Abstract
Using a many-state relaxation formalism and combining it with an accurate expression for the relaxation time of single-domain ferromagnetic particles and a log-normal size distribution we are able to fit Mössbauer spectra which span temperatures from where a pure hyperfine field distribution is present to where a single broad line occurs due to high relaxation rates. With this model, we have fitted the Mössbauer spectra of two Fe3O4 ferrofluids, from 12 to 180 K. Fits to the ferrofluid with a mean particle size of 4.5 nm yield an anisotropy energy (K) of 3.0±0.1×104 J/m3 with the onset of magnetic relaxation at 30±5 K while data for a mean particle size of 6.0 nm give K=2.4±0.2×104 J/m3 with magnetic relaxation starting at 55±5 K. These blocking temperatures agree with those extrapolated from frequency dependent χac data.
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