Recently, large-capacity magneto-resistive random access memories (MRAMs) have been attracting attention, with expectations. An MRAM employs a magnetic tunnel junction (MTJ) element which exploits the tunnel magneto-resistive (TMR) effect. Each MTJ element in an MRAM comprises two ferromagnetic layers (CoFeB) between which a tunnel barrier layer (MgO) is interposed, one of the two ferromagnetic layers being a magnetization fixed layer (reference layer) in which the direction of magnetization is fixed and so does not change, and the other being a magnetization free layer (memory layer) the direction of magnetization of which is capable of being easily changed. The states in which the directions of magnetization of the reference layer and memory layer are mutually parallel and anti-parallel are respectively defined as binary 0 and binary 1 on the basis of which data can be stored.
More specifically, when the directions of magnetization of the reference and memory layers are parallel, the resistance of the tunnel barrier layer (that is, the barrier resistance) is low, and the tunnel current is greater than that when the directions of magnetization are antiparallel. The MR ratio is defined as: (resistance in antiparallel state−resistance in parallel state)/resistance in parallel state. Because stored data is read by detecting differences in resistance due to the TMR effect, it is preferable when reading data that the ratio of resistive difference (MR ratio) by the TMR effect should be high.