Artificial Spin Ice: Designer Matter Far From Equilibrium

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Our project is a collaborative one between two Universities and a national laboratory working together across a combined theoretical and experimental programme. The experiments are based in both conventional laboratories and large-scale facilities. The work programme also involves continued international collaboration with colleagues in the US at Brookhaven National Laboratory, who have helped us make some of our most recent breakthroughs in creating and understanding nanostructured magnets, as well as adding new ones in the form of the unique expertise and facilities available for transmission x-ray imaging at the Advanced Light Source in Berkeley. Both of these are DOE-supported US national laboratories.Our goal is to understand and control non-equilibrium dynamics in a new class of magnetic materials: strongly correlated arrays of sub-micron sized magnets. Examples have been studied recently, and given the name "artificial spin ice". These materials are important examples of new metamaterials with unique properties not realised in naturally occurring magnetic materials. The artificial magnetic ice are systems composed of strongly interacting magnetic moments, and the moments are arranged geometrically in order to produce metastable configurations with a high degree of degeneracy in energy. We will use artificial spin ice as a paradigm for a systematic exploration of non-equilibrium dynamics. This is a model system for which the free energy is specified by design and hence completely determined, and the exact microstate--and its evolution in time--can be observed directly for detailed comparison with mathematical predictions. Inter-element interactions can be specified to a large extent by design through control of geometry. In this way it is possible to create competitions between ordering that result in geometrical frustrations responsible for complex response to applied magnetic fields. The resulting magnetic properties are analogous to those of traditional thin film or bulk magnetic systems, but with key differences that can be of especial importance for applications. For example, there are two ground state configurations in a square ice that form domains with zero net moment. These are separated by magnetised boundary walls along which magnetic charges can move.Magnetic charges (sometimes called emergent monopoles) are of particular interest since they can be readily detected and can be moved through an array by applied magnetic fields. We will develop methods to inject and detect charges, and control their flow through array geometries. Our goal is to identify structures and techniques for the design of circuits through which "magnetricity" can flow and be usefully employed for technological applications. Our idea is to use thermal fluctuations to aid magnetic charge mobility. We can do this by using nanoscale particles in our arrays, such that the particles are near their superparamagnetic blocking temperature. An important distinction with prior work is that we shall use materials with phase transitions close to room temperature to allow us to tune simply between thermally equilibrated and athermal non-equilibrium states. This will mean that the individual moments can reverse spontaneously, thus enabling thermally driven motion of magnetic charges through a fluctuating array. Through a combination of applied fields and temperature control we will be able to start, stop, and direct magnetic charge dynamics. These systems may also give us new experimental models for studies of critical dynamics at phase transitions since they can be modelled by well known exactly soluble Ising systems, as well as providing new paradigms for information processing architectures.