Micromagnetic Simulation of Ferromagnetic Nanowires

Computational simulations are widely used across the engineering and science disciplines to complete experimental and theoretical studies. In the research and development of magnetic devices, computational micromagnetic simulations are a powerful tool that allows not only to predict the macroscopic magnetic properties of a sample, but also to understand the microscopic distribution of magnetic moments (the magnetic texture) that is otherwise difficult to explore with experimental tests. The present work makes use of the micromagnetic simulation program OOMMF (Object Oriented MicroMagnetic Framework), which is an open source domain program written in C++. The input for the program is a text file that is used to define the sample geometry and all the simulation conditions (model). The aim of this study was to develop a model to simulate the magnetic response of a nanowire, either made by a ferromagnetic material, or made by a ferromagnetic core and a shell of a different material. By running some simulations, it was possible to explore how the wire geometry and material parameters influence the magnetic response and the local magnetic moment distribution. Nanowires are 1D nanostructure that exhibit an enhanced aspect ratio. This high value of shape anisotropy leads to unique magnetic properties and makes nanowires promising candidates for applications in nanoelectronics, for example as recording media in magnetic storage. Two ferromagnetic materials that show a different magnetic response were studied. Firstly, Nickel nanowires were used to show the correlation between wire diameter and magnetic response. Then a Cobalt nanowire was modelled to study the anisotropy of the magnetic response and texture due to crystal structure. The model allows to simulate the interaction between the nanowire and the surrounding material and it was designed to study the phenomenon of exchange bias. This phenomenon is a surface effect that happen when two magnetic materials are coupled at the interface. At the macroscopic level, the main consequence is the displacement of the hysteresis loop. By performing some simulation on a wire made by a Co core and a CoO shell, it was possible to prove that the displacement is controlled by the coupling at the interface. It was also possible to verify that the exchange bias is a surface effect, hence its efficacy depends on the ration interface/volume.