THERMOREFLECTANCE, PHOTOCONDUCTANCE AND RAMAN SCATTERING OF FERROMAGNETIC SPINELS MCr 2X4 (M=Cd, Hg ; X=S, Se) NEAR THE MAGNETIC PHASE TRANSITION

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Abstract
The results of recent experiments on thermoreflectance, photoconductance and resonant Raman scattering of CdCr2Se4, CdCr2S4 and HgCr2Se4 at photon energies near and above the absorption edge are reviewed. The critical behaviour near the magnetic phase transition of the thermoreflectance (TR) spectral features associated with the bands of itinerant electrons provide evidence of a strong coupling between electrons and spins. The latter is supported by the temperature variation of both the photoconductivity spectra and the resonance of the Raman scattering cross section. The actual mechanism of spin-dependent electronic processes seems to be more complex than predicted by the simplest models. The Raman spectra and their dependence on temperature and incident photon energy show that the concept of spin dependent Raman scattering in the ferromagnetic spinels has to be revised. The results allow to conclude that the intensities of the Raman-active phonons are determined mainly by the line shapes of the resonance curves and their changes with temperature

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