Enhancement of Thermal Stability Using Ferromagnetically Coupled Synthetic Free Layers in MgO-Based Magnetic Tunnel Junctions

Very high thermal stability 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(d(CoFeB) = 1-4 (nm unit). The MTJs were elliptical with approximate dimensions of 60 nm 170 nm. Resistance-field (R - H) loops were measured repeatedly; the H-dependence of switching probability P-SW was observed. From the distribution of switching field H-C, thermal stability factor was evaluated based on the Sharrock equation. The values increased concomitantly with increasing thickness of the upper CoFeB layer d(CoFeB). We obtained very large values of about 248 when d(CoFeB) = 4 cm. Such a large Delta(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 antiferro-magnetically. The F-coupled Sy with a thicker upper magnetic layer is suitable for use in nonvolatile memory cells in magnetoresistive random access memory.