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

TL;DR

This paper investigates the thermal stability, critical current requirements, and spin torque efficiency of skyrmion racetrack memory, revealing that pinning sites significantly influence current thresholds and error rates, with implications for device reliability.

Contribution

It provides new insights into the thermal stability and current-driven dynamics of skyrmions, including analytic estimates of error rates and spin torque efficiency in racetrack memory.

Findings

Pinning sites increase critical current density to about 0.62 TA/m².

Spin torque efficiency estimated at 0.19 k_BT/μA, comparable to MRAM.

Depinning time distribution must be narrower than 6% to ensure low error rates.

Abstract

In this paper the thermal stability of skyrmion bubbles and the critical currents to move them over pinning sites is 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 $j_{crit}$ = 0.62 TA/m$^2$. These currents are similar to those obtained experimentally to move stable skyrmions at room temperature. By calculating the thermal stability as well as the critical current, we can derive the spin torque efficiency $\eta$ = $\Delta/I_c = 0.19 k_BT_{300}~/\mu$A, 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 extremely narrow depinning time distribution smaller than ~6% of the current pulse length to reach bit error rates smaller than ${10^{ - 9}}$.