Exploring the use of two-photon lithography and deposition to produce arrays of magnetic elements

The 3D nanostructuring of magnetic materials provides unique opportunities, both in realisation of novel spin textures in 3D geometries as well as perturbing well-known configurations seen in 2D systems. We show a new and simple means to take any planar spin texture and apply 3D defects which can take the form of simple Gaussian bumps or more complex geometries. In this study, the focus is on the much-studied vortex state which is usually encountered in planar ferromagnetic elements. Two-photon lithography is used with a negative resist, to produce arrays of the structures. Two types of samples have been fabricated. In the first case, simple elements are produced with a flat surface. In the second case, the upper planar surface is perturbed with a 3D hemispherical defect. Subsequent evaporation allows Ni81 Fe19 (40 nm) to take the geometry of the underlying polymer. Samples were subject to standard physical characterisation including scanning electron microscopy and atomic force microscopy. These showed structures with low edge roughness and an accurate representation of the designed defect. The magnetic reversal of both samples was studied using MFM. For the case of unperturbed samples, a vortex state was observed, with central core. Application of a field yielded vortex core translation, annihilation and re- nucleation as seen in conventional planar systems. Interestingly, for perturbed samples a vortex state was seen but with a significantly different core position at remanence and a modified trajectory during magnetic reversal. The results are discussed in the context of micro-magnetic simulations