Ultrafast spin torque memory based on magnetic tunnel junctions with combined in-plane and perpendicular polarizers

Since the initial prediction and experimental demonstration of magnetization reversal by spin transfer torque (STT), there has been continuous progress toward the development of nonvolatile magnetic random access memory based on STT switching (STT-RAM) in nanoscale magnetic tunnel junctions (MTJs). In the most common STT-RAM configuration shown in Fig. 1(a), the magnetic moments of the free layer and the pinned polarizing layer of an MTJ lie collinear to one another in the plane of the junction. In this configuration (in-plane STT-RAM or IST-RAM), STT is small during the initial stages of the free layer's magnetic moment reversal, resulting in a relatively long nanosecond-scale switching time. Switching can be greatly accelerated in an alternative STT-RAM configuration, in which a second polarizer with magnetic moment perpendicular to the MTJ plane is added to the magnetic multilayer (orthogonal STT-RAM or OST-RAM). The initial STT from the perpendicular polarizer is large and has been predicted to induce ultrafast precessional switching of the free layer's magnetization on a time scale of 100 ps. The differences in the reversal modes expected for the IST-RAM and OST-RAM devices are illustrated in Figs. 1(c) and 1(d), wherein magnetization switching trajectories are shown for the two types of memory.