Electrical transport properties of Fe3−xMgxO4 ferrite films on MgO (001) and SrTiO3 (001) grown by molecular beam epitaxy

A series of Fe3−xMgxO4 ( 0 ≦ x ≦ 1.5 ) films were grown by plasma-oxygen- assisted molecular beam epitaxy (MBE), and systematical measurements, including structure and resistivity of these films, were studied. These films are grown on MgO and SrTiO3 (STO), which have small (∼−0.3%) and large (∼7.5%) lattice mismatch in order to obtain either strained or relaxed films, respectively. The X-ray diffraction data for the films grown on the MgO substrates show (004) and (008) peaks, indicating single crystalline quality, but extra (l l l) peaks are also observed for the films grown on the STO substrates, indicating polycrystalline structure. Resistance as a function of temperature (78–345 K) for all films is carried out. The resistivity of the films grown on both substrates presents a typical Arrhenius temperature dependence with ρ = ρ 0 exp ( E p / k B T ) . The prefactor ρ 0 increases linearly as a function of x in Fe3−xMgxO4, which indicates that Mg2+ ions have replaced Fe2+ ions at both the A and B sites, and have formed a mixed spinel structure. The activation energy remains a constant ∼60 meV in the range 0 < x < 0.9 in Fe3−xMgxO4, but abruptly rises to 110 meV at x > 0.9 , suggesting a possible semiconductor–insulator transformation. The cation distribution, which plays a major role in determining the electrical properties, is discussed and compared to the results from previous magnetization measurements.