Spin-Torque Effects In Mgo-Based Magnetic Tunnel Junctions
This thesis work focuses on understanding the fundamentals of the spintorque effect in MgO-based high tunneling magnetoresistance (TMR) magnetic tunnel junction (MTJ) nanopillar geometry devices. In order to achieve this goal, I have successfully established a reliable and repeatable high TMR and low resistance-area (RA) product MgO sputtering process at Cornell. Currently, we are capable of sputtering TMR (>100% with RA<10Ω/[MICRO SIGN]m2 ). In addition to speeding up the nanopillar two-terminal structure fabrication process, I have also worked on improving the nanopillar lift-off process, which has enabled me to finish the nanopillar fabrication process within a week. We have full control over high TMR MgO MTJ materials, nanofabrication and spin-torque analysis schemes at Cornell. I have shown that the spin-torque effect in asymmetric MgO-based MTJ could be significantly affected by the electronic structure in the electrodes or electrode/barrier interface in the MTJ ("asymmetric" MTJs indicate bottom and top electrodes materials are different ferromagnetic materials and "symmetric" MTJs have identical bottom and top electrode ferromagnetic materials.) Briefly, I have shown that the field-like torque in the asymmetric MTJs exhibit non-zero torkance at zero bias and would further reverse torkance asymmetric when the electrode was reversed from Fe40 Co40 B20 /MgO/Fe80 B20 to Fe80 B20 /MgO/Fe40 Co40 B20 . In addition, the field-like torque exhibits a constant and weak voltage dependence compared to symmetric Fe40 Co40 B20 /MgO/Fe40 Co40 B20 MTJs. Symmetric Fe80 B20 /MgO/Fe80 B20 MTJs exhibit opposite in-plane torkance compared to symmetric Fe40 Co40 B20 /MgO/Fe40 Co40 B20 . I also further probe the differences in spin-transfer torque in high TMR MgO-based MTJs, I have developed a pulse-biased microwave emission measurement to understand the spin-torque behavior under high current densities for real ultra-fast (<1ns) spin-torque-based magnetic switching devices. Here, I have observed that the microwave emissions are highly asymmetric under positive and negative voltage polarities. Within the bi-polar stable switching region, negative polarity (electrons flowing from fixed to free) induced strong microwave emission, but there is no microwave emission with positive voltage. This phenomena could be understood by a strong field-like torque effect affecting in-plane torque driven dynamics. Macromagnetic power phase diagram simulations capture the major features observed in the pulse-biased experiments. This result indicated that field-like torque could play a dominant role inducing unreliable switching even in the symmetric MTJ, plus the strong asymmetric in-plane torque with negative polarities could introduce chaotic dynamics and resulting in back-hopping switching.
spin-torque; mgo magnetic tunnel junctions
Buhrman, Robert A
Van Dover, Robert B.; Ralph, Daniel C
Ph.D. of Applied Physics
Doctor of Philosophy
dissertation or thesis