Large exchange coupling and high thermal stability in synthetic antiferromagnet with ultrathin seed layer

A synthetic antiferromagnet (SAF) consisting of two ferromagnetic layers antiferromagnetically coupled is a key component for a magnetoresistive element. By using an SAF as a free layer of conventional MRAMs, the switching characteristics and scalability can be improved. Toggle MRAMs must use SAF free layers. When the SAFs that use a spacer layer of Ru are grown on the AlO-barrier layer of magnetic tunnel junctions (MTJs), the antiferromagnetic coupling (AFC) significantly deteriorates. Until now, a thin Ru layer( < 1 nm) has been mostly used for the SAFs to obtain a strong AFC, which is, however, significantly deteriorated by high-temperature( > 300degC) annealing. This is a serious problem for the MgO-barrier MTJs that need high-temperature annealing. To solve these issues, we developed SAFs with an ultrathin seed layer and we used them in toggle MRAMs.