Thermodynamic evidence of a second skyrmion lattice phase and tilted conical phase in Cu$_2$0SeO$_3$

TL;DR

This study provides thermodynamic evidence for a second skyrmion lattice phase and a tilted conical phase in Cu$_2$OSeO$_3$, revealing their distinct hysteretic and dissipative behaviors through magnetization and susceptibility measurements.

Contribution

It reports the discovery of a second skyrmion phase and tilted conical phase in Cu$_2$OSeO$_3$ using thermodynamic measurements, complementing neutron scattering findings.

Findings

Identification of a second skyrmion phase with hysteretic boundaries.

Observation of a tilted conical phase with dissipation.

Agreement of anisotropy estimates with phase stabilization mechanisms.

Abstract

Precision measurements of the magnetization and ac susceptibility of Cu$_2$0SeO$_3$ are reported for magnetic fields along different crystallographic directions, focussing on the border between the conical and the field-polarized state for a magnetic field along the $\langle 100 \rangle$ axis, complemented by selected specific heat data. Clear signatures of the emergence of a second skyrmion phase and a tilted conical phase are observed, as recently identified by means of small-angle neutron scattering. The low-temperature skyrmion phase displays strongly hysteretic phase boundaries, but no dissipative effects. In contrast, the tilted conical phase is accompanied by strong dissipation and higher-harmonic contributions, while the transition fields are essentially nonhysteretic. The formation of the second skyrmion phase and tilted conical phase are found to be insensitive to a vanishing demagnetization factor. A quantitative estimate of the temperature dependence of the magnetocrystalline anisotropy may be consistently inferred from the magnetization and the upper critical field and agrees well with a stabilization of the low-temperature skyrmion phase and tilted conical state by conventional cubic magnetic anisotropies.