Influence Of Oxygen Vacancies And Strains On The Magnetic Properties Of Cobalt Ferrite Thin Films

Cobalt ferrite(Co_1?xFe_2+xO₄,CFO)has been widely studied in the fields of spin filtering and ferroelectromagnet with its large magnetostriction and ferrimagnetism characteristics.In this dissertation,the CFO thin films with the tailored oxygen vacancy densities were deposited by DC magnetron reactive co-sputtering at various O2 flow rates.Then they were annealed in vacuum and Ar+O2 atmosphere,respectively.The influences of oxygen vacancies on the magnetic and structure properties of the CFO thin films have been investigated.The CFO/La_0.67Sr_0.33MnO₃?CFO/LSMO?heterostructures were further fabricated with the different thickness CFO films.Their structure and magnetic properties were studied.The CFO films have polycrystalline structures and clearly grains.For the CFO films deposited at low O₂ flow rate and annealed in vacuum,the small saturation magnetization?M_S?accompanied with the large magnetic coercive field?H_C?were observed,which can be explained by the spin canting.The oxygen vacancies can be formed for the films deposited at low O₂ flow rate and annealed in vacuum,and can weaken the superexchange interactions between cations and result in spin canting for magnetic cations.The similar situation was observed for the films deposited at high O₂ flow rate,due to the formation of cation vacancies in the excess O₂ atmosphere.The spin canting can also be resulted by the cation vacancies.For the CFO/LSMO heterostructures with thinner CFO film,the rough surface and the large component of soft magnetic phase are attributed to that the CFO film suffers from more strains and misfit dislocations.Due to the potential applications in flexible spintronics,the various thicknesses?40,125,and 250 nm?CFO/muscovite?CFO/mica?heterostructures were prepared.For the flexible epitaxial CFO?111?/mica?001?heterostructures,sharp and well-organized interface with¹-nm-thick diffusion can be observed,which suggests that the strains in CFO films have been significantly relaxed on the mica substrates.For the heterostructures under bending states,the M_S gets larger while the H_C and magnetic anisotropy get smaller,compared with the unbent states.This observation can be attributed to the change of magnetocrystalline anisotropy caused by the distortion of the oxygen octahedron occupied by Co²⁺with the strong spin-orbit coupling for the heterostructures under bending states.