THERMAL ORDERING AND DEFECTS IN ARTIFICIAL MAGNETIC SQUARE ICE
Artificial spin ices (ASIs) are 2D arrays of single domain nanomagnets, designed for the user-defined exploration of the physics of competing interactions and collective ordering [1-3]. A periodic lattice with strong local anisotropies captures the essence of geometrically frustrated materials such as pyrochlore spin ice and water ice , with Ising-like nanobar magnets converging at interlinked vertices. Furthermore, they are realisations of well known models in statistical mechanics. Crucially, the governing dipolar interactions can be tailored via nanopatterning and real-space observation of magnetic order is allowed via microscopy.
Until recently, attention has largely focussed on the response of athermal systems to applied fields. For example, ac demagnetisation yields "icy" short-range correlated states however fails to access the true square ice ground state (GS) . Furthermore, the generation and manipulation of magnetic "monopole" charge defects, analogous to ionic conduction defects in water ice , has also become a topic of intense interest.
In this presentation I will discuss my previous work conducted at the University of Leeds, which reports the first experimental observations and subsequent studies of true thermal ordering in ASIs [2,3,6]. This is achieved via an one-shot early-fabrication-stage anneal process, which can allow for extensive GS ordering to be frozen into such systems. This picture is supported by the identification of a thermal distribution of magnetic excitations, within which evidence for charge-charge interactions can be identified. I will show how the strength of magnetic ordering can be controlled by the competing effects of dipolar coupling strength and quenched disorder, and parameterised using an effective temperature formalism.
To close, future directions for this fascinating
field will be discussed.
 R. F. Wang et al., Nature 439, 303 (2006); X. Ke et al., PRL 101, 037205 (2008)
 J. P. Morgan, A. Stein, S. Langridge & C.H. Marrows, Nature Physics 7, 75 (2011)
 Z. Budrikis, K. Livesey, J. P. Morgan, J. Akerman, A. Stein, S. Langridge, C. H. Marrows & R. L. Stamps, New Journal of Physics 14, 035014 (2012)
 M. J. Harris et al., Phys. Rev. Lett. 79(13), 2554 (1997)
 C. Castelnovo et al., Nature 451,42 (2008)
 J. P. Morgan, J. Akerman, C. Phatak, A. Stein, S. Langridge & C. H. Marrows, Phys. Rev. B 87, 024405, (2013)
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Feb 14, 2013
02:30 PM to
Mott Seminar Room
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