Effects of the shape of elongated magnetic particles on the coercive field

Magnetic nanowires could be the building bricks in the fabrication of composite magnetic materials because of their large intrinsic shape anisotropy. We investigate the relation between the detailed shape of a magnetic nanowire and its magnetic coercivity. We have performed three-dimensional micromagnetic simulations on various types of nanowires synthesized during the past few years such as cylinders, dumbbells, or diabolos. The calculations, performed on individual model objects, show that the wire tip plays a key role in the reversal mechanism and on the magnitude of the coercive field and that the aspect ratio plays a much lesser role. Ellipsoidal or cylindrical shapes favor a coherent rotation of the magnetization and thus large coercive fields. Complex tip shapes act as nucleation points and significantly reduce the coercive field. Thus, in order to optimize the shape of magnetic nanowires for permanent magnet applications, the focus should be put on the detailed shape of the wire tips and thus on the growth mechanism rather than on the aspect ratio. The numerical modeling results on individual wires are compared with the experimental data obtained on various types of wires synthesized by soft chemistry methods.