Skyrmions are chiral swirling magnetic spin textures that exhibit many characteristics: they can be small, they are stable at room temperature, we can create and annihilate them using a gate voltage and they are efficiently manipulated using an electrical current. These properties make skyrmions promising candidates for many applications such as skyrmion racetrack memories, skyrmion based logic devices or neuromorphic computing.​​
Realizing these technologies, however, requires precise control over the creation, stability, and motion of skyrmions. Recent studies have shown that gate voltages can not only nucleate and delete skyrmions but also modulate the Dzyaloshinskii–Moriya interaction (DMI), a key factor responsible for their stabilization and their chirality, ie. the sense of rotation of spins within their domain wall. In this PhD work, we investigated the evolution of the DMI chirality as a function of material parameters in a Ta/FeCoB/TaOx trilayer system, with the two top layers deposited in a wedge to tune interfacial properties. In addition, to better understand the interaction at the origin of the stabilization of skyrmion, we discovered, using ab initio calculations a new mechanism possibly explaining the inversion of the DMI chirality with respect to the thickness of the ferromagnetic layer, linked to structural relaxation in the ultrathin regime.​​
Then, we also experimentally demonstrate the magneto-ionic origin of the non-volatile and strong anisotropy variation induced by gate voltage in a Pt/Co/AlOx system, which is similar to the Ta/FeCoB/TaOx stack but exhibits larger DMI and magnetic anisotropy. A comparison between reference and gated samples showed the evolution of the oxidation state at the Co/Oxide interface a result further supported by Hard X-ray Photoelectron Spectroscopy (HAXPES) measurements. Additionally, we focused on the gate voltage control of skyrmion chirality, with the goal of achieving single-skyrmion manipulation and detection. Skyrmion stabilization in patterned tracks was thus achieved, where their behavior under gate voltage control was subsequently studied.​​
Finally, to better understand skyrmion dynamics under electrical currents, we conducted micromagnetic simulations under small (1010 A/m2) and large (1011 A/m2 ) current. We have in particular observed that for large currents, the type of skyrmions, ie. Néel, Bloch or intermediate, is modified. It induces a bending of their trajectory so that, in the stationary regime, initially nearly-Bloch skyrmions move under current similar to Néel skyrmions. This is of importance since the current induced motion is very often used to deduce skyrmion type.

Plus d'information :https://www.spintec.fr/phd-defense-gate-voltage-control-of-magnetic-skyrmions/
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Pour suivre la soutenance ​​​en visioconférence : https://univ-grenoble-alpes-fr.zoom.us/j/98769867024?pwd=dXNnT3RMeThjYStybGVQSUN0TVdJdz09

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