One-D neutron-polarization analysis on magnetic nanostructures

in Europhysics News (2010), 41(5), 15-15

Longitudinal polarization analysis in small-angle neutron scattering

in European Physical Journal B -- Condensed Matter (2010), 76(2), 209-213

Spin disorder at Gd grain boundaries

in Europhysics News (2009), 40/3

Grain-size-dependent magnetic susceptibility of nanocrystalline terbium

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 ▲]

Magnetic-field-dependent small-angle neutron scattering on random anisotropy ferromagnets

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 ▲]

Magnetic Nanorods: Genesis, Self-Organization and Applications

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 ▲]

Spin structure of nanocrystalline gadolinium

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 ▲]

Characterisation of the polarised neutron beam at the small angle scattering instrument SANS-I with a polarised proton target

in Nuclear Instruments & Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment (2008), 586

A transmission neutron polariser (Fe/Si supermirror) has been successfully implemented in the small angle neutron scattering instrument SANS-I at the SINQ neutron source. The polariser is needed for ... [more ▼]

A transmission neutron polariser (Fe/Si supermirror) has been successfully implemented in the small angle neutron scattering instrument SANS-I at the SINQ neutron source. The polariser is needed for investigations of magnetic nanostructures as well as for spin contrast variation techniques relying on the spin-dependent neutron scattering length of polarised nuclei. The V-shaped polariser is installed in the first section of the collimator system of the SANS instrument and its performance is optimised for neutrons with a wavelength between 0.5 and 1.0 nm. For a precise polarisation analysis of a beam with selectable incident divergence, such as in SANS experiments, an opaque spin filter is ideal. We used a solid polarised proton target exploiting the strong spin-dependent neutron scattering cross-section of hydrogen and determined the neutron beam polarisation to a precision of 0.5% for different collimations in a broad wavelength band. [less ▲]

Temperature dependence of dipole-field scattering in Nanoperm

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 ▲]

Resonant microwave absorption determination of characteristic magnetic length in magnetic-field-annealed Vitroperm

in Applied Physics Letters (2007), 91

The first direct resonant microwave absorption determination of the thermal renormalization of exchange stiffness, average magnetic anisotropy constant, and characteristic magnetic length in “field ... [more ▼]

The first direct resonant microwave absorption determination of the thermal renormalization of exchange stiffness, average magnetic anisotropy constant, and characteristic magnetic length in “field-annealed” Vitroperm samples with an initial magnetic permeability of μi = 20 000 and 150 000 has been presented and discussed. [less ▲]