Field-free spin-orbit torque switching of composite perpendicular CoFeB/Gd/CoFeB layers utilized for three-terminal magnetic tunnel junctions

Spin-orbit torque (SOT) induced magnetization switching has become a research focus in spintronics because it enables energy-efficient switching. There have been several experiments realizing field-free SOT-induced magnetization switching of materials with perpendicular magnetic anisotropy (PMA) in a bilayer system, either using thin Co(Fe) and CoFeB layers with interfacial PMA or using Co/Ni multilayers. All of these stacks are ferromagnets with large saturation magnetization (M_S). Here, we demonstrate SOT switching in a multilayer stack of CoFeB/Gd/CoFeB. This stack shows a good PMA and a low M_S (370 ± 20 emu/cm³), where CoFeB and Gd layers are antiferromagnetically exchange-coupled with each other. SOT induced magnetization switching has been demonstrated in this stack at zero magnetic field with a switching current density of ∼9.6 × 10⁶ A/cm² by using antiferromagnetic PtMn as the spin Hall channel material. The spin Hall angle of PtMn was also determined to be ∼0.084 ± 0.005 by performing a second harmonic Hall measurement. This layer structure is compatible with perpendicular magnetic tunnel junctions (p-MTJs), which could enable field-free three-terminal p-MTJs and lead to memory and logic devices based on SOT.