  Nanoscale Fe3O4 epitaxial thin film has been synthesized on MgO/GaAs(100)
spintronic heterostructure, and studied with X-ray magnetic circular dichroism
(XMCD). We have observed a total magnetic moment of (3.32 +- 0.1) uB/f.u.,
retaining 83% of the bulk value. Unquenched orbital moment of (0.47 +- 0.05)
uB/f.u. has been confirmed by carefully applying the sum rule. The results
offer direct experimental evidence of the bulk-like total magnetic moment and a
large orbital moment in the nanoscale fully epitaxial Fe3O4/MgO/GaAs(100)
heterostructure, which is significant for spintronics applications.
