Spin Torque Efficiency and Analytic Error Rate Estimates of Skyrmion Racetrack Memory

Abstract

In this paper, the thermal stability of skyrmion bubbles and the critical currents to move them over pinning sites were investigated. For the used pinning geometries and the used parameters, the unexpected behavior is reported that the energy barrier to overcome the pinning site is larger than the energy barrier of the annihilation of a skyrmion. The annihilation takes place at boundaries by current driven motion, as well as due to the excitation over energy barriers, in the absence of currents, without forming Bloch points. It is reported that the pinning sites, which are required to allow thermally stable bits, significantly increase the critical current densities to move the bits in skyrmion-like structures to about jcrit = 0.62 TA/m². The simulation shows that the applied spin transfer model predicts experimentally obtained critical currents to move stable skyrmions at room temperature well, which is in contrast to simulations based on spin orbit torque that predict significantly too low critical currents. By calculating the thermal stability, as well as the critical current, we can derive the spin torque efficiency η = ΔE/Ic = 0.19 kBT300/μA, which is in a similar range to the simulated spin torque efficiency of MRAM structures. Finally, it is shown that the stochastic depinning process of any racetrack-like device requires an extremely narrow depinning time distribution smaller than ~6% of the current pulse length to reach bit error rates smaller than 10−9.

Temporal Memory With Magnetic Racetracks

“…Finite temperature and pinning: At finite temperature, skyrmions in close to ideal racetracks undergo rapid diffusion and Brownian motion [31] that would make the racetracks unusable. At the other extreme, both intentional defects such as notches [32] or unintentional ones such as material non-uniformities give rise to pinning centers that can keep skyrmions pinned, preventing thermal diffusion but also preventing the current induced motion necessary for the operation of the racetrack. For the proposed racetrack to be viable, this pinning needs to be strong enough that the skymion remains localized with high probability for a sufficient time.…”

Coexistence of distinct skyrmion phases observed in hybrid ferromagnetic/ferrimagnetic multilayers

“…One solution to this problem is the use of antiferromagnetically coupled skyrmions 11 , 23 , 24 . The second problem is not having stable/constant inter-skyrmion distances, which leads to not having a controlled distance among bits 25 . One potential solution was proposed by Müller as a two-lane racetrack device, in which the information is stored in the lane number of each skyrmion 26 .…”

Controlled motion of skyrmions in a magnetic antidot lattice

“…Unfortunately, skyrmions driven by the electric current migrate towards the edges of a magnetic strip, due to the skyrmion Hall effect, which leads to an unstable transverse position and annihilation. Furthermore, the skyrmion motion is randomized by the thermal diffusion and the presence of pining centers in real samples [21,22]. Therefore, key challenges regarding skyrmion motion are related to stabilization, confinement, and control of the movement of skyrmions at room temperature [23,24].…”