Experimental evidence of a change of exchange anisotropy sign with temperature in Zn-substituted Cu2OSeO3

We report small-angle neutron scattering from the conical state in a single crystal of Zn-substituted Cu 2 OSeO 3 . Using a 3D vector-field magnet to reorient the conical wave vector, our measurements show that the magnitude of the conical wave vector changes as a function of crystallographic direction. These changes are explained using the anisotropic exchange interaction (AEI) within the continuum model, whose magnitude in free-energy transitions from a maxima to a minima along the ⟨ 111 ⟩ and ⟨ 100 ⟩ crystallographic directions respectively. We further find that the AEI free-energy constant undergoes a change of sign from positive to negative with decreasing temperature. Unlike in the related compound FeGe, where similar behavior of the AEI induces a reorientation of the helical wave vector, we show that the zero field helical wave vector in ( Cu 0.98 Zn 0.02 ) 2 OSeO 3 remains along the ⟨ 100 ⟩ directions at all temperatures due to the competing fourth-order magnetocrystalline anisotropy becoming dominant at lower temperatures

Structure and magnetism of the skyrmion hosting family GaV4S8−ySey with low levels of substitutions between 0≤y≤0.5 and 7.5≤y≤8

Polycrystalline members of the GaV4S8−ySey family of materials with small levels of substitution between 0 ≤ y ≤ 0.5 and 7.5 ≤ y ≤ 8 have been synthesized in order to investigate their magnetic and structural properties. Substitutions to the skyrmion hosting parent compounds GaV4S8 and GaV4Se8 are found to suppress the temperature of the cubic-to-rhombohedral structural phase transition that occurs in both end compounds and to create a temperature region around the transition where there is a coexistence of these two phases. Similarly, the magnitude of the magnetization and temperature of the magnetic transition are both suppressed in all substituted compounds until a glassy-like magnetic state is realized. There is evidence from the ac susceptibility data that skyrmion lattices with similar dynamics to those in GaV4S8 and GaV4Se8 are present in compounds with very low levels of substitution, 0 < y < 0.2 and 7.8 < y < 8, however, these states vanish at higher levels of substitution. The magnetic properties of these substituted materials are affected by the substitution altering exchange pathways and resulting in the creation of increasingly disordered magnetic states

Stability and metastability of skyrmions in thin lamellae of Cu2OSeO3

We report small-angle x-ray scattering measurements of the skyrmion lattice in two 200-nm-thick Cu2OSeO3 lamellae aligned with the applied magnetic field parallel to the out of plane [110] or [100] crystallographic directions. Our measurements show that the equilibrium skyrmion phase in both samples is expanded significantly compared to bulk crystals, existing between approximately 30 and 50 K over a wide region of magnetic field. This skyrmion state is elliptically distorted at low fields for the [110] sample, and symmetric for the [100] sample, possibly due to crystalline anisotropy becoming more important at this sample thickness than it is in bulk samples. Furthermore, we find that a metastable skyrmion state can be observed at low temperature by field cooling through the equilibrium skyrmion pocket in both samples. In contrast to the behavior in bulk samples, the volume fraction of metastable skyrmions does not significantly depend on cooling rate. We show that a possible explanation for this is the change in the lowest temperature of the skyrmion state in this lamellae compared to bulk, without requiring different energetics of the skyrmion state

Position-dependent stability and lifetime of the skyrmion state in nickel-substituted Cu2OSeO3

We report spatially resolved small-angle neutron-scattering measurements of the conical and skyrmion states of a bulk single crystal of nickel-substituted Cu2OSeO3, with a nominal concentration of Ni of 14%. We observe a significant spatial dependence of the structure of these magnetic states, characterized by increased disorder and misalignment with respect to the applied field as we approach the edge of the sample. Remarkably, the edge skyrmion state is also characterized by an extended stability towards lower temperatures. Surprisingly, in the same region of the sample, the metastable skyrmion state did not show simple decay. Instead, only a fraction of the scattered intensity appeared to decay, and the intensity therefore did not approach zero during our measurements. We suggest that the increased local disorder and the coexistence of conical and skyrmion states, induced by demagnetization effects at the edge of the sample, are responsible for the increased stability of this skyrmion state. We also infer that the unclear metastable behavior of the skyrmion lattice at the edge of the sample is due to the local geometry of the sample, which induces coexistence of different skyrmion states whose lifetimes are superimposed and difficult to separate in the data

Spin dynamics in bulk MnNiGa and Mn1.4Pt0.9Pd0.1Sn investigated by muon spin relaxation

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Megahertz dynamics in skyrmion systems probed with muon-spin relaxation

We present longitudinal-field muon-spin relaxation (LF μ SR ) measurements on two systems that stabilize a skyrmion lattice (SkL): Cu 2 OSeO 3 , and Co x Zn y Mn 20 − x − y for ( x , y ) = ( 10 , 10 ) , (8, 9), and (8, 8). We find that the SkL phase of Cu 2 OSeO 3 exhibits emergent dynamic behavior at megahertz frequencies, likely due to collective excitations, allowing the SkL to be identified from the μ SR response. From measurements following different cooling protocols and calculations of the muon stopping site, we suggest that the metastable SkL is not the majority phase throughout the bulk of this material at the fields and temperatures where it is often observed. The dynamics of bulk Co 8 Zn 9 Mn 3 are well described by ≃ 2 GHz excitations that reduce in frequency near the critical temperature, while in Co 8 Zn 8 Mn 4 we observe similar behavior over a wide range of temperatures, implying that dynamics of this kind persist beyond the SkL phase

Observation of the chiral soliton lattice above room temperature

Magnetic chiral soliton lattices (CSLs) emerge from the helical phase in chiral magnets when magnetic fields are applied perpendicular to the helical propagation vector, and they show great promise for next-generation magnetic memory applications. These one-dimensional structures are previously observed at low temperatures in samples with uniaxial symmetry. Here, it is found that in-plane fields are the key to stabilizing the CSL in cubic Co8Zn10Mn2 over the entire temperature range from 15 K to below the Curie temperature (365 K). Using small-angle resonant elastic X-ray scattering, it is observed that the CSL is stabilized with an arbitrary in-plane propagation vector, while its thin plate geometry plays a deciding role in the soliton wavelength as a function of applied field. This work paves the way for high temperature, real world applications of soliton physics in future magnetic memory devices