Field-free switching of perpendicular magnetization via out-of-plane spin-polarization induced by Ta with crystalline phase gradient

Contrary to the conventional spin current with the in-plane spin polarization, spin current with the out-of-plane spin polarization can achieve efficient all electrical control of perpendicular magnetization by spin-orbit torques (SOTs) without an external magnetic field. This feature is essential for high-density magnetic memory and logic devices. While such spin current with the out-of-plane spin polarization has been demonstrated in a limited number of high-quality low crystal or magnetic symmetry systems, its generation in conventional heavy metal is challenging but crucial for field-free spintronic devices. Here, we show that overcoming this challenge is possible by fabricating a crystalline phase gradient in heavy metal tantalum, where the out-of-plane spin polarization was induced and triggers an out-of-plane SOT providing the field-free switching for perpendicularly magnetized ferromagnet in Pt/Co/Ta (crystalline phase gradient) heterojunction. Meanwhile, the critical switching current density is lower than that of the Pt/Co/Ta (non-crystalline phase gradient) control case, and the polarity of field-free switching is determined by the sign of the crystalline phase gradient. Our work demonstrates that the crystalline phase gradient is a promising platform to generate the spin current with the out-of-plane spin polarization and achieve efficient field-free magnetization switching based on heavy metal tantalum, thus opening an avenue toward energy-efficient spintronic devices.