Magnetic viscosity in advanced metal particle pigments

Five specially prepared elongated metal particle pigments have been analysed to determine the detailed switching behaviour and magnetic viscosity. The particles varied in length from about 130 nm down to 40 nm. Hysteresis loops and time dependence measurements were made which led to a calculation of the activation volume of reversal. The activation volumes reduce with the particle size and are comparable for the smallest pigments. Physical particle sizes were measured by TEM. The objective of the work is to find the limiting size of the current technology determined by superparamagnetic behaviour.