Magnetic skyrmions are topologically nontrivial magnetic domains where the magnetization rotates in a fixed sense (chirality). They are usually created by magnetic fields or by non-homogeneous electric currents. Here S.G. Je et al. have shown that hexagonal skyrmion bubble lattices can be locally generated with ultrafast (30 fs) laser pulses. Their original optical approach to control skyrmions in commonly accessible materials paves the road towards the emerging skyrmion-based memory and synaptic devices.

Magnetic skyrmions are circular domain walls surrounded by a single chirality domain wall. This nontrivial topology implies that an energy barrier has to be overcome for their nucleation and annihilation.
This same energy barrier allows their stabilization and makes them promising as dense information carriers in spintronic devices at the nanoscale. One of the major challenges for developing novel skyrmion-based memory, logic and neuromorphic devices is fast and controlled creation of magnetic skyrmions at ambient conditions.

Ultrashort laser impulsions generation of skyrmion lattice in a strip of ultrathin heavy metal/ferromagnet/oxide trilayer, measured by magneto-optic Faraday effect microscopy.
The lateral size of the strip is 60 mm. (Insets: when illuminated, the uniform spins are transformed into skyrmions, topologically non-trivial swirling spin textures)

In this study published in Nanoletters, S.G. Je et al. have demonstrated, at room temperature, a controlled generation of skyrmion bubbles and hexagonal skyrmion bubble lattices from a ferromagnetic state by a single ultrafast (35 fs) laser pulse (see Figure).

This study, resulting from a collaboration between Spintec in Grenoble and Institut Jean Lamour in Nancy, was performed in Ta/FeCoB/TaOx ultrathin trilayers where the magnetic field alone does not lead to skyrmion bubble formation due to a high nucleation energy barrier. The measurements were performed by magneto-optic Faraday effect microscopy after the application of single ultra-short laser pulses.
All light polarizations give similar effects, illustrating the importance of the local heating of the sample allowing to overcome the nucleation energy barrier.

The authors moreover show that the skyrmion bubble lattices possess enhanced immunity against magnetic field perturbations as compared to the isolated skyrmion bubbles, suggesting the promising prospects of skyrmion lattices for data storage.
These results highlight the role of thermal effect in the creation of the skyrmion bubble lattice. It also offers an alternative way of ultrafast excitation and massive writing of clusters of skyrmions that decouples the writing operation from the skyrmion driving operation in the skyrmion-based shift register and that could be used in the emerging skyrmion-based synaptic devices.

Team(s):Spin-orbitronics, Sensors

Collaboration: Institut Jean Lamour, Université  de Lorraine, Nancy  France

Funding:  This work was supported by the ANR, by the Institut Carnot ICEEL for the project Optic-switch and Matelas, and by the French PIA project Lorraine Université d’Excellence.

Further reading: Creation of Magnetic Skyrmion Bubble Lattices by Ultrafast Laser in Ultrathin Films, S.-G. Je, P. Vallobra, T. Srivastava, J.-C. Rojas-Sánchez, T. H. Pham, M. Hehn, G. Malinowski, C. Baraduc, S. Auffret, G. Gaudin, S. Mangin, H. Béa and O. Boulle, Nanoletters (2018) DOI: 10.1021/acs.nanolett.8b03653

Contact : Olivier.boulle@cea.fr, helene.bea@cea.fr