Raman scattering study of delafossite magnetoelectric multiferroic compounds

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an R3m * C2=m pseudoproper ferroelastic transition [1]. 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 !Eg = 352 cm 1 and !Ag = 692 cm 1 , while these modes are detected at !Eg = 457 cm 1 and !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.