R. Rosman

Neutron depolarization in particulate media: A review of theory and experimental results

Abstract The polarization change of a polarized neutron beam due to transmission through a magnetic medium is analyzed in a neutron depolarization (ND) experiment. This change yields useful information about the micro-magnetic state of the medium, such as the correlation length and the mean local orientation of fluctuations in the local magnetization. These fluctuations are related to the intrinsic particle properties and magnetic interparticle correlations. A review of the ND theory and in particular of its application to particulate media is given. Furthermore, the results of ND experiments on several types of particulate media are discussed.

Neutron depolarization theory in the Larmor and the scattering approach

The neutron depolarization theory in the Larmor and the scattering approach is presented. An extension of the former is proposed, as a result of which both approaches result in basically the same formulas. The intrinsic anisotropy, which is the phenomenon that for magnetically isotropic media the depolarization is still dependent on the mutual orientation of the polarization vector and the propagation direction of the neutron beam, is discussed furthermore. It is shown that this phenomenon is caused by local stray fields around magnetic inhomogeneities as well as eventual correlations between the shape of these inhomogeneities and the orientation of the local magnetization.

Interparticle correlations in Fe3O4 ferrofluids, studied by the small‐angle neutron scattering technique

Aggregation in sterically stabilized Fe3O4 ferrofluids has been studied with the small‐angle neutron scattering technique. The influence on the aggregation of the particle volume fraction, varying between 0.02 and 7 vol % Fe3O4, and the influence of an applied magnetic field is examined. Contrast variation enabled the scattering of particles and surfactants to be measured independently. The particle size distribution derived from the scattering is in good agreement with the field dependence of the magnetization and the optical anisotropy. From the anisotropy in the neutron scattering patterns of fluids subjected to a magnetic field of 0.10 T perpendicular to the neutron beam, it is derived that at all volume fractions below 1 vol % Fe3O4 most of the particles are located in small chainlike aggregates oriented parallel to the field direction. Larger, three‐dimensional aggregates arise at higher volume fractions, both in the applied field and in zero field. The interparticle distance within an aggregate corresponds to surface‐to‐surface contact of the surfactant coatings.

Interparticle correlations in Fe3O4 ferrofluids, studied by the small angle neutron scattering technique

Abstract Aggregation in sterically stabilised Fe 3 O 4 ferrofluids has been studied using the SANS technique. Contrast variation allows the scattering of particles and surfactants to be measured independently. The presence of small chain-like aggregates at all volume fractions below 1 vol% Fe 3 O 4 and larger three-dimensional aggregates at higher volume fractions is derived.

Simulation of magnostatics in random magnetic particle system

Abstract A simple computer simulation model is presented enabling one to get detailed insight in the role of static dipolar interactions, anisotropy, texture and stacking of the particles on the static magnetization hysteresis curve of a particulate medium. In particular the effect of the dipole interactions on the correlations between the particle magnetizations are considered as a function of the applied field, the magnetic anisotropy of the particles and the average texture of the particle system. The coercive field is furthermore considered as a function of the anisotropy and the texture of the system.

Particulate recording tapes studied with neutron depolarization

The size and shape of the magnetic inhomogeneities in various particulate recording tapes have been studied using neutron depolarization (ND). The tapes are in the remanent magnetic state either during the dc‐demagnetization reversal process or after bulk erasure, ac erasure, or thermal annealing. The magnetic correlation length ζ measured along the direction perpendicular to the plane of the tape (Z direction) is at most twice the particle diameter in the dc‐(de)magnetized tapes, indicating the existence of clusters and switched volumes containing only a few particles along Z. The switched volumes are the magnetic inhomogeneities in the demagnetized state. A cluster is a group of particles in which the mean particle orientation or density differs from the mean particle orientation and density within the tape. The size along Z of the switched volumes after bulk or ac erasure is slightly smaller than that of switched volumes after dc demagnetization. After thermal annealing, the magnetic correlation length...

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

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...

Switching volumes in magnetic pigments studied with neutron depolarization

Abstract The size and shape of switching volumes in various magnetic pigments after DC- or AC-demagnetization is studied using neutron depolarization. The switching volumes in the DC-demagnetized pigments strongly exceed those in the AC- demagnetized pigments. Both exceed the particle volume.

Ni containing solid Kr bubbles studied with neutron depolarization and small-angle neutron scattering

Abstract Neutron depolarization and SANS measurements on Ni containing solid Kr bubbles are presented. The existence of a large fraction of small (∼3 nm) Kr bubbles and a small fraction of much larger (≳30 nm) nuclear inhomogeneities is deduced. The latter, which may be large Kr bubbles, effect the local magnetization.