We study the current-induced spin-orbit torque (SOT) switching and out-of-plane (OOP) effective field in a CuPt/CoPt heterostructure with interface oxidation. By introducing the oxidized copper at the interface, we found their switching abilities show remarkable improvement which is attributed to an unconventional OOP effective field. The OOP effective field is three times stronger than that without oxide treatment. Our observation identified the important role of the interface oxidation in the SOT process of CuPt/CoPt heterostructure.
The spin current can trigger a torque in ferromagnetic (FM) layer to electrically manipulate the magnetization. It is promise for an Magnetoresistive random-access memory (MRAM) device due to its endurance and speed [1-3]. To efficiently manipulate the magnetism in MRAM, the key point is to increase the spin accumulation in ferromagnetism (FM). In additiona, for traditional SOT devices, the external magnetic field is required to break the symmetry. Thus, the field-free switching has attracted attention.
We believe there could be a different path to enhance the field-free magnetization switching from material perspective. Previously, we studied the symmetry dependent field-free SOT switching in a CuPt/CoPt heterostructure [4]. The intrinsic broken mirror symmetry related SOT process is also reported in WTe2 [5]. The L11 CuPt deposited on SrTiO3 (111) substrate possesses a 3-fold symmetry, as shown in figure 1. The field-free switching emerges when the current is injected along the low symmetry axes ([1-10] and the other two equivalent axes).
In this work, we show the 3-fold symmetry unconventional out-of-plane torque is from the CuPt and it can be manipulated by the oxidation at the surface. Comparing to unoxidized sample, the oxidized sample approve a much larger switching ratio, since the large out-of-plane torque as the figure 2 shown. Regarding the oxidation degree of the copper is controllable by the time of oxidation treatment, thus a tuneable and efficient MRAM based on SOT induced magnetization switching is promised. 
**References** [1] K. Garello et al., 2018 IEEE Symposium on VLSI Circuits., p.81-82 (2018).
[2] F. Oboril, R. Bishnoi, M. Ebrahimi and M. B. Tahoori, IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems., Vol. 34.3, p.367-380 (2015).
[3] G. Prenat et al., IEEE Transactions on Multi-Scale Computing Systems., Vol. 2.1, p.49-60 (2016).
[4] L. Liu et al., Nat. Nanotechnol., Vol. 16.3, p.277-282 (2021).
[5] MacNeill, D., Stiehl, G., Guimaraes, M. et al., Nature Phys., Vol. 13, p.300–305 (2017).
 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/3b1d18361fb08bb804bb6ce897009839.png)

Unconventional spin orbital torque enhanced magnetization switching by interface oxidation in CuPt/CoPt bilayer

The combination of inversion symmetry breaking and spin-orbit coupling (SOC) give rise to spin-orbit torque (SOT), charge pumping, and chiral magnetism. In conventional SOT studies, the inversion symmetry breaking usually comes from the structure or the geometry, e.g. crystal inversion assymmety in the bulk of diluted ferromagnet with zinc-blende structure (Fig. 1a) and space inversion assymmetry in a heavy metal/ferromagnet bilayer (Fig. 1b). It was known that the SOT strength can be controlled by the magnitude of the SOC, therefore, heavy metals with large SOC has been widely used for SOT-based magnetic random-access memory (MRAM).
In our study, we demonstrated that the vertical composition gradient in a single FePt layer acts as a new type of inversion symmetry breaking (Fig. 1c). The SOT efficiencies scale almost linearly with the composition gradient (G), which is defined to be the change of the Co/Pt ratio per 1 nm. We found that by changing G from 0.31 %/nm to 0.71 %, the SOT efficiency increases by 300 %. In experiments, the composition gradient can be easily controlled during sample growth. Therefore, our work provides a powerful tool to tune the SOT strength.
Then we demonstrated the current-induced magnetization switching in a FePt single layer (Fig. 1e). However, this switching requires an in-plane external magnetic field to break the symmetry. For real application, it is pursued to realize magnetic field-free switching. To achieve this goal, we move our attention from L10 FePt (P4/mmm) to fcc CoPt3. Both of them possess good perpendicular magnetic anisotropy (PMA) and hold promise in magnetic storage (media and memory). We found that the CoPt3 has two structural properties, as illustrated in Fig. 2(a). One is the composition gradient along the thickness direction which can break the inversion symmetry and allows for the generation of the in-plane damping-like torque. The other is the formation of the Co platelets in the Pt-rich matrix near the substrate during growth. We can make a symmetry analysis of the Co platelet/Pt structure, and we can easily find that the mirror symmetry is broken relative to the (11-2) plane and preserved relative to the (1-10) plane. Therefore, when the current is applied along the [1-10] direction (low-symmetry axis), the lateral mirror symmetry is broken so that the out-of-plane SOT can be allowed. Then we can achieve the field-free magnetization switching (0 deg in Fig. 2c). In contrast, when the current is applied along the [11-2] direction (high-symmetry axis), the lateral mirror symmetry is preserved and there is no field-free switching (90 deg in Fig. 2c). We also found that the current-induced switching exhibit a three-fold angular dependence on the current angle (θI), which is in perfect agreement with the three-fold rotational symmetry of the crystal structure in Fig. 2b.
To summarize, we observed the field-assisted magnetization switching in FePt single layer by introducing a new type of inversion symmetry breaking: composition gradient. Furthermore, we demonstrated the symmetry-dependent field-free magnetization switching in the CoPt3 single layer. The composition gradient along the film's normal direction gives rise to the in-plane damping-like torque while the low symmetry (3m1) property at the interface of Co platelet/Pt gives rise to the 3m torque in CoPt3. The cooperation of these two effects leads to a three-fold field-free switching in the CoPt3 single layer. Our result of the self-switching in CoPt3 has provided one of the most simplified structures for field-free switching of perpendicular magnetization. The good endurance and high thermal stability make it a good candidate for magnetic memory devices and other spintronic applications.

