Magnetism in frustrated nanostructures

Artificial Spin Ice (ASI), comprised of ferromagnetic nanobars in a honeycomb geometry, attracts much attention since it is a directly imageable frustrated system which exhibits rich physics including ice-rule behaviour and magnetic monopole excitations. ASI’s nanobars undergo domain wall mediated magnetic reversal in external fields. Understanding and indeed controlling the trajectories of field driven domain walls and hence the order in which ASI’s nanobars are reversed is a crucial step towards mapping out ASI’s full functionality for potential applications. In this thesis, trajectories of domain walls during the early stages of ASI’s magnetic reversal are studied. Data showing domain walls executing non-random walks in the transverse domain wall regime and random walks in the vortex domain wall regime is presented. The former behaviour is linked to the domain wall’s chirality, and as such, attempts to control a domain wall’s initial chirality via triangular injection pads are discussed. In addition, ways in which a vortex domain wall’s chirality may be controlled with ellipsoidal hole obstructions are shown. Artificial Dipolar 2D-XY, a complementary system to ASI, also promises interesting behaviour. In this new frustrated architecture, individual nanobars are replaced with single domain nanodiscs whose magnetisations can point in any in-plane direction. In this thesis, intriguing results from preliminary experiments on this new system are presented. One of the best techniques used to study the magnetisations of nanostructures such as those described above is Lorentz Transmission Electron Microscopy (LTEM). Since the contrast yielded for unusual magnetic states was not well documented, software called Micromagnetic Analysis to Lorentz TEM Simulation (MALTS) was developed in order to aid in analysis of LTEM images. MALTS can simulate the LTEM contrast of any magnetic object of any size, shape or state. A description of its full functionality is also included in this thesis.Open Acces