In recent work, the time evolution of the magnetization reversal caused by spin-transfer torque in small perpendicular MRAM cells was investigated experimentally and theoretically 1, 2. If the magnetization reversal is incoherent, that is, it does not follow a macro-spin model, intermediate states in between magnetization up or down can be found. These states can last up to tens of ns for cell sizes of the order of 50nm, thus lengthening the reversal time.
We investigated time resolved current measurements for our MRAM cells with sizes smaller than 20nm in the expectation that no longer any phenomena of this kind should be observed for very small cells that are below the “single domain limit”. However, it turns out that even MRAM cells smaller than 20nm (see Fig. 1) show typical indications of domain wall pinning, albeit with shorter pinning times. Fig. 2 shows a clear trend that the reversal times – which have been carefully separated from the incubation times - decrease with size. The reversal times are stochastic in nature and for each size a substantial range for the reversal times exists. Even for the smallest cells, we can show that the observed features are not an artifact caused by noise. In the full paper, details of the measurements and interpretations of the data will be given.
References 1 P. Bouquin et al., Phys. Rev Appl, 15, 024037 (2021) 2 I. Volvach et al., Appl. Phys. Lett. 116, 192408 (2020)