Field-free Magnetization Switching by Utilizing the Spin Hall Effect and Interlayer Exchange Coupling of Iridium

Magnetization switching by spin-orbit torque (SOT) via spin Hall effect represents as a competitive alternative to that by spin-transfer torque (STT) used for magnetoresistive random access memory (MRAM), as it doesn’t require high-density current to go through the tunnel junction. For perpendicular MRAM, however, SOT driven switching of the free layer requires an external in-plane field, which poses limitation for viability in practical applications. Here we demonstrate field-free magnetization switching of a perpendicular magnet by utilizing an Iridium (Ir) layer. The Ir layer not only provides SOTs via spin Hall effect, but also induce interlayer exchange coupling with an in-plane magnetic layer that eliminates the need for the external field. Such dual functions of the Ir layer allows future build-up of magnetoresistive stacks for memory and logic applications. Experimental observations show that the SOT driven field-free magnetization reversal is characterized as domain nucleation and expansion. Micromagnetic modeling is carried out to provide in-depth understanding of the perpendicular magnetization reversal process in the presence of an in-plane exchange coupling field.

because the exchange coupling layer and spin Hall layer are separated. Although the scenario of using a Pt spacer was also demonstrated, the mechanism behind its interlayer exchange coupling still remains unclear.
Here, we demonstrate the realization of field-free magnetization switching by utilizing an Ir interlayer layer. Ir has been shown capable of generating spin Hall effect 26,27 , and equally important for this study, it provides interlayer exchange coupling when sandwiched by two ferromagnetic layers, e.g. Co 28,29 at adequate thicknesses. Combing these two properties, we are able to achieve deterministic magnetization switching without an external in-plane field. We also investigate the switching process in our devices and characterize it as domain nucleation followed with SHE-induced domain wall motion (DWM).

Results
Magnetic properties. The film stack for field-free magnetization switching is substrate/Co(2)/Ru(0.85)/ Co(2)/Ir(1.35)/Co(1.2)/Ta (2), with unit in nanometers. Here, the top thin Co layer is naturally perpendicularly magnetized due to the interfacial perpendicular anisotropy arising from the interface with the Ir layer 28,29 . The thickness of the Ir layer corresponds to the second antiferromagnetic coupling peak in the Ruderman-Kittel-Kasuya-Yosida (RKKY) thickness dependence curve (see supplementary information Fig. S1). The Co/Ru/Co below the Ir layer is a flux-matched in-plane synthetic antiferromagnetic (SAF) structure. The purpose of adopting the SAF instead of a single in-plane magnetized layer is to minimize the effect of the stray field on the perpendicular Co layer. The in-plane hysteresis loop of a fabricated SAF structure ( Fig. 1(b)) shows strong interlayer exchange coupling and the loop shape indicates the single domain characteristics of the in-plane Co layers.
The film shown in Fig. 1(a) is patterned into Hall-cross devices with 4-μm wide current channel and 1-μm wide voltage channel. Figure 1(d) shows the measured Hall voltage arising from anomalous Hall effect (AHE) with varying external perpendicular fields. The squared shape of the AHE loop indicates well defined perpendicular magnetic anisotropy of the top Co layer.

