CoFeB/MgO based perpendicular magnetic tunnel junctions with stepped structure for symmetrizing different retention times of “0” and “1” information

We investigated perpendicular magnetic tunnel junctions (p-MTJs) with a stepped structure for spin-transfer torque random access memory (SPRAM). In conventional p-MTJs, the retention time for storing “1” is shorter than that for storing “0”, because of the mangetostatic energy difference between the two states caused by dipole interaction. To counter this, one had to develop materials with low magnetization. Here we show, by employing a stepped structure, that the retention time can be made equivalent regardless of the stored information (“0” or “1”), without resorting to the employed materials. This is because this structure reduces the dipole interlayer coupling between two ferromagnetic layers as the diameter difference of the free and reference layers of MTJ increases.