Intrinsic Properties of Doped Lanthanum Manganite

Abstract

AbstractAn investigation designed to display the intrinsic properties of perovskite manganites was accomplished by comparing the behavior of bulk samples with that of thin films; the results show the "colossal magneto resistance" at very low temperatures is not an intrinsic property of the thermodynamically stable 1/3 doped material. Epitaxial 1500 Å films of perovskite La0.67Ca0.33MnO3 and La0.67Sr0.33MnO3 were grown by solid source chemical vapor deposition on LaA1O3 and post annealed in oxygen at 950°C. Crystals were prepared by LASER heated pedestal growth. The magnetic and electrical transport properties are essentially the same. Below Tc/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). We identify three distinct types of negative magnetoresistance. The largest effect is observed near the Curie temperature and is likely to be due to magnetic critical scattering. There is also magnetoresistance associated with the net magnetization of polycrystalline samples. The high temperature (above Tc) resistivity of La0.67Ca0.33MnO3 is consistent with small polaron hopping conductivity with a transition at 750K, while La0.67Sr0.33MnO3 does not exhibit activated conductivity until about 500K, well above Tc. The limiting low and high temperature resistivities may place a limit on the maximum possible magnetoresistance of these materials.