arXiv subjects

Joachim Wollschläger

Publications and source records attributed to Joachim Wollschläger.

25 records · Page 2Linked to original sources

arXiv2017

Electro-Oxidation of Ni42 Steel: A highly Active Bifunctional Electrocatalyst

Janus type Water-Splitting Catalysts have attracted highest attention as a tool of choice for solar to fuel conversion. AISI Ni 42 steel was upon harsh anodization converted in a bifunctional electrocatalyst. Oxygen evolution reaction- (OER) and hydrogen evolution reaction (HER) are highly efficiently and steadfast catalyzed at pH 7, 13, 14, 14.6 (OER) respectively at pH 0, 1, 13, 14, 14.6 (HER). The current density taken from long-term OER measurements in pH 7 buffer solution upon the electro activated steel at 491 mV overpotential was around 4 times higher (4 mA/cm2) in comparison with recently developed OER electrocatalysts. The very strong voltage-current behavior of the catalyst shown in OER polarization experiments at both pH 7 and at pH 13 were even superior to those known for IrO2-RuO2. No degradation of the catalyst was detected even when conditions close to standard industrial operations were applied to the catalyst. A stable Ni-, Fe- oxide based passivating layer sufficiently protected the bare metal for further oxidation. Quantitative charge to oxygen- (OER) and charge to hydrogen (HER) conversion was confirmed. High resolution XPS spectra showed that most likely gamma-NiO(OH) and FeO(OH) are the catalytic active OER and NiO is the catalytic active HER species.

physics.chem-ph↗

arXiv2017

Real-time monitoring of the structure of ultra thin Fe$_3$O$_4$ films during growth on Nb-doped SrTiO$_3$(001)

In this work thin magnetite films were deposited on SrTiO$_3$ via reactive molecular beam epitaxy at different substrate temperatures. The growth process was monitored in-situ during deposition by means of x-ray diffraction. While the magnetite film grown at 400$^\circ$C shows a fully relaxed vertical lattice constant already in the early growth stages, the film deposited at 270$^\circ$C exhibits a strong vertical compressive strain and relaxes towards the bulk value with increasing film thickness. Furthermore, a lateral tensile strain was observed under these growth conditions although the inverse behavior is expected due to the lattice mismatch of -7.5%. Additionally, the occupancy of the A and B sublattices of magnetite with tetrahedral and octahedral sites was investigated showing a lower occupancy of the A sites compared to an ideal inverse spinel structure. The occupation of A sites decreases for a higher growth temperature. Thus, we assume a relocation of the iron ions from tetrahedral sites to octahedral vacancies forming a deficient rock salt lattice.

cond-mat.mtrl-sci↗

arXiv2017

Quantitative disentanglement of spin Seebeck, proximity-induced and intrinsic anomalous Nernst effect in NM/FM bilayers

We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe$_2$O$_{\textrm{4-x}}$ ($4\geq x \geq 0$) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect (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$_{\rm x}$ ($x>0$) samples. This is verified by the investigation of static magnetic proximity effects via x-ray resonant magnetic reflectivity.

cond-mat.mtrl-sci↗

arXiv2016

Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited NiFe2O4 and on Fe of Various Thicknesses Investigated by XRMR

The longitudinal spin Seebeck effect is detected in sputter-deposited NiFe2O4 films using Pt as a spin detector and compared to previously investigated NiFe2O4 films prepared by chemical vapor deposition. Anomalous Nernst effects induced by the magnetic proximity effect in Pt can be excluded for the sputter-deposited NiFe2O4 films down to a certain limit, since x-ray resonant magnetic reflectivity measurements show no magnetic response down to a limit of 0.04 μB per Pt atom comparable to the case of the chemicallydeposited NiFe2O4 films. These differently prepared films have various thicknesses. Therefore, we further studied Pt/Fe reference samples with various Fe thicknesses and could confirm that the magnetic proximity effect is only induced by the interface properties of the magnetic material.

cond-mat.mes-hall↗

arXiv2015

Static magnetic proximity effect in Pt/NiFe2O4 and Pt/Fe bilayers investigated by x-ray resonant magnetic reflectivity

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity was measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be $(0.6\pm0.1)\,μ_{B}$ for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of $0.02\,μ_{B}$ per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.

cond-mat.mes-hall↗

arXiv2015

Static magnetic proximity effect in Pt/Ni$_{1-x}$Fe$_x$ bilayers investigated by x-ray resonant magnetic reflectivity

We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive tool to detect proximity induced interface spin polarization in Pt/Fe, Pt/Ni$_{33}$Fe$_{67}$, Pt/Ni$_{81}$Fe$_{19}$ (permalloy), and Pt/Ni bilayers. We demonstrate that a detailed analysis of the reflected x-ray intensity gives insight in the spatial distribution of the spin polarization of a non-magnetic metal across the interface to a ferromagnetic layer. The evaluation of the experimental results with simulations based on optical data from ab initio calculations provides the induced magnetic moment per Pt atom in the spin polarized volume adjacent to the ferromagnet. We find the largest spin polarization in Pt/Fe and a much smaller magnetic proximity effect in Pt/Ni. Additional XRMR experiments with varying photon energy are in good agreement with the theoretical predictions for the energy dependence of the magnetooptic parameters and allow identifying the optical dispersion $δ$ and absorption $β$ across the Pt L3-absorption edge.

cond-mat.mtrl-sci↗

arXiv2015

Sign change in the tunnel magnetoresistance of Fe3O4/MgO/Co-Fe-B magnetic tunnel junctions depending on the annealing temperature and the interface treatment

Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe3O4/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the preparation of the MTJs have been studied regarding their effect on the performance of the MTJs. A maximum TMR of up to -12% could be observed using both argon bombardment and annealing in oxygen atmosphere, despite exposing the magnetite surface to atmospheric conditions before the deposition of the MgO barrier.

cond-mat.mtrl-sci↗