Collective behavior of artificial spin ice in the presence of a magnetic field

Artificial spin ice systems are arrays of single-domain ferromagnetic islands that are user designed to study the collective behavior of interacting magnetic systems. I will report on the macro- and microscopic behavior of square artificial spin ice in the presence of an external magnetic field. First, using SQUID magnetometry, I will show the evolution of the magnetic properties of an extended array of islands with temperature and magnetic field and extract not only macroscopic magnetic features of the arrays, but also detailed microscopic information on the mechanisms behind the magnetic reversal of individual island moments. We found that arrays with weakly interacting islands rotate coherently, closely following the Stoner-Wohlfarth model, while arrays exhibiting strong island interactions exhibit a stiffening of the magnetic moments at the ends of the islands which leads to a less coherent island moment rotation. In addition, I will show that the overall shape of the individual islands also plays a critical role in the rotation mechanisms by utilizing bespoke micromagnetic simulation software. Lastly, using magnetic force microscopy, I will show how magnetic-field induced 1D avalanches are affected by nearest-neighbor interactions and defects. These results were used to develop a new model, based on the canonical 1D random field Ising model, to not only predict avalanche behavior in these systems but also as a way to determine and engineer defects in artificial spin ice arrays functioning as a tuning knob for future studies. These results on the fundamental properties of artificial spin ice materials have useful applications for other systems which exhibit avalanches or interacting objects in general.