In situ control of the helical and skyrmion phases in Cu2OSeO3 using high-pressure helium gas up to 5 kbar

Abstract

We report a small-angle neutron scattering study of the helical and skyrmion lattice order in singlecrystal Cu 2 OSeO 3 under quasihydrostatic helium gas pressures up to 5 kbar. By using helium gas as the pressure-transmitting medium (PTM) we ensure pressure application with improved hydrostaticity at cryogenic temperatures compared with previous reports where liquid PTMs were used. For 5-kbar He gas pressure we observe modest changes of the ambient pressure phase diagram; the critical temperature T c changes by +2.8(2)%, while in the low-T limit the helical propagation vector |q| changes by −0.5(2)%, the lower critical field H c1 changes by +2.5(1.0)%, and the upper critical field H c2 remains unchanged within uncertainty. The skyrmion phase also changes little under pressure; its largest T extent varies from T c − 2.5(5) K at ambient pressure to T c − 3.0(5) K at 5 kbar, and its location in the phase diagram follows the pressure-driven shift of T c. The weak pressure dependences of the critical magnetic fields and skyrmion phase contrast strongly with much stronger pressure-driven changes reported from previous quasihydrostatic pressure studies. Taking into account the present results and those of other uniaxial pressure data, we suggest that the results of previous quasihydrostatic pressure studies were influenced by inadvertent directional stress pressure components. Overall, our study represents a high-pressure study of the chiral magnetism in Cu 2 OSeO 3 under the most hydrostatic high-pressure conditions to date and serves also as a salient reminder of the sensitivity of chiral magnets to deviations from hydrostaticity in quasihydrostatic high-pressure studies.

Direct Visualisation of Skyrmion Lattice Defect Alignment at Grain Boundaries

“…One of the most prominent representatives of skyrmion hosting materials is the insulating B20-type compound Cu 2 OSeO 3 . A wide range of experiments have been performed on this compound, ranging from small-angle neutron scattering (SANS) [ 13 – 15 ], AC susceptibility [ 16 – 18 ] to various Lorentz transmission electron microscopy (LTEM) studies [ 19 , 20 ]. Furthermore, its magnetoelectric coupling allows to control its magnetic textures by external electric fields using the emerging electric polarisation as a handle.…”

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

“…Different approaches have been investigated to engineer the size and position of the skyrmion pocket, such as the application of electric fields [17][18][19], the application of uniaxial [20][21][22] and hydrostatic pressure [23][24][25][26][27], and the reduction of the sample dimensions into thin films [28]. In addition, it has been shown that it is possible to stabilize the skyrmion state over a wide temperature and field range via rapid field cooling (RFC) through the skyrmion pocket [17,18,[29][30][31][32].…”