UC Santa Cruz

The effects of synthesis method, Sr-doping, and Co₃O₄ on the rare-earth perovskite LaCoO₃ were examined and quantified. Structural and magnetic measurements were taken using neutron diffraction, X-ray diffraction, transmission electron microscopy, X-ray fluorescence and SQUID magnetometry. An optimal method for synthesizing LaCoO₃ nanoparticles is described, and the solid-state synthesis method for nominal LaCoO₃ is found to result in the formation of an extra Co₃O₄ phase. Bulk LaCoO₃ materials containing systematically varying amounts of the Co₃O₄ phase (denoted as La_wCoO₃, where w is the molar ratio La:Co) were also synthesized. As the amount of Co₃O₄

was increased, the ferromagnetic transition at T_c = 87 K was found to be sharper, the ferromagnetic moment larger, and the ferromagnetism more robust at high fields (H > 100 Oe). This is a similar effect to that from increased tensile strain in LaCoO₃ thin films and from increased surface area in nanoparticles. We propose that tensile strain also exists in the LaCoO₃-Co₃O₄ interfaces, which enhances the ferromagnetism. The lattice parameters for La_wCoO₃ exhibited thermal expansion behavior that was best fit with a power law, indicating a second-order structural transition at T_o = 37 K. A mathematical model for the magnetization, M/H, of

La_wCoO₃ was developed which successfully described the behavior at both low and high external fields. The model consists of three contributions to the magnetization: one ferromagnetic contribution and two paramagnetic contributions with different antiferromagnetic exchange interactions. The ferromagnetic contribution is found to have a critical exponent of β = 0.65, consistent with magnetic ordering of the surface.