Exchange bias switching in an antiferromagnet/ferromagnet bilayer driven by spin–orbit torque

Abstract

Electrical manipulation of the magnetization and exchange bias in antiferromagnet (AFM)/ferromagnet (FM) heterostructures[1,2] is expected to be of use in several high performance spintronic devices. An efficient method for electrical switching of the FM magnetization is to use the spin–orbit torque (SOT) generated from a heavy metal layer in AFM/FM/heavy metal structures[3]. The in-plane exchange field generated at the AFM/FM interface enables field-free switching of the perpendicular magnetization[4,5]. However, manipulation of the exchange bias is usually achieved by field cooling, which requires an external magnetic field and high temperature, hindering its application in practical devices. It has been demonstrated that the exchange bias field at the AFM/FM interface can be switched by the SOT generated in the Pt layer in a Pt/Co/IrMn structure, passing through the thin Co layer[2]. Strong SOTs can also be generated in certain AFM thin films (such as IrMn and PtMn) due to their giant spin Hall angle[6,7], allowing simpler spintronic devices to be created.Here, we report the current-induced switching of exchange bias field in a perpendicularly magnetized IrMn/CoFeB bilayer structure using a spin-orbit torque generated in the antiferromagnet IrMn layer[8]. By manipulating the current direction and amplitude, independent and repeatable switching of the magnetization and exchange bias field below the blocking temperature can be achieved. Figure 1a illustrates four states for the CoFeB magnetization and the interfacial IrMn spins. By applying a negative pulse current in the presence of an in-plane magnetic field ofμ0Hx = 100 mT, switching of the magnetization from up to down can be realized, as shown in Fig. 1c. Interestingly, the exchange bias field before the magnetization switching is -1.3 mT (Fig. 1b), but it changes to 1.6 mT after the magnetization switching (Fig. 1d), indicating reversal of the exchange bias field from negative to positive driven by the SOT current. Next, we find that when a relatively small current (e.g. 7 mA) is applied to a sample with DP state, only the magnetization switches, while the exchange bias field remains unchanged, leading to a transition from DP to UAP state, as shown in Fig. 1e. Then, when a pulse current with amplitude of 8 mA is applied to the samples with UAP state, the exchange bias field is reversed alone, but the magnetization remains unswitched, resulting in a transition from UAP to UP state (see Fig. 1f-h).X-ray magnetic circular dichroism (XMCD), polarized neutron reflectometry (PNR) measurements and micromagnetic simulations show that a small net magnetization within the IrMn interface plays a crucial role in these phenomena. Figure 2 illustrates the micromagnetic simulation results of exchange bias and its manipulation by SOT. Figure 2a shows the schematic of the AFM/FM exchange bias system. Initially, a positive exchange bias field is obtained, as shown in Fig. 2b. When we apply a SOT current of JSOT = 90 MA●cm-2, the FM magnetization can be switched while the exchange bias field is still positive, as indicated in Fig. 2c. Moreover, Fig. 2c also shows that a large SOT current ( JSOT= 270 MA●cm-2) can be used to reverse the exchange bias field. Figure 2d and 2e are snapshots of the z component of FM magnetization at selected times upon the application of different SOT currents. Through sweeping the SOT current density, it is clearly shown in Fig. 2f that the critical switching current density for the exchange bias is larger than that for the magnetization. Furthermore, to confirm the different critical current densities for magnetization and exchange bias field, temperature- and thickness-dependent switching are explored.In summary, we have reported current-induced exchange bias switching via the SOT originating from the AFM layer in an IrMn/CoFeB bilayer. The independent switching of magnetization and exchange bias field in a repeatable manner is demonstrated by manipulating the current direction and amplitude in the presence of an in-plane magnetic field. Experimental results from PNR and XMCD indicate a small net magnetization exists at the IrMn interface, comprised primarily of rotatable uncompensated Mn spins and a much smaller fraction (<10%) of pinned spins. Micromagnetic simulations and temperature-dependent measurements show that the critical current density to switch the interfacial pinned spins, and hence the exchange bias field, is larger than that of magnetization reversal, and this difference allows the FM layer and exchange bias to be individually manipulated by SOT currents, rather than via perpendicular magnetic fields. **