The characteristics of synthetic antiferromagnetic (SAF) nanostructures CoFeB/Ru/CoFe with a Ru layer thickness corresponding to the second antiferromagnetic (AF) maximum are obtained. The magnetization-reversal curves of SAF structures with a Ru thickness of 2.1, 2.3, and 2.5 nm are studied. The magnetization-reversal curves of CoFeB/Ru/CoFe structures under conditions of their fixation by an AF layer are presented. In addition, the magnetization-reversal curves of spin-tunnel magnetoresistive nanostructures with an AF layer and a SAF structure are presented, and an analysis of the thermal stability of structures with SAF is carried out.