Resistance drift of MgO magnetic tunnel junctions by trapping and degradation of coherent tunneling

Magnetoresistive Random Access Memory (MRAM) is a promising device for high-density (over Gbits scale), high-speed (equal to DRAM or better) non-volatile RAM, and much research has been done over several years with a view to overcoming the problems regarding practical use. Spin Torque Transfer switching MRAM (STT-MRAM) is considered to be the most promising candidate and there already are some papers on this new device. MgO is expected to be the best material for magnetic tunnel junction (MTJ) of STT-MRAM, because MgO-MTJ is known to show large Magnetoresistance (MR) and enhance spin polarization by the coherent tunneling effect, resulting in decrease of writing current of MTJ. MgO-MTJ has been shown to be an excellent barrier with little resistance drift compared with MTJ using alumina. Notwithstanding its excellent potential, the degradation mechanism of MgO-MTJ has not been well understood. In this paper, we will demonstrate for the first time the degradation of coherent tunneling and trapping phenomena of MgO-MTJ and discuss its mechanism.