Decreasing the size limit for a stable magnetic vortex in modified permalloy nanodiscs

This work proposes the use of tower-like structures (small diameter cylinders) embedded on nanodiscs in order to reduce the minimum size of disc diameter to sustain a vortex as ground state. By simulating the dynamics of nanostructured discs of Py-79, we demonstrate that the geometric modification of piling smaller diameter cylinders on top and bottom of a larger diameter disc, introduces a large out-of-plane anisotropy, which in turn allows for the tailoring of stable vortices, even as the diameter of disc and cylinders are greatly reduced, down to dozens of nanometers. This geometric modification is in contrast to the same result (of decreasing the overall size of structures containing vortices) if we introduce vacancies inside a nanodisc, turning it into a ring shaped structure. Such ring structures, while experimentally easy to achieve and also allowing for a stable vortex to form with a well defined chirality, act as an attractive site for vortices, effectively diminishing or completely eliminating the polarity component of the vortex. The proposed tower structures, however, are shown to not only preserve both vortex chirality and polarity, but also exacerbates the polarity, thus increasing the stability of the vortex ground state by pinning. This result introduces the possibility of creating devices that require a reliable magnetization profile, in the sense that the vortex becomes strongly pinned to the tower, which also has a well defined magnetization, rather than being bound to a vacancy which may potentially collapse the ground state. Upon constructing magnetization phase diagrams for regular discs and discs that contain these tower-like structures, we conclude that the latter are appropriate candidates for a high density, high reliability device, suitable for potential applications that require vortex pinning, such as in biomedicine and computation.