Magnetic dipole configurations in honeycomb lattices: order and disorder

Dipolar spin ice has attracted much attention because of its intriguing ground state ordering and elementary excitation properties. We present experimental realizations of magnetic dipolar spin ice on periodic lattices with honeycomb symmetry. We have analyzed in particular the evolution and distribution of excitations with magnetic charges +/-3 per vertex as a function of magnetic field and the distance b between the dipoles ranging from b = 0.4 to 1.7 mu m. In all the dipole patterns investigated, we observe a surprisingly high abundance of +-3 magnetic charges at coercivity in the descending and ascending branches of the magnetic hysteresis. At the same time, these +/-3 vertices form a charge ordered state with large domains, resembling an ionic crystal. Monte Carlo simulations of the magnetization reversal confirm in the framework of a macrospin model an enhanced abundance of +/-3 magnetic charges at the coercive field. But much better agreement is achieved by taking into account the micromagnetic reversal mechanism, which proceeds via nucleation and domain wall propagation for dipoles aligned with the field and via coherent rotation for all others.