Permanent magnet system to guide superparamagnetic particles

A new concept of permanent magnet systems for guiding superparamagnetic particles on arbitrary trajectories is proposed. The basic concept is to use one magnet system with a strong and homogeneous (dipolar) magnetic field to magnetize and orient the particles. A second constantly graded field (quadrupolar) is superimposed to the first to generate a force. In this configuration the motion of the particles is driven solely by the component of the gradient field which is parallel to the direction of the homogeneous field. Then the particles are guided with constant force in a single direction over the entire volume. The direction can be adjusted by varying the angle between quadrupole and dipole. Since a single gradient is impossible, the other gradient component determines the angular deviation of the force. The latter can be neglected if the homogeneous field is stronger than the local contribution of the gradient field. A possible realization is a coaxial arrangement of two Halbach cylinders. A dipole produces a strong and homogeneous field to evenly magnetize the particles, while the second cylinder is a quadrupole to generate the force. A simple prototype was constructed to demonstrate the principle on several nano-particles, which were moved along a rough square by manually changing the force angle. The observed velocities of particles were always several orders of magnitude higher than the theoretical value. This discrepancy is attributed to the observed formation of long particle chains as a result of their polarization by the homogeneous field. The magnetic moment of such a chain is then the combination of that of its constituents. A complete system will consist of another quadrupole (third cylinder) to additionally scale the strength of the force by another rotation. In this configuration the device could then also be used as a simple MRI machine to image the particles.