We investigated the low temperature performance of CoFeB/MgO based perpendicular magnetic tunnel junctions (pMTJs) by characterizing their quasi-static switching voltage, high speed pulse write error rate and endurance down to 9 K. pMTJ devices exhibited high magnetoresistance (>120%) and reliable (error rate<10-4) bi-directional switching with 2 to 200 ns voltage pulses. The endurance of the devices at 9 K surpassed that at 300 K by three orders of magnitude under the same write conditions, functioning for more than 10^12 cycles with 10 ns write pulses. The critical switching voltage at 9 K was observed to increase by 33% to 93%, depending on pulse duration, compared to that at 350 K. Ferromagnetic resonance and magnetization measurements on blanket pMTJ film stacks suggest that the increased switching voltage is associated with an increase in effective magnetic anisotropy and magnetization of free layer with decreasing temperature. Our work demonstrates that CoFeB/MgO based pMTJs have great potential to enable cryogenic MRAM and that their low temperature magnetization and effective magnetic anisotropy can be further optimized to lower operating power and improve endurance.
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