arXiv subjects

Kaito Nakamura

Publications and source records attributed to Kaito Nakamura.

3 recordsLinked to original sources

arXiv2026

Current-Induced Dynamics and Instability Pathways of Skyrmioniums in Chiral Magnets

We present a comprehensive study of current-driven dynamics, transformations, and instabilities of skyrmioniums in chiral magnetic films, considering both isolated objects and collective states forming skyrmionium-based meta-matter. Using micromagnetic simulations combined with an analytical description based on the generalized Thiele equation, we clarify how the internal structure of skyrmioniums governs their nonequilibrium response to electric currents. Despite having zero total topological charge, skyrmioniums exhibit a finite transverse velocity under applied currents. We show that this skyrmionium Hall effect originates from an imbalance between positive and negative topological contributions of the inner skyrmion and surrounding ring, which typically occupy different areas. Current-induced deformations further enhance this imbalance, yielding Hall angles comparable to those of skyrmions. At higher current densities, skyrmioniums undergo distinct instabilities depending on magnetic field and uniaxial anisotropy, including elongation, collapse into a skyrmion, transformation into a topologically trivial droplet, and expansion into stripe textures. We map these regimes in current--field and current--anisotropy phase diagrams and resolve their microscopic pathways via the evolution of topological charge and local rotational measures. Beyond isolated textures, mixed skyrmion--skyrmionium lattices display rich collective dynamics, including elastic transport, polymorphic transitions, soliton exchange, and stripe formation. Pulsed currents provide additional control, enabling access to regimes beyond continuous driving. Our results establish skyrmioniums and their meta-matter as tunable nonequilibrium systems probing the topological energy landscape far from equilibrium.

cond-mat.mes-hall↗

arXiv2025

Skyrmionium meta-matter: a topologically heterogeneous magnetic crystal with emergent hybrid dynamics

We introduce and systematically investigate a new class of topological magnetic textures, skyrmionium meta-matter, composed of skyrmioniums (Skm, $Q=0$) and skyrmions (Sk, $Q=-1$) arranged in periodic lattices mimicking the richness of atomic materials. Pure skyrmionium lattices are unstable against elongation distortions and relax into the spiral phase, but even a small fraction of skyrmions acts as topological "pins" that stabilize diverse mixed Skm--Sk crystals. We classify these states by topological stoichiometry (Skm$_n$Sk$_m$) and show that each composition hosts multiple metastable polymorphs with distinct plane-group symmetries. Structural transformations between polymorphs are achieved by varying the lattice spacing, suggesting experimental control via pressure or strain. The collective spin dynamics is explored for both in-plane and out-of-plane AC magnetic fields. The resulting absorption spectra show resonant modes beyond the two rotational and one breathing mode of conventional skyrmion lattices. We identify hybrid excitations unique to Skm--Sk crystals, including (i) deformation-assisted rotations, where skyrmions acquire polygonal shapes and rotate, and (ii) orbital modes, where breathing skyrmioniums induce circular motion of confined skyrmions without changing their size. Mode frequencies span sub-GHz to above 10 GHz, consistent with exchange and DMI energy scales. Our results establish skyrmionium-based meta-matter as a versatile platform for tunable, topologically heterogeneous magnetic lattices with rich structural and dynamical properties, paving the way for reconfigurable magnonic and spintronic applications.

cond-mat.mes-hall↗

arXiv2024

Communicating skyrmions as the main mechanism underlying skyrmionium (meta)stability in quasi-two-dimensional chiral magnets

We re-examine the internal structure of skyrmioniums stabilized in quasi-two-dimensional chiral magnets with easy-axis uniaxial anisotropy. Skyrmioniums are particle-like states of two nested skyrmions with opposite polarities contributing to zero topological charge. The physical principles of skyrmionium stability are drawn from both the analytical analysis with a trial function and from numerical simulations within the framework of micromagnetism. We deduce that the radii of the internal skyrmion with the positive polarity and the ring-shaped external skyrmion with the negative polarity are mutually dependent, which constitutes the paradigm of communicating skyrmions. For large central skyrmions, the skyrmionium transforms into a narrow circular domain wall, whereas for small internal radii, the ring expands, which occurs at the verge of collapsing into an ordinary isolated skyrmion. We show that skyrmioniums may form lattices of two varieties depending on the polarity of the internal skyrmion. At the phase diagram (magnetic field)-(uniaxial anisotropy), both skyrmionium lattices share the same area with one-dimensional spiral states and remain metastable solutions for the whole range of control parameters. By expanding at the critical line, skyrmionium lattices do not release isolated skyrmioniums. Isolated skyrmioniums of just one type exist apart from the corresponding lattice in a narrow field region restricted by the critical line of expansion from below and by the line of collapse above.

cond-mat.mes-hall↗