Growth, structural and magnetic properties of complex oxide thin film heterostructures by molecular beam epitaxy

Multifunctional oxide nanostructures combining properties of ferromagnetism/ferrimagnetism, antiferromagnetism and ferroelectricity are of scientific and technological interest. One approach to realize these materials is by forming nanocomposites consisting of layers with distinct order parameters. Ferrites---Fe 3O4 and CoFe2O4---are excellent candidates as the ferrimagnetic constituents in these multiferroic heterostructures due to their large magnetization and magnetostriction coefficients, and high Curie temperatures. However, the realization of these applications depends highly on their bulk properties. Ferrite heterostructures have magnetic properties differing from those of the bulk, where the difference is due to misfit strain and interlayer exchange interactions.;Towards attaining multiferroics, epitaxial Fe3O4 and CoFe2O4 thin films were deposited on ferroelectric and MgO substrates by molecular beam epitaxy. Systems of interest include Fe3O4/MgO, Fe3O4/SrTiO3, Fe3O4/BaTiO3, CoFe2O4/MgO, CoFe2O4/SrTiO3 and CoFe2O 4/PMNT. Their magnetic properties were measured by superconducting quantum interference device magnetometry, magneto-optic Kerr effect and magnetic force microscopy. The magnetic anisotropy of these epitaxial films depended on their surface roughness and interface coherency. Fe3O4/SrTiO 3 and Fe3O4/BaTiO3 structures exhibited enhanced perpendicular anisotropy, compared to Fe3O4/MgO. This substrate induced alteration in magnetic anisotropy was attributed to surface roughness. For CoFe2O4/MgO, misfit strain induced magnetoelasticity resulted in a large perpendicular anisotropy, leading to its out-of-plane easy axis. In contrast, semi-coherent interfaces in CoFe 2O4/SrTiO3 and CoFe2O4/PMNT lead to a reduced magnetoelasticity, resulting in in-plane easy axes.;Ferrite bilayers and 7-period superlattices were analyzed to determine the role of interlayer coupling and magnetoelasticity on the magnetization reversal of the constituent layers. Coercive field analyses on Fe3O 4/CoFe2O4 bilayers indicated that an exchange coupling was present at the Fe3O4/CoFe2O 4 interface that substantially increased the CoFe2O 4 magnetization reversal fields. The magnitude of the exchange coupling, ranging from 0.2x105 erg/ccm at 300K to 3.2x10 5 erg/ccm at 50K, had a linear dependence on the magnetocrystalline anisotropy constants of the ferrite constituents. An additional increase in CoFe2O4 magnetization reversal field was observed in the Fe3O4/CoFe2O4 superlattices. A change in magnetoelastic energy on the order of 106 erg/ccm, altered the CoFe2O4 easy axis in the superlattices, leading to a further increase in its magnetization reversal field.