References of "Michels, Andreas 50002669"
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See detailMeasurement of a magnetic-field dependent correlation length in nanocrystalline Ni using small-angle neutron scattering
Michels, Andreas UL; Weissmüller, J.; Erb, U. et al

in Physica Status Solidi A. Applied Research (2002), 189

We have analyzed magnetic-field dependent small-angle neutron scattering data on nanocrystalline electrodeposited Ni by means of the correlation function of the spin misalignment. The approach yields a ... [more ▼]

We have analyzed magnetic-field dependent small-angle neutron scattering data on nanocrystalline electrodeposited Ni by means of the correlation function of the spin misalignment. The approach yields a correlation length lC of the spin misalignment that is a measure for the characteristic dimension of regions in which the magnetic moments are misaligned coherently into a common direction. We find that lC varies strongly with the applied magnetic field Hi with values extending from about 50 nm (larger than the value for uniformly magnetized grains) at small Hi to about 10 nm (considerably smaller than the grain size) at large applied fields. [less ▲]

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See detailOn the influence of anisotropy on the magnetic small-angle neutron scattering of superparamagnetic particles
Michels, Andreas UL; Weissmüller, J.

in European Physical Journal B -- Condensed Matter (2002), 26

This paper presents a calculation of the magnetic small-angle neutron scattering cross-section resulting from a dilute ensemble of superparamagnetic particles exhibiting uniaxial magnetic anisotropy. We ... [more ▼]

This paper presents a calculation of the magnetic small-angle neutron scattering cross-section resulting from a dilute ensemble of superparamagnetic particles exhibiting uniaxial magnetic anisotropy. We focus on the two experimentally relevant scattering geometries in which the incident neutron beam is perpendicular or parallel to an applied magnetic field, and we discuss several orientations of the anisotropy axes with respect to the field. Magnetic anisotropy has no influence on the magnetic small-angle neutron scattering when the particles are mobile, as is the case e.g. in ferrofluids, but, when the particles are embedded in a rigid non-magnetic matrix and the orientations of the anisotropy axes are fixed, significant deviations compared to the case of negligible anisotropy are expected. For the particluar situation in which the anisotropy axes are parallel to the applied field, closed-form expressions suggest that an effective anisotropy energy or anisotropy-energy distribution can be determined from experimental scattering data. [less ▲]

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See detailTomographic characterization of grain-size correlations in polycrystalline Al-Sn
Krill III, C. E.; Döbrich, K. M.; Michels, D. et al

in Developments in X-Ray Tomography III (2002)

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See detailTomography with high resolution
Rau, C.; Weitkamp, T.; Snigirev, A. et al

in Developments in X-Ray Tomography III (2002)

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See detailA lower bound for the volume-averaged mean-square magnetostatic stray field
Michels, Andreas UL; Weissmüller, J.; Birringer, R.

in European Physical Journal B -- Condensed Matter (2002), 29

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See detailMagnetic small-angle neutron scattering by nanocrystalline terbium
Weissmüller, J.; Michels, D.; Michels, Andreas UL et al

in Scripta Materialia (2001), 44(8), 2357-2361

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent SANS data are analyzed quantitatively in terms of the correlation function of the ... [more ▼]

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent 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 particle. This internal structure can be suppressed by the applied field. [less ▲]

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See detailAnalysis of the small-angle neutron scattering of nanocrystalline ferromagnets using a micromagnetics model
Weissmüller, J.; Michels, Andreas UL; Barker, J. G. et al

in Physical Review. B, Condensed Matter and Materials Physics (2001), 63

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure ... [more ▼]

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure gives rise to strong elastic magnetic small-angle neutron scattering (SANS). The paper explores a method for analyzing field-dependent SANS data from such materials in terms of a model based on the theory of micromagnetics. Samples of cw Ni and of electrodeposited nc Ni and nc Co were characterized by x-ray scattering and magnetometry, and were investigated by SANS both with and without polarization of the neutron beam. The variation of the differential scattering cross section with the scattering vector and with the applied magnetic field is well described by the model. Also, experimental results for the exchange stiffness constant A and for the spin-wave stiffness constant D obtained from the analysis are found to agree with literature data obtained by inelastic neutron scattering on single-crystal specimens. The model supplies an “anisotropy field scattering function” that contains information on the magnitude of the magnetic anisotropy in the material, and on the characteristic length scales on which the anisotropy changes direction. The results suggest that the anisotropy may be strongly nonuniform in each crystallite, possibly due to twinning, and that some magnetic moments in the Ni samples are strongly pinned at defects. [less ▲]

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See detailComment on magnetic correlations in nanostructured ferromagnets
Weissmüller, J.; Michels, Andreas UL

in Physical Review Letters (2001), 87(14), 149701-149701

A Comment on the Letter by Jörg F. Löffler, Hans Benjamin Braun, and Werner Wagner, Phys. Rev. Lett. 85, 1990 (2000). The authors of the Letter offer a Reply.

