MAGNETIC AND CRYSTALLOGRAPHIC PROPERTIES OF MOLECULAR-BEAM EPITAXIALLY GROWN FE3O4 NIO SUPERLATTICES AND FE3O4 FILMS/

Ferromagnetic resonance, SQUID magnetometry, and x-ray diffraction hav e been used to characterize a set of [Fe3O4(68 angstrom)/NiO(17 angstr om)]N superlattices (SL) with N=3, 10, 30, and 100, as well as a 1.5-m um-thick Fe3O4 film. For this NiO thickness, Fe3O4 layers are strongly coupled and the in-plane anisotropy is much less than the 330-Oe ferr omagnetic resonance (FMR) linewidth at 35 GHz. Both in-plane and perpe ndicular FMR at 9.5 and 35 GHz have been used, with the 9.5-GHz data s howing significant hysteresis associated with the sample magnetization . X-ray diffraction indicates that both the film and SL's are nearly c ubic single-crystalline structures with long-range coherence. The 300 K magnetization data indicate the presence of small cubic anisotropy i n the SL's, although bulklike Fe3O4 magnetic ordering in the thick sin gle film. When the Fe3O4 film is cooled below the Verwey transition in a 10 kOe field (aligned along (100)), the FMR shows that the sample d evelops a large uniaxial (K(u) = 1.8 kOe) in-plane anisotropy with its easy axis along the field direction. Decreased microwave loss also oc curs abruptly at the film T(V).