Superspin relaxation in Fe3O4/hexane magnetic fluids: A dynamic susceptibility study

We have used frequency-resolved (100 Hz < f < 10,000 Hz) ac magnetic susceptibility measurements to directly determine the Neel and Brown relaxation times in 30-nm-size Fe3O4/hexane magnetic fluids at temperatures between 200 and 300 K. Our data collected on both powder and magnetic fluid samples allow the separation of the contributions from the Neel and Brown relaxation mechanisms that act concomitantly within the above-mentioned temperature range. At all temperatures we find that the Brown relaxation times (tau(B)) are shorter than their Neel counterparts (tau(N)), evidence that the Brown mechanism yields the major contribution towards the system's overall superspin dynamics. tau(B) exhibits a steep two-order-of-magnitude decrease upon heating, from tau(B) = 1 x 10(-3)s at T = 237 K to tau(B) = 1.5 x 10(-5)s at T = 270 K, a behavior mostly driven by the heating-induced reduction of the liquid carrier's viscosity. (C) 2012 Elsevier B.V. All rights reserved.