On Friday December 01^th at 10:00, Hélène Béa (SPINTEC) will defend her HDR entitled :
Tuning magnetic skyrmion stability and chirality by materials and gate voltage

Place : IRIG/SPINTEC, CEA Building 10.05, auditorium 445 (access needs authorization *)

visioconference link : https://univ-grenoble-alpes-fr.zoom.us/j/6286678422

Abstract : Magnetic skyrmions are chiral spin textures that gathered a lot of attention due to their topological properties, their nanometer size and the possibility to efficiently move them with currents. In our era of information, their use as data bits for ultrafast and power-efficient devices has opened many possibilities of applications in spintronics memories or logic, but also in non conventionnal computing, such as probabilistic and ultralow power computing or neuromorphic computing.
The chiral exchange called Dzyaloshinskii-Moryia interaction (DMI) is a key parameter to stabilize these quasi-particles. In heavy metal/ferromagnet/oxide ultrathin trilayers, DMI has an interfacial origin, similarly to surface magnetic anisotropy, and it is thus very sensitive to interface quality and material thicknesses.

In this habilitation, I present some results we obtained at Spintec, in collaboration with other teams about the stabilization and manipulation of magnetic skyrmions in heavy metal/ferromagnet/oxide trilayers. We performed a fine tuning of material parameters using crossed double wedges which allows to vary both the ferromagnetic thickness and the oxidation state at the ferromagnet/oxide interface. Skyrmion phases are obtained in perpendicular magnetic samples, when domain wall energy is decreased due to the vicinity of a transition, either towards paramagnetic phase or towards in-plane anisotropy behaviour.

By finely controlling material parameters, we have changed skyrmion speed and trajectories by the control of the sign of DMI, which inverts skyrmion chirality.
Using a gate voltage, we have also shown that it modifies not only anisotropy, magnetization and Curie temperature, but also Dzyaloshinskii-Moriya interaction. This allowed us, in Pt/Co/AlOx and Ta/FeCoB/TaOx stacks, to control skyrmion stability and realise a room temperature skyrmion switch.
Finally, in Ta/FeCoB/TaOx we have used a gate-voltage to invert the sign of the DMI and thus locally and dynamically reverse skyrmion chirality. Hence, this local, persistent and reversible chirality reversal allows the inversion of the motion of skyrmion under a current, offering a new degree of freedom to individually manipulate skyrmions with low power.

Jury :

• André THIAVILLE, Directeur de Recherche, Laboratoire de Physique des Solides, Rapporteur
• Pietro GAMBARDELLA, Professeur des Universités, ETH Zurich, Rapporteur
• Dafiné RAVELOSONA, Directeur de Recherche, Center for Nanoscience and Nanotechnology, Rapporteur
• Agnès BARTHELEMY, Professeure des Universités, UMR CNRS/Thales, Université Paris-Saclay, Examinatrice
• Liliana BUDA-PREJBEANU, Professeure des Universités, Spintec/Grenoble INP, Examinatrice

(*) Access to CEA requires an entry authorization. request it sufficiently in advance : before november 17^th at admin.spintec@cea.fr