First-order ferroelastic transition in a magnetoelectric multiferroic: CuCrO2

Abstract

We present the magnetic phase diagram of the multiferroic geometrically frustrated antiferromagnet CuCrO${}_{2}$ determined using dielectric constant and ultrasonic velocity measurements with the magnetic field parallel to the $[1\overline{1}0]$ direction. According to these measurements, at zero field the magnetoelectric phase induced by a proper screw magnetic ordering is observed below ${T}_{N1}=23.4$ K, while the velocity measurements reveal another transition at ${T}_{N2}=24.3$ K. As the dielectric and velocity measurements were performed simultaneously, our results confirm the presence of an intermediate nonferroelectric magnetic state between the magnetoelectric and paramagnetic phases. Moreover, our observations indicate that this intermediate phase persists up to 7 T for a field applied along the $[1\overline{1}0]$ direction. Based on similar observations obtained on the multiferric compounds CuO and MnWO${}_{4}$, this intermediate state is most likely a collinear antiferromagnetic phase. Apart from two phases observed at zero field, the ultrasonic velocity measured in the magnetoelectric phase as a function of the magnetic field reveals a spin-flop transition at ${H}_{\mathrm{flop}}=5$ T at 23 K, which is attributed to the flop of the spin spiral from the (110) plane to the $(1\overline{1}0)$ plane. In this paper, we also present a detailed analysis of the elastic properties of CuCrO${}_{2}$. The analysis of the data using a Landau-type free energy indicates that CuCrO${}_{2}$ undergoes a first-order pseudoproper ferroelastic transition leading to a $\overline{3}m\ensuremath{\rightharpoonup}2/m$ structural transition at ${T}_{N2}$. According to the model, the order parameter of the ferroelastic-antiferromagnetic transition at ${T}_{N2}$ belongs to the ${E}_{g}$ irreducible representation of the trigonal $\overline{3}m$ point group, and the magnetic moments must act as a secondary order parameter.