Ultrasound velocity measurements in perovskite cobaltites LaCoO₃ and La (Co_0.99Ni_0.01)O₃

Perovskite cobaltite LaCoO₃ has attracted much interest due to the possible occurrence of the spin-crossover phenomena. The spin state of Co³⁺ in this compound is considered to change from high-spin to low-spin state on cooling; from high-spin to “magnetic” low-spin state at ~ 100 K, and from “magnetic” to “nonmagnetic” low-spin state at ~ 20 K. Furthermore, for LaCoO₃, it was recently suggested that, in the low-temperature “nonmagnetic” low-spin state, the light Ni substitution for Co induces the formation of Ni-centered ferromagnetic spin-state clusters. To study the spin states of LaCoO₃ and lightly Ni-substituted LaCoO₃ from the viewpoint of the spin-lattice coupling, we performed ultrasound velocity measurements in single crystals of LaCoO₃ and La (Co_0.99Ni_0.01)O₃. In the temperature dependence of the elastic moduli, we observed elastic anomalies in these compounds, which suggest the coupling of lattice to local magnetic excitations. Additionally, for LaCoO₃, we observed a discontinuous elastic anomaly at ~ 100 K, which indicates the occurrence of phase transition.