**References:** 

[1] L. Liu, et.al., Electrical switching of perpendicular magnetization in a single ferromagnetic layer Physical Review B 101 (22), 220402 (2020). 

[2] L.Liu, et.al., Current-induced self-switching of perpendicular magnetization in CoPt single layer Nature Communication 13, 3539 (2022).

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/d17d241b8dd41a32df8b80ebc8770a92.jpg)

Spin orbit torque induced magnetization switching in single ferromagnetic layer

Modern magnetic-memory technology requires all-electric control of perpendicular magnetization with low energy consumption. While spin-orbit torque (SOT) in heavy metal/ferromagnet (HM/FM) heterostructures holds promise for applications in magnetic random access memory, till today, it is limited to the in-plane direction. Such in-plane torque can switch perpendicular magnetization only deterministically with the help of additional symmetry breaking, e.g., through the application of an external magnetic field, an interlayer coupling or an asymmetric design. Instead, an out-of-plane spin-orbit torque could directly switch perpendicular magnetization. In current presentation, we report that we observe an out-of-plane spin-orbit torque in an HM/FM bilayer of L11-ordered CuPt/CoPt and demonstrate field-free switching of the perpendicular magnetization of the CoPt layer.1 The low symmetry point group (3m1) at the CuPt/CoPt interface gives rise to this spin torque, herein after referred as 3m torque, which strongly depends on the relative orientation of current flow and crystal symmetry. We observe a 3-fold angular dependence in both the field-free switching and the current-induced out-of-plane effective field as shown in Figure 1. Because of the intrinsic nature of the 3m torque, the field-free switching in CuPt/CoPt shows good endurance in cycling experiments. Experiments with the wide variety of SOT bilayers with low-symmetry point groups at the interface may uncover further unconventional spin-torques in future.

![](https://assets.underline.io/uploads/markdown_image/1/image/e032f3aa438f0424fd4d0cd491e3db92.jpg)

Figure 1| Symmetry-dependent magnetic field-free magnetization switching. a, Schematic of the CuPt/CoPt Hall bar for electrical transport measurement. The red arrow represents the magnetization (M). The grey arrow represents the current (I) flowing. b, The definition of current flowing direction (θI). The current is applied along the Hall bar, which has an azimuth angle of θI with respect to the [1-10] direction. c, Anomalous Hall effect of the bilayer for θI = 0°. d, Current angle dependence of the SOT induced magnetization switching. The solid line is a cosine fit to the data. e, Current-induced magnetization switching with different θI.

