arXiv subjects

Kazuki Okigami

Publications and source records attributed to Kazuki Okigami.

6 recordsLinked to original sources

arXiv2026

Multiple-$Q$ spin textures induced by spiral--staggered interference in one-dimensional itinerant magnets

We theoretically investigate multiple-$Q$ magnetic states emerging from the interference between finite-$Q$ spiral and staggered spin modulations in a one-dimensional itinerant electron system. The multiple-$Q$ spin textures are characterized by a superposition of symmetry-unrelated ordering wave vectors in the same direction with distinct periodicities rather than rotationally symmetry-related ones. Motivated by recent experimental observations of broken helix magnetic structures in EuIn$_2$As$_2$, we focus on the microscopic interaction conditions in stabilizing such multiple-$Q$ states.We employ two effective spin models: One is the momentum-space-based model, and the other is the real-space-based model, both of which include bilinear and biquadratic easy-plane anisotropic interactions. By analyzing their ground state via simulated annealing, we find that a superposition of a spiral and a staggered modulation yields a robust double-$Q$ magnetic structure. Moreover, we demonstrate that the obtained double-$Q$ spin configuration exhibits an antisymmetric spin-split band structure even without the relativistic Dzyaloshinskii-Moriya interaction, and further reveals asymmetric band modulations when the magnetic field is applied along the out-of-plane direction. Our results provide a theoretical framework for understanding unconventional multiple-$Q$ magnetic textures in one-dimensional systems.

cond-mat.str-el↗

arXiv2025

Finite-temperature stability of skyrmion crystals in frustrated magnets: Role of sixfold anisotropy and uniform spin mode in momentum space

We study the finite-temperature stability of skyrmion crystals in frustrated magnets by analyzing the momentum-space exchange interaction of a classical Heisenberg model on a triangular lattice. Our analysis identifies two key momentum-space features that play a crucial role in stabilizing the skyrmion crystal phase. The first is the sixfold anisotropy in the momentum-space exchange interaction, which acts as a locking potential favoring triple-$Q$ skyrmion crystals. Monte Carlo simulations reveal that a larger anisotropy tends to enhance the stability region of the skyrmion crystal in the temperature--magnetic-field phase diagram. The second factor is the momentum-space energy related to the uniform spin mode, which correlates with the emergence of the skyrmion crystal phase at finite temperatures. These results provide a further understanding of the stabilization mechanism of the skyrmion crystal phase in frustrated magnets and will be useful for the design of skyrmion-hosting materials.

cond-mat.str-el↗

arXiv2025

Four-Spin Interactions as a Route to Multiple-Q Topological Magnetic Order

We investigate the role of four-spin interactions in stabilizing exotic multiple-$Q$ topological spin textures and demonstrate their ability to realize a skyrmion crystal. While such higher-order interactions are known to be important, their intricate nature makes systematic model construction significantly challenging. To address this issue, we develop a theoretical framework that connects microscopic real-space four-spin couplings to their effective interactions in momentum space, providing a clear route to engineer target magnetic phases. Applying this framework to a frustrated Heisenberg model with designed four-spin interactions, we identify the stabilization of the zero-field skyrmion crystal with a topological number of two via simulated annealing. Furthermore, our momentum-space analysis reveals the intrinsic mechanism by which the well-known ring-exchange interaction also favors the skyrmion crystal. Our findings not only present a concrete model for a higher-order skyrmion crystal but also offer a general methodology for understanding and designing a wide range of complex multiple-$Q$ magnetic orders driven by multi-spin interactions.

cond-mat.str-el↗

arXiv2025

Incommensurate broken helix induced by nonstoichiometry in the axion insulator candidate EuIn$_{2}$As$_{2}$

Zintl phase EuIn$_{2}$As$_{2}$ has garnered growing attention as an axion insulator candidate, triggered by the identification of a commensurate double-${\mathbf Q}$ broken-helix state in previous studies, however, its periodicity and symmetry remain subjects of debate. Here, we perform resonant x-ray scattering experiments on EuIn$_{2}$As$_{2}$, revealing an incommensurate nature of the broken-helix state, where both the wave number and the amplitude of the helical modulation exhibit systematic sample dependence. Furthermore, the application of an in-plane magnetic field brings about a fanlike state that appears to preserve the double-${\mathbf Q}$ nature, which might be attributed to multiple-spin interactions in momentum space. We propose that the itinerant character of EuIn$_{2}$As$_{2}$, most likely induced by Eu deficiency, gives rise to the helical modulation and impedes the realization of a theoretically-predicted axion state with the collinear antiferromagnetic order.

cond-mat.str-el↗

arXiv2024

Exploring topological spin order by inverse Hamiltonian design: A new stabilization mechanism for square skyrmion crystals

We propose a framework to construct a real-space spin model based on the inverse Hamiltonian design. The method provides an efficient way of realizing unconventional topological spin textures by optimizing the interaction parameters. In order to demonstrate its usefulness, we show that the tuning of the long-range exchange interactions can give rise to a square skyrmion crystal even without factors that have been previously identified as prerequisites for its stabilization, such as Dzyaloshinskii-Moriya interaction, multi-spin interaction, and bond-dependent magnetic anisotropy. Moreover, we elucidate the essence for the emergence of the square skyrmion crystal by classifying the parameter sets we get by the method. Since the present framework by adopting machine learning techniques can be universally applied to any magnetic system irrespective of lattice structures, it serves as the efficient construction of an effective spin model and the understanding of the stabilization mechanisms for unconventional topological spin orders.

cond-mat.str-el↗

arXiv2022

Engineering a skyrmion crystal in ferromagnetic/antiferromagnetic bilayers free from the DM interaction

We theoretically propose a new stabilization mechanism of a skyrmion crystal (SkX) in a bilayer triangular lattice system consisting of the ferromagnetic and the antiferromagnetic layers. By performing variational calculations and Monte Carlo simulations in a complementary way, we find that a magnetic frustration between the ferromagnetic and antiferromagnetic layers is a source of a finite-Q spiral state and the SkX in the strong interlayer coupling regime. We also show that the degree of frustration is related to the interlayer exchange interaction. The stronger interlayer coupling tends to make the effect of frustration larger, which results in the stabilization of the SkX. The present results not only provide a way of engineering the SkX in the ferromagnetic/antiferromagnetic domain and heterostructure but also imply the possibility of the SkX based on interorbital frustration scenario.

cond-mat.str-el↗