Colloquium: Thermalization, Defects, and Avalanches in Artificial Spin Ice

10/06/2017 - 3:30pm to 4:30pm
CP 155
Speaker(s) / Presenter(s): 
Professor John Cumings (University of Maryland)
Type of Event (for grouping events):
Artificial spin ice is a model nanomagnetic system, where nanoscale bar magnets are fabricated on a periodic lattice with dimensions and geometry intended to model magnetic materials. On some lattices, the bar magnets can assume a regular ordered ground state, but on other lattices, the configuration does not permit such orderings and the material is said to be frustrated. It has been show through thermalization that square lattice artificial spin ice structures composed of various specialized materials can obtain the antiferromagnetic ground state of the lattice, sometimes with the occurrence of domain walls separating domains of order.  Using this as a starting structure, we seek to understand the effect of the introduction of lattice defects, namely edge dislocations.  In two dimensions, an edge dislocation is a point defect, but we show that it can create an extended region of frustration, in the form of an antiphase domain wall terminating at the defect. I will discuss computational modelling of the behaviour of the ice structure and the behaviours observed when the lattice contains two defects as a function of their relative distance and orientation.  I will also discuss the reversal behaviour of frozen artificial spin ice structures on the kagome lattice.  In these specimens, we observe avalanches during the reversal process that take on various stochastic sizes, which fall along a power law distribution.  This behaviour suggests the occurrence of criticality in these systems, and I will present a statistical analysis within that context. Numerical simulation results and the possibility of universal behaviour will be presented and discussed.
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