Design and Control of Three Dimensional Magnetic Fields and Solitons in Helical Nanostructures

The interplay between geometric topology and magnetic spin topology can lead to novel phenomena such as effective anisotropies, formation and stabilization of intricate 3D spin textures, and emergent 3D magnetic fields consisting of knots [1]. Such effects can be easily studied in 3D nanostructures which can now be easily fabricated using focused electron beam induced deposition method [2]. Amongst them, nanohelices represent an important 3D nanostructure owing to their intrinsic geometric curvature and chirality [3]. Lorentz transmission electron microscopy (LTEM) provides a unique combination to characterize not only the microstructure but also the magnetic domain structure of such nanostructures at a high spatial resolution. In this work, we will present our work on various forms of nanohelices such as interconnected arrays of nanohelices, and helicoids. In interconnected nanohelices, we can create controllable topological stray field patterns by reconfiguring the magnetic state of the nanostructure [4]. By applying external magnetic fields, the magnetization of the nanostructure can be reconfigured, creating corresponding arrangements of magnetic charges in 3D space. These charges lead to unique forms of the emanating stray field localized in the gaps of the nanostructure. Furthermore, we will also discuss the formation of various solitons formed in such arrays during in-situ magnetization reversal. We will also discuss the fabrication of 3D helicoids with varying pitch and its effect on the resulting domain wall formation and the behavior under applied magnetic field. We will discuss the imaging of the stray fields and domain structure using off-axis electron holography and supported by micromagnetic simulations.