References of "Michels, Andreas 50002669"
     in
Bookmark and Share    
Full Text
See detailOne-D neutron-polarization analysis on magnetic nanostructures
Honecker, Dirk UL; Ferdinand, A.; Döbrich, Frank UL et al

in Europhysics News (2010), 41(5), 15-15

Detailed reference viewed: 46 (4 UL)
Full Text
Peer Reviewed
See detailNeutron spin-flip scattering of nanocrystalline cobalt
Honecker, Dirk UL; Döbrich, Frank; Dewhurst, C. D. et al

in Journal of Physics: Condensed Matter (2010), 23(1), 016003

Detailed reference viewed: 22 (0 UL)
Full Text
Peer Reviewed
See detailLongitudinal polarization analysis in small-angle neutron scattering
Honecker, Dirk UL; Ferdinand, A.; Döbrich, Frank UL et al

in European Physical Journal B -- Condensed Matter (2010), 76(2), 209-213

Detailed reference viewed: 85 (4 UL)
Full Text
See detailSpin disorder at Gd grain boundaries
Döbrich, Frank UL; Elmas, M.; Ferdinand, A. et al

in Europhysics News (2009), 40/3

Detailed reference viewed: 52 (2 UL)
Full Text
Peer Reviewed
See detailGrain-boundary-induced spin disorder in nanocrystalline gadolinium
Döbrich, Frank UL; Elmas, M.; Ferdinand, A. et al

in Journal of Physics : Condensed Matter (2009), 21(15), 1560031-1560035

Based on experimental magnetic-field-dependent neutron scattering data, we have calculated the autocorrelation function of the spin misalignment of nanocrystalline 160gadolinium. The analysis suggests the ... [more ▼]

Based on experimental magnetic-field-dependent neutron scattering data, we have calculated the autocorrelation function of the spin misalignment of nanocrystalline 160gadolinium. The analysis suggests the existence of two characteristic length scales in the spin system: the smaller one is about 5 nm and is attributed to the defect cores of the grain boundaries, whereas the larger length scale is of the order of the average crystallite size D = 21 nm and presumably describes the response of the magnetization to the magnetic anisotropy field of the individual crystallites. [less ▲]

Detailed reference viewed: 66 (1 UL)
Full Text
Peer Reviewed
See detailGrain-size-dependent magnetic susceptibility of nanocrystalline terbium
Philippi, S.; Markmann, J.; Birringer, R. et al

in Journal of Applied Physics (2009), 105(7), 7011-7013

This paper reports grain-size-dependent magnetic susceptibility data on nanocrystalline bulk Tb. We find that at small grain size Curie–Weiss behavior is not present for temperatures up to about 80 K ... [more ▼]

This paper reports grain-size-dependent magnetic susceptibility data on nanocrystalline bulk Tb. We find that at small grain size Curie–Weiss behavior is not present for temperatures up to about 80 K above the transition temperature and that the helical antiferromagnetic phase is absent. Possible origins for the suppression of the helix phase in nanoscaled Tb are discussed in terms of internal magnetostatic fields and competing length scales (grain size versus wavelength of the helix phase). [less ▲]

Detailed reference viewed: 84 (1 UL)
Full Text
Peer Reviewed
See detailPorosity-induced spin disorder in nanocrystalline inert-gas condensed iron
Michels, Andreas UL; Elmas, M.; Döbrich, Frank UL et al

in Europhysics Letters [=EPL] (2009), 85

We report magnetization and magnetic neutron scattering measurements on nanocrystalline Fe which was prepared by means of the inert-gas condensation technique. Depending on the compaction pressure applied ... [more ▼]

We report magnetization and magnetic neutron scattering measurements on nanocrystalline Fe which was prepared by means of the inert-gas condensation technique. Depending on the compaction pressure applied during the synthesis procedure (0.5–1.8 GPa), the resulting Fe samples contain porosity with volume fractions between about 20–35%. We provide evidence that the spin disorder which is associated with porosity has a strong influence on magnetic properties, and it gives rise to a characteristic clover-leaf–shaped angular anisotropy in the elastic-magnetic-scattering cross-section. [less ▲]

Detailed reference viewed: 67 (2 UL)
Full Text
Peer Reviewed
See detailMagnetic-field-dependent small-angle neutron scattering on random anisotropy ferromagnets
Michels, Andreas UL; Weissmüller, J.

