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.