Speaker
Description
Artificial Spin Ices (ASIs) are arrays of strongly correlated nano-scale magnetic islands. They function as an excellent physics playground in which to study the role of topology on critical phenomena. In this work, we investigate a variation on the canonical square ice system: namely, each island in an array is rotated about its centre through some angle. This rotation angle then defines a continuum of possible spin ice geometries and acts as a proxy for controlling the strength of interaction among classes of neighbouring spins in different arrays. This rotation has been shown [1] to weaken the nearest-neighbour coupling leading to a system dominated by long-range interactions, with a phase transition in ordering from antiferromagnetic - "square" - ice to ferromagnetic - "pinwheel" - ice. Here, we fabricate Co arrays using focused electron beam induced deposition. These arrays - of varying rotation angles - are used to map out this AFM-FM transition experimentally. The arrays are thin enough such that individual islands are thermally active close to room temperature. We use the Fresnel method of Lorentz transmission electron microscopy to image configurations after zero-field annealing. From this, we are able to extract experimental measures of the correlations, and note the different behaviour exhibited as the arrays melt. In particular, we observe that the correlation length decreases in the FM phase as short-range emergent vortex structures appear to form. We draw parallels with Monte Carlo results, and discuss the effect of cooling rate on accessing low-energy configurations.
