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La présentation sera faite en anglais.
Résumé : During operation of magnetic devices, the direction of magnetization is conventionally controlled by the application of external magnetic fields or, in nanoscale devices, by large spin-polarized electric currents. In either case, Joule heating and the associated energy dissipation present a severe challenge. Electric field manipulation of magnetic properties is a relatively new topic in magnetism, which may be an alternative solution to achieve low-power magnetization reversal.
The main tasks of my PhD project is to study the effect of electric fields on the magnetic properties (perpendicular magnetic anisotropy – PMA and Dzyaloshinskii-Moriya interaction – DMI) of magnetic thin film layers of Pt/Co/MOx. Samples with chiral domain walls and skyrmions stable at room temperature were grown, and the effect of electric field on the stability of these magnetic textures was studied. We studied Pt/Co/MOx (M = Al, Tb, Mg) covered with high-k dielectric layer (HfO2, ZrO2). To determine the thickness and roughness of each layer, we used X-ray reflectivity. To visualize the magnetic reversal processes and the magnetic domains of these samples, we used Kerr magneto-optical microscopy. The atomic layer deposition (ALD) method was used to deposit the high-k materials. These high-k materials were deposited under different experimental conditions (temperature, thickness) and studied by X-ray diffraction to determine the crystalline phase. Each magnetic stack was patterned by electron and optical lithography into capacitor-like micron sized devices. A thin platinum layer was used as top electrode. The devices were also studied by carrying out current-voltage and measurements to better understand their behavior in a circuit (e.g. capacitor, resistor).
By applying an electric field, we were able to locally modify the micromagnetic parameters and the dynamics of the domain walls in a reversible and non-volatile way. The effect was interpreted as being due to the migration of oxygen ions driven by the gate voltage within the high-k dielectric layer that acts as an ionic conductor. The effects of the electric field on the micromagnetic parameters was studied using Kerr microscopy, vibrating-sample magnetometry, magnetic force microscopy, and hard X-ray photoelectron spectroscopy.
The second part of my PhD details the study of the effect of He+ ion irradiation on the magnetic properties and the domain wall dynamics of Pt/Co/AlOx stacks. For relatively low fluences we observe a strong decrease of the PMA, associated to atomic displacements at both Pt/Co and Co/AlOx interfaces, without a deterioration of the strength of the interfacial DMI. The robustness of the DMI against interfacial intermixing, is beneficial for the dynamics of field-driven domain walls. Indeed, the decrease of the depinning field, associated to the decreasing PMA, together with the persisting large DMI, allow obtaining large DW velocities for much lower magnetic fields in the irradiated samples, compared with the pristine sample. Decoupling the PMA from the DMI can therefore be a route towards the conception of low energy devices based on field or current-driven domain wall dynamics.
