Xiaohua Lou

Demonstration of multilevel cell spin transfer switching in MgO magnetic tunnel junctions

Multilevel cell is an important concept to improve the density of spin transfer torque memory. We demonstrated two-bit multilevel cell spin transfer switching using MgO-based magnetic tunnel junctions. Two types of cell structural design are discussed. Multiple resistance levels are depicted in a current-driven spin transfer switching loop, which shows a switching current density of ∼2×106A∕cm2. Reversible transitions between high and low states of the soft bit are achieved using minor-loop spin transfer switching. The influence of external magnetic field to spin transfer switching is also discussed.

Dielectric breakdown of MgO magnetic tunnel junctions

We have investigated high-quality MgO tunnel junctions with a range of barrier thickness in order to identify the underlying physical mechanism responsible for dielectric breakdown. Two types of dielectric breakdown (“soft” and “hard”) were observed. Soft breakdown was observed in a few percent of the devices. This breakdown mode is not intrinsic and is attributed to tunnel junction imperfections. The hard breakdown occurs because a critical electric field is reached across the tunnel barrier. Other possible breakdown mechanisms, such as thermally driven mass diffusion or charge trapping, were not consistent with the hard dielectric breakdown data and were ruled out.

Transport properties of MgO magnetic tunnel junctions

We have studied transport properties of nanopillars with high-quality MgO tunnel barriers and with a range of MgO barrier thickness, and compared the bias-voltage dependence of tunneling magnetoresistance, differential conductivity, and resistance in parallel and antiparallel states with theoretical models. The differential conductance in the antiparallel state suggests that the dominant scattering mechanism for this state is inelastic scattering that mixes propagating Δ5 states in the electrode with evanescent Δ1 states in the MgO barrier. In addition, we observed in large populations of parts with both low- and high-voltage bias transport anomalies in the parallel state. Finally, in the junctions studied here, we observed a much weaker decrease in the tunneling magnetoresistance with bias voltage than reported in previous studies.