Claudio Scherer

Ferrofluids: properties and applications

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.

Direction implantation and radiation enhanced diffusion of tin into iron

Abstract The surface layers of pure iron samples treated by both direct ion implantation of Sn+ and radiation enhanced diffusion of tin are analyzed by means of Rutherford backscattering and 119Sn conversion electron Mossbauer scattering. It is argued that both processes produce amorphous alloys of iron and tin on the treated surfaces. The presence of amorphous phases is more pronounced on the directly implanted samples, while on the radiation enhanced diffused samples there is a marked tendency for the formation of intermetallic compounds. These results are consistent with some empirical rules currently developed for predicting the formation of amorphous intermetallic phases by ion beam mixing. The thermal evolution of the phases formed on the treated surfaces is consistent with the equilibrium phase diagram for the Fe-Sn system, in spite of a certain tendency for precocious decomposition. One unifying aspect is the fact that all the samples here studied show a similar CEMS spectrum after annealing in vacuum at 550°C, when only an unidentified singlet (δ = 1.60 mm·s−1) is observed; this is interpreted as being due to tin segregation at the grain boundaries of th substrate.

Stochastic Molecular Dynamics of colloidal particles

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.

Computer Simulation of the Stochastic Dynamics of Super-Paramagnetic Particles in Ferrofluids

Several papers have been written on the complex problem of the stochastic dynamics of the magnetic moments of super-paramagnetic particles, simultaneously with the stochastic rotation of these colloidal particles in a ferrofluid [1-3]. None of these works, however, is sufficiently general and conveniently simple and clear to be used in sumulational works to appropriately describe the experimental super-paramagntetic resonance lines. We have a new proposal for the equations of rotational motion, which is appropriate for simulations. Those equations are stochastic differential equations with multiplicative noise. Therefore, they have to be interpreted as Stratonovich-Langevin equations and the roles of stochastic calculus have to be used in the simulations. For this reason we will briefly present the essence of the numerical algorithms used in the solutions of Stratonovich equations. Finally, the simulational results for the magnetic response functions, and the corresponding dynamic susceptibilities, will be shown and their consequences will be analyzed.

Ion-induced energy propagating front and migration of point defects in metals

Abstract A Molecular Dynamics calculation, using the Embedded Atom Method, shows very clearly the emission and propagation of an energy front from a collision cascade induced by an ion impact in a metal. Additionally, this front is capable of producing change of site of interstitial atoms, situated far from the cascade region. This property entitles the phenomenon to be a candidate for a mechanism of Ion Beam Mixing, not considered in the previous works. The calculation was done at zero temperature.

Ion-induced energy propagating front and migration of point defects in metals – II

Abstract We present a molecular dynamics study of the emission and propagation of an energy front from the core of an ion-induced collision cascade. Cu metal and PtCu dilute alloy were used as model cases. Both systems were modeled using embedded atom method potentials with periodic boundary conditions. We also used 3D visualization to inspect the temporal evolution of the energy distribution in space. Additionally, we have calculated the temporal evolution of the atomic density and atomic migration inside the cascade core region and concluded that the interpretation of the chemical properties of IBM in terms of the known mechanisms is not satisfactory. We also concluded that the mechanism proposed in a previous paper, and examined more carefully here, is a good candidate to fulfill the conditions for the missing interpretation. We also observe that the emission of the energy front plays an important role in the evolution of the dynamics of the system inside the cascade core region.