We have developed a write-line-inserted magnetic tunneling junction (MTJ) (WLIM) for use in low-write-current magnetoresistance random access memory (MRAM). The write current of the WLIM was reduced to 0.98mA and its thermal stability factor was 85 for a 0.32×0.48μm2 MTJ. We evaluated the switching property of the WLIM in an external magnetic field (Hsw̱ext) and an internal magnetic field (Hsw̱int). We found that Hsw̱ext was larger than Hsw̱int when the aspect ratio of the MTJs was less than 1.5. Furthermore, we obtained a high write-current magnetic-field efficiency of 13.0Oe∕mA when the aspect ratio of the MTJs was low. These properties mean that the WLIM structure has advantages for use in low-write-current MRAM.
We have developed a shape-varying magnetic tunneling junction ͑MTJ͒ ͑SVM͒ which has a high MR ratio and low write current for use in high-speed magnetic random access memory ͑MRAM͒ 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: SFF͒, 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 ͑H sw ͒ of SFF was reduced as the thickness of NiFe increased. The H sw of SFF for 0.24ϫ 0.48 m 2 MTJ was 30 Oe when the thickness of CoFeB was 1.5 nm and that of NiFe was 3.0 nm. Furthermore H sw was reduced to 18 Oe by varying the shape of the MTJ of NiFe to 0.48ϫ 0.48 m 2 ; the shape of the MTJ of CoFeB was not changed ͑0.24ϫ 0.48 m 2 ͒. 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 H sw was 40 Oe and its thermal stability factor was 82. These properties are sufficient for operating MRAMs over 500 MHz.
We propose a magnetic random access memory (MRAM) cell that utilizes field-induced switching and is applicable to high-speed memories. The MRAM cell, called the shape-varying MRAM cell, has three free layers, each having different shapes and functions, and achieves low write-current switching with high thermal stability and high external field robustness. We show analytically that one of the layers contributes to the low write-current switching and another contributes to the thermal stability. We also show the results of a micromagnetic simulation, in which write current of <0.5 mA, write time of <2 ns, energy barrier (ΔE/kBT)>100, and external field robustness of >32 Oe were obtained.
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