Effects of SPin torques, oErsted and RAshba fields on DOmain walls dynamics – ESPERADO

Current-induced motion of magnetic domain walls through the spin-transfer torque (STT) effect has attracted a lot of attention in recent years, for its potential applications to magneto- logic and magnetic data storage as well as for the wealth of associated physical phenomena. In a previous ANR project (DYNAWALL - PNANO 2007) the partners of this project made considerable progress in the understanding of the STT effect as well as in the improvement of material properties used for domain wall motion. In particular, domain wall velocities of several hundreds of meters per second – the highest values reported in literature – have been demonstrated for Pt/Co/AlOx nanostructures and NiFe/Cu/Co spin-valves.
In this project, the novel physical phenomena that were highlighted in DYNAWALL will be investigated in order to reach a better understanding and to improve the materials used for STT. Theoretical papers have shown that strong spin-flip scattering of the conduction electrons leads to an important STT efficiency. In the bulk, the main source of spin-flip scattering is the spin-orbit interaction (SOI). We will study materials with a SOI larger than in the standard 3d magnetic materials, by doping metals or by using oxides so that the conduction electrons are more of p or d symmetry, with a higher SOI than purely s electrons. Another way to increase spin-flip scattering, revealed in DYNAWALL for the Pt/Co/AlOx system, is to use an ultrathin magnetic layer sandwiched between two different materials (a system with structure inversion asymmetry). In this case, a non-vanishing electric field is present in the magnetic layer. The resulting Rashba interaction leads to a strong spin-orbit term and thus to strong spin-flip scattering. However, the Rashba interaction acts also directly on the magnetization of the layer through the s-d interaction between the conduction electrons and the local magnetization. An important goal of this project is to reach a better understanding of how this Rashba interaction influences the magnetization dynamics and the domain wall propagation, by performing transport and microscopy measurements on new, optimized materials, as well as calculations and micromagnetic simulations.
Another important effect that we have demonstrated in NiFe/Cu/Co trilayers is the influence of the current geometry on the STT efficiency. On one hand, a component of the current flowing in the direction perpendicular to the layers can provide a supplementary, very efficient contribution to the domain wall motion. On the other hand, a non-vanishing magnetic (Oersted) field, transverse to the nanostructures, was shown to act on the NiFe magnetization. We will separately study the influence of these two phenomena of the high STT efficiency in these trilayers. The injection of a purely perpendicular current will be investigated in spin-valve systems and magnetic tunnel junctions. The effect of the Oersted field will be studied in nanostructured bilayers of NiFe with non-magnetic metal layers of different thicknesses, allowing the Oersted field to be varied for a fixed current and thus constant STT.
The success of DYNAWALL was built on our complementary expertise in material fabrication, transport and microscopy measurements, calculations and micromagnetic simulations. The use of high-resolution magnetic microscopies will allow observing directly the effect of the STT, the Rashba interaction and/or the Oersted field on the magnetization during the current pulses. The study of systems with original physical properties using unique characterisation and simulation tools will allow the partners to stay among the leading groups in the international community working on current-induced domain wall motion.