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Modelling of non-trivial spin textures in magnetic thin-films

Federica Nasr

Modelling of non-trivial spin textures in magnetic thin-films.

Rel. Mariagrazia Graziano. Politecnico di Torino, Corso di laurea magistrale in Nanotechnologies For Icts (Nanotecnologie Per Le Ict), 2021

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The observation of topological non-trivial whirling spin textures such as magnetic skyrmions has opened new avenues to realize spintronic devices at the nanoscale, built on the tuning of nanometric particle-like objects. Néel magnetic skyrmions may exist in ultra-thin films with Perpendicular Magnetic Anisotropy (PMA), e.g. of the type Heavy Metal/Ferromagnet/Insulator (HM/FM/MOx), due to an antisymmetric exchange interaction called interfacial Dzyaloshinskii-Moriya Interaction (iDMI). The present thesis aims to perform a micromagnetic study of the magnetic stack of the type Pt/Co(0.9nm)/MgO, already intensively characterized by the past, both experimentally and by modelling, since the possibility to stabilize tunable chiral bubbles at room temperature. This study focuses upon the heating-induced writing of topological solitons, e.g. heating due to an ultra-fast laser pulse. This represents an exotic experimental protocol that has been proved to make it easily accessible peculiar magnetic configurations, as the here studied skyrmion bubble lattice, inaccessible by simply sweeping the magnetic field or injecting spin-polarized current. Starting from a thermally demagnetized state, it is shown by micromagnetic simulations that an extremely confined 1-D skyrmion can be stabilized - despite thermal field and granularity - in the domain wall of an otherwise homochiral skyrmionic bubble (Sk) of topological charge Q = +1, resulting in a novel topological excitation wherefore called Double Skyrmion (DSk). The corresponding spin texture maps twice the possible orientations of magnetization, returning a total topological charge Q = +2. Such localized kink is an extremely confined region of the domain wall opposing the chirality promoted by the iDMI and perturbing the spherical symmetry of homochiral bubble. The systematic characterization of the double skyrmion static and dynamic properties reveals an overall behaviour analogous to what commonly observed for the conventional skyrmionic bubble. The DSk is a stable state that, despite the slightly larger energy than Sk, responds forcefully to Zeeman field and it is resilient against both DMI reduction and chirality switching. Particularly, the accomplishment of DSk size tuning operated by control parameters, i.e. magnetic field, and internal material parameters, such as the DMI strength, is numerically predicted. Current In-Plane Spin Transfer Torque (CIP-STT) and/or Spin Orbit Torque (SOT) are demonstrated to displace the Q = +2 topological charge in a track, with a characteristic transient re-orientation of the kink. However, a distinctive behaviour is observed in high-current regime, where the toughness of the SOT may induce a non-conservative manipulation of the total topological charge, either transforming or annihilating the DSk. Upon the demonstration of double skyrmion particle-like properties, stability, and tunability, one can open perspectives to conceive innovative high-density skyrmionic devices. The physical mechanism of Current Out-Of-Plane Spin Transfer Torque operated through a perpendicular spin polarizer is proposed as a potential candidate to nucleate such peculiar magnetic texture and further induce its spinning motion. This latter phenomenon consists of a distinctive DSk auto-oscillation, not observed for Sk, driven by the ultra-confined kink, where the 2-D rotation direction (ACW/CW) can be controlled by the specificity of 1-D skyrmion.

Relators: Mariagrazia Graziano
Academic year: 2020/21
Publication type: Electronic
Number of Pages: 93
Corso di laurea: Corso di laurea magistrale in Nanotechnologies For Icts (Nanotecnologie Per Le Ict)
Classe di laurea: New organization > Master science > LM-29 - ELECTRONIC ENGINEERING
Aziende collaboratrici: CEA-SPINTEC
URI: http://webthesis.biblio.polito.it/id/eprint/17854
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