Krzyk, Stephen

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Krzyk
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Stephen
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Domain-Wall Induced Large Magnetoresistance Effects at Zero Applied Field in Ballistic Nanocontacts

2013-02-08, von Bieren, Arndt, Patra, Ajit K., Krzyk, Stephen, Rhensius, Jan, Reeve, Robert M., Heyderman, Laura J., Hoffmann-Vogel, Regina, Kläui, Mathias

We determine magnetoresistance effects in stable and clean Permalloy nanocontacts of variable cross section, fabricated by UHV deposition and in situ electromigration. To ascertain the magnetoresistance (MR) effects originating from a magnetic domain wall, we measure the resistance values with and without such a wall at zero applied field. In the ballistic transport regime, the MR ratio reaches up to 50% and exhibits a previously unobserved sign change. Our results can be reproduced by recent atomistic calculations for different atomic configurations of the nanocontact, highlighting the importance of the detailed atomic arrangement for the MR effect.

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Magnetotransport effects of ultrathin Ni80Fe20 films probed in situ

2010, Krzyk, Stephen, Schmidsfeld, Alexander von, Kläui, Mathias, Rüdiger, Ulrich

We investigated the magnetoresistance of Permalloy (Ni80Fe20) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al2O3, MgO and SiO2 substrates. Growth and transport measurements were carried out at 80K in UHV. Applying in-plane magnetic vector fields up to 100mT, the magnetotransport properties were ascertained during growth. With increasing thickness the films exhibited a gradual transition from tunnelling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains, as well as the film s transition from superparamagnetic to ferromagnetic behaviour. Using an analysis based on a theoretical model of island growth, we found that the observed evolution of the magnetoresistance in the tunnelling regime originated from changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunnelling magnetoresistance and anisotropic magnetoresistance were found. We attributed this to an increasingly pronounced island growth, and to a slower percolation process of Permalloy when comparing growth on SiO2, MgO and Al2O3 substrates. The different growth characteristics resulted in a markedly earlier onset of both tunnelling magnetoresistance and anisotropic magnetoresistance for SiO2. For Al2O3 in particular the growth mode results in a structure of the film containing two different contributions to ferromagnetism, which lead to two distinct coercive fields in the high thickness regime.

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Direct observation of high velocity current induced domain wall motion

2010, Heyne, Lutz, Rhensius, Jan, Bisig, André, Krzyk, Stephen, Punke, P., Kläui, Mathias, Heyderman, Laura Jane, Le Guyader, Loic, Nolting, Frithjof

We study fast vortex wall propagation in Permalloy wires induced by 3 ns short current pulses with sub 100 ps rise time using high resolution magnetic imaging at zero field.We find a constant domain wall displacement after each current pulse as well as current induced domain wall structure changes, even at these very short timescales. The domain wall velocities are found to be above 100 m/s and independent of the domain wall spin structure. Comparison to experiments with longer pulses points to the pulse shape as the origin of the high velocities.

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Geometry-dependent scaling of critical current densities for current-induced domain wall motion and transformations

2009, Heyne, Lutz, Rhensius, Jan, Cho, Y.-J., Bedau, Daniel, Krzyk, Stephen, Dette, Christian, Körner, H. S., Fischer, J., Laufenberg, Markus, Backes, Dirk, Heyderman, Laura Jane, Joly, Loic, Nolting, Frithjof, Tatara, Gen, Kohno, Hiroshi, Seo, Sunae, Rüdiger, Ulrich, Kläui, Mathias

In a combined theoretical and experimental study, we investigate the critical current densities for vortex domain walls in magnetic nanowires. We systematically determine the critical current densities for continuous motion of vortex walls as a function of the wire width for different wire thicknesses and we find that the critical current density increases monotonously with decreasing wire width. Theoretically we present a mechanism that predicts a threshold current density based on wall transformations and this leads to a scaling of the critical current density jc1/width. The origin of this scaling is found to be the different dependence of the spin torque energy and the vortex nucleation energy on the wire width and good agreement with the experimental observations is found.

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Imaging of Domain Wall Inertia in Permalloy Half-Ring Nanowires by Time-Resolved Photoemission Electron Microscopy

2010, Rhensius, Jan, Heyne, Lutz, Backes, Dirk, Krzyk, Stephen, Heyderman, Laura Jane, Joly, L., Nolting, Frithjof, Kläui, Mathias

Using photoemission electron microscopy, we image the dynamics of a field pulse excited domain wall in a Permalloy nanowire. We find a delay in the onset of the wall motion with respect to the excitation and an oscillatory relaxation of the domain wall back to its equilibrium position, defined by an external magnetic field. The origin of both of these inertia effects is the transfer of energy between energy reservoirs. By imaging the distribution of the exchange energy in the wall spin structure, we determine these reservoirs, which are the basis of the domain wall mass concept.

