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F. Xue, S.-J. Lin, P. Li, W. Hwang, S. X. Wang, et al., Spin–orbit torques of an in-plane magnetized system modulated by the spin transport in the ferromagnetic co layer
Release time:2022-07-13
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- DOI number:
- 10.1063/5.0048917
- Journal:
- APL Materials
- Abstract:
- Spin–orbit torque (SOT) magnetoresistive random-access memory (MRAM) devices have been proposed for energy efficient memory and computing applications. 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. In this work, we report significant changes in SOTs due to a Co thin film inserted in the Pt/Co/Mg/CoFeB heterostructures. 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. Independent characterizations were performed to verify the presence of the changes in SOTs following spin modulation by the Co insertion layer. 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. 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. Spin–orbit torque (SOT) is an efficient and promising approach to manipulate magnetization of the ferromagnetic (FM) layer in NM (non-magnetic)/FM multilayers with ultra-high speed. Most SOT devices employ the spin Hall effect (SHE) and Rashba effect, which originate from spin–orbit coupling (SOC) within the NM layer1 and at the FM/NM interface.2,3 Spin currents generated within NM heavy metals (HMs) are significantly large due to strong SOC so that using SHE in HM is regarded as the most promising operating principle in spintronic devices.4,5 However, the figure of merit of charge-to-spin conversion, known as the spin-torque efficiency θ, is still not high enough in HM for efficient control of magnetization. Antiferromagnets,6,7 topological insulators,8 and semimetals9 interfaced with a ferromagnetic layer have been observed with SOTs as well and some of them have much higher θ than HM, such as MnPd3 of 0.35–0.44.10 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. 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 σy [as schematically shown in Fig. 1(a)]. In an HM with structure symmetry, σy dominates far over other spin polarization directions. 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.11 Comparably, σz (σx) exerts out-of-plane damping-like torque τDL,z ∼ m × (m × σz) [in-plane τDL,x ∼ m × (m × σx)], enabling field-free and low power switching of type-z (type-x). Therefore, a promising approach to achieve field-free SOT switching is to obtain significant σx and σz spins in the configurations of type-x and type-z. The macro-spin modeling indicates a field-free SOT switching in type-x, induced in only 1.7% of σx over σy, with its switching current lower than that of the corresponding type-y SOT switching.15 Spin currents with z-polarization have recently been observed in semimetal WTe2 and ferromagnet CoFeB,12,13 and x-polarized spins are additionally reported in single-phase Mn3GaN due to low magnetic symmetry.7 From both perspectives of improving spin-torque efficiency and generating unconventional spins, we studied in the current research the breaking of the symmetry of HM by involving magnetization, inserting a thin ferromagnetic layer to modulate the spins from HM.
- Document Code:
- 101106
- Volume:
- 9
- Translation or Not:
- no
- Date of Publication:
- 2021-10-12
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