Dynamics of magnetic monopoles in artificial spin ice

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Office of the Provost, Johns Hopkins University
While finding the elementary magnetic monopole seems not an easy task, recently, scientists have come out an alternative approach by studying emergent particles in spin systems. Spin ice is a magnet with frustrated interactions from which we ob- serve emergent magnetic charges. Two typical spin ice materials are Dy2Ti2O7, with tetrahedral lattice structure. Artificial spin ice is an array of magnetic nano-wires with similar frustrated interactions as spin ice. In this Senior Thesis Project, I studied both meso-scopic behavior and microscopic behavior of the dynamics of artificial spin ice in a honeycomb network of magnetic nanowires made with permalloy. The concept of magnetic charges is introduced for better visualization and interpretation of the magnetization behavior. Microscopic simulations on a single wire and on a honeycomb junction with high damping are presented in this thesis. The dynamics of magnetic charges is observed in our simulation. By finding the critical field that triggers the reversal process on a junction with respect to the angle of external field, an offset angle α is defined in the system to better estimate the critical field at different angle. This thesis also includes a detailed discussion on the avalanche length distribution when an external field is applied to a uniformly magnetized honeycomb lattice sheet. We found when external angle θ ∈ (90, 131), the avalanche length distribution decays exponentially; and when θ ∈ (132, 180), the avalanche length distribution decays as a power law. This work concludes with the introduction of inertia and its characteristic pa- rameter ε that helped us deal with the case when magnetic charges travels in low damping system.
Offset Angle, Avalanches, Magnetic Monopole, Kagome, Spin Ice