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 $0.15{Å}^{-1\mathrm{}}$. 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 $T\lesssim 40\mathrm{K}$, interparticle correlations affect the longitudinal superspin fluctuations.