Transport and magnetic anomalies due to A-site ionic-size mismatch in La0.5Ca0.5-xBaxMnO3

We present the results of measurements of the electrical resistivity (rho), magnetoresistance (MR) and de and ac susceptibility (chi) of polycrystalli ne samples of the type La0.5Ca0.5-xBaxMnO3, synthesized under identical hea t-treatment conditions. The substitution of larger Ba ions for Ca results i n a non-monotonic variation of the Curie temperature (Tc) as the system evo lves 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 hy steresis in the ac chi at around the Curie temperature (T-C = 120 K). The x = 0.2 and 0.3 compounds exhibit semiconducting-like behaviour as the tempe rature is lowered below 300 K, with a broad peak in rho at around 80-100 K; these compositions exhibit a weak increase in rho as the temperature is lo wered below 30 K, indicative of electron localization effects; these compos itions also undergo ferromagnetic transitions below about 200 and 235 K res pectively, though these are non-hysteretic; most significantly, for these c ompositions, the MR is large and conveniently measurable over the entire te mperature 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-sit e ionic-size mismatch plays a crucial role in determining these properties.