Magnetic nanostructures with structural inversion asymmetry
Roberto Lo Conte (Institut für Physik)

In the attempt of proposing new concepts for more efficient magnetic memories and logic devices, an intense scientific research is conducted in the field of Nanomagnetism and Spintronics. Accordingly, in the recent years a new discovery, going under the name of spin-orbit torques (SOTs), has attracted the attention of the scientific community. SOTs are expected to play a key role in the design of novel and more efficient spintronic devices based on the manipulation of the magnetic texture in ferromagnetic systems by electric currents. The origin of the SOTs is the strong spin-orbit interaction (SOI) in the material systems where such novel torques have been observed, i.e. magnetic multilayers of the type: heavy metalferromagnetoxide. Moreover, the structural inversion asymmetry (SIA) characterizing those magnetic heterostructures is the second key factor in the generation of the SOTs.
In this talk I present the observed SOT-driven magnetization manipulation in two different magnetic multilayers: TaCoFeBMgO and PtCoAlOx, employing Kerr microscopy. The two materials systems have an ultrathin ferromagnetic layer (1 nm for the CoFeB layer, 0.9-1.4 nm for the Co layer) and a spontaneous magnetization pointing out-of-plane. All the investigated material stacks were patterned into 1-1.5 μm-wide, 8-28 μm-long magnetic nanowires (NWs).
First, a deterministic current-induced magnetization switching effect is observed in TaCoFeBMgO NWs. The symmetry and the efficiency of the magnetization reversal are investigated. The experimental observations are in agreement with the presence of a strong SOT acting on the ferromagnet’s magnetization, which originates from the large spin-Hall effect (SHE) in the Ta layer.
Second, current-induced magnetic domain wall (DW) motion is studied in TaCoFeBMgO NWs. The observed current-induced DW motion (CIDWM) along the conventional current (j) points to SOTs as the driving mechanism. The material system is found to exhibit a positive Dzyaloshinskii-Moriya interaction (DMI), generated at the TaCoFeB interface. The observed dependence of the CIDWM velocity on an applied longitudinal magnetic field (HL) is in agreement with the scenario of a SHE-SOT-driven motion of DMI-stabilized right-handed homo-chiral DWs. Furthermore, the Boron is found to play a key role in the definition of the sign of the observed DMI.
Finally, fast CIDWM is observed in PtCoAlOx NWs. Here, the influence of the Co thickness (tCo) on the DW motion is at the center of the study. The general dependence of the DW velocity on j and on HL is in agreement with the interpretation based on DW moved by the SOT originating from the SHE in the Pt layer. A decrease in the magnitude of the effective negative DMI with an increasing tCo is extracted, justifying the interpretation based on an interfacial origin for the DMI in our system. Finally, the DW width is observed to play a key role in the definition of the DW velocity dependence on the longitudinal field.