Article

Heavy-Ion-Induced Displacement Damage Effects in Magnetic Tunnel Junctions With Perpendicular Anisotropy

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Abstract

We evaluate the resilience of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) to displacement damage induced by heavy-ion irradiation. MTJs were exposed to 3-MeV Ta <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions at different levels of ion beam fluence spanning five orders of magnitude. The devices remained insensitive to beam fluences up to $10^{11}$ ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , beyond which a gradual degradation in the device magnetoresistance, coercive magnetic field, and spin-transfer-torque (STT) switching voltage were observed, ending with a complete loss of magnetoresistance at very high levels of displacement damage (>0.035 displacements per atom). The loss of magnetoresistance is attributed to structural damage at the MgO interfaces, which allows electrons to scatter among the propagating modes within the tunnel barrier and reduces the net spin polarization. Ion-induced damage to the interface also reduces the PMA. This study clarifies the displacement damage thresholds that lead to significant irreversible changes in the characteristics of STT magnetic random access memory (STT-MRAM) and elucidates the physical mechanisms underlying the deterioration in device properties.

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... Displacement damage on modern STT-pMTJ arrays was studied recently by Xiao et al. [90], by subjecting set of about 25-70 MTJs without transistors, with dimensions of 80-115 nm to 3-MeV Ta fluences ranging from 10 9 to 10 14 ions/cm 2 . Following exposures of 10 11 ions/cm 2 and less, no effect was observed in the switching properties before and after exposure. ...
... Following 10 12 ions/cm 2 exposure, minor degradation of switching functionality is observed, which would not preclude normal memory functionality. At fluences of 10 13 ions/cm 2 and above, switching functionality is lost due to severe damage in the interface, FL, or PLs, with the exact nature of the degradation still under investigation [90]. ...
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