Straight motion of topological defects in thin films of FeN with stripe magnetic domains.

Dynamics of skyrmions[1] and of magnetic edge dislocations [2] in thin films is a new fascinating field of micromagnetism, where the comprehension of topology-related properties is challenging for fundamental studies and for spintronic applications. The dynamics and particularly the trajectories of these objects are intimately linked to the subjacent magnetic interactions of the system and to the consequent topology. As a matter of facts, skyrmion trajectories follow a “Hall-like” motion perpendicular the magnetic field gradient. This can be a drawback for the use of magnetic skyrmions as information carriers in spintronic devices, because touching the sample edges in constricted geometries can cause the skyrmion destruction. In this work, we study magnetic edge dislocations, whose topological properties makes them follow a straight trajectory when submitted to an external magnetic field. In thin magnetic films with perpendicular magnetic anisotropy, an up-down stripe-domain structure can be originated, on a mesoscopic scale comparable with film thickness (around 100 nm), due to the competition between shortrange exchange coupling and long-range magnetic dipolar interaction.[3] Even in the absence of quenched disorder, topological defects can develop in the stripe pattern due to large amplitude fluctuations occurring in two dimensions.[4] Here we investigate the dynamic behaviour of magnetic edge dislocations in a Nitrogen-implanted Fe film. Combining evidence from magnetic force microscopy data, micromagnetic simulations and a theory based on the Thiele equation, the defects were found to undergo a straight displacement when a moderate magnetic field is applied in plane along the stripes axis. The displacement may be either in the direction of the applied field or in the opposite direction, depending on whether the distribution of the in-plane magnetization in the topological magnetic defect is head-tohead or tail-to-tail, but irrespective of the out-of-plane magnetization in the domains. A relationship is established between the straight character of the motion and the vanishing of the topological charge for the stripe pattern, suggesting that topological defects in FeN films might be used for future spintronic devices intrinsically free from the undesired skyrmion Hall effect. [1] A. Dussaux, P. Schoenherr, K. Koumpouras, J. Chico, K. Chang, L. Lorenzelli, N. Kanazawa, Y. Tokura, M. Garst, A. Bergman, C. L. Degen & D. Meier Nature Communications 7, 12430 (2016) [2] Fert, A., Cros, V. & Sampaio, J. Skyrmions on the track. Nat. Nanotech. 8, 152–156 (2013). [3] S. Tacchi, S. Fin, G. Carlotti, G. Gubbiotti, M. Madami, M. Barturen, Massimiliano Marangolo, Mahmoud Eddrief, D. Bisero, A. Rettori, M. G. Pini Phys. Rev. B 89, 024411 (2014). [4] C. Blanco- Roldán, C. Quirós, A. Sorrentino, A. Hierro-Rodríguez, L. M. Álvarez- Prado, R. Valcárcel, M. Duch, N. Torras, J. Esteve, J. I. Martín, M. Vélez, J. M. Alameda, E. Pereiro & S. Ferrer Nature Communications 6, 8196 (2015) Magnetic force microscopy (MFM) images, showing the evolution of the domain pattern at the surface of the FeN film in the presence of a reversal field. Red crosses mark the imperfections of the topography. Two different types of magnetic edge dislocations (green and yellow rectangles, respectively) appear to translate in opposite directions along the stripes axis. The white bar denotes the unit length of 0.5 microns. Enlarged views of the dislocations are shown in the insets