Dirk Honecker

Magnetization reversal in Nd-Fe-B based nanocomposites as seen by magnetic small-angle neutron scattering

We have studied the magnetization-reversal process of a Nd2Fe14B/Fe3B nanocomposite using small-angle neutron scattering. Based on the computation of the autocorrelation function of the spin misalignment, we have estimated the characteristic size lC of spin inhomogeneities around the Nd2Fe14B nanoparticles. The quantity lC approaches a constant value of about 12.5 nm (∼average Nd2Fe14B particle radius) at 14 T and takes on a maximum value of about 18.5 nm at the coercive field of −0.55 T. The field dependence of lC can be described by a model that takes into account the convolution relationship between the nuclear and the magnetic microstructure.

Magnetic field dependent small-angle neutron scattering on a Co nanorod array: evidence for intraparticle spin misalignment

The results of magnetic field dependent small-angle neutron scattering measurements on a cobalt nanorod array are reported. The data provide evidence for the existence of intraparticle spin disorder.

Effect of Dzyaloshinski-Moriya interaction on elastic small-angle neutron scattering

The Dzyaloshinski-Moriya interaction (DMI) originates from the relativistic spin-orbit coupling, and in low-symmetry crystal structures without inversion centers it gives rise to antisymmetric magnetic exchange. DMI plays an important role in the formation of inhomogeneous spin structures such as long-wavelength spirals, vortex states, and skyrmion textures. Even in high-symmetry lattices, where the antisymmetric DM term would normally vanish, DMI is present in the vicinity of any lattice defect and can influence the magnetic microstructure of polycrystalline materials with a large defect density. In this work the authors have studied the impact of the DMI on the magnetic small-angle neutron scattering (SANS) cross section, which is able to probe long-wavelength magnetization fluctuations in the bulk and on the nanometer length scale. The analysis of the authors provides a promising way to characterize the DMI in defect-rich materials.

Analysis of magnetic neutron-scattering data of two-phase ferromagnets

(Received 16 July 2013; revised manuscript received 2 September 2013; published 23 September 2013)We have analyzed magnetic-field-dependent small-angle neutron scattering (SANS) data of soft magnetictwo-phase nanocomposite ferromagnets in terms of a recent micromagnetic theory for the magnetic SANScross section [Honecker and Michels, Phys. Rev. B

Configuration of the magnetosome chain: a natural magnetic nanoarchitecture

Magnetospirillum gryphiswaldense is a microorganism with the ability to biomineralize magnetite nanoparticles, called magnetosomes, and arrange them into a chain that behaves like a magnetic compass. Rather than straight lines, magnetosome chains are slightly bent, as evidenced by electron cryotomography. Our experimental and theoretical results suggest that due to the competition between the magnetocrystalline and shape anisotropies, the effective magnetic moment of individual magnetosomes is tilted out of the [111] crystallographic easy axis of magnetite. This tilt does not affect the direction of the chain net magnetic moment, which remains along the [111] axis, but explains the arrangement of magnetosomes in helical-like shaped chains. Indeed, we demonstrate that the chain shape can be reproduced by considering an interplay between the magnetic dipolar interactions between magnetosomes, ruled by the orientation of the magnetosome magnetic moment, and a lipid/protein-based mechanism, modeled as an elastic recovery force exerted on the magnetosomes.

Small-angle Neutron Scattering with One-dimensional Polarization Analysis

Magnetic small-angle neutron scattering (SANS) is a widely used technique for the investigation of nanoscale inhomogeneities in the magnetic microstructure of bulk materials [1,2]. However, up to now, magnetic SANS was almost exclusively utilized with an unpolarized or a polarized incident neutron beam – denoted as SANSPOL – and an analysis of the spin state of the neutron after the scattering process is generally not performed. Due to recent progress in the development of efficient 3He spin filters [3], it has only now become possible to perform routinely one-dimensional (longitudinal or uniaxial) neutron-polarization analysis (so-called POLARIS) in a SANS experiment, for instance, at V4 at HZB [4], at beamline NG3 at NIST [5 –7], or at the instrument D22 at the ILL [8,9]. The principle of operation of such an analyzing device relies on the spin-dependent absorption of neutrons by a nuclear-spin-polarized gas of 3He atoms. Essentially, the decisive advantages of 3He spin filters as compared to other polarizing/analyzing devices are (i) that they can be used over a rather broad wavelength band (from cold to thermal to hot neutrons) and (ii) that they allow for a rather large phase space (neutron-energy transfer and scattering angle) to be covered.