Magnetic birefringence of ferrofluids. II. Pulsed field measurements

For pt.I see ibid., vol.12, p.311 (1979). A method is described for measuring the rotary diffusion coefficient of ferrofluid particles, and hence their hydrodynamic size, by observing the decay of their birefringence after a magnetic field pulse. Details of the pulse amplifier used to produce field pulses of from 8*10 -3 to 8*10 -2 T with decay times of 3.6 to 36 mu s are given. The origin of the birefringence in ferrofluids is discussed and it is concluded that it is predominantly due to the shape anisotropy of the particles: particles in which Neel rotation occurs do not contribute to the birefringence. The existence of Faraday rotation in Co and Fe 3 O 4 ferrofluids is also reported.