Toggle MRAM with CoFeB-Based Synthetic Antiferromagnet Free Layers

For toggle MRAM, the free layer is a synthetic antiferromagnet (SAF) that must have specific magnetic properties such as: low magnetostriction, a repeatable saturation field (H_sat) that can be adjusted within a specific range, and a well-defined and reproducible intrinsic anisotropy axis. The results from fully-functional Mb-scale MRAM circuits using CoFeB SAF free layers optimized for toggle switching is reported. A 40% improvement in useable MR in 4Mb MRAM circuits using optimized CoFeB SAF free layers is demonstrated, with switching comparable to standard NiFe material. The critical properties optimized for switching, electrical properties of the devices, and thermal endurance are described.