Experimental Observation of Back-Hopping With Reference Layer Flipping by High-Voltage Pulse in Perpendicular Magnetic Tunnel Junctions

Low write error rate (WER) is an important requirement for spin-transfer torque magnetic random access memory to be developed as a product. However, there have been reports about back-hopping phenomena that disturb achieving low WER. We demonstrate the experimental observation of back-hopping from the behavior of the WER in perpendicular magnetic tunnel junctions. WER decreases as pulse voltage (Vp) increases, but increases back as Vp increases further. It is attributed to the back-hopping, which originates from flipping of the reference layer (RL) and consequent alternating reversal of RL and free layer. Thus, it is necessary for the RL to be more stable and robust to avoid the back-hopping and to achieve low WER. We performed switching experiments on the devices of different stacks, which reveal back-hopping was the most severe in a magnetic tunnel junction device with the weakest coupling strength of the RL, and was significantly suppressed with a strongly coupled RL.