Improved vortex nucleation in truncated soft magnetic cones

Magnetic vortex structures are promising for data storage applications due to the possibility to switch vortex chirality and core orientation independently. Subsequently two bits per element can be stored, which is favorable for high data storage densities. The magnetic vortex state of soft-magnetic nano-disks is stable over a large range of thickness and radii. Nevertheless the formation of a vortex state has to overcome an energy barrier in order to nucleate the vortex at the element’s edge. In the presented work the influence of a large edge tilt on the behavior of vortex nucleation is studied experimentally and by numerical simulations. Single elements and arrays of closely packed elements with a diameter of approx. 300 nm were fabricated by means of nanosphere lithography, by which an element edge tilt of 45° could be achieved. From the simulations it was found that with decreasing edge tilt from 90° (cylindrical dot) to 45° the probability to nucleate a vortex during magnetization reversal increases. For cylindrical elements at the onset of vortex nucleation the out of plane component of magnetization near the edge has opposite signs at the top and bottom of the element. For truncated cones, however, the magnetization points only in one direction and favors the nucleation of a magnetic vortex. Thus, the vortex formation in smaller elements is facilitated by engineering the shape of the dots. For narrowly packed structures it is shown that despite of strong dipolar interactions the magnetization reversal involves a vortex nucleation and annihilation process.