On Monday, the 15th Of March 2021 at 13h30, Arnaud De Riz will defence his thesis “Modeling of domain walls dynamics in circular cross-section nanowires” supervised by D. Gusakova and J.-Ch. Toussaint
Link to participate on-line:
This thesis studies theoretically the magnetic domain wall behavior in cylindrical wires subjected to the applied electron current. The concept of ordered and dense arrays of nanowires seems to be promising for the development of a three-dimensional magnetic memories in which the information may be encoded as magnetic domains separated by the domain walls. My work focuses on the domain wall dynamics and related critical phenomena such as domain wall pinning and the internal wall structure transformations. We combined micromagnetic simulations with simplified analytical models, to provide an overview of key parameters, useful in predicting and understanding experiments. In particular, the manuscript quantifies two critical phenomena. First, we discuss the control of the transverse domain wall position by introducing geometrical inhomogeneities in the moderate diameter wires (< 7 lex). Diameter modulations play the role of a potential barrier which implies that some threshold driving force must be applied to overcome the barrier. We calculated the threshold current-induced and field-induced driving force as a function of the geometrical parameters. The analytical model developed is a simple scaling law, which may be useful in resolving experimental and nanofabrication issues. Second, we quantify the effect of the spin-transfer torque together with the Oersted field generated by the electric current in large diameter wires (> 7 lex). For such diameters, the Bloch-point walls occur to be the most stable configurations and exhibit several interesting features. The Bloch-point wall is a micromagnetic singularity and is characterized by the magnetization curling direction (also called circulation). This micromagnetic object is expected to prevent the usual Walker breakdown, and thus enable high domain wall speed. In the frame of this thesis we showed that the previously overlooked OErsted field associated with an electric current is a key in experiments to stabilize the BPW and reach speed above 600m/s with spin-transfer. We investigate in detail this situation with micromagnetic simulations. The switching of the azimuthal circulation of the BPW to match that of the OErsted field occurs above a threshold current which we quantify as a function of geometry and material parameters. We also highlight the complexity of BPW transformation involving topological objects at the surface and in the volume.
André THIAVILLE, Rapporteur
François MONTAIGNE, Rapporteur
Mair CHSHIEV, Examinateur
Nicolas ROUGEMAILLE, Examinateur
Matthieu BAILLEUL, Examinateur