Electron spin resonance study of polycrystalline La_0.75(Ca_xSr_1−x)_0.25MnO₃ (x = 0, 0.45, 1)

Electron spin resonance (ESR) spectra of polycrystalline La_0.75(Ca_xSr_1−x)_0.25MnO₃ (x = 0, 0.45, 1) were studied within the temperature range 110 K≤T≤470 K. The temperature dependence of the ESR intensity for the samples is described by a thermally activated model in the paramagnetic regime. It is found that the activation energy in the orthorhombic phase is higher than that in the rhombohedral phase for La_0.75(Ca_0.45Sr_0.55)_0.25MnO₃. It is suggested that a higher energy is required to destroy the correlated polarons due to the fact that correlated polarons only exist in the orthorhombic phase. This proposition is confirmed by the analysis of the ESR linewidth data, which can be well fitted by the model of adiabatic hopping motion of small polarons. In addition, it is found that, at a fixed temperature, the linewidth decreases with increasing Sr doping, which reveals that the structural tolerance factor has a significant effect on the linewidth.