Transport and magnetic anomalies due to A-site ionic-size mismatch in La_0.5Ca_0.5−xBa_xMnO₃

Abstract

We present the results of measurements of the electrical resistivity (ρ), magnetoresistance (MR) and dc and ac susceptibility (χ) of polycrystalline samples of the type La_0.5Ca_0.5−xBa_xMnO₃, synthesized under identical heat-treatment conditions. The substitution of larger Ba ions for Ca results in a non-monotonic variation of the Curie temperature (T_C) as the system evolves from a charge-ordered insulating state for x=0.0 to a ferromagnetic metallic state for x=0.5. An intermediate composition, that with x=0.1, interestingly exhibits ferromagnetic, insulating behaviour with thermal hysteresis in the ac χ at around the Curie temperature (T_C=120 K). The x=0.2 and 0.3 compounds exhibit semiconducting-like behaviour as the temperature is lowered below 300 K, with a broad peak in ρ at around 80-100 K; these compositions exhibit a weak increase in ρ as the temperature is lowered below 30 K, indicative of electron localization effects; these compositions also undergo ferromagnetic transitions below about 200 and 235 K respectively, though these are non-hysteretic; most significantly, for these compositions, the MR is large and conveniently measurable over the entire temperature range of measurement below T_C and this experimental finding may be of interest from the applications point of view. We infer that the A-site ionic-size mismatch plays a crucial role in determining these properties.