Temperature Dependence of Lattice Frequencies in Magnetic Crystals MCr₂Se₄ (M=Hg, Cd)

Abstract

The vibrational frequencies of infrared-active modes in ferromagnetic crystals, HgCr₂Se₄ (Curie temperature T_c=120 K) and CdCr₂Se₄ (T_c=130 K), have been measured in the temperature range of 300 K to 85 K. To compare their behaviours with those in non-magnetic crystal, the same measurement has been made for CdIn₂S₄. All the measured frequencies increase continuously with decreasing temperature. In the magnetic crystals, the frequencies indicate some anomalous shifts to the higher frequency side (0.4∼0.6 cm⁻¹) near T_c. Such anomalous shifts are proportional to the magnetizations of magnetic crystals: this anomalous shift has not been observed in CdIn₂S₄. The value of the continuous frequency shift is estimated from the mode-Grüneisen parameter and the variation of volume determined by an X-ray technique. The anomalous shift is explained phenomenologically by using an expansion of the atomic potential with respect to the magnetization.