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Ta/MnN/CoFeB systems show high exchange bias of about 1800 Oe at room temperature; however, their thermal stability is limited by nitrogen diffusion that occurs during annealing processes [Quarterman et al., Phys. Rev. Mater. 3, 064413 (2019) and Dunz et al., AIP Adv. 8, 056304 (2018)]. In this study, we investigate the consequences of nitrogen diffusion in Ta/MnN/CoFeB exchange bias stacks in dependence on the Ta buffer layer thickness. Furthermore, we test the effects of introducing a TaN x layer between MnN and Ta as a diffusion barrier. Our findings show that the Ta buffer layer plays a decisive role in determining the exchange bias in the Ta/MnN/CoFeB system. It acts as a crystallographic seed layer for better growth of MnN and as a nitrogen sink during the annealing process. We show that both of these functions are crucial for the outcome of high exchange bias. Additionally, our results reveal that the measures decreasing nitrogen diffusion, even though being beneficial in terms of thermal stability, mostly lead to decreased crystallinity and thus weaker exchange bias. [ABSTRACT FROM AUTHOR]

We fabricated NiFe2O4 thin films on MgAl2O4 (001) substrates by reactive dc magnetron co-sputtering in a pure oxygen atmosphere at different substrate temperatures. The film properties were investigated by various techniques with a focus on their structure, surface topography, magnetic characteristics, and transport properties. Structural analysis revealed a good crystallization with epitaxial growth and low roughness and a similar quality as in films grown by pulsed laser deposition. Electrical conductivity measurements showed high room temperature resistivity (12Ωm), but low activation energy, indicating an extrinsic transport mechanism. A band gap of about 1.55 eV was found by optical spectroscopy. Detailed x-ray spectroscopy studies confirmed the samples to be ferrimagnetic with fully compensated Fe moments. By comparison with multiplet calculations of the spectra, we found that the cation valencies are to a large extent Ni2+ and Fe3+. [ABSTRACT FROM AUTHOR]

By submonolayer insertion of Au, Pt, or Pd into Ta/CoFeB/MgO/Ta heterostructures, we tune the perpendicular magnetic anisotropy and the coercive field of the ferromagnetic layer. We demonstrate that this has a major influence on the spin Hall switching current density and its dependence on the external magnetic field. Despite a rather small effective spin Hall angle of θSH ≈-0:07, we obtain switching current densities as low as 2×1010 A/m2 with a 2Å Au interlayer. We find that the Dzyaloshinskii-Moriya interaction parameter D is reduced with Au or Pd interlayers, and the perpendicular anisotropy field is reduced by an order of magnitude with the Pd interlayer. The dependence of the switching current density on the current pulse width is quantitatively explained with a domain wall nucleation and propagation model. Interface engineering is thus found to be a suitable route to tailor the current-induced magnetization switching properties of magnetic heterostructures. [ABSTRACT FROM AUTHOR]

The use of diesel-driven traction is an intrinsic part of the functioning of railway systems and it is expected to continue being so for the foreseeable future. The recent introduction of more restrictive greenhouse gas emission levels and other legislation aiming at the improvement of the environmental performance of railway systems has led to the need of exploring alternatives for cleaner diesel rolling stock. This paper focuses on assessing energy storage systems and the design of hybrid system architectures to determine their potential use in specific diesel-driven rail duty cycles. Hydrostatic accumulators, flywheels, Lithium-ion batteries and double-layer capacitors have been assessed and used to design hybrid system architectures. The potential of the different technology combinations has been analyzed using standardized duty cycles enhanced with gradient profiles related to suburban, regional and shunting operations. The results show that double-layer capacitors and Lithium-ion batteries have the highest potential to be successfully integrated into the system architecture of diesel-driven rail vehicles. Furthermore, the results also suggest that combining these two energy storage technologies into a single hybridisation package is a highly promising design that draws on their strengthens without any significant drawbacks. [ABSTRACT FROM AUTHOR]

Hybridisation of diesel-driven rail vehicles is already possible and promises significant reductions in fuel consumption and exhaust emissions (nitrogen oxides and particulate matter). However, current performance can be significantly improved by introducing technologies such as energy storage systems. The aim of this paper is to investigate the energy management strategies and technical life cycle cost aspects of railway-specific hybridisation architectures. To do so, twelve promising combinations of system architectures, energy storage systems and railway duty cycles have been selected, described and analyzed. A method based on a MATLAB/SIMULINK model has been used with three different approaches for the energy management strategy optimization (i) the Covariance Matrix Adaptation Evolution Strategy as baseline optimization; (ii) dynamic programming to determine the global optimum and (iii) a sensitivity analysis approach for the energy storage systems. Based on these boundary conditions, a hybridisation life cycle cost analysis has been carried out. As part of this analysis three different fuel price scenarios have been considered (i) low price of 0.78 €/l; (ii) medium price of 1.12 €/l; (iii) high price of 1.64 €/l. The results show that different energy storage technologies achieve diverse life cycle costs, triggering a range of investment, replacement and maintenance costs, as well as significantly reduced operational costs due to lower fuel consumption. In addition, the simulation results show that fuel consumption can be reduced up to 20%, in some cases rising to approximately 25% if energy management strategies are applied. Of these, Start/Stop and downsizing of the internal combustion engine are the most promising strategies for the emission reduction. Both require auxiliaries’ electrification and appropriate integration of energy storage systems. LCC results also indicate that operational conditions with high annual mileage are most beneficial when a hybrid consisting of an internal combustion engine and an energy storage system is used in a high fuel price scenario. [ABSTRACT FROM AUTHOR]

