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Title: Multi-channel tag based on fluid-suspended magnetic microparticles
Author: Cunningham, Lucy Katharine
ISNI:       0000 0004 8500 5504
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2020
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The applications of magnetism are far reaching, and much work has been done to make use of magnetic properties to develop viable applications. Two key areas are solid state memory, for example MRAM and hard discs, and the use of superparamagnetic nanoparticles in biological applications by using mechanical actuation. However, little has been done to bridge the gap between these two areas and bring to reality the possibility of carrying data in a liquid of magnetic particles. Ferrofluids to date are mostly simple iron oxide particles suspended in liquid, whereas this project uses micron lithography on thin film films of magnetic material which can be lifted off into solution to create an artificial ferrofluid of advanced materials. We hope to develop such a fluid with the same functionalisation which can be achieved through the thin film structures on a solid substrate. The particles can be redeposited onto a substrate to 'tag' it. By controlling the magnetic properties of batches of particles, we can detect the presence or absence of a given particle type hence providing a yes/no bit. In this thesis we first use the shape anisotropy of rectangular particles patterned from ultrathin films of Permalloy to control their magnetic properties. The extended shape introduces in-plane uniaxial anisotropy, with hysteresis along the easy axis. The dimensions of the rectangle determine the demagnetizing field so alter the coercivity. We can detect the presence of a particular particle shape by whether or not there is a switch measured at its specific coercivity. We characterize the particles and find that the coercivity and the ferromagnetic resonance peaks are specific to the particle dimensions. We also demonstrate that the particles can be lifted off into solution, redeposited under an applied field and detected in their dispersed form. However, the number of achievable channels is very low so we move on to an alternative system. We fabricate discs from magnetic multilayers consisting of ultrathin CoFeB films with interfacial perpendicular magnetic anisotropy which are antiferromagnetically interlayer exchange coupled via the Rudermann-Kittel-Kasuya-Yoshida interaction. They form synthetic antiferromagnets (SAFs) with uniaxial anisotropy along the surface normal. The coupling strength can be tuned by inserting ultrathin Pt layers between the CoFeB and the central Ru layer, which act to attenuate the exchange coupling. We characterize a range of multilayers each with specific coupling strengths and therefore different switching fields, which will be the basis of the channels of the tag. These are then patterned and lifted off into solution, then measured when redeposited to confirm the retention of their magnetic properties. We also investigate the time-dependence of the switching fields of individual CoFeB thin films and of the SAF bilayer films. In conclusion, we create a multi-channel tag from SAF discs with tuneable switching fields and demonstrate that the properties of the continuous film can be retained when patterned into discs which are lifted off into solution and redeposited onto a substrate.
Supervisor: Cowburn, Russell Sponsor: Ingenia Holdings Ltd
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: multi-channel ; tag ; magnetic ; microparticles ; fluid