Experimental demonstration of voltage-gated spin-orbit torque switching in antiferromagnet/ferromagnet structure

Abstract

Magnetic random-access memory (MRAM) has recently emerged as a promising way for next generation memory due to its nonvolatility, low power consumption and high density. How to achieve energy-efficient magnetization switching is one of the main challenges for MRAM. Recent studies propose spin orbit torque (SOT) as a promising method for data writing [1]. However, an external magnetic field along current direction is usually needed to realize deterministic switching. It can be overcome by using antiferromagnet/ferromagnet structures in which exchange bias can replace external field [2]. Voltage controll of magnetic anisotropy (VCMA) is another candidate method for lowering down the energy height by inducing electron accumulation at the ferromagnet/oxide interface[3]. Combination of VCMA and SOT effect may lead to decrease of power consumption by a gate voltage lowering the energy barrier height. For example, Yoda et.al propose a new spintronics-based memory employing the VCMA effect as a bit selecting principle and the SOT effect as a writing principle. [4]. Back et al reported that a gate voltage can be used to manipulate both perpendicular magnetic anisotropy and spin orbit torque[5]. Hence, it is essential to make it clear whether the voltage-controlled spin-orbit effective torques (VCSOT) or voltage controlled magnetic anisotropy causes the decrease of critical current, or both.In this work, we investigate VCSOT effect and VCMA effect in the IrMn/CoFeB/MgO structures. The film structures were thermally oxidized Si substrate/Ta(2)/IrMn(5)/CoFeB(0.94)/MgO(2.5)Al2O3(5), (with film thickness in nm in the parentheses). The films were patterned into Hall bar by ion beam etching (IBE) and lithography as shown in Fig. 1(a). Fig. 1(b) shows the micrograph of the Hall bar and the measurement setup. Furthermore, to explore voltage-gated SOT switching, a gate voltage is applied at the top electrode and the switching loops are obtained with no external field applied as shown in Fig. 1(d). Figure 1(f) presents critical current densities with 5mT field under different gate voltages. The critical current density Jc shows a decreasing tendency with increasing VMgO. These results indicate that a gate voltage can be used to effectively modulate SOT-driven perpendicular magnetization switching and this is a feasible writing method for low energy consumption.Next, spin-orbit effective fields at the different voltage were measured by using a second harmonic method as shown in Fig. 2 (a). The damping-like torque efficiency decreases with positive voltage increasing, whereas the field-like torque efficiency shows weaker voltage dependence. These results demonstrate that VMgO can modulate spin torques in IrMn/CoFeB/MgO system. The damping-like torque efficiency under VMgO of -0.4 V is 1.5 times larger than that under VMgO of 0.4 V. A larger JC is required for the magnetization switching under positive voltage due to the reduction of θSHE. Through VCSOT effect, positive voltage tends to increase Jc, which is reverse to the experimental results in Fig. 1(f). In-plane hysteresis loops at different gate voltages are obtained as shown in Fig. 2(b). The dependence of anisotropy energy Keff(VMgO) on gate voltage is shown in Fig.2 (c). By linear fitting, Keff decreases by 55% when VMgO changes from -0.6V to 0.6V. According to theoretical research from Ref.[6], the Jc for SOT switching is proportional to Keff. Therefore, the decrease of Keff will lead to significant reduction of Jc. Figure 2(d) shows the interfacial anisotropy coefficient Ki. We can see that the Ki changes linearly with VMgO due to VCMA effect. From the slope of the linear fitting, the VCMA coefficient ξ of 34.3 fJ/Vm is obtained for IrMn/CoFeB/MgO structure. Therefore, VCSOT effect is not the reason for the reduction of Jc in the voltage-gated SOT switching. The decrease of Jc in the voltage-gate SOT switching is mainly caused by the reduction of PMA due to VCMA effect.In summary, we demonstrate field-free switching in the IrMn/CoFeB/MgO structures and show the critical switching current can be modulated by the control voltage. Moreover, VCSOT effect and VCMA effect are explored. We measured spin-orbit effective torques under different voltage. We concluded that through VCSOT effect, positive voltage tends to increase Jc, which is reverse to the experimental results. Next, we measured anomalous Hall resistance at different voltage to explore VCMA effect. From quantitative calculation, the critical switching current at different voltage is consistent with experiments. Therefore, we concluded VCSOT effect hindered the reduction of Jc, while the VCMA effect made a main contribution to the Jc reduction. This work provides comprehensive understanding of voltage-gated spin torque switching. **