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Domain-Wall Depinning Assisted by Pure Spin Currents

2010, Ilgaz, Dennis, Nievendick, Jan, Heyne, Lutz, Backes, Dirk, Rhensius, Jan, Moore, Thomas A., Niño, Miguel Ángel, Locatelli, Andrea, Mentes, Tevfik Onur, Schmidsfeld, Alexander von, Bieren, Arndt von, Krzyk, Stephen, Heyderman, Laura Jane, Kläui, Mathias

We study the depinning of domain walls by pure diffusive spin currents in a nonlocal spin valve structure based on two ferromagnetic Permalloy elements with copper as the nonmagnetic spin conduit. The injected spin current is absorbed by the second Permalloy structure with a domain wall, and from the dependence of the wall depinning field on the spin current density we find an efficiency of 6 x 10 -14 T/(A/m²), which is more than an order of magnitude larger than for conventional current induced domain-wall motion. Theoretically we find that this high efficiency arises from the surface torques exerted by the absorbed spin current that lead to efficient depinning.

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Magnetoresistance measurement of tailored Permalloy nanocontacts

2010, Patra, Ajit, Bieren, Arndt von, Krzyk, Stephen, Rhensius, Jan, Heyderman, Laura, Hoffmann, Regina, Kläui, Mathias

We study the evolution of the magnetoresistance (MR) in Permalloy nanocontacts prepared by controlled low-temperature UHV electromigration in nanoring segment structures with constrictions. The ring geometry allows for the controlled and reproducible positioning of a domain wall in the nanocontacts. We observe three different resistance levels, corresponding to distinct domain-wall positions. A change in the sign of the MR difference, between a domain wall at the constriction and a domain wall next to the constriction, occurs with decreasing constriction width. This is in line with our micromagnetic simulations, where the MR is calculated based on the anisotropic MR (AMR) effect.

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Nonadiabatic Spin Torque Investigated Using Thermally Activated Magnetic Domain Wall Dynamics

2010, Eltschka, Matthias, Wötzel, Mathias, Rhensius, Jan, Krzyk, Stephen, Nowak, Ulrich, Kläui, Mathias, Kasama, Takeshi, Dunin-Borkowski, Rafal E., Heyderman, Laura J., Driel, Hedwig J. van, Duine, Rembert A.

Using transmission electron microscopy, we investigate the thermally activated motion of domain walls (DWs) between two positions in Permalloy (Ni80Fe20) nanowires at room temperature. We show that this purely thermal motion is well described by an Arrhenius law, allowing for a description of the DW as a quasiparticle in a one-dimensional potential landscape. By injecting small currents, the potential is modified, allowing for the determination of the nonadiabatic spin torque: ßt=0.010±0.004 for a transverse DW and ßv=0.073±0.026 for a vortex DW. The larger value is attributed to the higher magnetization gradients present.

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Current-induced domain wall motion in Ni80Fe20 nanowires with low depinning fields

2010, Malinowski, Gregory, Lörincz, Andreas, Krzyk, Stephen, Möhrke, Philipp, Bedau, Daniel, Boulle, Olivier, Rhensius, Jan, Heyderman, Laura Jane, Cho, Young Jin, Seo, Sunae, Kläui, Mathias

In this paper, we report on domain wall (DW) motion induced by current pulses at variable temperature in 900 nm wide and 25 nm thick Ni80Fe20 wires with low pinning fields. By using Ar ion milling to pattern our wires rather than the conventional lift-off technique, a depinning field as low as ∼2 3 Oe at room temperature is obtained. Comparison with previous results acquired on similar wires with much higher pinning shows that the critical current density scales with the depinning field, leading to a critical current density of ∼2.5 × 1011Am−2 at 250 K. Moreover, when a current pulse with a current density larger than the critical current density is injected, the DW is not necessarily depinned but it can undergo a modification of its spin structure which hinders current-induced DW motion. Hence, reliable propagation of the DW requires an accurate adjustment of the pulsed current density.

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Direct imaging of current induced magnetic vortex gyration in an asymmetric potential well

2010, Bisig, André, Rhensius, Jan, Kammerer, Matthias, Curcic, Michael, Stoll, Hermann, Schütz, Gisela, Wayenberge, Bartel van, Chou, Kang Wei, Tyliszczak, Tolek, Heydermann, Laura Jane, Krzyk, Stephen, Bieren, Arndt von, Kläui, Mathias

Employing time-resolved x-ray microscopy, we investigate the dynamics of a pinned magnetic vortex domain wall in a magnetic nanowire. The gyrotropic motion of the vortex core is imaged in response to an exciting ac current. The elliptical vortex core trajectory at resonance reveals asymmetries in the local potential well that are correlated with the pinning geometry. Using the analytical model of a two-dimensional harmonic oscillator, we determine the resonance frequency of the vortex core gyration and, from the eccentricity of the vortex core trajectory at resonance, we can deduce the stiffness of the local potential well.