Magnetic fluids may be classified as ferrofluids (FF), which are colloidal suspensions of very fine (∼ 10 nm) magnetic particles, and magnetorheological fluids, which are suspensions of larger, usually non-stable, magnetic particles. We review the general classification and the main properties of FF, some theoretical models and a few applications. We consider the stability of a FF in terms of various forces and torques on the magnetic particles. We discuss thermodiffusion, which is an important phenomenon in FF, and which gives rise to the Soret effect. We also consider the rotational dynamics of the magnetic moments of the particles. A large portion of this review is dedicated to applications of FF, including a few of the many technological applications. Among the uses of a FF in the study of materials, we have selected the doping of liquid crystals. Among the very promising uses in Medicine, we discuss drug targeting, hyperthermia, cell separation, and contrast in magnetic resonance imaging. We also make some comments on directions for future research on the properties of ferrofluids.

We examine the temperature and frequency dependence of the proton spin-lattice relaxation rate 1/T" as a probe of the electron-spin dynamics of the quasi-one-dimensional exchange-coupled paramagnet TMMC [(CH3),NMnC1$. The rate is measured from 5.5-16 MHz and from 1. 4'K to roomtemperature. An extension of Moriya's magnetic-relaxation theory to the linear-chain system, using the exact classical results for static spin-correlation functions, gives over-all good quantitative agreement between theory and experiment. A sharp minimum in 1/T, at T 18'K, an eo '" dependence on frequency at room temperature and co "~a t low temperature, with the external field parallel to the chain, and the frequency independence of 1/T, at room temperature with the field perpendicular ta the chain are all explained by the theory.

An approach for the rotational dynamics of magnetic particles and their magnetic moments, in fluid suspensions, is developed. A possible application is to magnetic resonance in ferrofluids. Based on a generalized Lagrangian formulation for the equations of motion of the particle, we introduce its interaction with the solvent fluid via dissipative and random noise torques, as well as the interaction between the particle and its magnetic moment, treated as an independent physical entity and characterized by three generalized coordinates: its two polar angles and its modulus. In the appropriate limits, it reduces to the cases of superparamagnetic particles or nonsuperparamagnetic ͑blocked magnetic moments͒ particles. It is also indicated how the dynamic complex susceptibility may be calculated from the equations of motion, and as an example the effect of the particles inertia on the susceptibility is numerically evaluated for some arbitrary values of the parameters.

Colloidal particles move in the carrier liquid under the action of several forces and torques. When the particles carry a dipole moment, electric or magnetic, as in ferrofluids, the rotational and translational motions are coupled because the field on a particle depends on the spatial and directional distribution of the others and the force and torque on it depends on the field. Moreover, there is Brownian, as well as dissipative forces and torques on each particle. Consequently, the numerical solution of the equations of motion requires, besides the techniques of Classical Molecular Dynamics, those of Stochastic Dynamics. The algorithm is explained in some detail and applied on a typical ferrofluid. For different values of the temperature, the possibility of the formation of structures is examined.