Exploring the Spin State of Cobalt in Oxide Perovskites

Abstract

Controlling the spin state of a magnetic ion is important in the growing market of spintronic devices. The temperature dependent low spin to high spin transition of Co\(^{3+}\) in LaCoO\(_{3}\) has been commonly studied, but the factors that determine the spin state of Co\(^{2+}\) and Co\(^{3+}\) in a particular environment are unclear. In attempt to induce a spin state transition of these ions, Co\(^{2+}\) and Co\(^{3+}\) were introduced in small amounts in various cubic and orthorhombic perovskites SrTiO\(_{3}\), LaAlO\(_{3}\), CaSnO\(_{3}\), SrSnO\(_{3}\), and BaSnO\(_{3}\). Here I report that 4% doping of Co into SrTiO\(_{3}\) shows a high spin state of Co\(^{2+}\) but an intermediate spin state of Co\(^{3+}\) with a graduate decrease of effective moment as the cobalt oxidation state is increased from +2 to +3. In a system of LaAlO\(_{3}\)–SrTiO\(_{3}\) doped with 5% Co\(^{3+}\), LaAl\(_{0.95}\)Co\(_{0.05}\)O\(_{3}\) is nonmagnetic, but a mixture of 70% LaAlO\(_{3}\) and 30% SrTiO\(_{3}\) shows a high spin state of Co\(^{3+}\). Finally, the effect of M-O bond length on the spin state of Co\(^{3+}\) was examined by doping 10% Co\(^{3+}\) into ASnO\(_{3}\) (A = Ca, Sr, and Ba). While CaSn\(_{0.8}\)Co\(_{0.1}\)Nb\(_{0.1}\)O\(_{3}\) showed Co\(^{3+}\) in intermediate spin, increasing the cation size to Ba induced a high spin state. The work presented in this thesis shows the effect of crystal structure, such as M-O bond length, on the spin state of Co\(^{2+}\) and Co\(^{3+}\). Future work would provide further understanding of factors that determine the spin state of Co ions, allowing for more targeted spintronic materials to be made for various electronic device applications.