  Since the discovery of the spin orbit torque (SOT) induced by spin Hall
effect in heavy metals, much effort has been devoted to understanding the
mechanism of the charge-to-spin conversion as well as to developing new schemes
for high speed, low energy magnetic recording technologies. While fast
switching has been demonstrated in three terminal magnetic tunnel junctions
(3T-MTJs) through applying short voltage pulses in the heavy metal channel,
detailed understanding of the switching mechanism is lacking due to the
complexity of the multi-layered magnetic structure and the three-terminal
geometry. We show in this letter that current-induced effective fields play a
key role in the fast switching and by tuning the applied external field we can
finely tune the symmetry of the pulse switching between two switching
polarities, namely parallel to anti-parallel (P-AP) and anti-parallel to
parallel (AP-P). These results show that the manipulation of detailed magnetic
configuration is the key to fast switching and is a useful way for future
optimization of SOT memory and further applications.
