Back hopping in spin transfer torque switching of perpendicularly magnetized tunnel junctions

We analyze 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\ensuremath{'} of conductance close to---but distinct from---that of the conventional parallel state. The P\ensuremath{'} state does not exist at remanence. It generally relaxes to the conventional antiparallel state if the current is removed. The P\ensuremath{'} 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.