Summary

We have simulated the magnetisation reversal in part-spherical and droplet nanodots. In droplet nanodots, we observe two different mechanisms for this reversal -- the single-domain state and the vortex state.

If the overall size of the droplet system is increased then we notice a distinct transition from the single domain state to the vortex state, which we identify for isotropic nickel (see appendix C) as being at a droplet bounding sphere diameter of 140nm (physical droplet diameter in this instance is around 90% of the bounding sphere diameter, approximately 126nm, and $h=$3$/$7$d =$ 60nm). This change occurs at a smaller diameter than for thin ($h \leq$ 30nm) circular nanodots (see figure 3.10).

The ``soft'' vortex behaviour -- i.e. the vortex will readily adjust its position to accommodate a change in applied field -- observed in the large droplets is a useful characteristic in sensor applications; smaller droplets have the square hysteresis loops desirable for data storage.

In part-spherical particles, three separate remanent states -- single-domain, out-of-plane vortex and in-plane vortex -- have been observed.

As the diameter of the part-spherical particle decreases, a larger $h/d$ is necessary for a vortex to form. As $h$ is increased, the magnetisation is more likely to form a vortex. Below a critical radius of 12.4nm for Ni$_{50}$Fe$_{50}$, all $h/d$ values will result in a single-domain remanent state.

The hysteresis loops observed experimentally agree well with the numerical results from the simulation.

We observe good agreement between the finite difference method in OOMMF and the hybrid finite element/boundary element model in magpar.