Material Developments and Domain Wall-Based Nanosecond-Scale Switching Process in Perpendicularly Magnetized STT-MRAM Cells

We investigate the Gilbert damping and the magnetization switching of\nperpendicularly magnetized FeCoB-based free layers embedded in tunnel junctions\nadequate for spin-torque operated memories. We study the influence of the boron\ncontent in MgO / FeCoB /Ta systems alloys on their Gilbert damping after\ncrystallization annealing. Increasing the boron content from 20 to 30\\%\nincreases the crystallization temperature, thereby postponing the onset of\nelemental diffusion within the free layer. This reduction of the interdiffusion\nof the Ta atoms helps maintaining the Gilbert damping at a low level of 0.009\nwithout any penalty on the anisotropy and the magneto-transport properties up\nto the 400$^\\circ$C annealing required in CMOS back-end of line processing. In\naddition, we show that dual MgO free layers of composition\nMgO/FeCoB/Ta/FeCoB/MgO have a substantially lower damping than their\nMgO/FeCoB/Ta counterparts, reaching damping parameters as low as 0.0039 for a 3\n\\r{A} thick Tantalum spacer. This confirms that the dominant channel of damping\nis the presence of Ta impurities within the FeCoB alloy. On optimized tunnel\njunctions, we then study the duration of the switching events induced by\nspin-transfer-torque. We focus on the sub-threshold thermally activated\nswitching in optimal applied field conditions. From the electrical signatures\nof the switching, we infer that once the nucleation has occurred, the reversal\nproceeds by a domain wall sweeping though the device at a few 10 m/s. The\nsmaller the device, the faster its switching. We present an analytical model to\naccount for our findings. The domain wall velocity is predicted to scale\nlinearly with the current for devices much larger than the wall width. The wall\nvelocity depends on the Bloch domain wall width, such that the devices with the\nlowest exchange stiffness will be the ones that host the domain walls with the\nslowest mobilities.\n