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(001) and MgAl2O4(001). 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. Mossbauer spectroscopy analysis of a 1.7 nm 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. (C) 2002 Elsevier Science B.V. All rights reserved.