Bias polarity dependent low-frequency noise in ultra-thin AlO_x-based magnetic tunnel junctions

We exploit bias polarity dependent low-frequency noise (LFN) spectroscopy to investigate charge transport dynamics in ultra-thin AlO_x-based magnetic tunnel junctions (MTJs) with bipolar resistive switching (RS). By measuring the noise characteristics across the entire bias voltage range of bipolar RS, we find that the voltage noise level exhibits an bias polarity dependence. This distinct feature is intimately correlated with reconfiguring of the inherently existing oxygen vacancies (V_O^.) in as-grown MTJ devices during the SET and RESET switching processes. In addition, we observe two-level random telegraph noise (RTN) with a longer and shorter tunneling length in the high resistance state (HRS) and low resistance state (LRS) at a low bias voltage. The intrinsic voltage fluctuations of RTN arise from the dynamics of electron trapping/de-trapping processes at the V_O^.-related trap sites. Notably, the RTN magnitude is similar in LRS but nonidentical in that of HRS for different bias polarity. These findings strongly suggest that the inherent V_O^. are distributed near the top CoFe/AlO_x interface in the HRS; in contrast, they are expanded to the middle region of the AlO_x in the LRS. More importantly, we demonstrate that the location and distribution of the inherent V_O^. can be electrically tuned, which plays an essential role in the charge transport dynamics in the ultra-thin AlO_x-based MTJs and have significant implications for developing emergent memory and logic devices.