References of "Weissmüller, J"
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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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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)

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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 ▲]

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See detailProbing dipolar angles in soft magnetic nanocomposites by small-angle neutron scattering
Michels, Andreas UL; Weissmüller, J.

in Neutron News (2006), 71(6), 34-35

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See detailSmall angle neutron scattering investigations of spin disorder in nanocomposite soft magnets
Vecchini, C.; Moze, O.; Suzuki, K. et al

in Journal of Alloys & Compounds (2006), 423(1-2), 31-36

The technique of SANS (small angle neutron scattering) furnishes unique information on the characteristic magnetic length scales and local magnetic anisotropies at the nanoscale in nanocomposite ... [more ▼]

The technique of SANS (small angle neutron scattering) furnishes unique information on the characteristic magnetic length scales and local magnetic anisotropies at the nanoscale in nanocomposite ferromagnets. Such information is not presently available using any other microscopic technique. The basic principles and results of the technique will be presented with regard to a unique and unexpected observation of a dipole field controlled spin disorder in a prototypical soft nanocomposite ferromagnet of the Nanoperm type. [less ▲]

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See detailDipolar correlations in a nanocomposite: A neutron scattering study of Nanoperm Fe89Zr7B3Cu
Michels, Andreas UL; Vecchini, C.; Moze, O. et al

in Physical Review. B, Condensed Matter and Materials Physics (2006), 74(13), 1344071-13440713

We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm ... [more ▼]

We present results for the magnetic-field, temperature, and neutron-polarization dependence of the small-angle neutron scattering intensity in the soft magnetic iron-based nanocomposite Nanoperm (Fe89Zr7B3Cu). An unusual “clover-leaf-shaped” intensity distribution on the detector is attributed to the dipolar stray fields around the nanosized iron particles, which are embedded in an amorphous magnetic matrix of lesser saturation magnetization. The dipole field induces spin disorder, correlating the spin misalignment of neighboring particles and matrix over several particle spacings. The clover-leaf-shaped anisotropy is observed over a wide range of applied magnetic field and momentum transfer. It persists up to several hundred degrees Kelvin above the Curie temperature of the matrix phase, indicating that some degree of magnetic coupling persists even when the matrix is paramagnetic. [less ▲]

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See detailA neutron-scattering investigation of the magnetic structure and magnetic excitations in nanocrystalline Tb
Vecchini, C.; Moze, O.; Hoser, A. et al

in Journal of Applied Physics (2006), 99(8), 5021-5023

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the ... [more ▼]

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the magnetic excitations in a nanocrystalline elemental rare earth. Refinement of the neutron-diffraction data reveals an “average” magnetic structure of each crystallite which contains a significant out-of-plane component to the magnetic moment as well as a suppression of the high-temperature antiferromagnetic phase observed for coarse-grained Tb. The inelastic-neutron-scattering measurements reveal the presence of a magnetic excitation of approximately 10 meV at 2.5 K. The excitation energy decreases with increasing temperature. The origins of this excitation are discussed with particular reference to the magnetic modes at the zone center observed for single-crystal Tb. [less ▲]

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See detailNeutron scattering and modeling of dipole-field-induced spin disorder in Nanoperm
Vecchini, C.; Moze, O.; Suzuki, K. et al

in Applied Physics Letters (2005), 87

We present magnetic-field-dependent small-angle neutron scattering data for the ferromagnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The spin-misalignment scattering in the approach-to-saturation regime ... [more ▼]

We present magnetic-field-dependent small-angle neutron scattering data for the ferromagnetic nanocomposite Nanoperm (Fe89Zr7B3Cu1). The spin-misalignment scattering in the approach-to-saturation regime unexpectedly reveals pronounced lobes of high intensity at angles ±30−40° relative to the magnetic-field axis. Based on numerical calculations, the four-fold angular symmetry of the scattering pattern can be explained in terms of local spin misalignment, which originates from dipolar stray fields due to the mismatch of the saturation-magnetization values between the bcc Fe particles and the amorphous magnetic matrix. [less ▲]

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See detailDipole-field-induced spin disorder in a nanocomposite soft magnet
Michels, Andreas UL; Vecchini, C.; Moze, O. et al

in Europhysics Letters [=EPL] (2005), 72(2), 249-255

We report on a study of a magnetic nanocomposite of the Nanoperm type (Fe89Zr7B3Cu1) by magnetic small-angle neutron scattering (SANS). The understanding of the magnetic microstructure of these materials ... [more ▼]

