Magnetic field dependence of nonlinear magnetic response and tricritical point in the monoaxial chiral helimagnet Cr1/3NbS2

Abstract

We present a comprehensive study of the magnetization dynamics and phase evolution in the chiral helimagnet ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$, which realizes a chiral soliton lattice (CSL). The magnetic field dependence of the ac magnetic response is analyzed for the first five harmonic components, ${M}_{\mathrm{n}\ensuremath{\omega}}(H)\phantom{\rule{0.16em}{0ex}}(n=1--5)$, using a phase sensitive measurement over a frequency range, $f=11--10\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}$. At a critical field, the modulated CSL continuously evolves from a helicity-rich to a ferromagnetic domain-rich structure, where the crossover is revealed by the onset of an anomalous nonlinear magnetic response that coincides with extremely slow dynamics. The behavior is indicative of the formation of a spatially coherent array of large ferromagnetic domains, which relax on macroscopic time scales. The frequency dependence of the ac magnetic loss displays an asymmetric distribution of relaxation times across the highly nonlinear CSL regime, which shift to shorter time scales with increasing temperature. We experimentally resolve the tricritical point at ${T}_{\mathrm{TCP}}$ in a temperature regime above the ferromagnetic Curie temperature which separates the linear and nonlinear magnetic regimes of the CSL at the phase transition. A comprehensive phase diagram is constructed which summarizes the features of the field and temperature dependence of the magnetic crossovers and phase transitions in ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_{2}$.