Intrinsic electrical transport and magnetic properties of La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃ MOCVD thin films and bulk material

An investigation designed to display the intrinsic properties of perovskite manganites was accomplished by comparing the behavior of bulk samples with that of thin films. Epitaxial 1500 Å films of perovskite La_0.67Ca_0.33MnO₃ and La_0.67Sr_0.33MnO₃ were grown by solid source chemical vapor deposition on LaAlO₃ and post annealed in oxygen at 950 °C. Crystals were prepared by laser heated pedestal growth. The magnetic and electrical transport properties of the polycrystalline pellets, crystals, and annealed films are essentially the same. Below T_C/2 the intrinsic magnetization decreases as T² (as can be expected for itinerant electron ferromagnets) while the intrinsic resistivity increases proportional to T². The constant and T² coefficients of the resistivity are largely independent of magnetic field and alkaline earth element (Ca, Sr, or Ba). Hall effect measurements indicate that holes are mobile carriers in the metallic state. We identify three distinct types of negative magnetoresistance. The largest effect, observed near the Curie temperature, is 25% for the Sr and 250% [ΔR/R(H)] for the Ca compound. There is also magnetoresistance associated with the net magnetization of polycrystalline samples which is not seen in films. Finally a small magnetoresistance linear in H is observed even at low temperatures. The high temperature (above T_C) resistivity of La_0.67Ca_0.33MnO₃ is consistent with small polaron hopping conductivity with a slight transition at 750 K, while La_0.67Sr_0.33MnO₃ does not exhibit activated conductivity until about 500 K, well above T_C. The limiting low and high temperature resistivities place a limit on the maximum possible magnetoresistance of these materials and may explain why the "colossal" magnetoresistance reported in the literature correlates with the suppression of T_C.