Projekte

During the past years we have developped - and continously improved - a two-dimensional colloidal model system to study the effects of structural ordering and dynamical response of magnetically interaction colloidal particles caused by an external magnetic field B. Our system consists of porous colloidal particles made superparamagnetic by doping the nano-sized pores with crystalline iron oxides. The magnetic field induced dipole moment gives rise to a magnetic dipole-dipole interaction between particles which was shown to dominate other colloidal interactions by several orders of magnitude. The interaction strength is calibrated providing a field dependent interaction parameter T which corresponds to an inverse effective system temperature. If the particles are confined in a plane - in our case at the horizontanl free water-air interface - hexagonal 2D mono-crystals can be prepared in a constant vertical homogeneous magnetic field. We have investigated the melting of such hexagonal crystals, and, by studying various physical quantities determined from the particle´s time dependent coordinates, we found first quantitative evidence or all essential features of the two-step melting scenario of Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY).

The goal of this project is to establish experimentally the phase behaviour of this system when (a) the hexagonal symmetry is broken by a B-field tilted of the normal, (b) a sinusoidal modulation of B is applied, (c) a sudden change of B(t) is performed and (d) a homogenous in-plane magnetic field gradient is used. Because of the reduced symmetry we expect a variety of fundamentally different phases and melting scenarii deviating from KTHNY, depending on amplitude, direction and time dependence of the applied external field. In order to keep the complexity as small as possible the direction of B will be kept (essentially) vertical for (b)-(d). All experiments will be done in close interrelation with another project where the very same questions will be treated theoretically, in order to compare and check exoperimental results with theory, and vice versa. During the past years we have developped - and continously improved - a two-dimensional colloidal model system to study the effects of structural ordering and dynamical response of magnetically interaction colloidal particles caused by an external magnetic field B. Our system consists of porous colloidal particles made superparamagnetic by doping the nano-sized pores with crystalline iron oxides. The magnetic field induced dipole moment gives rise to a magnetic dipole-dipole interaction between particles which was shown to dominate other colloidal interactions by several orders of magnitude. The interaction strength is calibrated providing a field dependent interaction parameter T which corresponds to an inverse effective system temperature. If the particles are confined in a plane - in our case at the horizontanl free water-air interface - hexagonal 2D mono-crystals can be prepared in a constant vertical homogeneous magnetic field. We have investigated the melting of such hexagonal crystals, and, by studying various physical quantities determined from the particle´s time dependent coordinates, we found first quantitative evidence or all essential features of the two-step melting scenario of Kosterlitz, Thouless, Halperin, Nelson and Young (KTHNY).

The goal of this project is to establish experimentally the phase behaviour of this system when (a) the hexagonal symmetry is broken by a B-field tilted of the normal, (b) a sinusoidal modulation of B is applied, (c) a sudden change of B(t) is performed and (d) a homogenous in-plane magnetic field gradient is used. Because of the reduced symmetry we expect a variety of fundamentally different phases and melting scenarii deviating from KTHNY, depending on amplitude, direction and time dependence of the applied external field. In order to keep the complexity as small as possible the direction of B will be kept (essentially) vertical for (b)-(d). All experiments will be done in close interrelation with another project where the very same questions will be treated theoretically, in order to compare and check exoperimental results with theory, and vice versa.

• Maret, Georg - Projektleiter*in

• FB Physik