Spin-orbit torques arising from the spin-orbit coupling of non-magnetic heavy metals allow electrical switching of perpendicular magnetization. However, the switching is not purely electrical in laterally homogeneous structures. An extra in-plane magnetic field is indeed required to achieve deterministic switching, and this is detrimental for device applications. On the other hand, if antiferromagnets can generate spin-orbit torques, they may enable all-electrical deterministic switching because the desired magnetic field may be replaced by their exchange bias. Here we report sizeable spin-orbit torques in IrMn/CoFeB/MgO structures. The antiferromagnetic IrMn layer also supplies an in-plane exchange bias field, which enables all-electrical deterministic switching of perpendicular magnetization without any assistance from an external magnetic field. Together with sizeable spin-orbit torques, these features make antiferromagnets a promising candidate for future spintronic devices. We also show that the signs of the spin-orbit torques in various IrMn-based structures cannot be explained by existing theories and thus significant theoretical progress is required.

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Spin-orbit torque facilitates efficient magnetization switching via an in-plane current in perpendicularly magnetized heavy-metal/ferromagnet heterostructures. The efficiency of spinorbit-torque-induced switching is determined by the charge-to-spin conversion arising from either bulk or interfacial spin-orbit interactions, or both. Here, we demonstrate that the spinorbit torque and the resultant switching efficiency in Pt/CoFeB systems are significantly enhanced by an interfacial modification involving Ti insertion between the Pt and CoFeB layers.Spin pumping and X-ray magnetic circular dichroism experiments reveal that this enhancement is due to an additional interface-generated spin current of the non-magnetic interface and/or improved spin transparency achieved by suppressing the proximity-induced moment in the Pt layer. Our results demonstrate that interface engineering affords an effective approach to improve spin-orbit torque and thereby magnetization switching efficiency.

in-plane (perpendicular) magnetic anisotropy. The spin current generated from the FM/Ti interface has a spin polarization along the m × y direction, where m is the magnetization direction of the bottom FM. For an FM with m aligned in the x-direction, a spin current with zspin polarization is generated. [22,23] This enables field-free SOT switching of the perpendicular magnetization of the top CoFeB layer.In this work, we investigate the SOTs in FM/Ta/CoFeB trilayers, in which the spin current generated by SHE in Ta can be combined with the interface-generated spin currents. Using various Ta thickness, we perform measurements of SOTinduced effective fields and of magnetization switching with various Ta thicknesses. We observe two interesting points for a sample with a thin Ta layer; first, the sign of the SOT is determined by the bottom FM layer, and is positive for NiFe and negative for CoFeB. Second, SOT-induced switching is achieved without an in-plane magnetic field. These results demonstrate that the interface-generated spin current of the FM/Ta bilayer governs the SOT of a sample with a thin Ta layer. On the other hand, as Ta thickness increases, the sign of the SOT becomes negative irrespective of the bottom FM and which is determined by the Ta, which has a negative spin Hall angle. This demonstrates that the SOT in the trilayer structure is composed of two contributions: SHE in HM and interface-generated spin current of the FM/HM bilayer, suggesting that the proper selection of an FM/HM combination and of the material thickness can enhance the SOT efficiency and induce field-free SOT switching. Results and Discussion Spin-Orbit Torques in FM/Ta/CoFeB TrilayersIn order to investigate SOT in FM/Ta/CoFeB trilayer structures, we employ Ta (5 nm)/FM (4 nm)/Ta (t Ta )/CoFeB (1.0 nm)/ MgO (3.2 nm) structures, where Ta thickness (t Ta ) ranges from 1.0 to 6.0 nm (Figure 1a). The bottom FM is in-plane magnetized CoFeB or NiFe. As the samples share identical material structures, except for the bottom FM, we refer to the trilayer CoFeB or NiFe samples according to the bottom FM layers. We first check the magnetic anisotropy of each layer by Spin-orbit torques (SOTs) in ferromagnet (FM)/Ta/CoFeB trilayers are investigated as a function of Ta thickness. When the Ta is thinner than 1.5 nm, the sign of the SOT exerting on the top perpendicularly magnetized CoFeB depends on the bottom FM layer; it is positive for NiFe and negative for CoFeB. As the Ta thickness increases, the sign becomes negative irrespective of the bottom FM, indicating that SOTs are dominated by Ta, which has a negative spin Hall angle. SOT-induced switching without an in-plane magnetic field is observed in the thickness ranges where the bottom FM or FM/Ta interface-generated SOT is dominant. The results herein demonstrate that proper design of an FM/heavy metal combination and the material thickness can lead to an enhancement of the SOT efficiency and allow for field-free SOT switching.