Static and dynamic neutron depolarization studies of ferromagnetic domain structures

Abstract

A polarized neutron beam passes through a ferromagnetic sample. In front of the sample a polarization turner turns the polarization towards the x, y or z direction. A second polarization turner behind the sample enables one to analyse any component of the polarization vector. In this way a (3×3) depolarization matrix can be measured from which various domain quantities are determined in the bulk, such as the mean magnetization in three dimensions (Mi, i=x,y,z), the mean domain size and the mean local direction of magnetization within the domains. Applying a periodic magnetic field on the sample and using a multi-channel analyser the time dependent depolarization matrix determines the mean periodic time dependence of the parameters mentioned. Among the possible applications of the method static and dynamic experiments are discussed. Static experiments have been carried out on an oblate Ni ellipsoid with a magnetic field in the y-direction in the [311] crystal direction in the plane of the ellipsoid. From the positive sign of the z-magnetization at decreasing temperature below the Curie temperature it is deduced that the [111] direction is the easy direction of magnetization. Dynamic experiments have been carried out applying a block shaped periodic field on one leg of a FeSi(3.5 wt% Si) picture frame crystal. After a field reversal eddy currents confine domain nucleation to the surface only, which depolarize partly the various polarization components. After a time the nuclei on each crystal side grow together forming two rather wavy domain walls parallel to the surface moving to the centre from which the position, velocity of the walls and the distance between the initial nuclei have been determined.