in Reports on Progress in Physics (2008), 71(6), 0665011-06650137

We report on the recently developed technique of magnetic-field-dependent small-angle neutron scattering (SANS), with attention to bulk ferromagnets exhibiting random magnetic anisotropy. In these ... [more ▼]

We report on the recently developed technique of magnetic-field-dependent small-angle neutron scattering (SANS), with attention to bulk ferromagnets exhibiting random magnetic anisotropy. In these materials, the various magnetic anisotropy fields (magnetocrystalline, magnetoelastic, and/or magnetostatic in origin) perturb the perfectly parallel spin alignment of the idealized ferromagnetic state. By varying the applied magnetic field, one can control one of the ordering terms which competes with the above-mentioned perturbing fields. Experiments which explore the ensuing reaction of the magnetization will therefore provide information not only on the field-dependent spin structure but, importantly, on the underlying magnetic interaction terms. This strategy, which underlies conventional studies of hysteresis loops in magnetometry, is here combined with magnetic SANS. While magnetometry generally records only a single scalar quantity, the integral magnetization, SANS provides access to a vastly richer data set, the Fourier spectrum of the response of the spin system as a function of the magnitude and orientation of the wave vector. The required data-analysis procedures have recently been established, and experiments on a number of magnetic materials, mostly nanocrystalline or nanocomposite metals, have been reported. Here, we summarize the theory of magnetic-field-dependent SANS along with the underlying description of random anisotropy magnets by micromagnetic theory. We review experiments which have explored the magnetic interaction parameters, the value of the exchange-stiffness constant as well as the Fourier components of the magnetic anisotropy field and of the magnetostatic stray field. A model-independent approach, based on the experimental autocorrelation function of the spin misalignment, provides access to the characteristic length of the spin misalignment. The field dependence of this quantity is in quantitative agreement with the predictions of micromagnetic theory. Experiments on nanocomposite ferromagnets reveal that the jump of the magnetization at internal phase boundaries leads to a significant magnetostatic perturbing field, with an unusual 'clover-leaf' SANS pattern as the experimental signature. Furthermore, SANS experiments have been used to monitor the orientation of magnetic domains as well as the length scale of intradomain spin misalignment. [less ▲]

Detailed reference viewed: 74 (4 UL)
Full Text
See detailLong range magnetic interactions in nanocrystalline ferromagnets
Weissmüller, J.; Herr, U.; Fecht, J. et al

in Schimmel, Th.; von Löhneysen, H.; Obermair, Ch. (Eds.) et al Nanotechnology—Physics, Chemistry, and Biology of Functional Nanostructures (2008)

Detailed reference viewed: 34 (4 UL)
Full Text
Peer Reviewed
See detailMagnetic Nanorods: Genesis, Self-Organization and Applications
Birringer, R.; Wolf, H.; Lang, C. et al

in Zeitschrift für Physikalische Chemie (2008), 222(2-3), 229-255

Magnetic-field-assisted self-assembly of magnetic dipole moment carrying iron nanoparticles is shown to result in the formation of magnetic and mechanically stiff nanoscale rods. The cooperative behavior ... [more ▼]

Magnetic-field-assisted self-assembly of magnetic dipole moment carrying iron nanoparticles is shown to result in the formation of magnetic and mechanically stiff nanoscale rods. The cooperative behavior of an ensemble of such rods and bundles thereof exhibits self-organized pattern formation on different length scales. Pattern formation on large length scales reveals great similarity with physical systems undergoing spinodal decomposition. Possible applications for dipolar magnetic nanorods in the field of perpendicular storage media are highlighted. We discuss an aerosol-synthesis-route allowing to prepare ferrofluids (FF) with shape-anisotropic particles constituting the magnetic phase immersed in the nonmagnetic carrier fluid. These so-called nanorod FF unveil a two orders of magnitude increase of viscosity enforced by an applied field of 10mT even at shear rates larger than 10-2s. This raises prospects for applications in microfluidics and MEMS. [less ▲]

Detailed reference viewed: 128 (1 UL)
Full Text
Peer Reviewed
See detailMagnetic domains and annealing-induced magnetic anisotropy in nanocrystalline soft magnetic materials
Suzuki, K.; Ito, N.; Saranu, S. et al

in Journal of Applied Physics (2008), 103(7), 7301-7303

The magnetic domains of nanocrystalline Fe84Nb6B10 annealed under static and rotating magnetic fields have been investigated by means of magneto-optical Kerr effect (MOKE) microscopy in order to clarify ... [more ▼]

