We studied annealing effects on perpendicular anisotropy in CoFeB-MgO magnetic tunnel junctions. The results show that annealing is an effective method to improve the perpendicular anisotropy of a CoFeB-MgO system. It is found that a thicker CoFeB layer requires a higher annealing temperature to buildup its perpendicular anisotropy. However, perpendicular anisotropy could be seriously degraded if the annealing temperature is more than 350 °C. Our study suggests that CoFeB thickness should be optimized so that the required annealing temperature window for perpendicular anisotropy could match the annealing temperature for high magnetoresistance. In this work, the perpendicular anisotropy energy density of 2.5 × 106 erg/cm3 was achieved with tunnel magnetoresistive value exceeding 70%. The use of CoFeB films will enable the development of high density nonvolatile memory with size down to 30 nm.

Spin transfer torque-based magnetic random access memory with perpendicular magnetic anisotropy (PMA) provides better scalability and lower power consumption compared to those with in-plane anisotropy. Spin transfer torque switching in magnetoresistive spin valves with PMA is investigated. The hard layer is made of (Co/Pd) multilayer, whereas the soft layer is a lamination of (CoFe/Pd) and (Co/Pd). By the insertion of an in-plane spin polarizer adjacent to the perpendicular anisotropy free layer, thus creating a modified-dual spin valve, a significant reduction of about 40% in the current density required for spin torque transfer switching was observed. By using a spin polarized current with different pulse widths down to 10 ns, the barrier energy EB in 100-nm-diameter devices was found to be reduced from 1.1 to 0.43 eV. Besides the reduction of switching current density in a device with PMA, the new structure shows a clear increase in magnetization switching speed as revealed by micromagnetic simulation.

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