Enhancement of writing margin for low switching toggle magnetic random access memories using multilayer synthetic antiferromagnetic structures

We developed synthetic antiferromagnets (SAFs) with a multilayer structure where more than three magnetic layers were coupled in an antiparallel configuration for use as a free layer of toggle magnetic random access memories, with the aim of reducing the switching field. The SAFs consisted of a magnetic layer of NiFeCoFe with a spacer layer of Ru and were deposited on the Al-oxide barrier layer in magnetic tunnel junctions. Conventional bilayer SAFs did not realize a large writing margin with a low spin flop field (Hflop). On the other hand, multilayer SAFs increased the saturation field (Hs) significantly with increases in both the number of magnetic layers (N) and the antiferromagnetic coupling (AFC) strength between the inner magnetic layers while maintaining a low Hflop. This is because strong AFCs involved in the inner magnetic layers and weak AFCs of the outer magnetic layers determine Hs and Hflop of these, respectively. To control an AFC of each spacer layer, we chose an appropriate thickness for each Ru layer in the multilayer SAFs according to the Ru crystalline structure that was responsible for its AFC strength.