The magnetic domains of nanocrystalline Fe84Nb6B10 annealed under static and rotating magnetic fields have been investigated by means of magneto-optical Kerr effect (MOKE) microscopy in order to clarify the origin of the dramatic magnetic softening brought about by rotating field annealing. The coercivity (Hc) values after static- and rotating-magnetic field annealings are 5.9 and 3.0  A/m, respectively. The MOKE image after static field annealing implies a highly coherent uniaxial anisotropy (Ku) in the sample whereas no sign of such a strong Ku is evident in the domain configuration after rotating field annealing. Our analytical solution of the random anisotropy model with additional Ku predicts that the fluctuating amplitude of the effective anisotropy (δK) in nanocrystalline Fe84Nb6B10 decreases from 20 to 11 J/m3 by removing Ku. The observed reduction of Hc may be attributed to this decrease in δK induced by rotating field annealing. [less ▲]

Detailed reference viewed: 138 (4 UL)
Full Text
Peer Reviewed
See detailSpin structure of nanocrystalline gadolinium
Michels, Andreas UL; Döbrich, Frank UL; Elmas, M. et al

in Europhysics Letters [=EPL] (2008), 81

We report on magnetic-field–dependent small-angle neutron scattering (SANS) experiments on nanocrystalline inert-gas condensed bulk Gd, which was synthesized using the low-capturing isotope 160Gd. The ... [more ▼]

We report on magnetic-field–dependent small-angle neutron scattering (SANS) experiments on nanocrystalline inert-gas condensed bulk Gd, which was synthesized using the low-capturing isotope 160Gd. The angular dependency of the scattering cross-section is in very good agreement with recent theoretical predictions. Rather unexpected for this type of material, we observe a "clover-leaf–shaped" anisotropy in the SANS signal, the origin of which is attributed to the existence of longitudinal magnetization fluctuations associated with atomic site disorder and modified coupling inside the defect cores of grain boundaries. [less ▲]

Detailed reference viewed: 86 (0 UL)
Full Text
Peer Reviewed
See detailUnraveling the nature of room temperature grain growth in nanocrystalline materials
Ames, M.; Markmann, J.; Karos, R. et al

in Acta Materialia (2008), 56

We report on the observation of real-time-resolved room temperature grain growth in nanocrystalline metals. We find that neither the time evolution of size can be modeled by standard growth theories nor ... [more ▼]

We report on the observation of real-time-resolved room temperature grain growth in nanocrystalline metals. We find that neither the time evolution of size can be modeled by standard growth theories nor are there any other systems aware to us that manifest a similar growth behaviour. We detect a transition from an initially self-similar slow growth to abnormal grain growth. Its onset seems to be associated with the simultaneous decrease of microstrain with increasing grain size. Abnormal grain growth is considered as a generic feature of nanocrystallinity but is a transient state since we observed in the late stage of coarsening, using orientational imaging microscopy, a monomodal grain size distribution. We empirically find a nonlinear-response-type of growth law which is in agreement with the observed coarsening kinetics. [less ▲]

Detailed reference viewed: 85 (3 UL)
Full Text
Peer Reviewed
See detailCharacterisation of the polarised neutron beam at the small angle scattering instrument SANS-I with a polarised proton target
Aswal, V. K.; van den Brandt, B.; Hautle, P. et al

in Nuclear Instruments & Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment (2008), 586

A transmission neutron polariser (Fe/Si supermirror) has been successfully implemented in the small angle neutron scattering instrument SANS-I at the SINQ neutron source. The polariser is needed for ... [more ▼]

A transmission neutron polariser (Fe/Si supermirror) has been successfully implemented in the small angle neutron scattering instrument SANS-I at the SINQ neutron source. The polariser is needed for investigations of magnetic nanostructures as well as for spin contrast variation techniques relying on the spin-dependent neutron scattering length of polarised nuclei. The V-shaped polariser is installed in the first section of the collimator system of the SANS instrument and its performance is optimised for neutrons with a wavelength between 0.5 and 1.0 nm. For a precise polarisation analysis of a beam with selectable incident divergence, such as in SANS experiments, an opaque spin filter is ideal. We used a solid polarised proton target exploiting the strong spin-dependent neutron scattering cross-section of hydrogen and determined the neutron beam polarisation to a precision of 0.5% for different collimations in a broad wavelength band. [less ▲]

