  Rotating all islands in square artificial spin ice (ASI) uniformly about
their centres gives rise to the recently reported pinwheel ASI. At angles
around 45$^\mathrm{o}$, the antiferromagnetic ordering changes to ferromagnetic
and the magnetic configurations of the system exhibit near-degeneracy, making
it particularly sensitive to small perturbations. We investigate through
micromagnetic modelling the influence of dipolar fields produced by physically
extended islands in field-driven magnetisation processes in pinwheel arrays,
and compare the results to hysteresis experiments performed in-situ using
Lorentz transmission electron microscopy. We find that magnetisation end-states
induce a Heisenberg pseudo-exchange interaction that governs both the
inter-island coupling and the resultant array reversal process. Symmetry
reduction gives rise to anisotropies and array-corner mediated avalanche
reversals through a cascade of nearest-neighbour (NN) islands. The symmetries
of the anisotropy axes are related to those of the geometrical array but are
misaligned to the array axes as a result of the correlated interactions between
neighbouring islands. The NN dipolar coupling is reduced by decreasing the
island size and, using this property, we track the transition from the strongly
coupled regime towards the pure point dipole one and observe modification of
the ferromagnetic array reversal process. Our results shed light on important
aspects of the interactions in pinwheel ASI, and demonstrate a mechanism by
which their properties may be tuned for use in a range of fundamental research
and spintronic applications.
