CPM Seminar

Flipping magnetic vortices on the picosecond time scale

Sebastian Gliga

Forschungszentrum Jülich

Magnetic vortices are naturally occurring structures in confined-geometry ferromagnetic materials. In a patterned magnetic nanostructure, the magnetization curls in the vicinity of its center, due to magnetostatic interactions. In order to prevent a singularity of the exchange energy density at the center of the structure, the magnetization points out of the plane. This region defines the vortex core and is typically of the order of 5 nm. While such a static magnetic structure has been predicted almost forty years ago [1], dynamic magnetic processes below the 10 nm scale only recently started attracting interest owing to the increase in resolution of magnetic imaging techniques. In particular, because of its high stability, it has until now been assumed that a vortex behaved like a rigid structure. In this respect, the reversal of the vortex core is one of the most dramatic modifications of the vortex structure. Indeed, it has been shown that it takes a static field of around a half a Tesla to “crush” the core and rebuild it in the opposite direction [2].

In this talk, I will present the study of the dynamics of controlled vortex core reversal using three-dimensional micromagnetic simulations based on finite-elements [3]. The simulations show that a single suitable in-plane magnetic pulse of intermediate strength (ca. 70 mT) can be used to reverse the orientation of a vortex core. We found that this unexpected process is mediated by the creation and annihilation of a vortex-antivortex pair in the sample. The annihilation process involves the propagation of a magnetic singularity through the sample [4] and produces bursts of spin-waves in the GHz range. Moreover, in contrast to previously studied magnetization switching mechanisms which exploit the demagnetizing field, we find that the magnetization dynamics is in this case driven by the exchange field. This allows the magnetization reversal process to unfold on the picosecond time scale, making it faster than any field-driven magnetization reversal process previously known from micromagnetic theory.

This new mechanism introduces the dual possibility of using vortex cores for data storage with a write speed in the THz range as well as sources of spin-waves that could drive magnetic logical circuits.

[1] E. Feldtkeller, H. Thomas, Phys. Kondens. Materie 4, 8 (1965).
[2] A. Thiaville, J. M. Garcia, R. Dittrich, J. Miltat, and T. Schrefl, Phys. Rev. B 67, 094410 (2003).
[3] R. Hertel, W. Wulfhekel, and J. Kirschner, Phys. Rev. Lett. 93, 257202 (2004).
[4] R. Hertel, C.M. Schneider, Phys. Rev. Lett. 97, 177202 (2006)