A cell structure of magnetoresistive RAM includes a synthetic anti-ferromagnetic (SAF) layer to serve as a pinned layer; a barrier layer, disposed on the SAF layer; and a magnetic free layer, disposed on the barrier layer. The SAF layer includes: a first magnetic layer; a second magnetic layer; and a spacer layer of a first metal element sandwiched between the first magnetic layer and the second magnetic layer. The first metal element is phase separated from a second metal element of the first magnetic layer and the second magnetic layer interfacing with the spacer layer.