Switching field distribution in magnetic tunnel junctions with a synthetic antiferromagnetic free layer

Abstract

By replacing the traditional single magnetic free layer in magnetic random access memory (MRAM) with a synthetic antiferromagnetic (SAF) layer composed of two antiferromagnetically coupled layers, the possibility of the kink generation has been suppressed to a minimum level and the normalized free-layer shift has been reduced from 15% to less than 5%. These phenomena are thought to be from the reduction of the effective thickness and magnetic moment by introducing a SAF free layer. Since the SAF free layer decreases the magnetostatic interaction with the pinned layer, free-layer shift is also decreased. A SAF free layer forms a closed magnetic loop between two antiferromagnetically coupled layers. Therefore, it increases a tendency to become a single domain and its switching field distribution is enhanced. With the optimized SAF free layer, array quality factor [AQF, σ(Hc)∕Hc] is increased to more than 10 and switching window could be obtained which is the area to be selectively switched in the memory cell array. When adopted in a 64-kb MRAM, it is confirmed that cells can be selectively switched without the disturbance to the other cells.