Magnetic inhomogeneity and Griffiths phase in Bi substituted La_0.65−xBi_xCa_0.35MnO₃ manganites

In the present communication, we report the effect of Bi substitution on structural, magneto-transport, magnetic and thermal properties of La_0.65−xBi_xCa_0.35MnO₃ (0≤x≤0.2) compounds. Rietveld refined XRD patterns confirm that all the samples are single phase and crystallize in rhombohedral symmetry with R-3C space group. It was observed from electrical and magnetic studies that with increasing Bi content both the metal-insulator transition temperature (T_MI) and ferromagnetic–paramagnetic (FM–PM) transition temperature (T_C) shift towards lower temperatures. Deviation of the temperature dependence of inverse susceptibility curves from the Curie–Weiss law suggests the existence of Griffiths-like phase. The electrical resistivity data were analyzed by utilizing various theoretical models. It is revealed that the electron–electron scattering is dominating in the metallic region, while the insulating region is well-described by the polaron hopping model. Analysis of thermoelectric power data further reveals that the small polaron hopping (SPH) mechanism is operative in the high-temperature insulating regime. The entropy change associated with the FM–PM transition is found to decrease with increasing x, which is presumably due to the increase in magnetic inhomogeneity with increasing Bi content.