Structural, magnetic and optical properties of y bFe₂O₄ films deposited by spin coating

Rare-earth iron oxides (RFe₂O₄) have attracting attention as new electronic device materials because of their numerous functionalities, such as electronic ferroelectricity, ferrimagnetism, and high infrared absorption. In this paper, nearly monophasic Y bFe₂O₄ films were prepared on α-Al₂O₃(001) substrates by the spin coating method using an aqueous-based Y bFe₂O₄ solution. The solution was composed of a stoichiometric ratio of Y b(CH₃COO)₃ and Fe(NO₃)₃ with excess chelating agents. After heat treatment above 800 °C, well-crystallized and highly (001)-oriented Y bFe₂O₄ started to epitaxially form on the substrate under controlled oxygen partial pressure with H₂/CO₂ gas mixtures. X-ray pole figure analysis confirmed the following epitaxial relationship: Y bFe₂O₄[100](001)//α-Al₂O₃[100](001). Moreover formation of an Fe₃O₄ interracial layer between Y bFe₂O₄ and α-Al₂O₃ was detected by high-resolution transmission electron microscopy. Presence of the Fe₃O₄ interracial layer seemed to release the lattice misfit with the substrate. The Fe²⁺/Fe³⁺ ratio in the obtained Y bFe₂O₄ films was nearly stoichiometric and the indirect bandgap assigned to Fe²⁺ → Fe³⁺ charge transfer excitation was found to be ∼0.4 eV by optical spectroscopy. A clear magnetic transition from the paramagnetic state to the ferrimagnetic state occurred at ∼230 K.