Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO2 and CuCrO2

TL;DR

This study used Raman scattering to investigate structural and magnetic transitions in CuFeO2 and CuCrO2, finding no Raman-active modes driving the ferroelastic transition, and identifying a magnon mode in CuCrO2.

Contribution

It provides the first Raman scattering analysis of CuFeO2 and CuCrO2, clarifying the role of phonon modes in their phase transitions.

Findings

Raman modes do not drive the ferroelastic transition.

Temperature dependence of modes shows anharmonic phonon interactions.

A broad band in CuCrO2 suggests magnon activity.

Abstract

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an R-3m -> C2/m pseudoproper ferroelastic transition (G. Quirion, M. J. Tagore, M. L. Plumer, O. A. Petrenko, Phys. Rev. B 77, 094111 (2008)). In that case, the group theory states that the order parameter associated with the structural transition must belong to a two dimensional irreducible representation Eg (x^2 - y^2, xy). Since this type of transition can be driven by a Raman Eg mode, we performed Raman scattering measurements on CuFeO2 between 5 K and 290 K. Considering that the isostructural multiferroic compound CuCrO2 might show similar structural deformations at the antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are observed at omega_Eg = 352 cm^-1 and omega_Ag = 692 cm^-1, while these modes are detected at omega_E_g = 457 cm^-1 and omega_Ag = 709 cm^-1 in CuCrO2. The analysis of the temperature dependence of modes shows that the frequency of all modes increases down to 5 K. This typical behavior can be attributed to anharmonic phonon-phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper ferroelastic transition observed at the N\'eel temperature TN1. Finally, a broad band at about 550 cm^-1 observed in the magnetoelectric phase of CuCrO2 below TN2 could be attributed to a magnon mode.