CPM Seminar

Micromagnetic investigations of current and field-induced vortex core reversal

Sebastian Gliga

Forschungszentrum Jülich

Micromagnetic simulations are routinely employed for the modeling of magnetic structures in mesoscopic ferromagnets. In recent years, the simulations have achieved a very high degree of reliability and accuracy, yielding in most cases perfect agreement with experiments, thereby demonstrating their predictive power. Micromagnetic simulations are now therefore being applied to investigate phenomena on time and length scales beyond the present limits of experimental resolution, where new dynamic processes are expected to be found.

One such recently discovered process is the ultrafast reversal of vortex cores in magnetic thin-film elements [1], which may find application in non-volatile storage media. Magnetic vortices are naturally-forming fundamental magnetization structures possessing a nanometric core around which the magnetization direction circulates in the film plane. In the core, the magnetization aligns perpendicular to the plane, pointing 'up' or 'down'. Recent experiments have shown that the core magnetization can be switched by means of weak in-plane pulses [2], but the details of the switching mechanism could not be resolved. Our three-dimensional micromagnetic finite-element simulations elucidate the complex dynamics of this micromagnetic process and describe the fastest field-induced switching process ever reported.

In this talk I will present recent results on the reversal of a vortex core induced by external fields and, more technologically relevant, by electric currents [3,4]. I will show that in both cases, the switching mechanism is identical. Moreover, two distinct routes exist through which vortex cores can be switched: a slow route, in which the reversal is achieved within nanoseconds and an ultrafast route unfolding on the picosecond time scale. We find that in both cases the switching occurs as soon as the internal energy exceeds well-defined threshold values, providing insight into the origin of the core reversal process.

Micromagnetic simulations have allowed furthering our understanding of the core switching process, which is arguably the most complicated micromagnetic switching process known to date, and allow for clear predictions for possible experimental studies.

[1] R. Hertel, S. Gliga, M. Fähnle, C. M. Schneider, Phys. Rev. Lett. 98, 117201 (2007)
[2] B. Van Waeyenberge, A. Puzic, H. Stoll , K. W. Chou, T. Tyliszczak, R. Hertel, et al., Nature (London) 444, 461 (2006)
[3] K. Yamada, S. Kasai, Y. Nakatani, K. Kobayashi, et al. , Nat. Mat. 6, 270-273 (2007)
[4] Y. Liu, S. Gliga, R. Hertel, and C. M. Schneider, Appl. Phys. Lett. 91, 112501 (2007)