Manganites show a variety of magnetic and structural phase transitions, which have a diverse influence on their thermal properties. With respect to this, thorough studies of temperature dependence of specific heat for several compositions of the series La1-xSrxMnO3 for x < 0.19 were performed using a relaxation method over the temperature range from 2 K to 385 K, in zero magnetic field, B, and in the field B=7 T.
As expected, the transition from the ferromagnetic to paramagnetic phase was visible on the temperature dependence of specific heat as a well-pronounced maximum, strongly influenced by magnetic field. Basing on these studies, the adiabatic change of temperature induced by a change in B, i.e., the main parameter characterizing the magnitude of the magnetocaloric effect, was determined. It was equal to ~3.5 K at a change of B by 7 T, which demonstrates unequivocally that the magnetocaloric effect in the studied compounds is small, despite of the fact that large B-induced isothermal entropy changes were found in numerous works, by analyzing the magnetization measurements.
An unusual behaviour was found for the structural transition from the rhombohedral (R) to the orthorhombic (O*) phase. For compositions for which the transition took place above the Curie temperature, e.g., for x=0.185, it was visible as a well-pronounced maximum, not affected by B, and not showing a temperature hysteresis. When the transition occurred below the Curie temperature, e.g., for x= 0.155, no anomaly at the transition temperature was observed. The only indication of the presence of the transition was a steep change of the temperature relaxation time, which indirectly demonstrated the change of the thermal conductivity of the sample.
A qualitatively different anomaly accompanied the other structural transition between two orthorhombic phases, O'-O*. At this transition, a bend on the temperature dependence of specific heat was observed, e.g., for x=0.155. |