First observed about ten years ago, magnetic skyrmions hold great promise for information storage and processing at the nanoscale and at low power.

These localized spin "textures" can wind up in two opposing directions, this sense of rotation being called chirality. Until now, these two chiralities could only be obtained in specific materials or by means of complex manipulations in a controlled environment (via chemisorption).

The Irig researchers, with their partners, sought to take up the challenge of reversing the chirality of skyrmions within the same material, through the application of a gate voltage.

For this, they chose skyrmions with a micrometric lateral extension and a nanometric thickness, stabilized at room temperature in a stack of Ta/FeCoB/TaO_x ultrathin layers. In their previous work, they observed that applying a gate voltage can modify the physical parameters which set the stability and the chirality of the skyrmions, the latter being controllable by the degree of oxidation of the TaO_x layer.

By precisely optimizing the thickness of the FeCoB ferromagnetic layer and the degree of oxidation of the adjacent TaO_x oxide, they succeeded in developing a device close to the "tipping point" between the two chiral forms. They then focused on moving the oxygen ions within the TaO_x layer by applying a gate voltage. In this way, they succeeded, for the first time, in inverting the chirality of skyrmions by applying a simple gate voltage.

However, the gate voltage also modifies the surface magnetic anisotropy of the stack, and consequently the energy of the skyrmions, as well as their stability and size. It must now be experimentally demonstrated that it is possible to invert the chirality of a unique skyrmion without destabilizing it, as suggested by the computational simulations performed in this study. For this, it will be essential to optimize even more finely the effects of the gate voltage. Finally, it will be necessary to reduce the lateral size of skyrmions to the nanometric level and to adapt the conditions necessary for their stabilization, depending on the targeted applications.

This advance opens the possibility of individually controlling and moving skyrmions. While scientists used to picture a "train" of skyrmions moving in the same direction under the effect of a current, they can now imagine skyrmions moving in both directions under the same current. It is therefore becoming possible to code the gap between two successive skyrmions or to selectively send skyrmions to different terminals (multiplexing).

This work was conducted in collaboration with the Institut Néel (CNRS) in Grenoble and the Laboratoire des Sciences des Procédés et des Matériaux (CNRS) in Villetaneuse.