On the Antiferromagneticx2013Ferromagnetic Phase Transition in Pinwheel Artificial Spin Ice

Nanopatterned magnetic thin films offer a platform for exploration of tailored magnetic properties such as emergent long-range order. A prominent example is artificial spin ice (ASI), where an arrangement of nanoscale magnetic elements, acting as macrospins, interact via their dipolar fields. In this study, we discuss the transition from antiferromagnetic (AF) to ferromagnetic (FM) long-range order in a square lattice ASI, as the magnetic elements are gradually rotated through 45 to a "pinwheel" configuration. The AFx2013FM transition is observed experimentally using synchrotron radiation x-ray spectromicroscopy and occurs for a critical rotation angle of the nanomagnets, contingent upon the dipolar coupling determined by their separation in the lattice. Large-scale magnetic dipole simulations show that the point-dipole approximation fails to capture the correct AFx2013FM transition angle. However, excellent agreement with experimental data is obtained using a dumbbell-dipole model which better reflects the actual dipolar fields of the magnets. This model explains the coupling-dependence of the transition angle, another feature not captured by the pointdipole model. Our findings resolve a discrepancy between measurement and theory in previous work on "pinwheel" ASIs. Control of the AFx2013FM transition and this revised model open for improved design of magnetic order in nanostructured systems.