We developed perpendicular magnetic tunneling junctions (MTJs) with four synthetic antiferromagnetically coupled Co/Pt layers (Quad-SyF) and investigated their magnetic and transport properties. The quad SyF comprised four Co/Pt layers and three 0.9 nm-thick Ru coupling layers, which consisted of Co/Co/Pta/Ru/Co/Co/Ptb/Ru/Co/Co/Ptc/Ru/Co/Co/Ptd from top to bottom. The exchange coupling field (Hex) reached a maximum of 1 T when the values of a, b, c, and d were 1, 2, 2, and 1, respectively. The tunnel magnetoresistance ratio of the MTJ with the Quad-SyF and the second-peak conventional Double-SyF increased as the annealing temperature was increased up to 400°C, whereas that of the MTJ with the first-peak conventional Double-SyF degraded at temperatures of more than 350°C in the blanket films. A 55 nm-diameter MTJ with Quad-SyF was found to be stable even against an external magnetic fields up to 300 mT. In contrast, in the conventional Double-SyF, the reference-layer magnetization direction flips at around 250 mT. The shift magnetic field of the MTJ with Quad-SyF becomes approximately zero when the values of a, b, c, and d were 1, 4, 1, and 2, respectively. No back-hopping of the MTJ with Quad-SyF was observed even for the write pulse width (tW) down to 10 ns. In contrast, an MTJ with conventional Double-SyF exhibited back-hopping. In the patterned MTJ with conventional Double-SyF, as the MTJ size decreases, the coercive field of Co/Pt significantly increases and Hex decreases, causing the m-H curve of the reference layer to cross the zero magnetic field. This enables both parallel and antiparallel configurations for the top and bottom Co/Pt layers in Double-SyF at the zero magnetic field, which could induce back-hopping. However, the m-H curves of the reference layer in the patterned MTJ with Quad-SyF are far from the zero magnetic field owing to the high Hex and low Hc, which could lead to the suppression of back-hopping.
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