References of "Michels, Andreas 50002669"
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See detailThe influence of spin-misalignment scattering on the SANS data evaluation of martensitic age-hardening steels
Bischof, M.; Staron, P.; Michels, Andreas UL et al

in Acta Materialia (2007), 55(8), 2637-2646

Small-angle neutron scattering has proved to be a valuable technique for probing precipitates in steels. The investigated sample is thereby exposed to a large magnetic field and is assumed to consist of a ... [more ▼]

Small-angle neutron scattering has proved to be a valuable technique for probing precipitates in steels. The investigated sample is thereby exposed to a large magnetic field and is assumed to consist of a single domain with all the magnetic moments perfectly aligned with the external field. In this situation the dominating magnetic scattering contrast is expected to stem from the differences between the magnetization values of matrix and particles. However, the present work shows that strong additional scattering can be present which is due to spin misalignment. The effects of this spin-misalignment scattering are discussed as being exemplary for differently heat-treated martensitic steel samples. It is revealed that the amount of spin-misalignment scattering is very sensitive to the applied heat treatment and is most pronounced in the as-quenched condition. In particular, when considering nuclear scattering curves the influence of spin-misalignment scattering may be very large and must not be neglected. [less ▲]

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See detailEffect of magnetic field annealing on the soft magnetic properties of nanocrystalline materials
Ito, N.; Michels, Andreas UL; Kohlbrecher, J. et al

in Journal of Magnetism & Magnetic Materials (2007), 316(2), 458-461

The effects of static- and rotating-magnetic-field annealing on the magnetic softness and the magnetization alignments in nanocrystalline Fe84Nb6B10 have been investigated by means of DC magnetometry and ... [more ▼]

The effects of static- and rotating-magnetic-field annealing on the magnetic softness and the magnetization alignments in nanocrystalline Fe84Nb6B10 have been investigated by means of DC magnetometry and small-angle neutron scattering (SANS). At the remanent state, the SANS pattern obtained from the static-field annealed sample exhibited an enhanced SANS signal in the direction normal to the uniaxial anisotropy (Ku) field direction, indicating a well-developed long-range magnetization alignment along the direction of Ku. Contrarily, the SANS pattern for the rotating-field annealed sample is virtually isotropic at zero field, reflecting the lack of anisotropic magnetization alignments. The soft magnetic properties of nanocrystalline Fe84Nb6B10 are improved dramatically by rotating-field annealing due to the suppression of Ku. [less ▲]

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See detailCreating local contrast in small-angle neutron scattering by dynamic nuclear polarization
van den Brandt, B.; Glattli, H.; Hautle, P. et al

in Journal of Applied Crystallography (2007), 40(s), 106-110

Low-resolution small-angle neutron scattering measurements can benefit from polarized protons to generate scattering contrast profiles. In a recently developed technique, time-resolved polarized SANS ... [more ▼]

Low-resolution small-angle neutron scattering measurements can benefit from polarized protons to generate scattering contrast profiles. In a recently developed technique, time-resolved polarized SANS tries to make use of spatial polarization gradients created around paramagnetic centres at the onset of dynamic nuclear polarization. The time constants which describe the build-up of polarization around the paramagnetic centre and the subsequent diffusion of polarization in the solvent were determined by analysing the temporal evolution of the nuclear polarization. The possible use and the limitations of this technique as a spectroscopic tool are discussed. [less ▲]

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See detailProbing dipolar angles in soft magnetic nanocomposites by small-angle neutron scattering
Michels, Andreas UL; Weissmüller, J.

in Neutron News (2006), 71(6), 34-35

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See detailSynthesis and magnetic properties of cobalt nanocubes
Gräf, C. P.; Birringer, R.; Michels, Andreas UL

in Physical Review. B, Condensed Matter and Materials Physics (2006), 73(21), 2124011-2124014

We report on the synthesis, structure, and magnetic properties of cobalt nanoparticles with cubic shape. The nanoparticles with an average cube-edge length of about 50 nm were prepared by wet-chemical ... [more ▼]

We report on the synthesis, structure, and magnetic properties of cobalt nanoparticles with cubic shape. The nanoparticles with an average cube-edge length of about 50 nm were prepared by wet-chemical processing and characterized by electron microscopy, x-ray diffraction, and magnetometry. The x-ray data indicate a mixture of hcp Co and ϵ-Co. From the magnetization measurements we deduce on the presence of a ferromagnet/antiferromagnet Co/CoO interface, which gives rise to a pronounced exchange-bias effect. Between 350 K and 5 K the coercivity HC increases by almost a factor of 6, whereas the exchange-bias field HE takes on a value of about 380 Oe at 5 K and vanishes above 75 K. The variation of HC with temperature cannot be understood in terms of the various conventional types of magnetic anisotropy. [less ▲]

