Performance of shape-varying magnetic tunneling junction for high-speed magnetic random access memory cells

We have developed a shape-varying magnetic tunneling junction (MTJ) (SVM) which has a high MR ratio and low write current for use in high-speed magnetic random access memory (MRAM) cells. Combining NiFe that has low anisotropy to CoFeB which has high anisotropy through the nonmagnetic layer by interlayer exchange coupling (synthetic ferromagnetic coupling free layer: SFF), the anisotropy of SFF was reduced much more than that of CoFeB, and the MR ratio was improved much more than that of NiFe. The switching magnetic field (Hsw) of SFF was reduced as the thickness of NiFe increased. The Hsw of SFF for 0.24×0.48 μ m2 MTJ was 30 Oe when the thickness of CoFeB was 1.5 nm and that of NiFe was 3.0 nm. Furthermore Hsw was reduced to 18 Oe by varying the shape of the MTJ of NiFe to 0.48×0.48 μ m2; the shape of the MTJ of CoFeB was not changed (0.24×0.48 μ m2). Combining the SVM and a write-line-inserted structure, we obtained a write current of 0.9 mA and an MR ratio of 140%. The Hsw was 40 Oe and its thermal stability factor was 82. These properties are sufficient for operating MRAMs over 500 MHz.