Spin-lattice coupling, Jahn-Teller effect and the influence of the measurement rate in La_0.7Ca_0.3−xSr_xMnO₃ manganites

This work presents a study of how the magnetic and temperature variables, as well as the measurement rate, affect magnetic and structural phase transitions for La_0.7Ca_0.3−xSr_xMnO₃ (x=0.15, 0.08, 0.07 and 0.06) manganites. A single phase rhombohedral crystal structure for Sr_0.15 compound and a single phase orthorhombic crystal structure for Sr_0.08, Sr_0.07, and Sr_0.06 compounds were identified by X ray diffraction. The Curie temperature adjustment to room temperature was reached for Sr_0.07 sample, in which a Jahn-Teller transition was observed. This Jahn-Teller transition showed a susceptible response to 0.5, 1, 2, 3, 5 and 7 K/min measurement rates, and was revealed without measurement sequence at 234, 257, 196, 233, 245 and 238 K respectively in thermomagnetic curves (10 mT). Also, Curie temperature increases sequentially around 2 K for the same measurement rates. To avoid magnetic field effect, a differential scanning calorimetry for Sr_0.07 sample was obtained in the same temperature region, for 2, 3, 5, 7 K/min showing the Jahn-Teller phase transition at 225 K and a vibrational like behavior in the heat flux below 225 K, for all the temperature rates. The latter, is related to the static to dynamic Jahn-Teller transition. Furthermore, Curie temperature appears around 284 K.