Spin–orbit torque magnetization switching in CoFeB/Ta/CoFeB tri-layer thin films

Spin–orbit torque magnetoresistive random-access memory (SOT-MRAM) with a perpendicularly magnetized ferromagnet (PMF) is a promising candidate for future low-power and high-performance non-volatile memory. However, conventional SOT devices require an external in-plane magnetic field to achieve deterministic magnetization switching, which is a major obstacle for high-density integration. To overcome this issue, we investigate field-free SOT switching in a CoFeB(PMF)/Ta/CoFeB(IMF) trilayer structure, where a Ta layer separates the PMF and an in-plane magnetized ferromagnet (IMF). We successfully demonstrate deterministic field-free magnetization switching in this trilayer system. The switching current density (Jc) was systematically investigated as a function of the Ta and CoFeB(IMF) layer thicknesses, revealing an optimal structure with a significantly reduced. At the optimal thickness of tTa = 5.0 nm and tIMF = 2.0 nm, the switching current density was minimized. To elucidate the origin of this high efficiency, we performed anomalous Hall effect measurements. A large current-induced loop shift was observed, corresponding to a giant effective magnetic field that assists the magnetization reversal. The efficiency of this current-induced field was found to be enhanced by a factor of 4.5 in the trilayer structure compared to a bilayer control sample. We attribute this giant assistance effect to the IMF layer, which enables both the symmetry breaking required for field-free switching and a dramatic reduction in the switching current. These findings provide a new and effective pathway for developing practical, field-free, and low-power SOT-MRAM devices.