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Chiral magnetic textures in epitaxial thin films with C2v symmetry

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 Dzyaloshinskii-Moriya 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 on sapphire. substrates, where inversion symmetry is broken and a large spin-orbit-coupling 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-plane anisotropy 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/Co/W(110) unit cell lattice parameter follows Vegard's law, decreasing monotonously 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 N'eel-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-backward-volume-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) at the synchrotron ELETTRA (Italy). 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 half-skyrmion, are observed in magnetic nanodots.
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Submitted on : Wednesday, January 19, 2022 - 10:40:23 AM
Last modification on : Friday, March 25, 2022 - 9:44:21 AM


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  • HAL Id : tel-03525781, version 2




Jose Antonio Pena Garcia. Chiral magnetic textures in epitaxial thin films with C2v symmetry. Materials Science [cond-mat.mtrl-sci]. Université Grenoble Alpes [2020-..], 2021. English. ⟨NNT : 2021GRALY046⟩. ⟨tel-03525781v2⟩



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