Discontinuous phases and phase coexistence in an artificial Potts lattice

The study of artificial spin ice makes strong allusions to Ising-model-like physics. This is intuitive because the state of each ferromagnetic island can be described by a magnetic domain with a binary degree of freedom. To move past the Ising-paradigm one can strongly couple pairs of islands to act as a composite object. We have fabricated a checkerboard array of island pairs with alternating horizontal or vertical orientation. We find that the quadrupole moment at each lattice site can be used to describe an effective spin state. The extra degrees of freedom allow the system to be described by a Potts Hamiltonian similar to the Blume-Emery-Griffiths model. We study this system by thermalizing the lattice in the presence of an in-situ applied magnetic field. We observe, for the first time, a field-induced, discontinuous phase transition. Our experimental method allows us to globally reconfigure an array of nanomagnets to a state which would be difficult to write using even the most advanced on-demand writing protocols.