Magnetic Reversal of Artificial Kagome Ice
Daunheimer, Stephen Allen
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Geometric frustration is a phenomenon where a crystalline material cannot satisfy all of its competing interactions, which can drastically change the behavior of a material. When water freezes into solid ice, the hydrogen atom positions are geometrically frustrated due to different interactions among neighboring oxygen atoms. Frustration is not limited to electrostatic interactions, though. Magnetic spin ice mimics the crystal structure and, therefore, the frustration of water ice. However, a problem with the spin ices is that the details of the magnetic state cannot be imaged which makes the dynamics difficult to probe. In 2006, a model system known as “artificial” spin ice was created to alleviate these problems. The artificial spin ices are also geometrically frustrated, but they are easier to fabricate, and the interactions in the system can be tailored to suit the experiment. They are made of lithographically defined arrays of interacting ferromagnetic elements, and the entire sample may be imaged to view the details of the magnetic state through a dynamic process. The research presented here focuses on artificial spin ices with a honeycomb shape known as artificial kagome ice. Low disorder samples are created to study the dynamics of the magnetic reversal process to better understand the statistics and kinetics of the reversal process of frustrated materials. Results indicate reversals are defined by a complex avalanche process with a randomness that can be tuned by crystal geometry and reversal angle. Magnetically reversing samples at field angles of 180°, 100°, and 120° allows us to directly measure the disorder in our samples. Many 180° reversals were performed to allow us to measure the randomness of the reversals. Reversals at 180° are highly random, whereas at 100° and 120° they are much less so. There appears to be a change in the nature of the reversals at an angle of θ = 130° where avalanches start appearing in the reversals. As the angle is increased, large avalanches spanning the entire crystal start to dominate the reversal process. The detailed image sequences of an artificial spin kagome ice sample allow us to simply model the behavior of the crystal as the motion of magnetic monopoles. Also, we make connections to the well-studied science of Barkhausen noise in magnetic materials noting that our samples exhibit the power law behavior typical of Barkhausen experiments.