Synthesis and characterization of barium iron oxide and bismuth iron oxide epitaxial films

Much interest exists in perovskite oxide materials and the potential they have in possessing two or more functional properties. In recent years, research on developing new materials with simultaneous ferromagnetic and ferroelectric behavior is the key to addressing possible challenges of new storage information applications. This work examines the fundamental properties of a perovskite oxide, namely BaFeO3, and the investigation of properties of a solid solution between BaFeO3 and BiFeO3.;The growth and properties of epitaxial BaFeO3 thin films in the metastable cubic perovskite phase are examined. BaFeO3 films were grown on (012) LaAlO3 and (001) SrTiO3 single crystal substrates by pulsed-laser deposition. X-ray diffraction shows that in situ growth at temperatures between 650-850°C yields an oxygen-deficient BaFeO 2.5+x pseudo-cubic perovskite phase that is insulating and paramagnetic. Magnetization measurements on the asdeposited BaFeO3 films indicate non-ferromagnetic behavior. Annealing these films in 1 atm oxygen ambient converts the films into a pseudo-cubic BaFeO3-x phase that is ferromagnetic with a Curie temperature of 235 K. The observation of ferromagnetism with increasing oxygen content is consistent with superexchange coupling of Fe +4-O-Fe+4.;The effects of anneal conditions on BaFeO3 are studied. X-ray characterization, such as reciprocal space maps, show more complex structure for as-grown BaFeO3-x epitaxial films. Epitaxial films grown at low laser energies are highly crystalline. However, they decompose after annealing. When grown at high laser energies, films exhibit complex structure which "cleans up" to a single pseudocubic or tetragonal structure upon ex situ anneal in oxygen ambient environment. Superlattices of BaFeO 3/SrTiO3 were synthesized to explore the nature of "cracking" in annealed BaFeO3, which occurs due to large change in lattice parameter. Magnetization of ex situ annealed BaFeO3-x epitaxial films were examined as a function of applied field direction and was not found to have a change in magnetization with direction of field, despite other research claims. Evidence supports that the unusually weak magnetization of BaFeO3-x is attributed to it being structurally and magnetically disordered.;Alloys of a solid-solution between BiFeO3 and BaFeO3-x have been successfully created. X-ray characterizations demonstrate alloy epitaxial films via two-target continuous rotation method have been carried all the way to 80% solubility. In addition, alloy films via solid-solution targets method have been successfully fabricated at near both end-member-points and at the half-point showing that the solubility is possible over the entire range of the solid-solution. Bi0.9Ba0.1FeO3 epitaxial films are of high crystalline quality with rocking curves widths of less than 0.22°, are fully strained, and have highly unusual in-plane and out-of-plane lattice parameters. TEM imaging illustrates that, despite extreme c/a ratios up to 1.26, the films are single phase with sharp interfaces with substrates. SQUID magnetometry was utilized, revealing that the samples are weakly ferromagnetic with a magnetization of 0.2microB per Fe, more than an order of magnitude larger than that of pure BiFeO3. Magnetic hysteresis loops show unfamiliar "pinching," signaling a possible breakdown of the helical magnetic ordering in the fully strained samples. BaFeO3-x, though it can be made ferromagnetic, it is a highly complex material. In studying BaFeO3-x's properties, conclusions can be made that its weak magnetization and unusual structure is highly disordered, magnetically and structurally. The creation of a new solid solution (Bi, Ba)FeO3 by two methods shows that a solid solution between BiFeO3 and BaFeO3-x can be synthesized. Specifically the creation of the alloy Bi0.9Ba0.1FeO3-delta , shows that one can improve on BiFeO3's magnetic properties, and more importantly supports the case that BaFeO3-x exhibits magnetic and structural disorder.