MAGNETIC BIREFRINGENCE OF FERROFLUIDS .1. ESTIMATION OF PARTICLE-SIZE

The magnetic birefringence of various samples of Fe3O 4 ferrofluids with particle sizes in the range of approximately 8.5-18.5 nm has been measured in field strengths of 5*10-4 to 5*10-1 T. For all the samples it has been found that in low fields the birefringence varies as the square of the field strength, indicating a classical behaviour of the particles. The size of the particles and the width of the particle size distribution curve have been estimated from an analysis of the variation of the birefringence with the applied magnetic field; the results obtained are in good agreement with results obtained from electron microscope and magnetic susceptibility measurements.