Field-tunable correlations in perpendicular artificial spin ice arrays*

Artificial spin ice (ASI) provides an effective platform for the study of custom designed frustration and its relationship with geometry, interaction, and stochasticity. Perpendicular ASI is particularly useful in this context, as the state of each element in a lattice is readily accessed using Kerr microscopy and the microstate of the entire lattice can be characterized through an applied field protocol. Unfortunately, studies of perpendicular ASI have been limited by weak interactions between elements. We have overcome this limitation by fabricating perpendicular ASI systems from Pt/Co islands on a soft-magnetic Ni80Fe20 (Py) underlayer to increase interactions. In the simplest case, the Py is saturated and serves to break the lateral symmetry in the arrays. We show that this configuration leads to a highly tunable system with unusual properties such as directionally-tunable interactions, preferred next-nearest neighbor coupling, and in situ adjustable coordination number.