Electrical manipulation of antiferromagnetic easy axis in IrMn/NiFe exchange-biased structures

Electrical control of antiferromagnetic moment antiferromagnet-based spintronics, which promises favourable device characteristics of ultrafast operation and high-density integration compared to conventional ferromagnet-based devices. To date, the manipulation of antiferromagnetic moments has been demonstrated in epitaxial antiferromagnets with broken inversion symmetry or antiferromagnets interfaced with a heavy metal, in which spin-orbit torque (SOT) drives the antiferromagnetic domain wall. Here, we report electrical manipulation of the antiferromagnetic easy axis in IrMn/NiFe bilayers without a heavy metal. We show that the direction of the antiferromagnetic easy axis and associated exchange between up to  22 degrees. The maximum rotation angle of the AFM easy axis achieved by SOT is independent of the NiFe thicknesses, indicating a critical role of interfacial uncompensated AFM moments that mediates the spin torque to the entire AFM and exchange-coupled FM moments. Moreover, the memristive behaviour, the gradual manipulation of the AFM easy axis according to the polarity and amplitude of the electric current, can be observed in a 500 nm device. Our results demonstrating the electrical control of the AFM moment in a reversible and non-volatile manner paves the way for the realization of nanoscale AFM-based spintronics for neuromorphic applications.

Notably, AFM switching typically shows multi-level characteristics 17,31 ; the (transverse) resistance of an AFM sample, representing the AFM moment direction, is gradually modulated by the magnitude and polarity of a writing current. This, however, relies on the AFM domain structure because the current-induced SOT controls the overall AFM moments by switching the AFM moment in some domains and/or by driving the AFM domain wall 10,28,32,39 . Therefore, to maintain the memristive behaviour in nanodevices, it is necessary to either engineer an AFM with nanometre-sized domains or find a way to control the AFM moment coherently.
In this article, we report electrical manipulation of the AFM easy axis in exchangebiased IrMn/NiFe bilayer structures. We observe that the planar Hall resistance of the bilayer is gradually modulated by an in-plane current and retains its value even after turning off the current. This demonstrates that the SOT caused by the spin Hall effect in IrMn effectively controls the AFM easy axis and the associated exchange bias between up to 22 degrees. To understand the switching mechanism, we investigate the dependence of the rotation angle of the AFM easy axis (AFM) on the IrMn and NiFe thicknesses; AFM diminishes with an increase in the IrMn thickness, indicating that the SOT-induced rotation of the AFM easy axis is hindered by the AFM anisotropy, which increases with its thickness. Interestingly, AFM remains constant regardless of the NiFe thickness. This implies that the SOT is not applied directly to the ferromagnetic NiFe layer. Instead, it is applied to uncompensated AFM moments at the IrMn/NiFe interface, subsequently triggering the coherent rotation of the magnetization of the exchange-coupled IrMn/NiFe bilayers. Furthermore, we show that the reversible memristive features of the SOT-induced AFM switching are maintained in a 500-nm-sized device, offering a route for developing nanoscale AFM spintronics devices for neuromorphic computing.

