Structural and magnetic properties in sputtered iron oxide epitaxial thin films -- Magnetite Fe$_3$O$_4$ and epsilon ferrite e-Fe$_2$O$_3$

Abstract: Epitaxial thin film fabrication of iron oxides including magnetite Fe3O4 and epsilon-ferrite epsilon-Fe2O3 with the potential for advancing electromagnetic devices has been investigated, which led to the first ever epsilon-ferrite epitaxial layer being synthesized in the conventional sputtering process. Concerning Fe3O4 (100) / MgO (100) films, a cube-on-cube epitaxial relationship and sharp rocking curves with FWHM of 50 - 350 arcsec were confirmed regardless of the small amount of Ge additions. Sputtering Ar gas pressure PAr heavily influenced their magnetic and transport properties. High PAr = 15 mTorr caused a high magnetization of 6.52 kG for the Ge added sample and the clear Verwey transition at 122 K for the non Ge addition case. Conversion electron Mossbauer spectroscopy (CEMS) measurements revealed that low PAr < 10 mTorr causes Fe/O off-stoichiometry on the oxidizing side for the non Ge addition case and the reductive side for the Ge addition case, respectively. Regarding the epsilon-Fe2O3 (001) / SrTiO3(111) epilayer synthesis, bilayer microstructure composed of an approximately 5nm thick initially grown epsilon-Fe2O3 (001) epilayer and subsequently grown e-Fe2O3 (001) epilayer was confirmed from cross-sectional TEM observations. The coexistence of magnetically hard and soft phases was confirmed from the magnetization measurements. As a possible application of the single nm thick epsilon-Fe2O3 layer, 4-resistive-state multiferroic tunnel junction (MFTJ) is considered.