We report on a study of a magnetic nanocomposite of the Nanoperm type (Fe89Zr7B3Cu1) by magnetic small-angle neutron scattering (SANS). The understanding of the magnetic microstructure of these materials leaves much to be desired since we lack techniques capable of resolving the spin structure in the bulk with nanoscale resolution. Here, we present an analysis of the SANS signal by which one cannot only characterise the nanoscale structure of the spin system, but which allows to identify origin and structure of the perturbing field. In Nanoperm, an unusual angular anisotropy of the scattering suggests that the local spin misalignment decorates, as the most important perturbing field, dipole stray fields around the crystalline phase of the composite. [less ▲]

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See detailCharacteristic magnetic length-scales in Vitroperm—Combining Kerr microscopy and small-angle neutron scattering
Grob, A.; Saranu, S.; Herr, U. et al

in Physica Status Solidi A. Applied Research (2004), 201

We present a study of the magnetic-field dependence of the magnetic microstructure of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic ... [more ▼]

We present a study of the magnetic-field dependence of the magnetic microstructure of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic magnetic anisotropy. The samples were analysed using Kerr microscopy and small-angle neutron scattering (SANS). As the applied magnetic field is decreased starting from saturation, we find that two processes occur independently: the formation of a domain structure with the net magnetisation of the domains aligned along the macroscopic easy axis and the increase in magnitude of static, nanometer-scale fluctuations of the spin orientation. The domain structure observations from Kerr microscopy are in excellent agreement with the results of the SANS experiments. [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 detailNeutronen sehen Spinorientierung auf Nanoskala
Weissmüller, J.; Michels, Andreas UL

in Physik in unserer Zeit (2004), 35

Die attraktiven magnetischen Eigenschaften nanokristalliner Weichmagnete werden darauf zurückgeführt, dass bei geringer Kristallitgröße die Orientierung der Spins den Sprüngen in den magnetisch leichten ... [more ▼]

Die attraktiven magnetischen Eigenschaften nanokristalliner Weichmagnete werden darauf zurückgeführt, dass bei geringer Kristallitgröße die Orientierung der Spins den Sprüngen in den magnetisch leichten Richtungen von Kristallit zu Kristallit nicht mehr folgen kann. Physiker am Forschungszentrum Karlsruhe konnten jüngst diese Vorhersage mit Neutronenstreuung überprüfen. [less ▲]

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See detailDomain formation and long-range spin disorder in Vitroperm
Michels, Andreas UL; Viswanath, R. N.; Weissmüller, J.

in Europhysics Letters [=EPL] (2003), 64

We present a study of the magnetic-field dependence of the small-angle neutron scattering of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic ... [more ▼]

We present a study of the magnetic-field dependence of the small-angle neutron scattering of the nanocrystalline soft magnet Vitroperm (Fe73Si16B7Nb3Cu1), which was treated so as to exhibit a macroscopic magnetic anisotropy. We show how the field dependences of i) the radial average of the scattering cross-section dΣ/dΩ and ii) the variation of dΣ/dΩ as a function of the orientation of the scattering vector can be analysed to supply separately information on the orientation of the net magnetisation of the domains and on the internal spin disorder within the domains. As the applied magnetic field is decreased starting from saturation, we find that two processes occur independently: the formation of a domain structure with the net magnetisation of the domains aligned along the macroscopic easy axis and the increase in magnitude of static, nanometer-scale fluctuations of the spin orientation. [less ▲]

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See detailA comparison of torque-strengths in micromagnetics
Michels, Andreas UL; Weissmüller, J.

in Journal of Magnetism & Magnetic Materials (2003), 258-259

In the approach-to-magnetic-saturation range, we present experimental data for the magnitude and applied magnetic-field dependence of the volume-averaged mean-square magnetostatic stray field |Hdb|^2 that ... [more ▼]

In the approach-to-magnetic-saturation range, we present experimental data for the magnitude and applied magnetic-field dependence of the volume-averaged mean-square magnetostatic stray field |Hdb|^2 that results from non-zero divergences of the magnetization from within the volume of bulk samples of nanocrystalline electrodeposited Ni and Co and coarse-grained polycrystalline cold-worked Ni. The magnitude of |Hdb|^2 is compared to the volume-averaged mean-square anisotropy field Hp|^2, which arises from the combined effect of magnetocrystalline and magnetoelastic anisotropy. [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 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 detailMagnetic microstructure and properties of the nanocrystalline hard magnet terbium
Weissmüller, J.; Michels, D.; Michels, Andreas UL et al

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

We present an experimental study of the magnetic properties and magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering (SANS) data suggest that up to ... [more ▼]

We present an experimental study of the magnetic properties and magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering (SANS) data suggest 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; as a consequence, domain-wall movement along the basal planes is eliminated as a mechanism for magnetization reversal, and the coercive field is substantially increased. [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 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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