Large spin-orbit torque efficiency and magnetic symmetry effect in the antiferromagnet IrMn INVITED

Abstract

Current induced spin-orbit torque (SOT) has been extensively investigated as a promising mechanism to switch magnetization electrically, which potentially allows higher energy efficiency in information technologies [1]. Metallic antiferromagnets (AFM) have been demonstrated to be an efficient source of SOT [2]. However, the relationship between the magnetic structure and SOT of AFM has been frequently overlooked, which not only clouds the origin of SOT in AFM but also hampers the engineering efforts for application development. Here we demonstrate that the magnetic structure of AFM IrMn has a strong influence on its SOT generation [3-4]. We fabricate epitaxial IrMn thin films of three phases, which have distinct crystal and magnetic structures, as shown in Fig. 1(a) and 1(b). The magnetic structures of the three phases have been carefully studied using the neutron diffraction technique. Using spin-torque ferromagnetic resonance technique (ST-FMR) [Fig. 1(c)] and IrMn/permalloy (Py) bilayers, we show that the measured SOT efficiencies () of L10-IrMn, L12-IrMn3 and γ-IrMn3 are 0.61±0.01, 1.01±0.03 and 0.80±0.01, respectively, as displayed in Fig. 1(d). These values are substantially larger than that of the polycrystalline IrMn, which is only 0.083±0.002. By altering the direction of electric current in the crystal lattice of L10-IrMn, we observe a 4-fold rotation symmetry in both the SOT efficiency and the in-plane magnetic anisotropy, as shown in Fig. 1(e). Fig. 1(f) shows that 4-fold symmetry disappears when a thin Cu spacer is inserted between L10-IrMn and Py while the measured SOT efficiency decreases to 0.22±0.03. These results suggest a magnetic structure enhanced SOT efficiency in L10-IrMn, which consists of a large isotropic bulk contribution and a comparable anisotropic interfacial contribution [3]. Moreover, we observe an anomalous out-of-plane damping-like torque in both L10-IrMn and L12-IrMn3 [4], as shown by the red curves in both Fig. 1(g) and 1(h). The microscopic origin of this torque appears to be an out-of-plane spin polarization. This is consistent with the intrinsic effects of the a laterally broken magnetic mirror symmetry () that is present commonly in both L10-IrMn and L12-IrMn3. We attribute the absence of this anomalous SOT in γ-IrMn3 to the lack of magnetic asymmetry due to a disordered magnetic structure.