References of "Michels, Andreas 50002669"
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See detailSmall-angle Neutron Scattering with One-dimensional Polarization Analysis
Michels, Andreas UL; Honecker, Dirk UL; Döbrich, Frank UL et al

in Neutron News (2011), 22(3), 15-19

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See detailNew Methodology for Micromagnetic Simulations of Nanocomposites With Applications to SANS Experiments
Erokhin, Sergey; Berkov, Dmitry; Gorn, Nataliya et al

in IEEE Transactions on Magnetics (2011), 47(10), 3044-3047

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See detailNeutron spin-flip scattering of nanocrystalline cobalt
Honecker, Dirk UL; Döbrich, Frank UL; Dewhurst, C. D. et al

in Journal of Physics : Condensed Matter (2011), 23(1), 1-4

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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

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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

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See detailCorrelation functions of the spin misalignment in magnetic small-angle neutron scattering
Michels, Andreas UL

in Physical Review. B, Condensed Matter and Materials Physics (2010), 82(2), 0244331-02443312

We have numerically calculated the autocorrelation function C(r) of the spin misalignment by means of micromagnetic theory. C(r) depends sensitively on the details of the underlying magnetic ... [more ▼]

We have numerically calculated the autocorrelation function C(r) of the spin misalignment by means of micromagnetic theory. C(r) depends sensitively on the details of the underlying magnetic microstructure and can be determined by Fourier inversion of magnetic small-angle neutron scattering data. The model system which we consider consists of a single isolated spherical nanoparticle that is embedded in an infinitely extended matrix. The particle is uniquely characterized by its magnetic anisotropy field Hp(x), whereas the matrix is assumed to be otherwise anisotropy-field free. In the approach-to-saturation regime, we have computed the static response of the magnetization to different spatial profiles of Hp(x). Specifically, we have investigated the cases of a uniform particle anisotropy, uniform core shell, linear increase, and exponential and power-law decay. From the magnetization profiles and the associated C(r), we have extracted the correlation length lc of the spin misalignment, and we have compared the applied-field dependence of this quantity with semiquantitative theoretical predictions. We find that for practically all of the considered models for the anisotropy field (except the core-shell model) the field dependence of the spin-misalignment fluctuations is quite uniquely reproduced by lc(Hi)=L+lh(Hi), where the field-independent quantity L is on the order of the particle size and lh(Hi) represents the so-called exchange length of the applied magnetic field. [less ▲]

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See detailCoulomb Cluster: Strukturen aus Staub
Michels, Andreas UL

Presentation (2010)

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See detailMagnetic interactions in nanomagnets: a neutron-scattering study
Michels, Andreas UL

Presentation (2010)

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See detailMagnetic interactions in nanomagnets: a neutron-scattering study
Michels, Andreas UL

Presentation (2010)

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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

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See detailSpin disorder at Gd grain boundaries
Döbrich, Frank UL; Elmas, M.; Ferdinand, A. et al

in Europhysics News (2009), 40/3

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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