Magneto-resistance and superparamagnetism in magnetite films on MgO and MgAl2O4

The present study is a comparison of the resistive and magnetic properties of ultrathin epitaxial Fe₃O₄ films grown on MgO(0 0 1) and MgAl₂O₄(0 0 1). Such films on MgO are known to contain anti-phase boundaries, which cause magneto-resistance in these films, an increased resistivity with respect to the bulk and superparamagnetic behaviour for films below 5 nm thickness. Ultrathin films on MgAl₂O₄ show a very similar behaviour as the resistivity is also increased with respect to the bulk, but is still lower than the resistivity of films grown on MgO. This indicates a larger domain size in the films grown on MgAl₂O₄. This is supported by the observed magneto-resistance that is larger than for films grown on MgO. Mössbauer spectroscopy analysis of a 1.7nm thick film on MgO indicates that all magnetic moments in the film are fluctuating, whereas for the film on MgAl₂O₄ a sextet with a parabolic background is observed. This shows that some domains are small enough for the moment to fluctuate while in larger ones, the moments are fixed; this also indicates that the domains in the film on MgAl₂O₄ are larger than in the film grown on MgO.