Phonons and magnetic order in ferromagnetic CdCr2Se4 and CdCr2S4

Measurements are reported of the Raman effect on single crystal ferromagnetic CdCr₂Se₄ and CdCr₂S₄ as a function of temperature. The lines observed at 158 cm^?1, 172 cm^?1 and 241 cm^?1 for CdCr₂Se₄ and 257 cm^?1, 281 cm^?1 and 396 cm^?1 for CdCr₂S₄ have been identified as due toГ ₁₂ ⁺ ,Г ₂₅ ⁺ andГ ₁ ⁺ phonons, respectively. Certain lines ofГ ₁ ⁺ orГ ₂₅ ⁺ symmetry, observed in both materials, show a temperature dependence of the intensity which resembles the behaviour of the spin correlation function. Some of these lines also exhibit a symmetry change at the Curie point. A group theoretical analysis of the eigenvectors of the Raman active modes is given. The results are interpreted in terms of a model proposed by Baltensperger which involves the ion position dependence of the magnetic exchange interaction in order to couple spin system and phonons. The weak frequency shift of the lines, which occurs predominantly in the ordered state, is interpreted in terms of the same model.