Direct Measurement of Superparamagnetic Fluctuations in Monodomain Fe Particles via Neutron Spin-Echo Spectroscopy

Using the technique of neutron spin-echo spectroscopy, we have conducted a direct measurement of the superparamagnetic relaxation of nanoscale, magnetic monodomain iron particles in the time range between 0.01 and 160 nsec and for momentum transfers q between 0.035 and 0.15^-1. Using a phenomenological model which includes the effect of a particle size distribution, we are able to determine the temperature T and q dependence of the longitudinal superspin relaxation time spanning 4 orders of magnitude. We find that for T40K, interparticle correlations affect the longitudinal superspin fluctuations.