Relaxation behaviour of spherical and cylindrical magnetic nanoparticles in liquid media

Markink, E.M. (2012) Relaxation behaviour of spherical and cylindrical magnetic nanoparticles in liquid media.

Abstract:Magnetic nanoparticles are very promising for various applications in medicine. In this research the relaxation behaviour of superparamagnetic particles is investigated. Magnetic relaxation is defined as the return or adjustment to equilibrium after a change of the external magnetic field. Superparamagnetic nanoparticles have the same high magnetic response as ferromagnets, but exhibit no hysteresis. This makes them particularly suited for relaxation experiments. The relaxation behaviour is in uenced by both characteristics of the particles and their environment. This is used in clinical applications, which include in vitro sensing and immunoassays. Physical models describing the relaxation behaviour of both spherical and cylindrical particles are made. Cylindrical particles are thought to be beneficial due to their shape anisotropy. Simulations show indeed that cylindrical particles are allowed to have larger volume at the superparamagnetic limit, indicating a higher magnetic response due to the increased amount of magnetic material. However, surface effects counteract this effect. Quantification of these surface effects was outside the scope of this research, but measurements of the magnetization curves of spherical and cylindrical particles showed that the surface effects are larger than the shape anisotropy effects in the superparamagnetic regime. The relaxation behaviour was measured with two different setups i.e. a superconducting quantum interference device (SQUID) magnetometer measuring the relaxation as a function of time and a differential transformer measuring the susceptibility as a function of frequency. The SQUID magnetometer setup was designed and built in our own lab, partly during this assignment. It uses a static magnetic field to align the magnetic particles. After the field is switched off, the particles will return to a random orientation. The corresponding decrease of magnetic moment as a function of time is measured with the SQUID. The setup using the differential transformer was already developed at Utrecht University, where the measurements took place as well. This setup uses an oscillating magnetic �eld to align the particles. At low frequencies, the particles are able to align to the magnetic field before it changes direction. As the frequency increases, full alignment is not possible anymore. This transition point corresponds to the relaxation time. Multiple samples were used to verify the simulation model with the experiments. Unfortunately, none of the samples was able to confirm or reject the simulation models. The relaxation time of samples containing iron oxide particles was outside the measurement regime for both setups. Samples containing spherical nickel particles showed sedimentation and clustering. At last, the cylindrical nickel particles could not be compared to the simulation model, since they turned out to be ferromagnetic instead of superparamagnetic.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics MSc (60436)
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