Results
Electrical manipulation of the AFM easy axis in IrMn/NiFe. To demonstrate electrical control of the AFM easy axis, we employ IrMn/NiFe exchange-biased bilayers. In such structures, a charge current induces a spin current through the spin Hall effect in the IrMn layer, exerting torques on the magnetization of the exchange-coupled IrMn/NiFe bilayer.
Note that IrMn, a widely used AFM material for exchange bias, exhibits a sizeable (inverse) spin Hall effect [40][41][42][43][44] . As schematically illustrated in Fig. 1a, when applying a charge current in the x-direction, a spin current flowing in the z-direction has spin polarization  in the y-direction, thus causing the AFM/FM magnetic moment to rotate in a direction parallel to . We fabricate an IrMn (5 nm)/NiFe (4 nm) layer by deposition and subsequent annealing under a magnetic field along the x-direction to induce an unidirectional exchange-bias field (BEB). Figure 1b shows the magnetization curves of the sample while sweeping the magnetic fields in the x-(blue squares) and y-(red circles) directions. The hysteresis loop shifts toward the negative field direction only for the measurement along the x-direction, demonstrating exchange bias developed in the IrMn (5 nm)/NiFe (4 nm) bilayer along the positive x-direction. The samples are then patterned into a 4-μm-wide Hall bar structure for electrical measurements. First, we measure the planar Hall resistance (RH) of the IrMn/NiFe bilayer while rotating the sample on the x-y plane under a magnetic field of 100 mT, which is sufficient to saturate the magnetization. Figure   1c shows the RH as a function of the azimuthal angle of the magnetic field B, which allows us to extract the magnetization direction of the IrMn/NiFe bilayer m from the measured RH value.
We next present the main result of this work; the manipulation of the AFM easy axis through the in-plane current-induced SOT in the IrMn/NiFe structure. Figure 1d shows the changes in RH of the IrMn/NiFe bilayer as a function of the in-plane current pulse IP with a width of 30 s. For each IP, RH is measured with a reading current of 100 A after applying IP. Initially, the magnetization direction is in the x-direction (φm = 0˚), and the corresponding RH value is set to zero by removing an offset. As IP increases positively (solid red symbols), RH remains unchanged until IP = 15 mA and gradually increases when IP exceeds 15 mA. Finally, for IP = ~ 30 mA, the RH value saturates to -0.14 , which corresponds to φm = -15˚. We observe similar behaviour of the RH and the corresponding φm but opposite signs when a negative IP is applied (solid blue symbols). This demonstrates that the magnetization of the IrMn/NiFe structure is rotated clockwise (counterclockwise) by a positive (negative) in-plane current. Moreover, when we sweep IP between 33 mA (open red/blue symbols), the RH value varies between 0.14 , demonstrating the electrical modulation of φm of 15˚ in a reversible manner.
We further investigate whether the change in the RH value manifests the rotation of the AFM easy axis of IrMn, given that the RH value of the IrMn/NiFe bilayer is mostly dominated by NiFe. To this end, we measure the dependence of RH on the magnetic field along the x-direction Bx. Prior to the measurement, we apply an IP of -30 mA to set RH = 0.14  (or φm = +15˚). Figure 1e shows that RH gradually decreases with an increase in Bx (red squares) and approaches zero when Bx = 200 mT. This indicates that the magnetization rotates towards the magnetic field direction as expected. Interestingly, when reducing Bx, RH is restored to its initial value (red line in Fig. 1e); i.e., the magnetization rotates back to +15˚, as depicted in the inset of Fig. 1e. The same behaviour is observed when RH is initialized to -0.14  (blue squares and line in Fig. 1e). The spontaneous recovery of RH (or φm) indicates that there is a bias field acting on NiFe in the direction of φ = 15˚, which we attribute to the exchange bias originating from the AFM IrMn. This result provides evidence that the AFM easy axis φAFM (//φm) is electrically manipulated in an IrMn/NiFe bilayer structure via the current-induced SOT. Note that the changes in RH with IP are not observed in a Ta/NiFe sample, in which spin current is effectively generated by the spin Hall effect in Ta 45,46 , though. In the structure without an AFM layer, the magnetization rotated by the SOT returns to its easy axis (that is in the x-direction) once the current is turned off, resulting in no variation of RH and φm (Supplementary Figure S1). This again confirms that AFM IrMn plays a critical role in the electrical modulation of φm in the IrMn/NiFe structure.

