Manipulating topological spin textures is a key for exploring unprecedented emergent electromagnetic phenomena. Whereas switching control of magnetic skyrmions, e.g., the transitions between a skyrmion-lattice phase and conventional magnetic orders, is intensively studied towards development of future memory device concepts, transitions among spin textures with different topological orders remain largely unexplored. Here we develop a series of chiral magnets MnSi 1− x Ge x , serving as a platform for transitions among skyrmion- and hedgehog-lattice states. By neutron scattering, Lorentz transmission electron microscopy and high-field transport measurements, we observe three different topological spin textures with variation of the lattice constant controlled by Si/Ge substitution: two-dimensional skyrmion lattice in x = 0–0.25 and two distinct three-dimensional hedgehog lattices in x = 0.3–0.6 and x = 0.7–1. The emergence of various topological spin states in the chemical-pressure-controlled materials suggests a new route for direct manipulation of the spin-texture topology by facile mechanical methods.

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A series of ferroelectric switching measurements are performed on the 75/25mol% vinylidene fluoride /trifluoroethylene copolymer thin films with various structures to elucidate the factors governing the switching characteristics of such films. It is shown that the spin-coated and well-annealed samples exhibit a remanent polarization P r of 85 mC/m 2 . Unlike that in previous reports, the switching time τ s of these samples is independent of thickness down to 50 nm if Au electrodes are used instead of Al electrodes. Thin films with different metal electrodes exhibit anisotropic τ s due to a potential difference between metal electrodes produced by their work function difference. A train of unipolar short on-off electric field pulses induce a slow but full polarization reversal whose time evolution supports the nucleation and growth mechanism. Continuous switching under bipolar on-off electric field pulses reveals an accelerated polarization reversal with decreasing pulse width. This means that τ s is a function of not only applied electric field but also the preceding poling conditions. Fatigue is significantly improved using Au electrodes instead of Al electrodes. The results are discussed in relation to the microscopic features of the VDF/TrFE copolymer and the underlying nucleation-growth mechanism.

We have investigated magnetic orders and excitations in a Y-type hexaferrite BaSrCo 2 Fe 11 AlO 22 (BSCoFAO), which was reported to exhibit spin-driven ferroelectricity at room temperature [Hirose et al., Appl. Phys. Lett. 104, 022907 (2014)]. By means of magnetization, electric polarization and neutron diffraction measurements using single-crystal samples, we establish H-T magnetic phase diagram for magnetic field perpendicular to the c axis (H ⊥c). This system exhibits an alternating longitudinal conical (ALC) magnetic structure in the ground state, and it turns into a non-coplanar commensurate magnetic order with spin-driven ferroelectricity under H ⊥c. The field-induced ferroelectric phase remains as a metastable state after removing magnetic field below ∼ 250 K. This metastability is the key to understanding of magnetic-field-reversal of the spin-driven electric polarization in this system. Inelastic polarized neutron scattering measurements in the ALC phase reveal a magnetic excitation at around 7.5 meV, which is attributed to spin components oscillating in a plane perpendicular to the cone axis. This phason-like excitation is expected to be electric-field-active magnon, i.e., electromagnon excitation, in terms of the magnetostriction mechanism.