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See detailDipolar correlations in a nanocomposite: A neutron scattering study of Nanoperm Fe89Zr7B3Cu
Michels, Andreas UL; Vecchini, C.; Moze, O. et al

in Physical Review. B, Condensed Matter and Materials Physics (2006), 74(13), 1344071-13440713

We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm ... [more ▼]

We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm (Fe89Zr7B3Cu). An unusual “clover-leaf-shaped” intensity distribution on the detector is attributed to the dipolar stray fields around the nanosized iron particles, which are embedded in an amorphous magnetic matrix of lesser saturation magnetization. The dipole field induces spin disorder, correlating the spin misalignment of neighboring particles and matrix over several particle spacings. The clover-leaf-shaped anisotropy is observed over a wide range of applied magnetic field and momentum transfer. It persists up to several hundred degrees Kelvin above the Curie temperature of the matrix phase, indicating that some degree of magnetic coupling persists even when the matrix is paramagnetic. [less ▲]

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See detailA neutron-scattering investigation of the magnetic structure and magnetic excitations in nanocrystalline Tb
Vecchini, C.; Moze, O.; Hoser, A. et al

in Journal of Applied Physics (2006), 99(8), 5021-5023

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the ... [more ▼]

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the magnetic excitations in a nanocrystalline elemental rare earth. Refinement of the neutron-diffraction data reveals an “average” magnetic structure of each crystallite which contains a significant out-of-plane component to the magnetic moment as well as a suppression of the high-temperature antiferromagnetic phase observed for coarse-grained Tb. The inelastic-neutron-scattering measurements reveal the presence of a magnetic excitation of approximately 10 meV at 2.5 K. The excitation energy decreases with increasing temperature. The origins of this excitation are discussed with particular reference to the magnetic modes at the zone center observed for single-crystal Tb. [less ▲]

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See detailTime-resolved nuclear spin-dependent small-angle neutron scattering from polarised proton domains in deuterated solutions
van den Brandt, B.; Glättli, H.; Grillo, I. et al

in European Physical Journal B -- Condensed Matter (2006), 49(2), 157-165

We have investigated the process of dynamic proton polarisation by means of time-resolved polarised small-angle neutron scattering (SANS) on frozen solutions of EHBA-CrV molecules in glycerol-water ... [more ▼]

We have investigated the process of dynamic proton polarisation by means of time-resolved polarised small-angle neutron scattering (SANS) on frozen solutions of EHBA-CrV molecules in glycerol-water mixtures as a function of the concentration of EHBA-CrV and for different degrees of deuteration of the solvent. In the EHBA-CrV complex, the spins of the 20 protons which surround the paramagnetic CrV can be oriented using the method of dynamic nuclear polarisation (DNP), thereby offering the possibility to create locally a nuclear spin-dependent contrast for SANS. The time constants which describe the build-up of polarisation around the paramagnetic centre and the subsequent diffusion of polarisation in the solvent were determined by analysing the temporal evolution of the nuclear polarisation, which in turn was obtained by fitting a core-shell model to the time-dependent SANS curves. The results on the spin dynamics obtained using the scattering function of a core-shell could be independently confirmed by evaluating the integrated SANS intensity. A thermodynamic one-centre model is presented which is able to reproduce the observed dependence of the proton polarisation times on the proton concentration of the solvent. [less ▲]

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See detailSmall angle neutron scattering investigations of spin disorder in nanocomposite soft magnets
Vecchini, C.; Moze, O.; Suzuki, K. et al

in Journal of Alloys & Compounds (2006), 423(1-2), 31-36

The technique of SANS (small angle neutron scattering) furnishes unique information on the characteristic magnetic length scales and local magnetic anisotropies at the nanoscale in nanocomposite ... [more ▼]

The technique of SANS (small angle neutron scattering) furnishes unique information on the characteristic magnetic length scales and local magnetic anisotropies at the nanoscale in nanocomposite ferromagnets. Such information is not presently available using any other microscopic technique. The basic principles and results of the technique will be presented with regard to a unique and unexpected observation of a dipole field controlled spin disorder in a prototypical soft nanocomposite ferromagnet of the Nanoperm type. [less ▲]

