Combined FORC and X-ray microscopy study of magnetisation reversal in antidot lattices

Magnetic nanostructures, that are patterned on the length scale of the dipole and exchange interaction, have gained significant scientific interest in the past years [1-6]. These nanostructures have great potential for technological applications in data processing and storage, and spintronics [1-6]. Magnonic crystals are a class of such nanostructures and are metamaterials with periodically alternating magnetic properties - similar to photonic crystals [2,3,4]. This periodic variation is achieved by creating holes in a magnetic host material to form a so-called antidot lattice. The introduction of the artificial antidot lattice changes the spin wave dispersion in the material and can be used to form a spin wave guide or filter [2,4]. To tune the spin wave dispersion, understanding the magnetisation states and the static magnetic properties is of great importance. These static properties like the anisotropy, the coercivity and the orientation of the easy axes are determined by the hole size and distance, the antidot lattice symmetry and its orientation, and the magnetic host material [1,2,5,6]. Here, we present new insights into the magnetisation reversal behaviour of nanoscaled hexagonal antidot lattices, patterned both in in-plane (Fe) and out-of-plane (GdFe) magnetised thin films. The antidots were prepared by polystyrene self-organisation lithography or FIB milling of the magnetic materials [6]. An approach combining first-order reversal curve (FORC) measurements and x-ray microscopy (XM) with magnetic contrast was used to identify irreversible processes and to subsequently image their microscopic origin. Using a fast laser magneto-optical Kerr effect (MOKE) based FORC technique [7], it was possible to individually measure specific sample areas (spatial resolution <2 μm) and to compare a large number of samples. Subsequent XM investigations allowed to reproduce, localise, and quantify the magnetic states involved in the reversal processes.