Domain-Wall Motion Driven by Laplace Pressure in Co−Fe−B/MgO Nanodots with Perpendicular Anisotropy

Abstract

We have studied the magnetization reversal of CoFeB-MgO nanodots with perpendicular anisotropy for size ranging from w=400 nm to 1 μm. Contrary to previous experiments, the switching field distribution is shifted toward lower magnetic fields as the size of the elements is reduced with a mean switching field varying as 1/w. We show that this mechanism can be explained by the nucleation of a pinned magnetic domain wall (DW) at the edges of the nanodots where damages are introduced by the patterning process. As the surface tension (Laplace pressure) applied on the DW increases when reducing the size of the nanodots, we demonstrate that the depinning field to reverse the entire elements varies as 1/w. These results suggest that the presence of DWs has to be considered in the switching process of nanoscale elements and open a path toward scalable spintronic devices.

Multi‐Level Switching and Reversible Current Driven Domain‐Wall Motion in Single CoFeB/MgO/CoFeB‐Based Perpendicular Magnetic Tunnel Junctions

“…Therefore, in contrast to the present design of p‐STT‐MRAMs with pillar aspect ratio close to 1, to realize multi‐level switching, ellipses are preferred. The γ = 2.5–5.9 mJ m −2[ 23,27 ] and DW width down to 10 nm [ 33,41 ] were previously reported, (see also in Table S4, Supporting Information), enabling to realize the multi‐domain in small size structures.…”

“…To obtain multi‐domain structure in smaller p‐MTJs, the FL properties should be carefully tuned. This is available since a large range of K eff (up to 2.5 × 10 5 J m −3 ), [ 27,30,35 ] A ex (0.8–9 × 10 −11 J m −1 ) [ 23,35 ] and M s (up to 1.3 × 10 6 A m −1 ) [ 35,43,44 ] has already been reported previously, as summarized in Tables S1–S3 (Supporting Information). By using K eff = 2.5 × 10 5 J m −3 , M s = 3.0 × 10 5 A m −1 and A ex = 1.0 × 10 −11 J m −1 , the critical size from the theoretical prediction based on Equation () for two and three‐domains are 239 and 740 nm, respectively, which are consistent with our experimental results.…”

“…By applying the negative current, a spin up domain ( A up ) is created first, which usually forms at the edge of the pillar since the presence of stray field leads to lower anisotropy. [ 27 ] With further increasing current, the DW is propagated through the sample, resulting in the change of the domain area and resistance. If the current is large enough (major loop), the DW fully sweeps the sample and the sample reaches the P state.…”

“…In our samples, the tunneling current induced STT acts as a driving force, where the current direction is perpendicular to the direction of DW motion. According to previous study, the STT induced pressure ( P STT ) on the DW is about 10 4 Pa. [ 27 ] By considering the simple case where the DW forms along the diameter, the DW area ( A DW ) is the product of d MTJ and FL thickness ( t FL ). Therefore, by using F STT = P STT × A DW , we estimate that the driving force in our samples is about 10 −12 N, which is in the same order of magnitude obtained from current‐driven DW motion in magnetic U‐shaped nanowires.…”

“…[23,24] Furthermore, the DW propagation with significant Dzyaloshinskii-Moriya interaction (DMI) was reported in fabricated 10 µm wide CoFeB bars, [25] leading to the proposal of p-MTJs with skyrmions free layer. [26] Currently, many issues hinder the realization of multidomain based switching in p-MTJs including magnetic domain compressibility, [24] the DW Laplace pressure [27] and the shape anisotropy. The demonstration of well-controlled current driven DW motion in p-MTJs is crucial for DW-based memories.…”