggesting both its topological character, and its role as a potential intermediary of metastable skyrmion decay. These findings provide new insights into the tunability of equilibrium and metastable topological spin textures via chemical composition and magnetic anisotropy, offering strategies for designing materials with customizable and dynamic skyrmion properties for advanced technological applications.

Current-driven dynamics of topological spin textures, such as skyrmions and antiskyrmions, have garnered considerable attention in condensed matter physics and spintronics. As compared with skyrmions, the current-driven dynamics of their antiparticles – antiskyrmions − remain less explored due to the increased complexity of antiskyrmions. Here, we design and employ fabricated microdevices of a prototypical antiskyrmion host, (Fe_0.63Ni_0.3Pd_0.07)₃P, to allow in situ current application with Lorentz transmission electron microscopy observations. The experimental results and related micromagnetic simulations demonstrate current-driven antiskyrmion dynamics confined within stripe domains. Under nanosecond-long current pulses, antiskyrmions exhibit directional motion along the stripe regardless of the current direction, while the antiskyrmion velocity is linearly proportional to the current density. Significantly, the antiskyrmion mobility could be enhanced when the current flow is perpendicular to the stripe direction. Our findings provide novel and reliable insights on dynamical antiskyrmions and their potential implications on spintronics.