  For over ten years, arrays of interacting single-domain nanomagnets, referred
to as artificial spin ices, have been engineered with the aim to study
frustration in model spin systems. Here, we use Fresnel imaging to study the
reversal process in 'pinwheel' artificial spin ice, a modified square ASI
structure obtained by rotating each island by some angle about its midpoint.
Our results demonstrate that a simple 45{\deg} rotation changes the magnetic
ordering from antiferromagnetic to ferromagnetic, creating a superferromagnet
which exhibits mesoscopic domain growth mediated by domain wall nucleation and
coherent domain propagation. We observe several domain-wall configurations,
most of which are direct analogues to those seen in continuous ferromagnetic
films. However, novel charged walls also appear due to the geometric
constraints of the system. Changing the orientation of the external magnetic
field allows control of the nature of the spin reversal with the emergence of
either 1-D or 2-D avalanches. This unique property of pinwheel ASI could be
employed to tune devices based on magnetotransport phenomena such as Hall
circuits.
