Investigation of Brownian and Neel relaxation in magnetic fluids

Relaxation of magnetic fluids was studied at room temperature with a DC-SQUID-magnetometer in a time window ranging from a few milliseconds to seconds. Freeze-dried samples and unstable liquid samples with sediments showed Neel relaxation signals which are in excellent accordance with the time dependance expected from theory. The Brownian relaxation of a stable ferrofluid was shorter than the observed time window. The relaxation of a stable magnetic fluid with increased viscosity is well described by a superposition of two exponential decays. The different time dependencies of the Neel and Brownian relaxation processes can be utilised to determine the type of relaxation process and hence allow conclusions on the mobility of the particles. (C) 1999 Elsevier Science B.V. All rights reserved.