Jeudi 06 avril à 9h30
Salle Rémy Lemaire, K223
Orateur : Alexis WARTELLE(Néel)
"Push ferromagnetic domain walls around... and make them transform ?!"
In so-called soft ferromagnetic nanowires, which can be grown by electrodeposition into templates with cylindrical pores, magnetization likes to lie along the wire axis. This gives two possible orientations ; now, what happens if both are present in a single nanowire ? Somehow, magnetization has to turn 180° at the boundary between two neighbouring sections with opposite orientations. It thus builds a domain wall between two domains with uniform magnetization.
Simulations and experiments show that domain walls in cylindrical nanowires come in two kinds, depending on the diameter. At low diameter, the simpler type of domain wall is essentially a magnetic dipole transverse to the wire axis. At large diameter, the other type of domain wall looks like a vortex. What makes it interesting is that it features the only known singularity of ferromagnetism : the so-called Bloch point. The Bloch point wall thus contains a topological defect, which raises questions regarding the stability of this magnetization configuration.
After introducing the frame of micromagnetism, in which these domain walls are investigated, I will describe the two types encountered in nanowires. Then, the focus will move to experimental observations of domain walls and the startling, unpredicted transformations from one type to the other (under magnetic field). If time allows, I will try and describe the latter from the point of view of topology.