The magnetic properties of ferromagnetic liquids containing iron particles in mercury

Ferromagnetic fluids have been prepared by the electrodeposition of iron on to an agitated mercury cathode. The mercury contained additives such as tin and sodium, the presence of which, has been shown to enhance particle stability. The particle sizes are approximately 45Å in diameter and fluids stable over many months have now been prepared. Magnetisation measurements have been made at 77K and 293K and the ageing effects of heating the fluids to 420K have been studied over the short and long term. The saturation magnetisation of these samples studied is typically 65 gauss and the fluids are visibly ferromagnetic. Magnetic measurements indicate that all fluids prepared are superparamagnetic at room temperature but possess properties of hysteresis in the solid state at 77K. The coercive force at 77K for those fluids with additives increased less rapidly on ageing than for fluids without additives. For those fluids containing tin and sodium, the coercive force becomes constant within 24 hours at an ageing temperature of 420K. The change in coercive force during ageing is considered to arise from an increase in particle size and the constant value obtained in the fluid containing tin and sodium is interpreted as an indication that particle growth has been inhibited. The magnetic properties are dependent upon the particle size distributions and good agreement is obtained between the experimental observations and the theoretical form of the coercive force against particle size curve obtained from a log normal distribution. Measurements of the coercive force and remanence not only allows the stability of the ferromagnetic fluid to be monitored but also enable predictions to be made concerning the particle size distribution. The results of such measurements on iron particles in mercury with tin and sodium as additives are presented in this paper.