Electronic measurement of single-domain magnetic particle concentration

A radio frequency oscillator with the coil of the resonant circuit wound around a tube containing a suspension of single-domain magnetic particles was employed to study the effect of the volume fraction of suspended particles on the bulk permeability with two types of suspending fluid and three different types of magnetic oxides. For a given particle and suspending fluid the permeability (measured as the inverse square oscillator frequency) of the suspension was linearly proportional to the volume fraction of particles in suspension. Volume fraction was maintained less than 10%. The level of the permeability was shifted by the suspending fluid, and the fractional increase in permeability with volume fraction was determined by the difference between the specific contribution of the particles and that of the suspending fluid to the permeability. Temperature effects on the oscillator frequency from 20 to 35 C were found to contribute less than 0.03% error in particle volume measurement by this method.