Pattern generation and symbolic dynamics in a nanocontact vortex oscillator

Top: Schematic illustration of a nanocontact vortex oscillator, showing gyration in the film plane and periodic core reversal. Bottom: Dynamics in the reconstructed phase space, with the different patterns generated shown in yellow, red, and blue. Reference: M.-W. Yoo et al., Nature Communications 11, 601 (2020).

Our latest work on chaotic dynamics in nanocontact vortex oscillators has just appeared in Nature Communications.

This represents the fruit of many years of labour, which began with a number of interesting discussions with Sébastien Petit-Watelot at the Institut Jean Lamour (CNRS/Univ Lorraine) in Nancy and Damien Rontani at the Laboratoire Matériaux Optiques, Photonique et Systèmes (CentraleSupélec/Univ Lorraine) in Metz. We had the idea to look at the possible waveform patterns that could be generated by the nanocontact vortex oscillator in its chaotic state. Myoung-Woo Yoo, whilst on his Marie Sklodowska-Curie fellowship with me on this topic, devised a clever pattern filtering algorithm to analyse the experimental time traces, which are quite noisy even at 77 K as a result of the relative low signal to noise ratios in these samples. This technique allowed us to really delve into the different patterns available, which turn out to be quite simple, as shown in the figure above.

Ultimately, we were able to show that the complexity and entropy of the signal can be controlled with the applied current, where the most interesting things occur in the chaotic regime. Basic benchmarks show that the oscillator can be a good random number generator with some interesting symbolic dynamics.

This work has been funded by the Agence Nationale de la Recherche as part of the CHIPMuNCS project.

Current-driven skyrmion dynamics in disordered films

Reference: J.-V. Kim and M.-W. Yoo, Appl. Phys. Lett. 110, 132404 (2017).

We studied how current-driven skyrmion motion in ultrathin films is affected by disorder. We modelled the disorder by assuming a grain structure, where the local perpendicular anisotropy fluctuates from grain to grain. We find that the velocity versus current curves are reminiscent of behavior in driven elastic interfaces in disordered media, such as domain wall creep. Moreover, we identify an extrinsic contribution to the skyrmion Hall effect due to disorder scattering, which is drive dependent. This work has just appeared in Applied Physics Letters.

Siphoning spins


A News & Views piece I wrote for Nature has just appeared.

Topological insulators are well-known for their exotic electron-transport properties. A Letter out in this week describes how the spin-polarized currents at the surface of a topological insulator can be used to generate large spin-transfer torques in a transition metal ferromagnet. These torques are at least an order of magnitude larger than those generated by the spin Hall effect with heavy-metal substrates.