We have investigated the influence of doping the antiferromagnetic MnN component in the polycrystalline MnN/CoFe exchange bias system, which shows high exchange bias of up to 1800 Oe at room temperature. The thermal stability of this system is limited by nitrogen diffusion that occurs during annealing. In order to predict improved thermal stability, defect energies of potential dopant elements which substitute for Mn were calculated via density functional theory. Elements calculated to have negative defect energies will bind nitrogen more strongly to the lattice, limiting diffusion. We prepared exchange bias stacks with dopant concentrations of a few percent by (reactive) co-sputtering, testing dopant elements with defect energies ranging from highly negative to slightly positive. We show that doping with elements calculated to have negative defect energies indeed improves the thermal stability. Y-doped MnN layers with dopant concentrations below 2% result in exchange bias fields higher than 1000 Oe for annealing temperatures up to 485 ° C. [ABSTRACT FROM AUTHOR]

Magnetron sputtered thin Co–Mn–Sb films were investigated with respect to their element-specific magnetic properties. Stochiometric Co1Mn1Sb1 crystallized in the C1b structure has been predicted to be half-metallic and is therefore of interest for spintronic applications. It should show a characteristic antiferromagnetic coupling of the Mn and Co magnetic moments and a transition temperature TC of about 480 K. Although the observed transition temperature of our 20 nm thick Co32.4Mn33.7Sb33.8, Co37.7Mn34.1Sb28.2, and Co43.2Mn32.6Sb24.2 films is in quite good agreement with the expected value, we found a ferromagnetic coupling of the Mn and Co magnetic moments which indicates that the films do not crystallize in the C1b structure and are probably not fully spin polarized. The ratio of the Co and Mn moments does not change up to the transition temperature and the temperature dependence of the magnetic moments can be well described by the mean-field theory. [ABSTRACT FROM AUTHOR]

We report an exchange bias of more than 2700 Oe at room temperature in MnN/CoFe bilayers after high-temperature annealing. We studied the dependence of exchange bias on the annealing temperature for different MnN thicknesses in detail and found that samples with tMnN > 32nm show an increase of exchange bias for annealing temperatures higher than TA = 400 °C. Maximum exchange bias values exceeding 2000 Oe with reasonably small coercive fields around 600 Oe are achieved for tMnN = 42, 48 nm. The median blocking temperature of those systems is determined to be 180 °C after initial annealing at TA = 525 °C. X-ray diffraction measurements and Auger depth profiling show that the large increase of exchange bias after high-temperature annealing is accompanied by strong nitrogen diffusion into the Ta buffer layer of the stacks. [ABSTRACT FROM AUTHOR]

We investigate the influence of the film thickness on the martensitic transformation for the example of Ni-Mn-Sn thin films. Epitaxial films with thicknesses ranging from 100 nm down to 10 nm were deposited on MgO by co-sputtering on heated substrates. The martensitic transformation is investigated using temperature dependent x-ray diffraction, magnetization and resistivity measurements. X-ray diffraction and transmission electron microscopy is used to study the growth and the martensitic structure of the films. We find that the martensitic transformation temperatures reduce and the transformation range increases with decreasing film thickness. We show that the transformation is still possible down to a film thickness of 10 nm. A systematic smdy on the resistance change caused by the martensitic Uransformation implies that the ti-ansformation is suppressed close to the interfaces to the MgO. [ABSTRACT FROM AUTHOR]

Highlights • MnN shows potential as a novel antiferromagnet for the replacement of IrMn. • The anisotropy constant of MnN is comparable to that of IrMn. • Strategies are suggested for the reduction in the MnN layer thickness. Abstract At the present time there is a requirement to identify new antiferromagnetic alloys or compounds which might be suitable for the production of exchange bias systems. The phenomenon of exchange bias remains crucial for the operation of all read heads in hard disk drives and also has potential for use in magnetic random access memory (MRAM) systems. There is also an increasing interest in the use of antiferromagnets themselves in spintronic devices. Generally for applications the alloy IrMn is used, however given that Iridium is one of the rarest, and therefore most expensive elements on Earth, there is a search for alternative materials. In this paper we report on a study of the compound MnN in terms of its thermal stability. We have produced polycrystalline films of this compound with sub 10 nm grains and examined the thermal stability in layers of thicknesses of up to 30 nm. Using thermal activation studies we have determined a room temperature value of the anisotropy constant of this compound in a tetragonal structure of up to (6.3 ± 0.3) × 106 erg/cm3. The antiferromagnetic grains can be aligned by thermal annealing at an optimum temperature of 380 K. Above this temperature the magnetic properties deteriorate possibly due to nitrogen desorption. [ABSTRACT FROM AUTHOR]

Magneto-optical and electronic properties of the Co2FeSi Heusler compound were studied by polar Kerr magneto-optical spectroscopy and ab-initio calculations. The thin-film samples were grown by dc/rf magnetron co-sputtering on MgO(100) substrates. A Cr seed layer was deposited prior to the Co2FeSi layer to achieve its epitaxial growth. The magneto-optical spectroscopy was carried out using generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.4 to 5.5 eV with an applied magnetic field of up to 1.2T. The polar Kerr spectra showed a smooth spectral behavior up to 5.5 eV indicating nearly free charge carriers. Experimental data were compared with ab-initio calculations based on density functional theory employing the full-potential linearized augmented plane wave method. [ABSTRACT FROM AUTHOR]

We investigated an out-of-plane exchange bias system that is based on the antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta/MnN/CoFeB/MgO/Ta were grown on SiOx by (reactive) magnetron sputtering and studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of the exchange bias and the perpendicular magnetic anisotropy were observed as functions of both film thicknesses and field cooling temperatures. In optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a coercive field of 350 Oe were observed at room temperature. The effective interfacial exchange energy is estimated to be Jeff = 0.24 mJ/m² and the effective uniaxial anisotropy constant of the antiferromagnet is Keff = 24 kJ/m³. The maximum effective perpendicular anisotropy field of the CoFeB layer is Hani = 3400 Oe. These values are larger than any previously reported values. These results possibly open a route to magnetically stable, exchange biased perpendicularly magnetized spin valves. [ABSTRACT FROM AUTHOR]

We investigated an out-of-plane exchange bias system that is based on the antiferromagnet MnN. Polycrystalline, highly textured film stacks of Ta/MnN/CoFeB/MgO/Ta were grown on SiOx by (reactive) magnetron sputtering and studied by x-ray diffraction and Kerr magnetometry. Nontrivial modifications of the exchange bias and the perpendicular magnetic anisotropy were observed as functions of both film thicknesses and field cooling temperatures. In optimized film stacks, a giant perpendicular exchange bias of 3600 Oe and a coercive field of 350 Oe were observed at room temperature. The effective interfacial exchange energy is estimated to be Jeff = 0.24 mJ/m² and the effective uniaxial anisotropy constant of the antiferromagnet is Keff = 24 kJ/m³. The maximum effective perpendicular anisotropy field of the CoFeB layer is Hani = 3400 Oe. These values are larger than any previously reported values. These results possibly open a route to magnetically stable, exchange biased perpendicularly magnetized spin valves. [ABSTRACT FROM AUTHOR]

We investigated the temperature dependence of the switching current for a perpendicularly magnetized CoFeB film deposited on a nanocrystalline tungsten film with large oxygen content: nc-W(O). The effective spin Hall angle |ΘSHeff| ≈ 0.22 is independent of temperature, whereas the switching current increases strongly at low temperature. The increase indicates that the current induced switching itself is thermally activated, in agreement with a recent theoretical prediction. The dependence of the switching current on the in-plane assist field suggests the presence of an interfacial Dzyaloshinskii--Moriya interaction with D≈0.23 mJ/m², intermediate between the Pt/ CoFe and Ta/CoFe systems. We show that the nc-W(O) is insensitive to annealing, which makes this system a good choice for the integration into magnetic memory or logic devices that require a high-temperature annealing process during fabrication. [ABSTRACT FROM AUTHOR]

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 is 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±0.1) μB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02 μB 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. [ABSTRACT FROM AUTHOR]

Superconducting transformers are an important innovation for future power transmission and transportation systems. Powerful, lightweight, energy-saving and environmentally friendly they offer enormous benefits compared to their conventional counterparts. Siemens is developing a 1-MVA demonstrator transformer for laboratory testing, exhibiting innovative features like horizontal design, cabled-conductor windings and a closed cooling cycle with sub-cooled nitrogen. Being one of the most promising applications Siemens has started a programme towards the development of on-board transformers for electrical rail vehicles. This paper summarises world-wide efforts in the development of superconducting transformers and reports on the progress achieved at Siemens. [Copyright &y& Elsevier]

We present x-ray resonant magnetic reflectivity (XRMR) as a very sensitive tool to detect proximity induced interface spin polarization in Pt/FM heterostructures. Different XRMR experiments are carried out and the results are evaluated for their dependence on the magneto-optical depth profile, the photon energy, the optical parameters, and the ferromagnetic material. We demonstrate that a detailed analysis of the reflected x-ray intensity gives insight into the spatial distribution of the spin polarization of a nonmagnetic 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. For a series with different ferromagnetic materials consisting of Pt/Fe, Pt/Ni33Fe67, Pt/Ni81Fe19 (permalloy), and Pt/Ni bilayers 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 magneto-optical parameters and allow identifying the optical dispersion δ and absorption β across the PtL3-absorption edge. [ABSTRACT FROM AUTHOR]