Defect dynamics and strain coupling to magnetization in the cubic helimagnet Cu2OSeO3

Small but significant static and dynamic strain coupling effects have been detected in Cu2OSeO3 through elastic and anelastic anomalies associated with magnetic phase transitions observed as a function of temperature (1.5–150 K) and magnetic field (0–300 mT). The magnetic transition near 60 K is accompanied by a small increase in single-crystal elastic constants which can be understood in terms of biquadratic coupling between shear strain and the magnetic order parameter, even though the shear strain itself is almost negligibly small. The conical-collinear transition is associated with distinct minima in the elastic properties, while weaker anomalies at lower fields may be related to changes in the configuration of magnetic domains. A distinctive acoustic loss peak at ∼42K, independent of magnetic field, is attributed to freezing of a defect which is coupled with shear strain, has an associated activation energy of ∼5kJmol−1, and may play a role in pinning the magnetic microstructures. Anomalies below ∼10K indicate the presence of some additional relaxation process which could signify a change in magnetic structure.