  We propose a mechanism for nucleation and high-density packing of 360{\deg}
domain walls (DWs) on planar ferromagnetic nanowires, of 100 nm width, by using
circular magnetic fields. The extent of the stray field from a 360{\deg} DW is
limited in comparison to 180{\deg} DWs, which allows them to be packed more
densely than 180{\deg} DWs in a potential data storage device. We use
micromagnetic simulations to demonstrate high-density packing of 360{\deg} DWs,
using a series of rectangular 16 x 16 nm2 notches to act as local pinning sites
on the nanowires. For these notches, the minimum spacing between the DWs is 240
nm, corresponding to a 360{\deg} DW packing density of 4 DWs per micron.
Understanding the topological properties of the 360{\deg} DWs allows us to
understand their formation and annihilation in the proposed geometry. Adjacent
360{\deg} DWs have opposite circulation, and closer spacing result in the
adjacent walls breaking into 180{\deg} DWs and annihilating.
