Giant adiabatic spin torque in magnetic tunnel junctions with a hybrid free layer structure
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.