Detailed reference viewed: 77 (1 UL)
Full Text
Peer Reviewed
See detailAngular dependence of coercivity and remanence of Ni nanowire arrays and its relevance to magnetoviscosity
Günther, Annegret UL; Monz, S.; Tschöpe, A. et al

in Journal of Magnetism & Magnetic Materials (2008), 320(7), 1340-1344

Ni nanowire arrays with varying wire dimensions (diameter d, length l) and center-to-center distances dCC were synthesized by pulsed electrodeposition of Ni in porous Al templates. The magnetization ... [more ▼]

Ni nanowire arrays with varying wire dimensions (diameter d, length l) and center-to-center distances dCC were synthesized by pulsed electrodeposition of Ni in porous Al templates. The magnetization-reversal behavior of the arrays was investigated by means of magnetometry for different angles θ between the wire axes and the applied magnetic field. The functional dependences of the characteristic parameters coercivity HC(θ) and reduced remanence mR/mS(θ) exhibit a strong dependence on the wire dimensions and the center-to-center distance. For instance, for nanowire arrays with d=40 nm, dCC=100 nm, and for θ=0°, the coercivity takes on a rather large value of μ0HC=85 mT and mR/mS=94%; reducing dCC to 30 nm and d to 17 nm results in μ0HC=49 mT and mR/mS=57%, an observation which suggests an increasing magnetostatic interwire interaction at increased (d/dCC)-ratio. The potential application of nanowires as the constituents of ferrofluids is discussed. [less ▲]

Detailed reference viewed: 106 (8 UL)
Full Text
Peer Reviewed
See detailTemperature dependence of dipole-field scattering in Nanoperm
Michels, Andreas UL; Vecchini, C.; Moze, O. et al

in Journal of Magnetism & Magnetic Materials (2007), 316(2), 448-450

We present small-angle neutron scattering (SANS) data for the temperature variation of the recently observed dipole-field-induced spin-misalignment scattering in the soft magnetic nanocomposite Nanoperm ... [more ▼]

We present small-angle neutron scattering (SANS) data for the temperature variation of the recently observed dipole-field-induced spin-misalignment scattering in the soft magnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The associated clover-leaf-shaped angular anisotropy of the SANS pattern, which is due to spin disorder arising from dipolar stray fields of the iron nanoparticles, persists up to several hundred Kelvin above the decoupling point of the intergranular amorphous matrix phase. This observation, in conjunction with the q-dependence of the scattering, suggests the existence of long-range magnetic correlations between the iron particles through the paramagnetic matrix, in agreement with previous investigations. The characteristic wavelength of the dipole-field-induced spin disorder appears to be temperature independent. [less ▲]

Detailed reference viewed: 59 (1 UL)
Full Text
Peer Reviewed
See detailSynthesis of a nanorod ferrofluid and characterisation by magnetic-field-dependent small-angle X-ray scattering
Döbrich, Frank UL; Michels, Andreas UL; Birringer, R.

in Journal of Magnetism & Magnetic Materials (2007), 316(e), 779-782

Compared to conventional ferrofluids, which contain mostly spherical particles, a dispersion of highly anisometric particles such as rods or chains is expected to give rise to an enhancement or ... [more ▼]

Compared to conventional ferrofluids, which contain mostly spherical particles, a dispersion of highly anisometric particles such as rods or chains is expected to give rise to an enhancement or modification of well-known ferrofluid properties. This contribution reports on the synthesis of a ferrofluid containing stable chains of iron nanoparticles and on its microstructural characterisation by means of transmission electron microscopy and small-angle X-ray scattering (SAXS). The SAXS measurements develop a pronounced anisotropy of the scattering pattern as a function of an increasing external magnetic field. Evaluation of the radially averaged SAXS curves in terms of basic scattering functions is discussed. [less ▲]

Detailed reference viewed: 89 (1 UL)
Full Text
Peer Reviewed
See detailResonant microwave absorption determination of characteristic magnetic length in magnetic-field-annealed Vitroperm
Basheed, G. A.; Kaul, S. N.; Michels, Andreas UL

in Applied Physics Letters (2007), 91

The first direct resonant microwave absorption determination of the thermal renormalization of exchange stiffness, average magnetic anisotropy constant, and characteristic magnetic length in “field ... [more ▼]

The first direct resonant microwave absorption determination of the thermal renormalization of exchange stiffness, average magnetic anisotropy constant, and characteristic magnetic length in “field-annealed” Vitroperm samples with an initial magnetic permeability of μi = 20 000 and 150 000 has been presented and discussed. [less ▲]

Detailed reference viewed: 63 (3 UL)
Full Text
Peer Reviewed
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 ▲]

Detailed reference viewed: 48 (0 UL)