SOT-driven magnetization switching.
To study the SOT-driven magnetization switching, current pulses are injected along the current channel with each pulse length of 100 μs. The Hall voltage was measured after each write pulse with a read current of 100 μA, less than 1/10 of the writing current. Figure 2  corresponding to the two perpendicular magnetization states. At H x = 20 mT, the switching hysteresis loop collapses, indicating no magnetization switching occurs. At zero external field, the switching completely recovers, however, with flipped loop shape. The same switching loops are maintained for H x < 0. Figure 2(b) shows the similar measurement sequence with the reversed magnetization of the bottom SAF structure (from  to ). In contrast to that shown in Fig. 2(a), the collapse of the switching hysteresis loop now occurs at H x = −20 mT instead. It is our interpretation that when charge current flows into the Ir layer, pure spin current is generated due to spin Hall effect and then injected to the top perpendicular Co layer. This pure spin current drives the observed magnetization reversals provided there exists an in-plane magnetic field. At zero applied field, this in-plane field arises from the interlayer coupling between the perpendicular Co and in-plane Co layers above and below the Ir interlayer via the RKKY interaction. When the externally applied field cancels the coupling field in magnitude and direction exactly, magnetization reversal no longer occurs. Our evidence suggests this in-plane coupling field has a magnitude of 20 mT. It should be noted that the 20 mT coupling field shown in the devices is significantly smaller than that measured at film level, which is around 95 mT.
The above argument is further confirmed by the measurements on the devices that have the film stack without in-plane Co layer below Ir (see Supplementary information Fig. S2). AHE hysteresis loops resulting from perpendicular magnetization switching can only be obtained with external in-plane field whereas no current-induced switching is observed at zero field.
Switching process. To understand the nature of current-induced magnetization switching in the presence of the in-plane coupling field in our devices, Kerr microscopy is utilized to visualize magnetization reversal process, as shown in Fig. 3, with current pulses of 100 ns in duration. It's observed that small opposite domains nucleate at the beginning of the reversal. We notice the nucleation sites mostly locate around the device edge, where the perpendicular anisotropy should be lower than other device areas due to the damages caused by the etching process during device fabrication. Continued application of subsequent current pulses results in the expansion of the reversed domains in all directions. With sufficient number of current pulses, the region along the current pulses can be mostly reversed. We should note that there are always some small residual domains located sparsely along the current path that are difficult to eliminate, even with perpendicular magnetic field.
It's widely observed that SHE can induce domain wall motion (DWM) in heavy metal/ferromagnet systems [30][31][32][33][34] . One of the keys to the SHE-driven DWM is the Dzyaloshinskii-Moriya interaction (DMI) at heavy metal/ferromagnet that not only makes Neel-type domain walls more energetically favorable than Bloch-type ones but also introduces chirality to the domain wall structure [35][36][37] . Normally, up-down and down-up domain walls of an opposite domain possess the same chirality (either left-or right-handed), as shown in Fig. 3(b). These domain walls move in the same direction when receiving the SOTs from the neighboring heavy metal layer and thus domains can only shift forwards or backwards. With applying external in-plane field that's larger than the effective DMI field, up-down and down-up domain walls can have different chiral structures. As a result, a domain can either expand or shrink depending on the direction of injected current 38 , which further leads to deterministic magnetization switching. In our case, we measured the effective DMI field at Ir/Co interface to be  S4). Hence, the exchange coupling field (20 mT) in our devices is able to overcome the effective DMI field and give rise to the domain walls with opposite chirality (Fig. 3(c)). Based on our observations, it's the two types of domain walls moving in opposite directions that enables field-free magnetization switching. Therefore, the magnetization switching process in our devices can be interpreted as domain nucleation followed with SHE-driven DWM till the expansion of reversed domains produces full reversal.

Micromagnetic simulation.
To provide further understanding of the experimental observations, the effect of pure spin current in the perpendicular Co layer is modeled by Slonczewski spin transfer torque included Landau-Lifshitz-Gilbert damped gyromagnetic equation of motion. The full stack is modeled with the bottom SAF structure set as ⇄. The modeling is carried out by initially creating a reversed domain, or domains, in a perpendicularly saturated top Co layer. Figure 4 shows sets of time evolution of domain configurations during the expansion of the initial reverse domains under the pure spin current. It is found that the reversed domains expand in all directions. Domain walls that are parallel to the interlayer exchange coupling field (as in the top left) move as fast as those orthogonal to the field (top right). It is interesting to note that as the reverse domains expand, the wall fronts often become curved during their motion (except when the wall front is perpendicular to the exchange field, as the case of top right in the Fig. 4 shows). This is because the wall moving speed is correlated with the orientation of magnetization component at the center of the wall. Figure 5 shows the wall speed as a function of the along-wall projection  of the magnetization at the center of the wall with various interlayer exchange coupling strength. The simulation results suggest that the wall moves faster when the magnetization at the center of the wall becomes more parallel to the tangent of the wall. Furthermore, it can also be seen that not only the speed of DWM but also the slope of the speed angular dependence increase with higher interlayer exchange coupling field.

