Towards high-density low-power spin-transfer-torque random access memory
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In this work, we investigate the prospects for spin-transfer-torque random access memory (STTRAM) as the new generation low-power high-density non-volatile memory. Possible means to lower the switching current and increase the packing density of STTRAM are proposed. In an STTRAM cell, the logical value of the memory bit is stored as orientation of magnetic moment in its ferromagnetic ``free'' layer. The bit typically consists of two thin film ferromagnets (FM) separated by an insulating tunnel barrier as in a magnetic tunnel junction (MTJ) structure. One of the two FM layers has fixed magnetization direction, while the other layer is free to be switched. We first study STT-assisted switching in spin valve structures with in-plane, perpendicular, and canted magnetizations in free and (or) reference layers using point contact measurements to explore the use of non-collinear magnetizations in free and fixed FM to reduce both switching current and time. Next, we consider the possibility of storing two memory bits within a single MTJ with a cross-shaped free layer that could still be addressed by one selection transistor. We provide a detailed discussion of the switching dynamics and associated regions of reliable switching currents, in addition to illustrating the effects of varying device geometry on the latter. Moving on from the standard MTJ structure, we then consider the possible use of topological insulators, as opposed to the fixed FM, as the spin-polarizer layer. It has been established that spin and momentum are locked helically in the surface states of a three-dimensional topological insulator (TI). Suggestions of possible use of the TI spin-polarized surface states in spintronic devices to induce reversal of a magnet have been made using theoretical and experimental studies. Here, we simulate magnetization reversal of a metallic nanomagnet by an underlying TI. The TI, thanks to the spin-momentum helical locking of the surface states, causes a spin-polarized current injection to the FM above it. We study the efficiency of the spin injection as a function of varying transparency of the TI-FM interface. The transport in the TI and the FM is assumed to be diffusive at room temperature for the assumed resistance values. Finally, we take into account random thermal fluctuations leading to write error rate (WER) in STTRAM write operation and use Fokker-Planck method and stochastic Landau-Lifshitz-Gilbert-Slonczewski equation to model WER in STTRAM. We conclude with possible future research directions.