Neutron spin echo study of low-T-c ferrofluid

We studied the diffusion of single domain particles of Mn0.3Zn0.7(Fe2O4) (R -p approximate to 100 Angstrom) stabilized by oleic acid in dodecane near t he transition temperature T-c, of particles from superparamagnetic to param agnetic state. The structure of the ferrofluid has been studied by small an gle neutron scattering. We found a strong density fluctuation near T-c, 2 2 0 degrees C and H = 0, induced by critical spin diffusion. In addition, we studied this system using neutron spin echo (NSE) inelastic scattering. The temperature variation of the self-correlation function G(S)(T, q, t) = exp [ - q(2)D (t/tau)(beta(T))] at momentum transfer q = 0.07 Angstrom (-1) in the time interval t = 0.5-40 ns reveals strong influence of magnetic fluctu ations on the mobility: these fluctuations decrease the diffusion rate D an d induce stretched relaxation (beta approximate to 0.6) at T < T-c approxim ate to 16 degrees C. Above T,, Stokes diffusion dominates (beta approximate to 1) (C) 1999 Published by Elsevier Science B.V, All rights reserved.