Growth and Characterization of Stoichiometry-Varied Mn_xCo_3-xO₄ Manganese-Cobalt Spinel Thin Films

The cobalt-manganese spinels MnCo₂O₄ and CoMn₂O₄ have shown strong catalysis behavior through oxygen reduction reactivity (ORR). These materials can perform ORR at levels on par with ubiquitous but expensive materials such as platinum. Despite their impressive catalytic properties, little has been done in the way of thin film characterization on these materials. Additionally, phase segregation has been observed in nanoparticle studies due to a miscibility gap between Jahn-Teller active tetragonal CoMn₂O₄ and cubic MnCo₂O₄. Thin film studies enable further analysis of phase composition, degree of inversion, and charge transfer within the crystal. We have grown and studied the properties of cobalt-manganese spinel thin films at varying cation stoichiometries (Mn_xCo_3-xO₄ from x = 0 to 2.1). Films were grown using molecular beam epitaxy. In-situ X-ray photoelectron spectroscopy, X-ray diffraction and K-edge/L-edge X-ray absorption spectroscopy were performed. The experiments allowed us to determine information such as cation valence states, spinel inversion parameters, cation-oxygen bond lengths and magnetization properties. The results allow us to create a map of material properties as a function of cation stoichiometry for the entire cobalt-manganese spinel system.