Small Angle Neutron Scattering and Neutron Depolarization by magnetic small Particles

Abstract

Because of its magnetic moment the neutron interacts both with magnetic and nuclear inhomogeneities smultaneously. In a small angle neutron scattering experiment(SANS) information about the size of an inhomogeneity is obtained from the momentum transfer of the neutrons to the sample. By contrast variation in the nuclear/magnetic scattering amplitude and/or polarization analysis, one of the two interactions can be amplified or distinguished from the other. SANS experiments on a Fe 3 O 4 ferrofluid, well and not deuterated, to study aggregation in a magnetic field, will be discussed. From the anisotropy in the scattering parallel and perpendicular to the applied field, even in very diluted ferrofluids the particles in the field occur mainly as dimers and not in longer chains. At higher concentrations 3-dimensional aggregates are formed in the presence of a field. In a depolarization experiment only information about the magnetic inhomogeneity is obtained. In such an experiment a polarized neutron beam is transmitted through a system of magnetic particles. During transmission the beam changes its polarization direction n and degree of polarization P. From a 3-dimensional analysis of n and P the mean magnetization vector, the magnetic correlation length (domain size) and the mean square direction cosines of the local magnetization of the magnetically correlated volume are determined. Static depolarization experiments to study magnetic recording materials will be discussed. Applied to recording materials three kinds of correlation are distinguished. Orientation correlations arise in needle shaped particles in a dense packing. Density variations occur automatically by a random stacking of particles, but they may vary dependent on the stacking procedure. Magnetic interaction between particles cause an increase or decrease of the correlation length parallel or perpendicular to the local magnetization direction respectively. Finally the possibilities of dynamic depolarization experiments will be discussed.