Magnetic interparticle correlations in ferroxdure, magnetic pigments, and micrometer Ni and Fe particles, studied with neutron depolarization

Abstract

Neutron-depolarization (ND) measurements on ferroxdure compact and slurry, several magnetic pigments, and compacts of micrometer Ni and Fe particles in different remanent magnetic states are discussed. The pigments involve CrO2, Co-modified γ-Fe2O3, CoZn ferrite, Fe, and mixtures of p-wt % Co-modified γ-Fe2O3 and (100−p)-wt % α-Fe2O3 (1≤p≤100). The depolarization yields information about the local magnetization during the magnetization reversal process and about particle orientational correlations and density variations within the particulate media. From the ND measurements on ferroxdure it follows that at most weak correlations exist in the compact while strong correlations exist in the slurry. The magnetization reversal process in the slurry takes place by a coherent rotation of groups of particles. The correlation length in the virgin pigments ζ0v is in agreement with magnetically uncorrelated single-domain particles. Its value in the state of maximum remanence ζm is more than tenfold ζ0v, being due to the presence of clusters of particles in which the mean particle density and/or orientation differs from the mean particle orientation and/or density within the pigments. The dependence of ζm on p in the Co-modified γ-Fe2O3/α-Fe2O3 mixtures shows that the Co-modified γ-Fe2O3 and the α-Fe2O3 particles are badly dispersed. The magnetic correlation length in the compacts of micrometer Ni and Fe particles is in agreement with magnetically uncorrelated (quasi-) single-domain particles. The maximum value of the remanent reduced magnetization of the compacts is relatively low. This is a consequence of the fact that the demagnetizing fields of a particle magnetized to saturation highly exceed the coercive field of these relatively soft materials. From ND measurements on the pigments after dc or ac demagnetization it follows that most of the switched volumes are elongated along the direction of the applied field. The switched volumes are the magnetic inhomogeneities in the demagnetized remanent state. The switched volumes after dc demagnetization are much larger than those after ac demagnetization. The latter only slightly exceeds the particle volume.