  The spin and orbital magnetic moments of the Fe3O4 epitaxial ultrathin film
synthesized by plasma assisted simultaneous oxidization on MgO(100) have been
studied with X-ray magnetic circular dichroism (XMCD). The ultrathin film
retains a rather large total magnetic moment, i.e. (2.7+-0.15) uB/f.u., which
is ~ 70% of that for the bulk-like Fe3O4. A significant unquenched orbital
moment up to (0.54+-0.05) uB/f.u. was observed, which could come from the
symmetry breaking at the Fe3O4/MgO interface. Such sizable orbital moment will
add capacities to the Fe3O4-based spintronics devices in the magnetization
reversal by the electric field.
