Enhanced interaction strength in perpendicular artificial spin ice arrays*

Perpendicular implementations of artificial spin ice are appealing as a custom designed, frustrated system due to the optical accessibility of the magnetic components. This allows for complete microstate characterization in situ with an applied field using magneto-optical Kerr effect microscopy. We have fabricated arrays of lithographically patterned, single-domain Pt/Co multilayer islands. Studies on this system thus far have been limited by the weak interactions between islands that fit the constraints necessary for optical accessibility. New samples fabricated with a permalloy underlayer show an enhancement in correlation and allow for studies beyond the weakly interacting regime. We study both the quasi-dynamic hysteresis and demagnetized states of these systems to gain an understanding of how the coupling is affected by the soft underlayer.