To increase the storage density, multi-level MRAM has been reported using complex magnetoresistive spin valve with perpendicular anisotropy []. The structure consists of two FLs and one reference layer, all based on Co/Pd multilayer exhibiting different four resistance levels. The selection of free-layer for storing data can be done by distinct level of resistance and their respective STT switching current []. Writing improvements As discussed earlier, spin-torque transfer (STT) principle has been widely accepted as the method to write information. Any new methods implemented in the design of MRAM must improve the writing speed, reduce the writing current and consumed power. This section discusses, in detail, the methods in which writability or speed can be increased. Equation (2) indicates that the critical current to switch the magnetization of the free layer depends on the damping factor Therefore, it is essential to find materials that have a  low damping factor, in order to reduce writing current. Materials such as FePt or Co/Pd, which were once researched for pMTJ applications due to their high Ku, have a large damping factor. Therefore, even though these materials could be scaled down to smaller devices with high thermal stability, writing information on these devices would consume significant power. In this context, the invention of interface-pMTJ based on Ta/CoFeB/MgO came as a boon for the STT-MRAM. CoFeB has a much lower damping factor and could be written with much lower currents, as compared to FePt or Co/Pd. Interface-pMTJ devices could be used down to 20 nm sizes, beyond which one may consider materials such as Co/Pd, Co/Pt or FePt, although the writing issues on such
To increase the storage density, multi-level MRAM has been reported using complex magnetoresistive spin valve with perpendicular anisotropy []. The structure consists of two FLs and one reference layer, all based on Co/Pd multilayer exhibiting different four resistance levels. The selection of free-layer for storing data can be done by distinct level of resistance and their respective STT switching current []. Writing improvements As discussed earlier, spin-torque transfer (STT) principle has been widely accepted as the method to write information. Any new methods implemented in the design of MRAM must improve the writing speed, reduce the writing current and consumed power. This section discusses, in detail, the methods in which writability or speed can be increased. Equation (2) indicates that the critical current to switch the magnetization of the free layer depends on the damping factor  Therefore, it is essential to find materials that have a low damping factor, in order to reduce writing current. Materials such as FePt or Co/Pd, which were once researched for pMTJ applications due to their high Ku, have a large damping factor. Therefore, even though these materials could be scaled down to smaller devices with high thermal stability, writing information on these devices would consume significant power. In this context, the invention of interface-pMTJ based on Ta/CoFeB/MgO came as a boon for the STT-MRAM. CoFeB has a much lower damping factor and could be written with much lower currents, as compared to FePt or Co/Pd. Interface-pMTJ devices could be used down to 20 nm sizes, beyond which one may consider materials such as Co/Pd, Co/Pt or FePt, although the writing issues on such
MRAM_Review_Ver1_Mat_Today.docx