Direct observation of vortex states in an antiferromagnetic layer have been recently reported [Wu, et al, Nature Phys. 7, 303 (2011)]. In contrast to their analogues in ferromagnetic systems, namely in nanomagnets, the vortex core of antiferromagnets
are not expected (and have not been observed) to present gyrotropic or any other remarkable dynamics, even when external fields are applied. Using simulated annealing and spin dynamics techniques we have been able to describe a number of properties of such a vortex state. Besides of being in agreement with reported results, our results also indicate, whenever applied to antiferromagnetic nanodisks, that the presence of holes in the sample may induce two types of motions for this vortex. Its dynamics depends upon the relative separation between its core and the hole: when they are very apart the vortex core oscillates near the nanodisk center (its equilibrium position); while, if they are sufficiently close, the core moves towards the hole where it is captured and remains static.
We consider a nanodisk possessing two coupled materials with different ferromagnetic exchange constant. The common border line of the two media passes at the disk center dividing the system exactly in two similar half-disks. The vortex core motion cr
ossing the interface is investigated with a simple description based on a two-dimensional model which mimics a very thin real material with such a line defect. The main result of this study is that, depending on the magnetic coupling which connects the media, the vortex core can be dramatically and repeatedly flipped from up to down and vice versa by the interface. This phenomenon produces burst-like emission of spin waves each time the switching process takes place.
We investigate the influence of artificial defects (small holes) inserted into magnetic nanodisks on the vortex core dynamics. One and two holes (antidots) are considered. In general, the core falls into the hole but, in particular, we would like to
remark an interesting phenomenon not yet observed, which is the vortex core switching induced by the vortex-hole interactions. It occurs for the case with only one hole and for very special conditions involving the hole size and position as well as the disk size. Any small deformation in the disk geometry such as the presence of a second antidot changes completely the vortex dynamics and the vortex core eventually falls into one of the defects. After trapped, the vortex center still oscillates with a very high frequency and small amplitude around the defect center.
Defects introduced in ferromagnetic nanodisks may deeply affect the structure and dynamics of stable vortex-like magnetization. Here, analytical techniques are used for studying, among other dynamical aspects, how a small cylindrical cavity modify th
e oscillatory modes of the vortex. For instance, we have realized that if the vortex is nucleated out from the hole its gyrotropic frequencies are shifted below. Modifications become even more pronounced when the vortex core is partially or completely captured by the hole. In these cases, the gyrovector can be partially or completely suppressed, so that the associated frequencies increase considerably, say, from some times to several powers. Possible relevance of our results for understanding other aspects of vortex dynamics in the presence of cavities and/or structural defects are also discussed.
