Modulation of thermal stability and spin-orbit torque in IrMn/CoFeB/MgO structures through atom thick W insertion

Abstract

Spin-orbit-torque- (SOT-) driven switching of ferromagnets (FM) with perpendicular magnetic anisotropy (PMA) has attracted extensive attention as it allows for lower-power and faster magnetization switching compared with the conventional spin-transfer torques.[1] However, the requirement of a static magnetic field collinear with the current to achieve deterministic switching poses a challenge to the SOT device application. By introducing antiferromagnets (AFM), recent studies have shown that the in-plane exchange-bias field generated at the AFM/FM interface enables field-free switching of perpendicular magnetization, which makes AFM/FM structure a promising candidate for future spintronic devices.[2,3]On the other hand, CoFeB/MgO-based magnetic tunnel junctions are attractive in achieving the state-of-the-art tunnel magnetoresistance (TMR) ratio, which is important for the development of advanced nonvolatile spintronics memories and logics.[4] Therefore, field-free magnetization switching in AFM/CoFeB/MgO structures has been drawing intense attention.[5] In these structures, however, the PMA was not strong enough to guarantee the long-term data storage for magnetoresistive random-access memory and was reported to be deteriorated upon annealing at temperatures above ~200 °C due to the diffusion of Mn.[6] Besides, the spin-orbit torque of these AFM/CoFeB/MgO systems was not strong enough to implement low-power consumption for magnetization control. All the mentioned bottlenecks hinder the development of these structures in SOT devices.In this work[7], we investigated the modulation of thermal stability and SOT in IrMn/CoFeB/MgO structures through atom thick W insertion. The films consisting of Ta(2 nm)/IrMn(5 nm)/W(tW)/Co40Fe40B20(1 nm)/MgO(2.5 nm)/Ta(3 nm) were deposited on thermally oxidized Si substrate by magnetron sputtering, followed by annealing at 300°C for 30 minutes under an in-plane magnetic field of 1.5T. Subsequently, the films were patterned into Hall bar devices with the longitudinal channel and transverse channel width of 5 µm and 3 µm, respectively. Figure 1(b) presents magneto optical Kerr effect (MOKE) measurements, which shows that samples with W insertion exhibit sharp PMA signals while the magnetic signal of reference sample without W insertion vanished. This W layer serves as a thermal barrier to prevent the diffusion of Mn during annealing, which is confirmed by the results of spherical aberration corrected transmission electron microscope (ACTEM) and atomic-resolution electron energy-loss spectroscopy (EELS). Figure 1(d) shows field-dependent Hall resistance (RHall) measurements with in-plane applied magnetic fields μ0Hx. The magnetization could not be fully pulled into the in-plane direction with magnetic field even up to 1000 mT, indicating the strong PMA of the samples with W insertion which is much larger than that in our previous report without W insertion, the PMA field of which is only ~300 mT.[6]Figure 2(b) presents W layer thickness dependence of SOT effective fields. Both damping-like effective field △μ0Hx and field-like effective field △μ0Hy are tuned by W layer thickness. Focusing on the variation trend of effective fields with W layer thickness, we find that the changes in SOT fields and PMA fields (Fig. 2a) track each other fairly closely. In order to investigate visually this relation, we plot SOT fields as a function of PMA fields in Fig. 2c. SOT fields scale roughly linearly with PMA fields, which is in consistence with theoretical calculations and experimental demonstration in other systems.[8] That is, the existence of interfacial spin-orbit coupling (ISOC) at heavy metal/FM interfaces quantified by PMA fields would reduce, in a linear manner, the effective SOT exerted on the FM layer. Finally, field-free magnetization switching was implemented in the sample with W insertion, and the obtained exchange bias field of 1.5 mT is comparable to that shown in our previous work without W insertion [6].In summary, we have demonstrated that the thermal stability and PMA fields in IrMn/CoFeB/MgO heterostructures can be effectively enhanced by incorporating a thin W layer at the IrMn/CoFeB interface via suppressing the diffusion of Mn during annealing. Moreover, SOT fields scaling linearly with ISOC in IrMn/W/CoFeB quantified by PMA fields indicates that efficient SOT could be achieved by minimizing the ISOC via the tuning of insertion W layer thickness. Finally, field-free switching of perpendicular magnetization is achieved when a thin W layer is inserted. This work demonstrates an efficient scheme for thermally robust technological application based on AFM spintronics.[7] **