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See detailMeasuring the exchange-stiffness constant of nanocrystalline solids by elastic small-angle neutron scattering
Michels, Andreas UL; Weissmüller, J.; Wiedenmann, A. et al

in Philosophical Magazine Letters (2000), 80

In ferromagnets with a non-uniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline or cold-worked polycrystalline materials, the static magnetic microstructure gives rise to ... [more ▼]

In ferromagnets with a non-uniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline or cold-worked polycrystalline materials, the static magnetic microstructure gives rise to elastic magnetic small-angle neutron scattering (SANS). The paper explores a method for determining the exchange-stiffness constant A by analysis of the dependence of the elastic SANS cross-section on the applied magnetic field. Experimental results for A and for the spin-wave stiffness constant D in cold-worked or nanocrystalline Ni and Co are found to agree with literature data obtained by inelastic neutron scattering on single-crystal specimens. [less ▲]

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See detailExchange-stiffness constant in cold-worked and nanocrystalline Ni measured by elastic small-angle neutron scattering
Michels, Andreas UL; Weissmüller, J.; Wiedenmann, A. et al

in Journal of Applied Physics (2000), 87(9), 5953-5955

We present a new method for determining the exchange-stiffness constant A of a ferromagnetic bulk material by field-dependent elastic small-angle neutron scattering (SANS). In the limit of high applied ... [more ▼]

We present a new method for determining the exchange-stiffness constant A of a ferromagnetic bulk material by field-dependent elastic small-angle neutron scattering (SANS). In the limit of high applied magnetic field H, for which the scattering volume is a single magnetic domain and the magnetization is nearly aligned with the direction of the applied field, a combination of micromagnetics theory with neutron scattering formalism suggests closed-form expressions for the differential scattering cross section as a function of the scattering vector and of H. Based on these results it is suggested that the exchange-stiffness constant can be extracted from experimental SANS data recorded as a function of H. At ambient temperature we have applied this method to polycrystalline cold-worked Ni and nanocrystalline electrodeposited Ni, finding exchange-stiffness constants of (8.2±0.2)×10−12 and (7.6±0.3)×10−12  J/m, respectively. Measurement at 5 K yields a value of (9.2±0.2)×10−12 J/m for the nanocrystalline sample, a temperature dependence that agrees qualitatively with data in the literature. In addition to the value of A, the technique supplies information on the spatial structure of the magnetic anisotropy field. [less ▲]

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See detailSmall-angle neutron scattering by the magnetic microstructure of nanocrystalline ferromagnets near saturation
Weissmüller, J.; McMichael, R. D.; Michels, Andreas UL et al

in Journal of Research of the National Institute of Standards & Technology (1999), 104(3), 261-275

The paper presents a theoretical analysis of elastic magnetic small-angle neutron scattering (SANS) due to the nonuniform magnetic microstructure in nanocrystalline ferromagnets. The reaction of the ... [more ▼]

The paper presents a theoretical analysis of elastic magnetic small-angle neutron scattering (SANS) due to the nonuniform magnetic microstructure in nanocrystalline ferromagnets. The reaction of the magnetization to the magnetocrystalline and magnetoelastic anisotropy fields is derived using the theory of micromagnetics. In the limit where the scattering volume is a single magnetic domain, and the magnetization is nearly aligned with the direction of the magnetic field, closed form solutions are given for the differential scattering cross-section as a function of the scattering vector and of the magnetic field. These expressions involve an anisotropy field scattering function, that depends only on the Fourier components of the anisotropy field microstructure, not on the applied field, and a micromagnetic response function for SANS, that can be computed from tabulated values of the materials parameters saturation magnetization and exchange stiffness constant or spin wave stiffness constant. Based on these results, it is suggested that the anisotropy field scattering function SH can be extracted from experimental SANS data. A sum rule for SH suggests measurement of the volumetric mean square anisotropy field. When magneto-crystalline anisotropy is dominant, then a mean grain size or the grain size distribution may be determined by analysis of SH. [less ▲]

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See detailModelling the influence of grain-size-dependent solute drag on the kinetics of grain growth in nanocrystalline materials
Michels, Andreas UL; Krill, C. E.; Ehrhardt, H. et al

in Acta Materialia (1999), 47(7), 2143-2152

The large relative change in total grain-boundary area that accompanies grain growth in a nanocrystalline material has a potentially strong influence on the kinetics of grain growth whenever grain ... [more ▼]

The large relative change in total grain-boundary area that accompanies grain growth in a nanocrystalline material has a potentially strong influence on the kinetics of grain growth whenever grain-boundary migration is controlled by solute (impurity) drag. As the grain-boundary area decreases, the concentration of solute or impurity atoms segregated to the boundaries is expected to increase rapidly, introducing a grain-size dependence to the retarding force on boundary migration. We have modified the Burke equation—which assumes the drag force to be independent of the average grain size—to take into account a linear dependence of grain-boundary pinning on grain size. The form of the resulting grain-growth curve is surprisingly similar to Burke's solution; in fact, a constant rescaling of the boundary mobility parameter is sufficient to map one solution approximately onto the other. The activation energies for grain-boundary motion calculated from the temperature dependence of the mobility parameter are therefore identical for both models. This fact provides an explanation for the success of Burke's solution in fitting grain-growth data obtained in systems, such as nanocrystalline materials, for which the assumption of grain-size-independent solute drag is incorrect. [less ▲]

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See detailGrain-growth kinetics in nanocrystalline iron prepared by ball milling
Michels, Andreas UL; Krill, C. E.; Natter, H. et al

in Grain Growth in Polycrystalline Materials III (1998)

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