Cluster demagnetizing effects in particulate dispersions

Abstract

We describe a study of a dispersion of metal pigments. We have used a high field pulsed magnetometer to examine the acquisition of remanence and have observed anomalous behavior as a function of both the magnitude of the pulsed field and its duration. The remanence increases monotonically until a critical field or a critical time is reached and then suddenly decreases at a highly reproducible value of field, and then subsequently increases when a larger pulsed field is applied. At very high fields a second drop in remanence is observed. By comparison with a computer simulation we infer that this behavior may arise due to strong demagnetising dipolar interactions in aggregates of a certain critical size.