References of "Barker, J. G"
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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 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 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 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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