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Séminaire QUEST

Jeudi 27 Octobre à 9h30,
Salle Remy Lemaire, K223

Orateur : Ales Hrabec (Laboratoire de Physique du Solide, Orsay)
"From domain walls towards functional skyrmion devices"


The Dzyaloshinskii-Moryia interaction (DMI) [1] has been recently demonstrated in thin ferromagnet/heavy metal systems where the interplay between exchange, dipolar, and anisotropy energies is perturbed by a new energy term with profound consequences. The DMI term is expressed as Di,j×(Si × Sj), where Di,j is the DMI vector, and Si and Sj are spin moments sitting on neighbouring atoms. When D is sufficiently high, a non-uniform ferromagnetic state has a lower energy giving rise to exotic textures like cycloids, helicies, or skyrmions. Thin films with broken inversion symmetry therefore offer a way to prepare Néel-like skyrmions [2-4] which are sensitive to spin-orbit torques and can be potentially used in racetrack memory based devices [5].

In my talk I will show how we can use the inherited knowledge of physics of DWs to design new magnetic films to obtain stable isolated skyrmions suitable for their efficient current-induced motion at room temperature. We use finely tailored thin films composed of Pt/FM/Au based bilayer (FM=Ni\Co) to obtain a state where the DW energy is significantly lowered by engineering the perpendicular anisotropy, DMI and dipolar coupling. This leads to a formation of a non-uniform magnetic state in a zero magnetic field. As soon as a small magnetic field is applied this state collapses down to isolated skyrmions. I will demonstrate systematic nucleation and displacement of skyrmions by nanosecond pulses of electric current and show that the skyrmions are moved by a net spin current originating from the spin Hall effect. The nature of the skyrmions is confirmed by the gyrotropic force which serves as a topological filter. These results set the ground for emerging spintronic technologies where issues concerning skyrmion stability, nucleation, and propagation are paramount.

[1] A. Crépieux and C. Lacroix, J. Magn. Magn. Mater. 182, 341 (1998).
[2] S. Heinze et al., Nature Phys. 7 713-718 (2011).
[3] O. Boulle et al., Nature Nanotech. 11 449-454 (2016).
[4] C. Moreau-Luchaire et al., Nature Nanotech. 11 444-448 (2016).
[5] A. Fert et al., Nature Nanotech. 8 152-156 (2013).

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