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Register a new userRaman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO2 and CuCrO2
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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 Neel 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.
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This paper presents results of a recent study of multiferroic CCO by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at $T sub{N1}=24.2$ K and $T sub{N2}=23.6$ K. The low temperature magnetic structure below $T sub{N2}$ is unambiguously determined to be a fully 3-dimensional proper screw. Between $T sub{N1}$ and $T sub{N2}$ antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the ($H,K$) plane, whereas nearest neighbors along the $L$-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.
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Polycrystalline samples of CuCrO2 were synthesized by solid state reaction method. Temperature dependent dielectric measurements, synchrotron x-ray diffraction (SXRD), pyroelectric current and Raman measurements have been performed on these samples. Evidences of the presence of relaxor type ferroelectricity, which otherwise have gone unnoticed in CuCrO2 system (a member of delafossite family) near room temperature, have been presented. Presence of broad maximum in dielectric permittivity and its frequency dispersion indicates relaxor-type ferroelectricity in CuCrO2 near room temperature. Careful analysis of temperature dependent SXRD data and Raman spectroscopic data indicates that the distorted CrO6 octahdera, is giving rise to strain in the sample. Due to this strain, polar regions are forming in an otherwise non-polar matrix, which is giving rise to relaxor type ferroelectricity in the sample. Regularization of CrO6 octahedra and disappearance of disorder induced peak in Raman spectra at high temperatures could be the reason behind observed dielectric anomaly in this sample. Present investigations propose that relaxor type ferroelectricity near room temperature is an inherent property of the CuCrO2 system, making it a fascinating material to be explored further.
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