Imaging spin textures in advanced magnetic nanostructures using Lorentz microscopy

[摘要] This thesis focuses on the imaging of nanomagnetic systems using Lorentz transmission electron microscopy (LTEM), through which their complex structure, magnetisation processes and interactions may be understood.Magnetic domain walls (DWs) in cylindrical and square nanowires, that possess a complex three dimensional structure, were studied initially. Micromagnetic simulations were used to characterise the structure and stability of cylindrical nanowires and nanotubes across a range of dimensions thus calculating a phase diagram. The phase diagram showed that the less well known asymmetric transverse DW is a metastable state. Square nanowires were fabricated using electron beam lithography and imaged using LTEM. Three different DW types were identified, which were shown to be metastable states.The effect of anti-symmetric exchange induced at the interface of a magnetic thin film, the Dzyaloshinkii-Moriya interaction (DMI), was simulated for magnetic vortices in permalloy (Py) discs. The results were used to assess methods by which its effect could be measured. The DMI was found to cause the magnetisation around the vortex core to become divergent, depending on its strength. In addition, the DMI modified the magnetisation processes of the disc, causing the vortex to be more easily expelled with increasing DMI strength. By depositing Py thin films capped with Pt and a control sample with Cu, the vortex expulsion field was determined for both samples using LTEM in-situ magnetising experiments. This allowed the strength of DMI to be estimated.Finally, the magnetostatic interactions of an array of nanomagnets, a variation of artificial spin ice, was investigated. By using LTEM to image the entire array, a reversal sequence was obtained. This showed that the array behaved in a similar manner to a ferromagnetic thin film with strong anisotropy, since the net moment of the array preferred to be directed along the same direction. Micromagnetic simulations revealed that, contrary to artificial spin ice systems, the system did not display frustration. Further simulations suggested that the behaviour of the array could be changed from ferromagnetic to anti-ferromagnetic by reducing the space between elements.

[发布机构] University:University of Glasgow;Department:School of Physics and Astronomy

[关键词] Nanomagnetism, Lorentz microscopy