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

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an R3-bar → C2/m pseudoproper ferroelastic transition. 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 (x2 − y2, xy). Since this type of transition can be driven by a Raman Eg mode, we performed Raman scattering measurements on CuFeO2 between 5 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 ωEg = 352 cm−1 and ωA1g = 692 cm−1, while these modes are detected at ωEg = 457 cm−1 and ωA1g = 709 cm−1 in CuCrO2. The analysis of the temperature dependence of the modes in both compounds shows that the frequencies of all modes increase with decreasing temperature. This typical behavior is 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 transitions observed at the Néel temperature TN1. Finally, a broad band at about 550 cm−1 observed in the magnetoelectric phase of CuCrO2 below TN2 could be associated with magnons. (paper)