S Krzyk

Universität Konstanz, Constance, Baden-Württemberg, Germany

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Publications (18)85.09 Total impact

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    ABSTRACT: 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.
    Physical Review Letters 02/2013; 110(6):067203. DOI:10.1103/PhysRevLett.110.067203 · 7.51 Impact Factor
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    ABSTRACT: 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.
    Physical review. B, Condensed matter 10/2010; 82(13-13). DOI:10.1103/PhysRevB.82.134447 · 3.66 Impact Factor
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    ABSTRACT: 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 6×10{-14}  T/(A/m{2}), 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.
    Physical Review Letters 08/2010; 105(7):076601. DOI:10.1103/PhysRevLett.105.076601 · 7.51 Impact Factor
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    ABSTRACT: 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 quasi-particle in a 1D potential landscape. By injecting small currents, the potential is modified, allowing for the determination of the non-adiabatic spin torque: the non-adiabatic coefficient is 0.010 +/- 0.004 for a transverse DW and 0.073 +/- 0.026 for a vortex DW. The larger value is attributed to the higher magnetization gradients present.
    Physical Review Letters 07/2010; 105(5):056601. DOI:10.1103/PhysRevLett.105.056601 · 7.51 Impact Factor
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    ABSTRACT: 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.
    Applied Physics Letters 04/2010; 96(15). DOI:10.1063/1.3373590 · 3.52 Impact Factor
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    ABSTRACT: 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.
    Journal of Physics D Applied Physics 02/2010; DOI:10.1088/0022-3727/43/4/045003 · 2.72 Impact Factor
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    ABSTRACT: 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.
    Physical Review Letters 02/2010; 104(6):067201. DOI:10.1103/PhysRevLett.104.067201 · 7.51 Impact Factor
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    ABSTRACT: 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.
    Applied Physics Letters 01/2010; 96(3):032504-032504-3. DOI:10.1063/1.3291067 · 3.52 Impact Factor
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    ABSTRACT: 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 80 K in UHV. Applying in-plane magnetic vector fields up to 100 mT, 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.
    New Journal of Physics 01/2010; 12(1):013001. DOI:10.1088/1367-2630/12/1/013001 · 3.67 Impact Factor
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    ABSTRACT: 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 jc∝1/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.
    Physical Review B 11/2009; 80(18):184405. DOI:10.1103/PhysRevB.80.184405 · 3.74 Impact Factor
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    ABSTRACT: Herein, different concepts for domain wall propagation based on currents and fields that could potentially be used in magnetic data storage devices based on domains and domain walls are reviewed. By direct imaging, we show that vortex and transverse walls can be displaced using currents due to the spin transfer torque effect. For the case of field-induced wall motion, particular attention is paid to the influence of localized fields and local heating on the depinning and propagation of domain walls. Using an Au nanowire adjacent to a permalloy structure with a domain wall, the depinning field of the wall, when current pulses are injected into the Au nanowire, was studied. The current pulse drastically modified the depinning field, which depended on the interplay between the externally applied field direction and polarity of the current, leading subsequently to an Oersted field and heating of the permalloy at the interface with the Au wire. Placing the domain wall at various distances from the Au wire and studying different wall propagation directions, the range of Joule heating and Oersted field was determined; both effects could be separated. Approaches beyond conventional field- and current-induced wall displacement are briefly discussed.
    Journal of Magnetics 06/2009; 14(2). DOI:10.4283/JMAG.2009.14.2.053 · 0.32 Impact Factor
  • D Bedau · M Kläui · M T Hua · S Krzyk · U Rüdiger · G Faini · L Vila
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    ABSTRACT: Using microwave currents, we excite resonances of geometrically confined pinned domain walls, detecting the resonance by the rectification of the microwave current. By applying magnetic fields, the resonance frequency of the domain wall oscillator can be tuned over a wide range. Increasing the power leads to a redshift due to the nonlinearity of the system. From this frequency shift, we directly deduce the quantitative shape of the potential, so that a complete characterization of the pinning potential is obtained.