Mechanisms of SOT-induced AFM switching.
To understand the underlying mechanism of the electrical manipulation of the AFM easy axis, we investigate various IrMn/NiFe structures. Assuming that the spin current is only generated in the IrMn layer, there are two possible scenarios; first, the spin current exerts torques on entire AFM moments, similar to the Néel SOT in CuMnAs 9,16 , although this phenomenon is expected to be absent in polycrystalline IrMn layers. Second, the spin current induces spin accumulation on the IrMn/NiFe interface, giving torques to NiFe and controlling its magnetization. This is followed by the rotation of the exchange-coupled IrMn moment. In the latter case, where opposite spins are accumulated on the top and bottom interfaces, the rotation direction will reverse when changing the stacking order, whereas it is independent of the stacking order in the former case. To verify this, we measure the RH versus IP curves using a NiFe (4 nm)/IrMn (5 nm) sample, an inverted structure of the IrMn/NiFe, which is shown in Fig. 1d. Figure 2a shows that the AFM extracted from the RH value rotates reversibly within ±15˚ by the in-plane current, but its polarity is reversed. This indicates that the spin current in the IrMn and associated spin accumulation at the IrMn/NiFe interface are primarily responsible for the electrical manipulation of the magnetization of IrMn/NiFe bilayers. We note that the sample temperature increases due to the current injection, which may reduce the AFM anisotropy and thereby assists the rotation of the AFM moment. Furthermore, to rule out the possible effects of Oersted fields generated by a current flowing into the IrMn layer, we examine an IrMn (5 nm)/NiFe (4 nm)/Ta (1.5 nm) structure, where the Ta layer diminishes the Oersted field effect, but it provides additional spin currents injected into NiFe layer since Ta has a negative spin Hall angle opposite to IrMn 40,43 . Figure 2b shows the results; by introducing the Ta layer, the current-induced variation of AFM is enhanced to ±22˚ while switching polarity remains the same. This indicates that the Oersted field contribution is not significant in this measurement, accentuating that the spin current in IrMn and the associated SOT is the main cause of the electrical manipulation of the AFM moment.
We subsequently study the thickness dependence of the SOT-induced manipulation of the AFM easy axis. Figure 3a shows the hysteresis loops of the IrMn (tIrMn)/NiFe (4 nm) bilayers, where the IrMn thickness (tIrMn) ranges from 5 to 25 nm, demonstrating that the exchange bias field (BEB) increases with an increase in tIrMn. This is attributed to the enhancement of the AFM anisotropy for a thicker tIrMn 47,48 . Figure 3b plots the AFM versus current density JP curves for samples with different tIrMn; the maximum value of AFM achieved by SOT is gradually reduced as tIrMn is increased (See Supplementary Figure S2 for the full set of data). Figure 3c  Memristive behaviour of AFM switching. We finally discuss memristive characteristics based on SOT-induced AFM switching in a reversible and non-volatile manner. Figure 4a shows minor φAFM -IP curves of the IrMn (5 nm)/NiFe (4 nm)/Ta (1.5 nm) structure with a 4-μm-wide Hall bar. As shown in the measurement sequence illustrated in Fig. 4b, we first apply an initializing current pulse IP,ini, of -32 mA to set φAFM = +22˚ and then measure RH while sweeping IP between -32 mA and the positive maximum IP [IP(+max)]. The measurement is repeated as we increase IP(+max) from +22 mA to +31 mA. This result demonstrates that multiple φAFM values between 22˚ can be obtained according to the magnitude of IP(+max). Similar results of minor loops are observed when sweeping IP between +32 mA and the negative maximum IP (Supplementary Figure S3). We test whether the memristive feature is maintained in nanoscale devices. Figure 4c shows the minor φAFM -IP curves of the IrMn (5 nm)/NiFe (5 nm) structure with a 500-nm-wide Hall bar, as measured with experimental procedures similar to that shown in Fig. 4a,b. Multilevel φAFM values are successfully achieved in the 500 nm device, comparable to those in the 4 μm sample. This implies that the gradual change of φAFM in IrMn/NiFe bilayers is due to the collective rotation of the AFM easy axis and the exchange-coupled FM moment, which is distinct from the previous results based on AFM domain wall motions 28,29,39,50 .
The scalable memristive characteristics can facilitate electrically controlled multi-level spintronic devices for neuromorphic computing.

Discussion
We have demonstrated reversible electrical manipulation of the AFM easy axis in IrMn/NiFe exchange-biased structures. We observe that the SOT caused by the spin Hall effect in IrMn effectively controls the AFM easy axis and the associated exchange bias between up to 22 degrees. The maximum rotation angle of the AFM easy axis achieved by SOT is independent of the NiFe thicknesses, indicating a critical role of interfacial uncompensated AFM moments that mediates the spin torque to the entire AFM and exchange-coupled FM moments. Moreover, the memristive behaviour, the gradual manipulation of the AFM easy axis according to the polarity and amplitude of the electric current, can be observed in a 500 nm device. Our results demonstrating the electrical control of the AFM moment in a reversible and non-volatile manner paves the way for the realization of nanoscale AFM-based spintronics for neuromorphic applications. The constant offset of RH, which may be caused by the misalignment of the Hall cross, is removed such that RH = 0 corresponds to the magnetization aligned to the x-direction (φm = 0˚). All switching measurements were performed at room temperature without an external magnetic field.

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.