Phase shift in skyrmion crystals

The magnetic skyrmion crystal is a periodic array of a swirling topological spin texture. Since it is regarded as an interference pattern by multiple helical spin density waves, the texture changes with the relative phase shifts among the constituent waves. Although such a phase degree of freedom is relevant to not only magnetism but also transport properties, its effect has not been elucidated thus far. We here theoretically show that a phase shift in the skyrmion crystals leads to a tetra-axial vortex crystal and a meron-antimeron crystal, both of which show a staggered pattern of the scalar spin chirality and give rise to nonreciprocal transport phenomena without the spin-orbit coupling. We demonstrate that such a phase shift can be driven by exchange interactions between the localized spins, thermal fluctuations, and long-range chirality interactions in spin-charge coupled systems. Our results provide a further diversity of topological spin textures and open a new field of emergent electromagnetism by the phase shift engineering.

Skyrmion crystals in centrosymmetric itinerant magnets without horizontal mirror plane

We theoretically investigate a new stabilization mechanism of a skyrmion crystal (SkX) in centrosymmetric itinerant magnets with magnetic anisotropy. By considering a trigonal crystal system without the horizontal mirror plane, we derive an effective spin model with an anisotropic Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction for a multi-band periodic Anderson model. We find that the anisotropic RKKY interaction gives rise to two distinct SkXs with different skyrmion numbers of one and two depending on a magnetic field. We also clarify that a phase arising from the multiple-Q spin density waves becomes a control parameter for a field-induced topological phase transition between the SkXs. The mechanism will be useful not only for understanding the SkXs, such as that in Gd$$_2$$ 2 PdSi$$_3$$ 3 , but also for exploring further skyrmion-hosting materials in trigonal itinerant magnets.

Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry

CuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated—both experimentally and theoretically—since it is stable for only a few tenths of a kelvin just below the Néel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian.