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Author: Nielsch, Kornelius × Start date: 2020 × End date: 2019 × Start date: 2014 × Type: Text × WGL Institute: IFWD ×

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Now showing 1 - 4 of 4
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    Temperature gradient-induced magnetization reversal of single ferromagnetic nanowires
    (Bristol : IOP Publ., 2017-11-17) Michel, Ann-Kathrin; Niemann, Anna Corinna; Boehnert, Tim; Martens, Stephan; Moreno, Josep M. Montero; Goerlitz, Detlef; Zierold, Robert; Reith, Heiko; Vega, Victor; Prida, Victor M.; Thomas, Andy; Gooth, Johannes; Nielsch, Kornelius
    In this study, we investigate the temperature- and temperature gradient-dependent magnetization reversal process of individual, single-domain Co39Ni61 and Fe15Ni85 ferromagnetic nanowires via the magneto-optical Kerr effect and magnetoresistance measurements. While the coercive fields (HC) and therefore the magnetic switching fields (HSW) generally decrease under isothermal conditions at elevated base temperatures (Tbase), temperature gradients (ΔT) along the nanowires lead to an increased switching field of up to 15% for ΔT  = 300 K in Co39Ni61 nanowires. This enhancement is attributed to a stress-induced, magneto-elastic anisotropy term due to an applied temperature gradient along the nanowire that counteracts the thermally assisted magnetization reversal process. Our results demonstrate that a careful distinction between locally elevated temperatures and temperature gradients has to be made in future heat-assisted magnetic recording devices.
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    Face Centred Cubic Multi-Component Equiatomic Solid Solutions in the Au-Cu-Ni-Pd-Pt System
    (Basel : MDPI, 2017-4-12) Freudenberger, Jens; Rafaja, David; Geissler, David; Giebeler, Lars; Ullrich, Christiane; Kauffmann, Alexander; Heilmaier, Martin; Nielsch, Kornelius
    A single-phase solid solution is observed in quaternary and quinary alloys obtained from gold, copper, nickel, palladium and platinum. The lattice parameters of the alloys follow the linear rule of mixture when considering the lattice parameters of the elements and their concentration. The elements are a priori not homogeneously distributed within the respective alloys resulting in segregations. These segregations cause a large broadening of X-ray lines, which is accessed in the present article. This correlation is visualized by the help of local element mappings utilizing scanning electron microscopy including energy dispersive X-ray analysis and their quantitative analysis.
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    Polyethenetetrathiolate or polytetrathiooxalate? Improved synthesis, a comparative analysis of a prominent thermoelectric polymer and implications to the charge transport mechanism
    (Cambridge : RSC Publ., 2018) Tkachov, Roman; Stepien, Lukas; Grafe, Robert; Guskova, Olga; Kiriy, Anton; Simon, Frank; Reith, Heiko; Nielsch, Kornelius; Schierning, Gabi; Kasinathan, Deepa; Leyens, Christoph
    1,1,2,2-Ethenetetrathiolate (ett4-) coordination polymers, such as poly[Kx(Ni-ett)], have been known for decades for their excellent thermoelectric properties. However in reality, ett4- is neither a "true" comonomer which participates in the polymerization, nor represents a "true" repeat unit of the target polymer. Indeed, poly[K2(Ni-ett)], which is formally the product of Ni-induced polymerization of ett4-, has a poor conductivity and needs to be oxidized to show attractive thermoelectric characteristics. The polymerization and oxidation processes are poorly controllable which causes irreproducibility of the polymer properties. To improve the synthesis reproducibility, we studied polymerization of potassium tetrathiooxalate (K2tto), the convenient synthesis of which was developed in our recent work. Because K2tto is the "true monomer", and not its precursor, a high quality product is reproducibly formed simply by mixing K2tto with NiCl2 at room temperature. The procedure does not require additional components (bases), or special conditions (prolonged heating), which are usually needed for the preparation of this polymer from the monomer precursor 1,3,4,6-tetrathiapentalene-2,5-dione (TPD). Furthermore, as tto2- is formally the product of two-electron oxidation of ett4-, the poorly controllable oxidation process is avoided and poly[Ni-tto] almost free from K is directly formed upon the complexation of Ni2+ and tto2-. Thus-obtained poly[Ni-tto] possesses conductivity in the range of 27-47 S cm-1 and a Seebeck coefficient in the range of -38 to -55 μV K-1, which are superior thermoelectric properties compared to poly[Kx(Ni-ett)] samples obtained by the previously reported methods. Redox and structural properties of poly[Ni-tto] were compared with those of poly[Kx(Ni-ett)] obtained by the reported methods. Furthermore, DFT calculations were performed to shed more light on generally promising properties of this class of materials. Particularly, possible packing models have been predicted for polymers, and the molecular dynamics simulations have been used to simulate the molecular arrangements under ambient conditions.
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    Transition to the quantum hall regime in InAs nanowire cross-junctions
    (Bristol : IOP Publ., 2019) Gooth, Johannes; Borg, Mattias; Schmid, Heinz; Bologna, Nicolas; Rossell, Marta D.; Wirths, Stephan; Moselund, Kirsten; Nielsch, Kornelius; Riel, Heike
    We present a low-temperature electrical transport study on four-terminal ballistic InAs nanowire cross-junctions in magnetic fields aligned perpendicular to the cross-plane. Two-terminal longitudinal conductance measurements between opposing contact terminals reveal typical 1D conductance quantization at zero magnetic field. As the magnetic field is applied, the 1D bands evolve into hybrid magneto-electric sub-levels that eventually transform into Landau levels for the widest nanowire devices investigated (width = 100 nm). Hall measurements in a four-terminal configuration on these devices show plateaus in the transverse Hall resistance at high magnetic fields that scale with (ve 2 /h) -1 . e is the elementary charge, h denotes Planck's constant and v is an integer that coincides with the Landau level index determined from the longitudinal conductance measurements. While the 1D conductance quantization in zero magnetic field is fragile against disorder at the NW surface, the plateaus in the Hall resistance at high fields remain robust as expected for a topologically protected Quantum Hall phase. © 2019 IOP Publishing Ltd.