Thermal study of the interplay between spin and lattice in CoCr2O4and CdCr2O4

Abstract

The interplay between spin and lattice degrees of freedom in the spinels CoCr${}_{2}$O${}_{4}$ and CdCr${}_{2}$O${}_{4}$ has been investigated by heat capacity and thermal expansion measurements. In CoCr${}_{2}$O${}_{4}$, sharp anomalies are observed at the magnetic phase transition temperatures of 13, 26, and 93 K. A large pressure dependence of the transition temperature at 13 K, which is calculated from the heat capacity and thermal expansion, indicates strong spin-lattice coupling in the incommensurate phase. This result provides the possibility of the reduction of lattice symmetry from cubic at the phase transition. In CdCr${}_{2}$O${}_{4}$, negative thermal expansion due to spin-lattice coupling emerges in temperature from 140 to 45 K, which is followed by strong positive thermal expansion at lower temperature. The magnetostructural transition at 7.8 K is observed as a large anomaly with an indication for a large pressure dependence of the transition temperature. The unusual thermal expansion suggests the importance of other perturbations besides exchange interactions between Cr ions for the formation of the spin clusters. Our results demonstrate that thermal measurements can help to further understand the frustrated systems with strong spin-lattice coupling.