2ND-ORDER PHASE-TRANSITION IN FECR2S4 INVESTIGATED BY MOSSBAUER-SPECTROSCOPY - EXAMPLE OF ORBITAL PARA-TO-FERROMAGNETISM TRANSITION

Mössbauer studies on stoichiometric samples of the spinel ferrimagnet FeCr2S4 are reported with special attention to the second-order phase transition at 13 K. Magnetic spectra were first solved as a superposition of singlets. This gives the experimental linewidths and intensities of each line. These as-deduced linewidths and intensities were assumed in computing the magnetic spectra for a single-site solution. The transition at 13 K is associated to a discontinuity of the barycenter of the spectra. Moreover, anomalies of the linewidth variations versus T as well as differences between the theoretical and the experimental intensities of the peaks cannot be resolved in the single-site solution. The singleness of this solution is discussed. A more reliable two-site solution is suggested. The experimental mean linewidth, the characteristics of the electric-field-gradient (EFG) (Vzz, and and the hyperfine magnetic field of the sites show discontinuities at 13 K. These features are explained assuming narrow d bands in FeCr2S4. The sixth 3d electron of A-site Fe2+ is assumed to occupy an orbital with and Cr2 symmetry due to the hybridization of |with the B-site Cr2+ state. FeA2+ and CrB2+ in thiospinel lie almost at the same energy level. At T>13 K, the sixth 3d electron shares the | and the hybridized | orbital statistically. This gives two sites with the same intensity. At T<13 K, only the hybridized | orbital is occupied. This corresponds to an orbital paramagnetism (T>13 K) to a ferromagnetic ordering (T<13 K) transition. Recently Cyrot showed that an Hubbard Hamiltonian in the atomic limit leads not only to spin ordering but also to orbital ordering. Furthermore, the transition at T<13 K is associated to a slow relaxation of the EFG. The coupling of the band with the phonons at T<13 K is different from a linear and weak dynamic Jahn-Teller effect as is the case for A-site diluted Fe2+. © 1979 The American Physical Society.