Effects of array shape and disk ellipticity in dipolar-coupled magnetic metamaterials*

Two-dimensional lattices of dipolar-coupled thin film ferromagnetic nanodisks can give rise to emergent ‘superferromagnetic’ (SFM) order when the spacing between dots becomes sufficiently small. This allows for the design of metamaterials, which can be tailored to have a specific type of long-range order magnetic order by tuning the lattice symmetry. We have previously shown experimentally that a square lattice symmetry gives rise to antiferromagnetic order, and a hexagonal lattice gives rise to ferromagnetic order. In the present study, we pattern the arrays into micron sized hexagons, squares, and rectangles in order to investigate shape anisotropy as an effect of the finite-size for such arrays. The magnetic domain patterns were examined using XPEEM and were found to be below their blocking temperature at room temperature. Due to lithographic defects, the dots were found to feature a slight ellipticity, thus giving rise to a shape anisotropy that overwhelmed the anisotropy of the array shape. Distinct differences were found in the magnetic switching characteristics of horizontally and vertically oriented rectangular arrays which was corroborated by micromagnetic simulations.