Magnetic Imaging of Artificial Spin Ice in an Electron Microscope

John Cumings (University of Maryland)

Abstract: Most materials are expected to become perfectly ordered at low temperature, but some cannot because their ordered state is not a unique ground state. A famous example of this is ice, where competing interactions prevent protons from ordering at low temperature. Spin ices exhibit the same physics, with an analogous crystalline structure but residual entropy due to magnetic disorder. Many questions remain for these systems, partly because the role of crystal imperfections is not understood. Toward this end, a new model system has recently been proposed and demonstrated that takes advantage of nanolithography to create synthetic magnetic lattices, dubbed "artificial spin ice". Interactions within these patterns are expected to reveal the same physics, but prior results show only a statistical trend toward obedience of an ice rule. I will present extensions of this work to a new lattice and show that the resulting magnetic arrangements exhibit the perfect frustration one would expect for a T=0 spin ice. With this, the system is poised to answer basic questions of frustrations, such as the role of crystal imperfections in sustaining or removing the disordered state. Additionally, it illuminates the role frustration and entropy might play in technologically important magnetic systems, such as patterned-media magnetic memory.

Bio: Dr. Cumings earned his BA in physics from Boston University in 1997, and his Ph.D. from the University of California at Berkeley in 2002. He held a postdoctoral research position at Stanford for two years before becoming an Assistant Professor at the University of Maryland in 2005. His research focuses on understanding the dynamic properties of nanoscale systems through transmission electron microscopy and other imaging techniques. He was the first to fabricate a nanoscale bearing from a multiwall carbon nanotube and has made many other contributions to nanoscience. Among other awards, he received an IBM Research Fellowship (2001-2002) and The MSGS Award for Outstanding Advising in Materials Science and Engineering (2007).

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