Back-hopping in Spin-Transfer-Torque switching of perpendicularly magnetized tunnel junctions

TL;DR

This paper investigates back-hopping in spin-torque switching of perpendicularly magnetized tunnel junctions, revealing it as a sequential process involving the fixed layer, and explains its dependence on layer coupling.

Contribution

It provides a detailed analysis of back-hopping mechanisms, identifying the weak coupling of the spin-polarizing layer as a key factor, and offers insights for mitigating this phenomenon in MRAM devices.

Findings

Back-hopping involves two sequential switching events.

The final P' state is distinct from the conventional parallel state.

Back-hopping occurs when the spin-polarizing layer is weakly coupled.

Abstract

We analyse the phenomenon of back-hopping in spin-torque induced switching of the magnetization in perpendicularly magnetized tunnel junctions. The analysis is based on single-shot time-resolved conductance measurements of the pulse-induced back-hopping. Studying several material variants reveals that the back-hopping is a feature of the nominally fixed system of the tunnel junction. The back-hopping is found to proceed by two sequential switching events that lead to a final state P' of conductance close to --but distinct from-- that of the conventional parallel state. The P' state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P' state involves a switching of the sole spin-polarizing part of the fixed layers. The analysis of literature indicates that back-hopping occurs only when the spin-polarizing layer is too weakly coupled to the rest of the fixed system, which justifies a posteriori the mitigation strategies of back-hopping that were implemented empirically in spin-transfer-torque magnetic random access memories.