References of "Birringer, R"
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See detailCritical behaviour of nanocrystalline gadolinium: evidence for random uniaxial dipolar universality class
Ferdinand, A.; Probst, A.-C.; Michels, Andreas UL et al

in Journal of Physics : Condensed Matter (2014), 26

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See detailMagnetic ordering in nanocrystalline gadolinium: A neutron diffraction study
Ryan, D. H.; Michels, Andreas UL; Döbrich, Frank UL et al

in Physical Review. B, Condensed Matter and Materials Physics (2013), 87(064408), 1-6

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See detailInfluence of crystallite size and temperature on the antiferromagnetic helices of terbium and holmium metal
Michels, Andreas UL; Bick, Jens-Peter UL; Birringer, R. et al

in Physical Review. B, Condensed Matter and Materials Physics (2011), 83

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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 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-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 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 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 ▲]

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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 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 ▲]

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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 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 ▲]

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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 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 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 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 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 detailElastic properties of single-crystalline and consolidated nano-structured yttrium oxide at room temperature.
Baller, Jörg UL; Krüger, Jan-Kristian UL; Birringer, R. et al

in Journal of Physics : Condensed Matter (2000), 12(25), 5403-5409

High resolution Brillouin spectroscopy was used to characterize the elastic stiffness properties of consolidated nano-crystalline yttrium oxide as well as of the related single-crystalline state. Defect ... [more ▼]

High resolution Brillouin spectroscopy was used to characterize the elastic stiffness properties of consolidated nano-crystalline yttrium oxide as well as of the related single-crystalline state. Defect enriched grain boundaries are discussed as sources for the extremely soft elastic properties of the nano-crystalline state. [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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