Magnetic properties and magnetization reversal in Co nanowires with different morphology

In the present study, magnetic properties and demagnetization processes of Co nanowires with various morphologies (ellipsoids, capped cylinders, and cylinders) are investigated by means of Object Oriented Micromagnetic Framework (OOMMF) software package with finite difference micromagnetic simulation. The results show that the aspect ratio, morphology, and dipolar interactions of the nanowires play a key role in determining the switching modes and hence the magnetic properties. For the Co nanowires with the same morphology and diameter, the coercivity increases substantially with increasing aspect ratio of the nanowires due to the increased shape anisotropy when the aspect ratio is less than 3. For a single nanowire with a fixed diameter of 15 nm and a length of 200 nm, the coercivity of ellipsoids of 14.45 kOe is noticeably higher than those of capped cylinders of 12.55 kOe and cylinders of 11.15 kOe, indicating the considerable influence of the morphology on the coercivity of the wires. The magnetization reversal mechanism of a single Co nanowire is mainly described by a nucleation propagation process, which starts at the ends of the nanowires. For the nanowires assembly, the magnetic moment reversal of some nanowires occurs first, and the rest reverse gradually with increasing applied field and ultimately the reversal of the whole assembly is achieved. This process could be theoretically interpreted based on the nucleation of closure domains at the ends of the wires as well as the subsequent depinning and propagation of domain walls. Meanwhile, the squareness of the demagnetization curve and coercivity of the nanowires assembly deteriorate faster than those of the single nanowires due to the dipole interactions.