Magnon and phonon Raman scattering of the planar ferromagnet Rb2CrCl4

Abstract

Magnon and phonon Raman scattering is studied in the canted ferromagnet Rb2CrCl4. The phonon spectra obtained at about 8 and 200 K are discussed on the basis of the exact crystal structure (D2h18) and of the appropriate quadratic one (D4h5) previously proposed for the isostructural crystal K2CuF4. Among the twelve modes predicted in D4h5 symmetry, only one (belonging to Gamma 4+) is missing. The most important feature of this work is the first direct observation of the optic gap predicted by the theory at about 170 cm-1 and the study of its magnetic field dependence for B along the easy direction (110) and the hard direction (001). The behaviour of the line is discussed in relationship with spin wave calculations by considering a possible slight misorientation of the field off the principal axes. The acoustic magnon line, previously observed directly via ferromagnetic resonance, is also seen for B>2T along (110). Polarisation measurements on the two lines indicate that the acoustic and optic modes must be assigned to gamma 2+ and gamma 1+ (C2h) respectively. A quantitative analysis of the authors results shows that the average Lande factor is close to the spin-only value and it suggests that gperpendicular to is slightly larger than g/sub ///. It also leads to an estimate of P+2D where P and D stand for the two anisotropy parameters.