Abstract
We have used frequency-resolved (100Hz<f<10,000Hz) ac magnetic susceptibility measurements to directly determine the Néel and Brown relaxation times in 30-nm-size Fe3O4/hexane magnetic fluids at temperatures between 200 and 300K. Our data collected on both powder and magnetic fluid samples allow the separation of the contributions from the Néel and Brown relaxation mechanisms that act concomitantly within the above-mentioned temperature range. At all temperatures we find that the Brown relaxation times (τB) are shorter than their Néel counterparts (τN), evidence that the Brown mechanism yields the major contribution towards the system’s overall superspin dynamics. τB exhibits a steep two-order-of-magnitude decrease upon heating, from τB=1×10−3s at T=237K to τB=1.5×10−5s at T=270K, a behavior mostly driven by the heating-induced reduction of the liquid carrier’s viscosity.
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