Small angle scattering of orientated magnetic structures and applications to magnetic colloids

SANS on orientated nanometer scale structures generally leads to anisotropic detector images. In the case of nonmagnetic materials models of different particle shapes and orientation can be used to fit the 2 D patterns and obtain information about the scattering objects and their mean orientation. When polarized neutrons SANSPOL are used to investigate anisotropic magnetic structures the situation is more complex. Here the external magnetic field, defining the direction of the neutron spin, leads to a second striking direction. The emergence of a local magnetisation which might be different from the direction of the external field can be a source of a third asymmetry. In our contribution we present the theoretical and experimental treatment involving these three specific directions. We apply the theory to the analysis of SANSPOL experiments on magnetic colloids. Locally ordered structures formed by magnetic particles are modelled by orientated cylinders or ellipsoids and used to explain the different scattering patterns under different external magnetic field and shear conditions. We discuss to which extent SANS techniques can discriminate between different structure and orientation models. The examples illustrate how polarised neutrons improve the information content and strongly contribute to the interpretation of the field and shear induced structures in magnetic liquid