Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO sub(2) and CuCrO sub(2)

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO sub(2) reveal that the antiferromagnetic transition observed at T sub(NI) = 14 K might be induced by an R3m [rharu] 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 E sub(g) (x super(2) - y super(2), xy). Since this type of transition can be driven by a Raman E sub(g) mode, we performed Raman scattering measurements on CuFeO sub(2) between 5 and 290 K. Considering that the isostructural multiferroic compound CuCrO sub(2) might show similar structural deformations at the antiferromagnetic transition T sub(NI) = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO sub(2) are observed at omega E sub(g) = 352 cm super(-1) and omega A1 sub(g) = 692 cm super(-1), while these modes are detected at omega E sub(g) = 457 cm super(-1) and omega A sub(1g) = 709 cm super(-1) in CuCrO sub(2). 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 CuFeO sub(2) and CuCrO sub(2) drive the pseudoproper ferroelastic transitions observed at the Neel temperature T sub(N1) . Finally, a broad band at about 550 cm super(-1) observed in the magnetoelectric phase of CuCrO2 below T sub(N2) could be associated with magnons.