Discussion
Recently, CB Seung-heon, et al. 39 reported that spin current generated by the interface of non-magnetic metal/ in-plane magnetized ferromagnet can contain some perpendicular polarization. Such perpendicular component of the spin current further gives rise to the SOTs that can achieve field-free magnetization switching. This mechanism can't be applied to explain our observations since the spin diffusion length of Ir is only 0.5 nm 40 . Therefore, even if such spin current is generated at Ir/in-plane Co interface it can't travel through the 1.35 nm-thick Ir layer to reach the top perpendicular Co layer.

Conclusion
To summarize, we achieved robust field-free magnetization switching by utilizing the SHE and interlayer exchange coupling of Ir. The switching process is dominated by SHE-induced domain wall propagation to achieve the full expansion of reserved domains. Combined modeling study shows that in the presence of the in-plane coupling field, the nucleated domains can expand in all directions and the higher the coupling field, the higher the expansion speed of the reversed domains. The domain wall speed also increases when the magnetization at the center of the wall becomes more parallel to the wall during the motion. It's noteworthy that the film stack we used is easy to be built up with magnetoresistive layers on the top. Meanwhile, the device size should be able to scale down to the dimension of tens of nanometers, which shows its potential for practical applications in memory and logic.

Methods
All films are deposited at room temperature by magnetron sputtering with base pressure <2 × 10 −8 Torr. Film-level hysteresis loops are measured by Alternating Gradient Field Magnetometer (AGFM). The films are patterned into Hall-cross devices with 4-µ m wide current channel and 1-μm wide voltage channel utilizing e-beam lithography, optical lithography and ion beam etching. To study the SOT-driven magnetization switching, current pulses are injected along the current channel with each pulse length of 100 μs. The amplitude of the current at milliamp (mA) level is swept from negative to positive values and back to negative again. The Hall voltage was measured after each write pulse with a read current of 100 μA. Prior to writing, the moments of bottom SAF is initially set as ⇄ (or ⇆) by applying 1 T magnetic field in + x (or −x) direction. The current density is calculated with taking into the account both the resistivity and cross-section area of each layer. For studying the domain wall propagation during magnetization switching process, 100 ns current pulses are injected and the dynamics of magnetization reversal was tracked by Kerr microscope. For DW modeling in our devices, the interlayer exchange coupling strength between the perpendicular Co layer and the in-plane Co layer sandwiching the Ir layer is varied throughout the study. The top perpendicular Co layer is assumed to have 2 nm thickness with interfacial perpendicular uniaxial anisotropy of K s = 4 × 10 −3 J/m 2 and saturation magnetization M s = 1.2 T. The two Co layers in the Co/Ru/Co SAF are assumed to be flux-matched without perpendicular anisotropy and to have antiparallel exchange coupling of σ ex,SAF = −1.0 × 10 −3 J/m 2 . The film stack is meshed laterally with each mesh cell of 2 × 2 nm 2 size. The exchange stiffness constant A = 1.6 × 10 −11 J/m is assumed for all three Co layers. A spatially uniform pure spin current with spin polarization along the positive y-direction is assumed corresponding to a charge current density of J c = 10 12 A/m 2 and a spin Hall angle of 10%. The top Co layer of the SAF is always magnetized along the positive x-direction with essentially uniform magnetization. Zero DMI is assumed for simplification.

Data Availability
All data gathered and/or analyzed in this study are included in the main article as well as the Supplementary Information.