Frustration and topological defects in hexagonal networks with weak Perpendicular Magnetic Anisotropy

Abstract

An artificial spin ice is an array of nanomagnets that exhibit frustrated magnetic states due mainly to their magnetostactic interactions, and so highly dependent on the lattice geometrical features, with typical lateral dimensions of hundreds of nanometers. Despite it has been a lot of effort over the past decade to construct and understand the static and dynamic behaviour of such systems on magnetical materials with in-plane anisotropy (e.g. permalloy or cobalt) few has been explored on out-of-plane anisotropic materials. In our experiment, we study this through amorphous alloy NdCo5 which exhibits an stripe phase weak out-of-plane domain behaviour, that also allows us for a direct observation of magnetic domain reversal. Sets of continuous kagome lattices were fabricated by electron-beam lithography and further magnetron sputtering deposition. To analyze the role of the out-of-plane anisotropy, a 65 nm thick NdCo5 layer and an analogous 40nm thick Co sample were deposited. To avoid edge effects and to guarantee enough number of elements lattices, the size of the array area is over 200x200 μm2. In order to study size related effects branch width was varied in the range 0.3-3 μm and three increasing length/widht aspect ratios were studied for each case. Magnetic behaviour was characterized by Magnetic Force Microscopy and Magneto-Optical Kerr Effect and a set of micromagnetic simulations were launched to support the observations. Different kinds of topological defects are found to nucleate in these hexagonal networks depending on bar aspect ratio and width in comparison with stripe domain spatial periodicity, and on magnetic history. For bar widths below 400 nm a spin ice regime is found after out of plane demagnetization. The interplay between shape anisotropy, rotatable anisotropy, size effects and geometrical frustration will be discussed in order to understand the experimental results.