Structural and Material Exploration of Magnetic Tunnel Junctions

Spin-Torque-Transfer Magnetic Random Access Memory (STT-RAM) has the potential to become a universal memory due to its superior properties compared with conventional memory technologies. However, numerous challenges still remain before this technology becomes practical. Efforts to improve the performance of magnetic tunnel junctions (MTJs), the building block of STT-RAM, were made both from the structural and material aspects, with the results and discussions presented in this dissertation. MTJ structures with AlOx tunnel barriers were deposited using the Bias Target Ion Beam Deposition (BTIBD) technique. A CoFe/FeMn/CoFe trilayer with exchange bias (EB) coupling was developed and the underlying physics was studied. By controlling the sputtering voltage, the ferromagnetic (FM) electrode/barrier interface profiles could be tuned to improve the tunneling efficiency as well as other important electrical and magnetic properties. During the MTJ fabrication process, formation of the tunnel barrier is the most critical step. An effective way of detecting the bottom electrode oxidation was developed by simple low temperature magnetic measurement. MTJs using CoFeB/MgO/CoFeB were successfully fabricated by BTIBD with tunneling magnetoresistance (TMR) around 70%. MTJ performance was optimized by tuning the annealing conditions and other junction stack parameters. Preliminary work on doublebarrier MTJs (DBMTJs) demonstrated the capability of BTIBD to make DBMTJs with the two pinned layers either parallel or antiparallel for different applications. From a material perspective, MnAl thin film with PMA and other unique properties was explored for its potential application as the FM electrode for MTJs. The successful growth of magnetic τ-MnAl with the L10 structure and a record high saturation magnetization value on MgO(001) substrate confirmed the candidacy of MnAl as a promising MTJ FM electrode. Further magnetic and structural analysis of τ-MnAl revealed certain correlations between the chemical ordering and saturation magnetization. Post-deposition heat treatment, rapid thermal annealing (RTA) in particular, was found to be a very effective way of tuning the properties of thin-film τ-MnAl.