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José PENA GARCIA presents

Chiral magnetic textures in epitaxial thin films with C2v symmetry

Monday 12th of July 2021 at 2.00 pm

Link : https://univ-grenoble-alpes-fr.zoom.us/j/98761618171?pwd=bWxWdDlyZVc5d2NzL0hleUptSDRxdz09

Amphi of the A Building: number of person limited to 30

The presentation will be in English.


Abstract :

Chirality is a fascinating property of nature, observed for the first time in 1848 by Louis Pasteur, in the optical response of molecules. More generally, chiral textures often appear in physics as a result of symmetry breaking, either spontaneously, for example in macroscopic quantum systems, or when stabilized by a chiral interaction, as in liquid crystals or magnetism. In magnetism, chirality emerges naturally due to a chiral anti-symmetric exchange interaction called DzyaloshinskiiMoriya interaction (DMI), in systems with a breaking inversion symmetry and a large spin-orbit coupling. In competition with other magnetic interactions, the DMI promotes chiral non-collinear magnetic textures whose symmetry depends on the crystal symmetry.

In this thesis, we study the Au1-xPtx/Co/W(110) stack grown epitaxially onsapphire. substrates, where inversion symmetry is broken and a large spin-orbitcoupling arises due to the large atomic number of the buffer and capping layers.The particularity of the Au1-xPtx/Co/W(110) stack is that it possess a C2v symmetry, which results into an anisotropic DMI, with a Néel and a Bloch component,and an uniaxial strain-induced in-plane anisotropy, in addition to the out-of-planeanisotropy promoted by the spin-orbit-coupling.The crystal structure and symmetry of the Au1-xPtx/Co/W(110) stack is investigated by grazing X-ray diffraction and reflection high energy electron diffraction. We found that the C2v symmetry is conserved as the Pt content increases,and that the Au1-xPtx unit cell lattice parameter follows Vegard’s law, decreasingmonotonously as the Pt content increases.

The evolution of the magnetic parameters as a function of the Au-to-Pt content in the Au1-xPtx/Co/W(110) stack is investigate with Brillouin light scattering in the Damon-Eshbach and the magnetostatic-backward-volume-waves configuration. The interfacial anisotropy is found to follow Vegard’s law, decreasing monotonously as the Pt content increases. On the other hand, the Néel-type DMI is found to increase non-monotonously up to a certain value with the increasing of Pt content, and to decrease above. The in-plane anisotropy energy density is found to remain constant for all the different Pt compositions and to possess a two-fold symmetry. A non-zero Bloch-type DMI is observed using the magnetostatic-backwardvolume-waves configuration for Au0.58Pt0.42/Co/W(110) where the anisotropic Néel-type DMI was found to be maximum.

The different chiral magnetic configurations that can be stabilized in the magnetic stacks are investigated with X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM). The energy difference of domain walls (DWs) aligned along the two main in-plane directions depends on the the Au-to-Pt content, as explained by a one-dimensional model and micromagnetic simulations. When the DW energy difference is high, this leads to the stabilisation of self-organised stripe domains or to elliptical magnetic skyrmions. Finally, when approaching the spin-reorientation transition and under specific circumstances, magnetic merons, topologically equivalent to halfskyrmion, are observed in magnetic nanodots

Thesis Directors: Stefania PIZZINI and Jan VOGEL