Current-driven domain wall dynamics in ferromagnetic layers synthetically exchange-coupled by a spacer: A micromagnetic study

Abstract

The current-driven domain wall motion along two exchange-coupled ferromagnetic layers with perpendicular anisotropy is studied by means of micromagnetic simulations and compared to the conventional case of a single ferromagnetic layer. Our results, where only the lower ferromagnetic layer is subjected to the interfacial Dzyaloshinskii-Moriya interaction and to the spin Hall effect, indicate that the domain walls can be synchronously driven in the presence of a strong interlayer exchange coupling, and that the velocity is significantly enhanced due to the antiferromagnetic exchange coupling as compared with the single-layer case. On the contrary, when the coupling is of ferromagnetic nature, the velocity is reduced. We provide a full micromagnetic characterization of the current-driven motion in these multilayers, both in the absence and in the presence of longitudinal fields, and the results are explained based on a one-dimensional model. The interfacial DzyaloshinskiiMoriya interaction, only necessary in this lower layer, gives the required chirality to the magnetization textures, while the interlayer exchange coupling favors the synchronous movement of the coupled walls by a dragging mechanism, without significant tilting of the domain wall plane. Finally, the domain wall dynamics along curved strips is also evaluated.These results indicate that the antiferromagnetic coupling between the ferromagnetic layers mitigates the tilting of the walls, which suggest these systems to achieve efficient and highlypacked displacement of trains of walls for spintronics devices. A study, taking into account defects and thermal fluctuations, allows to analyze the validity range of these claims.2

Domain Wall Motion in Magnetic Nanostrips

“…where represents the dynamical equilibrium angle (or steady state), which is the result of compensation between and the shape anisotropy field ( ). As a consequence, the DW moves at constant velocity, given by ≡̇= Δ 0 (69) where the ratio / defines the DW mobility, = Δ 0 . However, there is a limit for this rigid DW motion since sin(2 ) < 1, which implies that…”

Readable High-Speed Racetrack Memory Based on an Antiferromagnetically Coupled Soft/Hard Magnetic Bilayer

“…For simplicity, we only considered the “inter-DW” coupling as shown in Figure 7. We have derived a group of Thiele equations depicting the dynamic behavior of the central coordinates ( q ) and the azimuthal angle ( φ ) of the upper and lower layers (The more specific information about the derivation of these equations are depicted in detail in the Supplementary Materials S1) [2,18,28,29].…”

Magnetic Racetrack Memory: From Physics to the Cusp of Applications Within a Decade

“…Hence, an asymmetric pulse pattern would be required to avoid shifting errors. In contrast, in RTM 4.0 the tilting in the two AF coupled layers exactly compensates [69]. Consequently, shifting bits comes with almost no inertia and is symmetric for the positive and negative shift direction.…”

“…A DW that travels orthogonally to the wire experienced greater driving torques and moved faster in contrast to a DW that accumulates a tilt during its motion. Remarkably, this problem was found to be eliminated in curved nanowires that were composed of SAF structures [68], [69]. The driving torques in such magnetic structures are largely derived from an ECT that is insensitive to the tilting behavior of the DWs.…”