Abstract
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 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.
Highlights
Transition metal oxides are one of the most interesting material classes, providing a huge variety of structural, magnetic, and electronic properties ranging from metallic to insulating, from ferro- to antiferromagnetic, as well as ferroelectric states [1]
Materials 2018, 11, 1122 platinum show huge thermoelectric effects [11] based on the recently observed spin Seebeck effect in magnetite [12] pushing the development of more efficient thermoelectric nanodevices [13]
Epitaxial Fe3 O4 /NiO ultra-thin bilayer systems with thicknesses between 5 nm and 20 nm were grown via reactive molecular beam epitaxy (RMBE) on
Summary
Transition metal oxides are one of the most interesting material classes, providing a huge variety of structural, magnetic, and electronic properties ranging from metallic to insulating, from ferro- to antiferromagnetic, as well as ferroelectric states [1]. Have attracted intensive research interest in the last decade in the field of spintronics [2] and spin caloritronics [3,4]. Due to their anticipated half-metallic behavior with complete spin polarization at the Fermi level [5] and high (bulk) Curie temperature of 858 K [6], thin magnetite films are promising candidates for room temperature spintronic devices such as highly spin-polarized electrodes for magnetic tunneling junctions [7,8,9] or spin injectors [10]. The reduction of the crystal symmetry leads to a spontaneous ferroelectric polarization, and to multiferroicity [16,17]
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