  The discovery of spin torque transfer (STT) has lead to a significant advance
in the development of spintronic devices. Novel structures and materials have
been studied in order to improve the performance of the magnetic tunnel
junctions (MTJs) performances and understand the fundamental physics in spin
torque transfer. The adiabatic spin torque effect, which is due to the spatial
non-uniformity of magnetic properties, has been predicted in theory and
demonstrated experimentally in magnetic nanowires. However, this important spin
torque has been rarely concerned in the magnetic tunnel junctions (MTJ) because
of its extremely weak effect in conventional MTJs. This paper reports for the
first time a giant adiabatic spin torque in MTJ devices with a hybrid free
layer structure. The generation of the giant adiabatic spin torque was realized
through the introduction of a spatial magnetic non-uniformity in a hybrid free
layer along the current direction. It is observed that the giant adiabatic spin
torque can substantially promote the current-induced switching process in the
MTJ devices: the adiabatic spin torque can be larger than the in-plane spin
torque, which allows for the switching with a single-polar current under
different bias fields. Moreover, the adiabatic spin torque, which is
proportional to the level of spatial non-uniformity, increases during the
switching process. The observed effects are confirmed by numerical simulations.
These results have far-reaching implications for the future of high-density
STT-MRAM devices.