    Physical Review Letters 01/2009; 101(25):256602. DOI:10.1103/PhysRevLett.101.256602 · 7.51 Impact Factor
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    ABSTRACT: Using low temperature magnetoresistance measurements, the possibility to selectively move a domain wall locally by applying current pulses through a Au nanowire adjacent to a permalloy element is studied. We find that the domain wall depinning field is drastically modified with increasing current density due to the Joule heating and the Oersted field of the current, and controlled motion due to the Oersted field without any externally applied fields is achieved. By placing the domain wall at various distances from the Au wire, we determine the range of the Joule heating and the Oersted field and both effects can be separated.
    Applied Physics Letters 10/2008; 93(13-93):132503 - 132503-3. DOI:10.1063/1.2990629 · 3.52 Impact Factor
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    ABSTRACT: Details are presented of a single shot focused magneto-optic Kerr effect (MOKE) magnetometer which is used to capture the movement of single domain walls (DWs) in permalloy (Ni80 Fe20) nanowires (≥400 nm width and ≥20 nm thickness) in real time. By probing the DW motion within the 1 µm diameter laser spot of the instrument, DW velocity and pinning field distributions were obtained. An external field was ramped up linearly, and depinning of a DW from the same start position was observed at three different fields, indicating the stochastic nature of the DW motion.
    Journal of Physics D Applied Physics 07/2008; 41(16):164009. DOI:10.1088/0022-3727/41/16/164009 · 2.72 Impact Factor
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    ABSTRACT: In this paper, we present a complete three-dimensional characterization of vortex core spin structures, which is important for future magnetic data storage based on vortex cores in disks and in wires. Using electron holography to examine vortices in patterned Permalloy devices we have quantitatively measured the in-plane and out-of-plane magnetization of a vortex core. Observed core widths and integrated phase shifts agree well with those derived from micromagnetic simulations. (c) 2008 American Institute of Physics.
    Applied Physics Letters 03/2008; 92(11):112502-112502-3. DOI:10.1063/1.2829601 · 3.52 Impact Factor
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    ABSTRACT: By direct imaging we determine spin structure changes in Permalloy wires and disks due to spin transfer torque as well as the critical current densities for different domain wall types. Periodic domain wall transformations from transverse to vortex walls and vice versa are observed, and the transformation mechanism occurs by vortex core displacement perpendicular to the wire. The results imply that the nonadiabaticity parameter beta does not equal the damping alpha, in agreement with recent theoretical predictions. The vortex core motion perpendicular to the current is further studied in disks revealing that the displacement in opposite directions can be attributed to different polarities of the vortex core.
    Physical Review Letters 03/2008; 100(6):066603. DOI:10.1103/PhysRevLett.100.066603 · 7.51 Impact Factor
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    ABSTRACT: We report the direct transmission electron microscopy observation of spin structure transformations in nanoscale Permalloy zigzag wires due to Joule heating during the injection of current pulses. This heating is sufficient to overcome the energy barriers separating the different metastable domain wall spin structures. Due to the large energy barriers these are stable and observable at room temperature by off-axis electron holography and Fresnel imaging. The interaction between different domain walls is probed and the main pinning mechanism is determined to be the edge roughness. In addition to transformations, we also report on thermally assisted domain wall hopping between two pinning sites and structural changes that occur when the samples are subjected to even higher current pulses. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    Physica Status Solidi (A) Applications and Materials 12/2007; 204(12):3922 - 3928. DOI:10.1002/pssa.200777193 · 1.62 Impact Factor
  • D Bedau · M Kläui · S Krzyk · U Rüdiger · G Faini · L Vila
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    ABSTRACT: Magnetic domain walls are found to exhibit quasiparticle behavior when subjected to geometrical variations. Because of the spin torque effect such a quasiparticle in a potential well is excited by an ac current leading to a dip in the depinning field at resonance for current densities as low as 2 x 10(10) A/m2. Independently the resonance frequencies of transverse walls and vortex walls are determined from the dc voltage that develops due to a rectifying effect of the resonant domain wall oscillation. The dependence on the injected current density reveals a strongly nonharmonic oscillation.
    Physical Review Letters 11/2007; 99(14):146601. DOI:10.1103/PhysRevLett.99.146601 · 7.51 Impact Factor