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

Very high thermal stability ¿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(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 ¿₀ was evaluated based on the Sharrock equation. The ¿₀ values increased concomitantly with increasing thickness of the upper CoFeB layer d_CoFeB. We obtained very large ¿₀ values of about 248 when d_CoFeB = 4 nm. Such a large ¿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.