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Very high thermal stability Â¿0 was demonstrated in MgO-based magnetic tunnel junctions (MTJs) with CoFeB/Ru/CoFeB ferromagnetically coupled synthetic free layers (F-coupled Sy). Samples had a structure of underlayer/PtMn(15)/CoFe(2.5)/Ru(0.85)/CoFeB(3)/MgO(1)/F-coupled Sy/Ta(5)/Ru(7), where F-coupled Sy was CoFeB(2)/Ru(1.5)/CoFeB(dCoFeB = 1-4 (nm unit). The MTJs were elliptical with approximate dimensions of 60 nm Ã 170 nm. Resistance-field (R - H) loops were measured repeatedly; the H-dependence of switching probability PSW was observed. From the distribution of switching field HC, thermal stability factor Â¿0 was evaluated based on the Sharrock equation. The Â¿0 values increased concomitantly with increasing thickness of the upper CoFeB layer dCoFeB. We obtained very large Â¿0 values of about 248 when dCoFeB = 4 nm. Such a large Â¿0 originates in enhanced shape magnetic anisotropy in the F-coupled Sy. The shape anisotropy is lost because of the low effective magnetization if the magnetizations are coupled antiferromagnetically. The F-coupled Sy with a thicker upper magnetic layer is suitable for use in nonvolatile memory cells in magnetoresistive random access memory.