The macro-spin modeling indicates a field-free SOT switching in type- x, induced in only 1.7% of σ x over σ y, with its switching current lower than that of the corresponding type- y SOT switching. Therefore, a promising approach to achieve field-free SOT switching is to obtain significant σ x and σ z spins in the configurations of type- x and type- z. 11 Comparably, σ z (σ x) exerts out-of-plane damping-like torque τ DL, z ∼ m × ( m × σ z), enabling field-free and low power switching of type- z (type- x). The y-polarized spins can deterministically switch type- y magnetizations by taking a few precessions before polarity changes without external fields to break the symmetry. In an HM with structure symmetry, σ y dominates far over other spin polarization directions. Generally, the charge current injected into the NM layer along the x-direction generates a spin current flowing along the z-direction with spin polarization σ pointing along y, notated as σ y. In addition to the high spin-torque efficiency, the absence of external magnetic fields in SOT switching is also required to achieve high-density magnetic memory devices and low energy consumption at the system level. Our work has experimentally demonstrated a new avenue to modulate SOTs with physically sputtered metal layers, and this finding is promising to enable flexible and efficient spin polarizations for MRAM devices. In addition, we found that the dynamic spin pumping coupling between Pt/Co with weak PMA and the in-plane CoFeB could significantly modulate the effective SOTs in the heterostructure, and this effect is dependent on the thickness of the spacer Mg through long-range spin-wave mediated coupling. Independent characterizations were performed to verify the presence of the changes in SOTs following spin modulation by the Co insertion layer. Remarkably, the damping-like effective field has been enhanced by 7.4 times after inserting a thin Co layer with weak perpendicular magnetic anisotropy (PMA), while the field-like effective field is reduced to near zero value. In this work, we report significant changes in SOTs due to a Co thin film inserted in the Pt/Co/Mg/CoFeB heterostructures. New classes of materials such as antiferromagnets, topological insulators, and semimetals can generate spins with unconventional polarization and improve the efficiency of field-free SOT switching. Spin–orbit torque (SOT) magnetoresistive random-access memory (MRAM) devices have been proposed for energy efficient memory and computing applications.
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