Magnetic anomalies in single crystal Fe3O4 thin films

Abstract

Fe₃O₄ thin films were reactively sputtered onto 〈100〉 and 〈110〉 MgO substrates. X‐ray diffraction data indicate single crystal Fe₃O₄ films under a tensile stress. Magnetization data show good agreement with bulk Fe₃O₄ for M_s, but also show the presence of a large anisotropy component manifest in the lack of saturation in fields up to 70 kOe. Conversion electron Mössbauer spectroscopy data taken at zero field also show good agreement with bulk Fe₃O₄ for the values of the hyperfine fields and isomer shifts of the two Fe₃O₄ sites, indicate good stoichiometry, but indicate the presence of a large anisotropy component randomizing the moments. In‐plane torque measurements on 〈100〉 oriented Fe₃O₄ are consistent with bulk Fe₃O₄ magnetocrystalline anisotropy. However, in‐plane torque measurements on 〈110〉 oriented Fe₃O₄ show the presence of a uniaxial anisotropy superimposed on the crystalline terms. This anisotropy, modeled as being due to the stress induced by epitaxy, changes the easy axis from the 〈111〉 directions to the 〈110〉 directions in the (110) plane. Since the directional dependence of the crystal and stress anisotropy are measurable the ‘‘large anisotropy’’ component is considered to be random.