Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO₂and CuCrO₂

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 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 ωE g = 352 cm −1 and ωA g = 692 cm −1 , while these modes are detected at ωE g = 457 cm −1 and ωA g = 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 Néel 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.

Lattice dynamics and thermal transport in multiferroicCuCrO2

“…Based on our experiments and modeling, we evaluated the effect of spin-phonon coupling on the phonons and lattice thermal conductivity. Our IXS and INS measurements, sampling a large range of wavevectors, show little effect of spin-phonon coupling on the phonon frequencies across T N , in agreement with prior Raman reports for zone-center phonons [16]. Our INS and neutron diffraction measurements revealed strongly anisotropic Cu vibrations, with larger amplitudes parallel to the basal plane of the delafossite structure than normal to it.…”

“…We show that lattice distortions across T N do not discontinuously alter the lattice dynamics, as is expected considering the very small magnitude of the distortion (∆L L ∼ −3 × 10 −4 [7]). Raman measurements also found no discontinuity across T N [16]. However, we find that low-energy spin fluctuations do lead to increased phonon scattering rates above T N .…”

“…Park et al [26] focused on spin-waves in CuCrO 2 and attributed an intensity change across T N for a 12.5 meV excitation to a mixed magnon-phonon character. Furthermore, ultrasonic measurements in delafossite compounds have indicated substantial softening of elastic constants near T N [16,27]. A possible signature of spin-phonon coupling can also be found in reported thermal conductivity measurements of YMnO 3 or CaMnO 3 [28][29][30], motivating studies of the role of the spin-phonon interaction on phonon scattering rates.…”

“…Thus, it presents an opportunity to study the direct coupling between the magnetic and ferroelectric order parameters [3,4]. A number of studies have investigated the crystal structure, magnetic excitations, magnetization, and electric polarization in CuCrO 2 [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], but little is known about the phonons and their coupling or response to the magnetic/ferroelectric ordering.…”

Multiferroic CuCrO2 under high pressure: In situ X-ray diffraction and Raman spectroscopic studies

“…The O atoms are situated at 6c (0,0,u) positions [21,22]. Some of these compounds with B as magnetic trivalent cation (Fe or Cr) includes CuFeO 2, CuCrO 2, AgCrO 2, belong to the magnetoelectric multiferroics and at low temperature they undergo a series of magnetic phase transitions as a result of geometrical frustration of magnetic ions on triangular lattice [23,24]. Interestingly a few of these compounds like CuScO 2 , CuLaO 2 , CuInO 2 , CuAlO 2 show negative thermal expansion (NTE) behaviour as revealed by neutron scattering measurements [25,26].…”