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See detailNeutron scattering and modeling of dipole-field-induced spin disorder in Nanoperm
Vecchini, C.; Moze, O.; Suzuki, K. et al

in Applied Physics Letters (2005), 87

We present magnetic-field-dependent small-angle neutron scattering data for the ferromagnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The spin-misalignment scattering in the approach-to-saturation regime ... [more ▼]

We present magnetic-field-dependent small-angle neutron scattering data for the ferromagnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The spin-misalignment scattering in the approach-to-saturation regime unexpectedly reveals pronounced lobes of high intensity at angles ±30−40° relative to the magnetic-field axis. Based on numerical calculations, the four-fold angular symmetry of the scattering pattern can be explained in terms of local spin misalignment, which originates from dipolar stray fields due to the mismatch of the saturation-magnetization values between the bcc Fe particles and the amorphous magnetic matrix. [less ▲]

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See detailDipole-field-induced spin disorder in a nanocomposite soft magnet
Michels, Andreas UL; Vecchini, C.; Moze, O. et al

in Europhysics Letters [=EPL] (2005), 72(2), 249-255

We report on a study of a magnetic nanocomposite of the Nanoperm type (Fe89Zr7B3Cu1) by magnetic small-angle neutron scattering (SANS). The understanding of the magnetic microstructure of these materials ... [more ▼]

We report on a study of a magnetic nanocomposite of the Nanoperm type (Fe89Zr7B3Cu1) by magnetic small-angle neutron scattering (SANS). The understanding of the magnetic microstructure of these materials leaves much to be desired since we lack techniques capable of resolving the spin structure in the bulk with nanoscale resolution. Here, we present an analysis of the SANS signal by which one cannot only characterise the nanoscale structure of the spin system, but which allows to identify origin and structure of the perturbing field. In Nanoperm, an unusual angular anisotropy of the scattering suggests that the local spin misalignment decorates, as the most important perturbing field, dipole stray fields around the crystalline phase of the composite. [less ▲]

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See detailProduction of ultracold neutrons from a cold neutron beam on a 2H2 target
Atchison, F.; van den Brandt, B.; Brys, T. et al

in Physical Review. C : Nuclear Physics (2005), 71

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See detailHigh-accuracy measurement of the spin-dependent neutron scattering length of the deuteron
van den Brandt, B.; Hautle, P.; Kohlbrecher, J. et al

in Proceedings of the 16th International Spin Physics Symposium and Workshop on Polarized Electron Sources and Polarimeters (2005)

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See detailAn experimental approach to the dynamics of nuclear polarisation
van den Brandt, B.; Glättli, H.; Grillo, I. et al

in Nuclear Instruments & Methods in Physics Research Section A (2004), 526

In the past 3 years a series of experiments have shed new light on the mechanism of dynamic nuclear polarisation (DNP). Time-resolved polarised small-angle neutron scattering and nuclear magnetic ... [more ▼]

In the past 3 years a series of experiments have shed new light on the mechanism of dynamic nuclear polarisation (DNP). Time-resolved polarised small-angle neutron scattering and nuclear magnetic resonance have been used simultaneously to study the nuclear polarisation build-up around paramagnetic centres during DNP. This approach, which aims at visualising the nuclear polarisation process on a microscopic scale, shall be exemplified and the experimental techniques used described in some detail. [less ▲]

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See detailCharacteristic magnetic length-scales in Vitroperm—Combining Kerr microscopy and small-angle neutron scattering
Grob, A.; Saranu, S.; Herr, U. et al

in Physica Status Solidi A. Applied Research (2004), 201

We present a study of the magnetic-field dependence of the magnetic microstructure of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic ... [more ▼]

We present a study of the magnetic-field dependence of the magnetic microstructure of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic magnetic anisotropy. The samples were analysed using Kerr microscopy and small-angle neutron scattering (SANS). As the applied magnetic field is decreased starting from saturation, we find that two processes occur independently: the formation of a domain structure with the net magnetisation of the domains aligned along the macroscopic easy axis and the increase in magnitude of static, nanometer-scale fluctuations of the spin orientation. The domain structure observations from Kerr microscopy are in excellent agreement with the results of the SANS experiments. [less ▲]

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See detailSpin structure of nanocrystalline terbium
Weissmüller, J.; Michels, Andreas UL; Michels, D. et al

in Physical Review. B, Condensed Matter and Materials Physics (2004), 69

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering (SANS) data are analyzed quantitatively in terms of ... [more ▼]

