Reordering of magnetic colloid structures in external magnetic fields

Abstract

Field-induced local ordering within a cobalt-based magnetic colloid has been studied with polarized small-angle scattering by turning the direction of the external magnetic field. By analysing the nuclear-magnetic cross term we were able to separate the scattering contributions from dipolar chains and domains of pseudo-crystalline hexagonal arrangements. The magnetic moments were found to be aligned along the external magnetic field and to follow the change of the field direction. The observed variation of the particle moments must give rise to a rearrangement of the local hexagonal structure with respect to the new field direction to explain the scattering pattern. We confirm this by two-dimensional simultaneous least-squares fits for different Q ranges [where, Q = 4π sin(θ)/λ, λ is the X-ray wavelength and 2θ is the scattering angle] with a model containing a core-shell particle form factor and a structure factor composed of Gaussian peaks and contributions from polydisperse cylinders. © International Union of Crystallography 2007.