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 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.
ABSTRACT
We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe_{2}O_{x} (4≥x≥0) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect (ANE) contributions and even disentangle the ANE in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating NiFe_{2}O_{4} to metallic Ni_{33}Fe_{67}. A proximity-induced ANE could only be identified in the metallic Pt/Ni_{33}Fe_{67} bilayer in contrast to Pt/NiFe_{2}O_{x} (x>0) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity.
ABSTRACT
We prepared magnesia, tantalum oxide, and barium titanate based tunnel junction structures and investigated their memristive properties. The low amplitudes of the resistance change in these types of junctions are the major obstacle for their use. Here, we increased the amplitude of the resistance change from 10% up to 100%. Utilizing the memristive properties, we looked into the use of the junction structures as artificial synapses. We observed analogs of long-term potentiation, long-term depression and spike-time dependent plasticity in these simple two terminal devices. Finally, we suggest a possible pathway of these devices toward their integration in neuromorphic systems for storing analog synaptic weights and supporting the implementation of biologically plausible learning mechanisms.
ABSTRACT
Praseodymia films have been exposed to oxygen plasma at room temperature after deposition on Si(111) via molecular beam epitaxy. Different parameters as film thickness, exposure time and flux during plasma treatment have been varied to study their influence on the oxygen plasma oxidation process. The surface near regions have been investigated by means of X-ray photoelectron spectroscopy showing that the plasma treatment transforms the stoichiometry of the films from Pr2O3 to PrO2. Closer inspection of the bulk properties of the films by means of synchrotron radiation based X-ray reflectometry and diffraction confirms this transformation if the films are thicker than some critical thickness of 6 nm. The layer distance of these films is extremely small verifying the completeness of the plasma oxidation process. Thinner films, however, cannot be transformed completely. For all films, less oxidized very thin interlayers are detected by these experimental techniques.
