Anomalous moment and anisotropy behavior inFe3O4films

Abstract

${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ films were grown on Si, $〈100〉$ and $〈110〉$ MgO, $〈111〉$ Mg${\mathrm{Al}}_2$ ${\mathrm{O}}_4$, and $〈0001〉{\mathrm{Al}}_2{\mathrm{O}}_3$ by reactive sputter deposition. X-ray diffraction and TEM studies of the films grown on MgO show they are uniformly strained epitaxial single crystal specimens. Conversion electron Mössbauer spectroscopy (CEMS) spectra for films on all substrates show the presence only of the stoichiometric ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ phase, and values for the hyperfine fields and isomer shifts of the $A$ and $B$ sites consistent with bulk ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$. However, the CEMS spectra exhibit an anomalous out-of-plane moment distribution, although the moments are expected to be in the plane of the film due to the large shape anisotropy. Furthermore, the magnetization remains unsaturated in fields as large as 70 kOe, and torque measurements of films grown on MgO remain unsaturated at 21 kOe. The extrapolated values for the anisotropy, derived from torque curves taken both in and out of the film plane, are much smaller than that required to cause the lack of saturation in high fields and the anomalous CEMS spectra, and are fairly well explained as a combination of crystalline, magnetoelastic, and shape anisotropy of bulk single crystal ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ subjected to in-plane tensile stress. The magnetoelastic anisotropy derived for ideal epitaxial ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ films grown on $〈100〉$ and $〈110〉$ MgO, using bulk values for the magnetostriction and elastic constants, agrees well with values determined experimentally. A comparison of films with thicknesses ranging from 0.01 to 6 μm indicates the anomalous behavior to be a volume, as opposed to a surface, effect. The anomalous behavior is exhibited in films grown on other substrates and by other techniques (evaporation and molecular beam epitaxy), and is independent of thickness and deposition conditions. It appears to be an intrinsic property of all ${\mathrm{Fe}}_3$ ${\mathrm{O}}_4$ films. Possible origins of this behavior are discussed.