We present a comparative study of the morphology and structural as well as magnetic properties of crystalline Fe₃O₄/NiO bilayers grown on both MgO(001) and SrTiO₃(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 SrTiO₃. In the case of Fe₃O₄, the misfit strain on MgO(001) and SrTiO₃(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, Fe₃O₄ films grown on NiO/SrTiO₃ 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 SrTiO₃ the NiO films showed strong strain relaxation. Fe₃O₄ films also exhibited strong relaxation, even for films of 5 nm thickness on both NiO/MgO and NiO/SrTiO₃. The magnetite layers on both substrates showed a fourfold magnetic in-plane anisotropy with magnetic easy axes pointing in 100 directions. The coercive field was strongly enhanced for magnetite grown on NiO/SrTiO₃ due to the higher density of structural defects, compared to magnetite grown on NiO/MgO.
Keywords: magnetic anisotropy; magnetite; nickel oxide; strain relaxation.