Experimentally demonstrated in the early 2010’s, spin-orbit torques (SOTs) very quickly generated a very strong interest in the magnetism and spin electronics community. Indeed, they allow, in a heavy metal / ferromagnetic metal / oxide (HM/FM/Ox) multilayer, to manipulate the magnetization of the ferromagnetic layer (FM) by injecting an in-plane current. Noting that the FM/Ox bilayer corresponds to half of a typical stack used in MRAM memory cells (Magnetic Random Access Memory), we understand that this mechanism is very interesting for writing the free layer of these cells. Indeed, the writing current no longer crosses the tunnel barrier, which naturally responds to some of the limitations of current MRAMs. However, the physical interpretation of these phenomena has proved to be particularly complex. These torques have two components, generally called "Field-like", FL, and "Damping-like", DL. While initially, theoretical studies predicted that the DL component was mainly due to a volume effect in the HM layer, and the FL component was mainly due to an interface effec t, more recent experimental studies have shown that it is not so simple to separate these two contributions. In this thesis work, we have chosen an original approach that allows us to study only one of the two contributions. To do so, we have chosen to focus on the interfacial contribution by studying Ox1/FM/Ox2 samples. We were thus able to highlight in these stacks the presence of SOTs, which was not so obvious in a structure that did not contain heavy metal and also had a strong symmetry. On the other hand, we were able to show that only the FL component of these couples was present. The unexpected behaviour of this FL-SOT as a function of the thickness of the FM layer led us to propose a model based on the combination of a Rahsba interfacial effect and a quantum confinement effect due to the very thin thickness of conductive material in these multilayers.

Read more : http://www.spintec.fr/phd-defense-spin-orbit-torques-in-an-ultra-thin-ferromagnetic-metal-layer-between-two-oxides-quantum-confinement-and-rashba-effect/

Contact : alain.marty@cea.fr