Impact of Strain and Morphology on Magnetic Properties of Fe(3)O(4)/NiO Bilayers Grown on Nb:SrTiO(3)(001) and MgO(001)

We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe(3)O(4)/NiO bilayers grown on both MgO(001) and SrTiO(3)(001) substrates by reactive molecular beam epitaxy. These structures were investigated by means of X-ray photoelectron spectroscopy, low-energy electron diffraction, X-ray reflectivity and diffraction, as well as vibrating sample magnetometry. While the lattice mismatch of NiO grown on MgO(001) was only 0.8%, it was exposed to a lateral lattice mismatch of −6.9% if grown on SrTiO(3). In the case of Fe(3)O(4), the misfit strain on MgO(001) and SrTiO(3)(001) amounted to 0.3% and −7.5%, respectively. To clarify the relaxation process of the bilayer system, the film thicknesses of the magnetite and nickel oxide films were varied between 5 and 20 nm. While NiO films were well ordered on both substrates, Fe(3)O(4) films grown on NiO/SrTiO(3) exhibited a higher surface roughness as well as lower structural ordering compared to films grown on NiO/MgO. Further, NiO films grew pseudomorphic in the investigated thickness range on MgO substrates without any indication of relaxation, whereas on SrTiO(3) the NiO films showed strong strain relaxation. Fe(3)O(4) films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO(3). The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in [Formula: see text] directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO(3) due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.