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering (SANS) data are analyzed quantitatively in terms of the correlation function of the spin misalignment. We find that up to applied fields of several tesla the magnetization remains “locked in” to the basal planes of the hcp crystal lattice of each individual crystallite, but that the in-plane orientation of the spins is highly nonuniform within each grain. This spin disorder at the nanoscale can be suppressed by a large applied field, but in the remanent state the disorder reduces the magnetization to values considerably below the Stoner limit. In field-dependent SANS, the intragrain spin disorder gives rise to a crossover of the scattering curves, and to the unusual finding that the scattering cross section at small scattering vector increases with increasing magnetic field. As the origin of the internal spin disorder within the grains, we propose an extra magnetic anisotropy energy at small grain size, presumably due to microstrain, a suggestion which is supported by analysis of ac-susceptibility data in the paramagnetic state. Our finding of a reduced remanence at small grain size is contrary to the remanence enhancement that is observed in other nanocrystalline hard magnets. We also report an unusual logarithmic field dependence of the magnetization over wide ranges of the applied field and temperature. [less ▲]

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See detailNeutronen sehen Spinorientierung auf Nanoskala
Weissmüller, J.; Michels, Andreas UL

in Physik in unserer Zeit (2004), 35

Die attraktiven magnetischen Eigenschaften nanokristalliner Weichmagnete werden darauf zurückgeführt, dass bei geringer Kristallitgröße die Orientierung der Spins den Sprüngen in den magnetisch leichten ... [more ▼]

Die attraktiven magnetischen Eigenschaften nanokristalliner Weichmagnete werden darauf zurückgeführt, dass bei geringer Kristallitgröße die Orientierung der Spins den Sprüngen in den magnetisch leichten Richtungen von Kristallit zu Kristallit nicht mehr folgen kann. Physiker am Forschungszentrum Karlsruhe konnten jüngst diese Vorhersage mit Neutronenstreuung überprüfen. [less ▲]

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See detailRange of magnetic correlations in nanocrystalline soft magnets
Michels, Andreas UL; Viswanath, R. N.; Barker, J. G. et al

in Physical Review Letters (2003), 91

We have obtained the magnetic field dependence of static ferromagnetic correlations in nanocrystalline electrodeposited Co and Ni by means of the correlation function of the spin misalignment, determined ... [more ▼]

We have obtained the magnetic field dependence of static ferromagnetic correlations in nanocrystalline electrodeposited Co and Ni by means of the correlation function of the spin misalignment, determined from small-angle neutron scattering data. The approach yields a correlation length lC, which is a measure for the spatial extent of inhomogeneities in the magnetization distribution. The correlation length depends strongly on the applied magnetic field with values ranging from 94 nm in nanocrystalline Co at low fields to about 15 nm at saturation. The results for lC indicate that in Co the main source of nonuniformity in the spin system is the anisotropy field of each individual crystallite, whereas in nanocrystalline Ni the main sources of spin disorder originate from twin faults or from the defect cores of grain boundaries. [less ▲]

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See detailA comparison of torque-strengths in micromagnetics
Michels, Andreas UL; Weissmüller, J.

in Journal of Magnetism & Magnetic Materials (2003), 258-259

In the approach-to-magnetic-saturation range, we present experimental data for the magnitude and applied magnetic-field dependence of the volume-averaged mean-square magnetostatic stray field |Hdb|^2 that ... [more ▼]

In the approach-to-magnetic-saturation range, we present experimental data for the magnitude and applied magnetic-field dependence of the volume-averaged mean-square magnetostatic stray field |Hdb|^2 that results from non-zero divergences of the magnetization from within the volume of bulk samples of nanocrystalline electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The magnitude of |Hdb|^2 is compared to the volume-averaged mean-square anisotropy field Hp|^2, which arises from the combined effect of magnetocrystalline and magnetoelastic anisotropy. [less ▲]

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See detailDomain formation and long-range spin disorder in Vitroperm
Michels, Andreas UL; Viswanath, R. N.; Weissmüller, J.

in Europhysics Letters [=EPL] (2003), 64

We present a study of the magnetic-field dependence of the small-angle neutron scattering of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic ... [more ▼]

We present a study of the magnetic-field dependence of the small-angle neutron scattering of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic magnetic anisotropy. We show how the field dependences of i) the radial average of the scattering cross-section dΣ/dΩ and ii) the variation of dΣ/dΩ as a function of the orientation of the scattering vector can be analysed to supply separately information on the orientation of the net magnetisation of the domains and on the internal spin disorder within the domains. As the applied magnetic field is decreased starting from saturation, we find that two processes occur independently: the formation of a domain structure with the net magnetisation of the domains aligned along the macroscopic easy axis and the increase in magnitude of static, nanometer-scale fluctuations of the spin orientation. [less ▲]

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