Symmetry Dependent Field-free Switching of Perpendicular Magnetization

The combination of inversion symmetry breaking and spin-orbit coupling gives rise to symmetry-dependent spin physics, such as spin-orbit torque (SOT). The lack of vertical inversion symmetry in multilayer structures gives rise to a conventional SOT (or in-plane SOT), which can be used to switch the perpendicular magnetization with the assistance of an external magnetic field. To realize the magnetic field-free switching of the perpendicular magnetization, one solution is to generate an unconventional SOT (or out-of-plane SOT) by using the lack of lateral inversion/mirror symmetry. In the presentation, I will present our recent progress in engineering the out-of-plane SOT through the crystal symmetry at the interface of CuPt/CoPt bilayer.

Perpendicular magnetization switching through crystal symmetry-engineered spin-orbit torque

The spin-orbit torque (SOT) has been used to efficiently manipulate the magnetization in the ferromagnet (FM)/heavy metal (HM) heterostructures for non-volatile storage applications. Engineering of the interface in FM/HM has attracted great attention due to its crucial role in the spin transfer and generation. However, the underlying mechanism for the interfacial contribution to SOT generation is still a subject of debate. Interfacial transparency has been widely used to illustrate the interfacial contribution to the SOT efficiency, for example, the Pt/Co bilayer has a larger interfacial transparency and thus a larger SOT efficiency than Pt/Py bilayer despite the same spin Hall effect of Pt. However, this explanation has been questioned since the Pt/Co interface also has a large spin memory loss, which can lead to a substantial drop of the SOT efficiency [1]. The orbital hybridization is another phenomenon at the FM/HM interface, which is particularly prominent for Pt/Co. Theoretical studies have predicted a strong effect of hybridization on the SOT generated at the Pt/Co interface [2,3], which has not been proved experimentally.
In this work, we perform the STFMR on the epitaxially grown Pt/Ni81Fe19 (as known as permalloy, Py) and Pt/Co/Py heterostructures with (111) crystalline orientation to investigate both the strength and components of their SOT. We demonstrate that inserting an ultra-thin Co layer between epitaxial Pt and Py substantially enhances both the in-plane (IP) and out-of-plane (OP) damping-like SOTs, both of which can be attributed to the strong interfacial hybridization between Co and Pt.

Role of interfacial orbital hybridization in spin-orbit torque generation in Pt-based heterostructures

Antiferromagnetic (AFM) materials have attracted wide attention in spin-orbit torque (SOT)-based spintronic due to its abundant spin-dependent properties and
unique advantage of immunity against external field perturbations. To act as the charge-to-spin conversion source in energy-saving spintronic devices, it is of great importance for the AFM material to possess a large spin torque efficiency (ξDL). In this work, using the spin torque ferromagnetic resonance (ST-FMR) technique and a Mn2Au/NiFe(Py) bilayer system, we systemically study the ξDL of AFM Mn2Au films with different crystal structures. Compared with polycrystalline Mn2Au with effective ξDL < 0.051, we show a much larger ξDL of ~0.333 in single-crystal Mn2Au, which arises from the large spin Hall conductivity instead of electrical resistivity. Moreover, with a further contribution of interfacial effects, the effective ξDL of single-crystalline Mn2Au/Py system increases to 0.731, which is more than two times larger than the value of ~0.22 reported for the Mn2Au/CoFeB system. By utilizing the large ξDL of Mn2Au in a perpendicularly magnetized MnGa/Mn2Au system, energy-efficient deterministic magnetization switching with a current density at ~10^6 A cm−2 is achieved. Our results reveal a significant potential of Mn2Au as an efficient SOT source and shed light on its application in future AFM material-based SOT integration technology.

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

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.References: [1] A. Manchon et al, Rev. Mod. Phys. 91, 035004 (2019). [2] W. Zhang et al, Phys. Rev. Lett. 113, 196602 (2014). [3] J. Zhou et al, Sci. Adv. 5, eaau6696 (2019). [4] J. Zhou et al, Phys. Rev. B 101, 184403 (2020).

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

EOF: Spin-Orbitronics II

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Presentations

Unconventional spin orbital torque enhanced magnetization switching by interface oxidation in CuPt/CoPt bilayer

Spin orbit torque induced magnetization switching in single ferromagnetic layer

Symmetry Dependent Field-free Switching of Perpendicular Magnetization

Perpendicular magnetization switching through crystal symmetry-engineered spin-orbit torque

Role of interfacial orbital hybridization in spin-orbit torque generation in Pt-based heterostructures

Giant spin torque efficiency in single-crystalline antiferromagnet Mn2Au films

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

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