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Title: Exquisitely balanced magnetic switching in microscopic ferromagnetic rings
Author: Hayward, T. J.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2007
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Initially, the switching behaviours of single layer NiFe rings are investigated. It is demonstrated that their magnetic switching is a competition between local energy barriers and the total energy of the rings’ magnetic states. In 5μm diameter NiFe rings the vortex state formed during magnetic switching alternates stochastically from one field cycle to the next. Micromagnetic simulations are used to demonstrate that this behaviour is caused by the thermal excitation of a nanoscopic region of edge spins which have a disproportionate effect on the switching of the ring as a whole. Co/Cu/NiFe pseudo-spin-valve rings are also characterised. It is found that their switching is dominated by the magnetostatic field emanating from head-to-head domain walls in the cobalt layer. This field nucleates reverse domains in the NiFe layer, allowing it to reverse its magnetisation at much lower applied fields than in an equivalent single layer structure. The reproducibility of the magnetic switching of these multilayer rings is also probed. It is shown that although the rings’ switching fields always adopt well defined values, a variety of different reversal paths can be taken which lead to dramatically different switching fields being recorded in consecutive field cycles. Finally, the suitability of the rings for device based applications is investigated. A simple method of reading and writing the circulation of vortex states in pseudo-spin-valve ring structures is presented. Furthermore, it is shown that by choosing the direction in which the ring is saturated prior to obtaining the vortex state, a desired vortex circulation may be selectively written. The read and write mechanisms of a working memory element are therefore demonstrated. A design for a new type of ring-shaped digital biosensor is also presented.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available