Ryo Ozawa

Zero-Field Skyrmions with a High Topological Number in Itinerant Magnets

Magnetic Skyrmions are swirling spin textures with topologically protected noncoplanarity. Recently, Skyrmions with the topological number of unity have been extensively studied in both experiment and theory. We here show that a Skyrmion crystal with an unusually high topological number of two is stabilized in itinerant magnets at a zero magnetic field. The results are obtained for a minimal Kondo lattice model on a triangular lattice by an unrestricted large-scale numerical simulation and variational calculations. We find that the topological number can be switched by a magnetic field as 2↔1↔0. The Skyrmion crystals are formed by the superpositions of three spin density waves induced by the Fermi surface effect, and hence, the size of Skyrmions can be controlled by the band structure and electron filling. We also discuss the charge and spin textures of itinerant electrons in the Skyrmion crystals which are directly obtained in our numerical simulations.

Effective bilinear-biquadratic model for noncoplanar ordering in itinerant magnets

Noncollinear and noncoplanar magnetic textures including Skyrmions and vortices act as emergent electromagnetic fields and give rise to novel electronic and transport properties. We here report a unified understanding of noncoplanar magnetic orderings emergent from the spin-charge coupling in itinerant magnets. The mechanism has its roots in effective multiple spin interactions beyond the conventional Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, which are ubiquitously generated in itinerant electron systems with local magnetic moments. By carefully examining the higher-order perturbations in terms of the spin-charge coupling, we construct a minimal effective spin model composed of the bilinear and biquadratic interactions with particular wave numbers dictated by the Fermi surface. Taking two-dimensional systems as examples, we find that our effective model captures the underlying physics of the instability toward noncoplanar multiple-

Engineering chiral density waves and topological band structures by multiple- Q superpositions of collinear up-up-down-down orders

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Simulated floating zone method

states discovered recently: a single-

Shape of magnetic domain walls formed by coupling to mobile charges

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Surface and interface effects on a magnetic Chern insulator

helical state expected from the RKKY theory is replaced by a double-

Reconstruction of Chiral Edge States in a Magnetic Chern Insulator

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Erratum: Effective bilinear-biquadratic model for noncoplanar ordering in itinerant magnets [Phys. Rev. B 95, 224424 (2017)]

vortex crystal with chirality density waves even for an infinitely small spin-charge coupling on generic lattices [R. Ozawa et al., J. Phys. Soc. Jpn. 85, 103703 (2016)], and a triple-

Vortex Crystals with Chiral Stripes in Itinerant Magnets

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22pAH-5 Spin Scalar Chiral Stripes in the Kondo Lattice Model on a Square Lattice

Skyrmion crystal with a high topological number of two appears while increasing the spin-charge coupling on a triangular lattice [R. Ozawa, S. Hayami, and Y. Motome, Phys. Rev. Lett. 118, 147205 (2017)]. We find that by introducing an external magnetic field, our effective model exhibits a plethora of multiple-