Spin-orbit-torque- (SOT-) driven switching of ferromagnets (FM) with perpendicular magnetic anisotropy (PMA) has attracted extensive attention as it allows for lower-power and faster magnetization switching compared with the conventional spin-transfer torques.[1] However, the requirement of a static magnetic field collinear with the current to achieve deterministic switching poses a challenge to the SOT device application. By introducing antiferromagnets (AFM), recent studies have shown that the in-plane exchange-bias field generated at the AFM/FM interface enables field-free switching of perpendicular magnetization, which makes AFM/FM structure a promising candidate for future spintronic devices.[2,3] On the other hand, CoFeB/MgO-based magnetic tunnel junctions are attractive in achieving the state-of-the-art tunnel magnetoresistance (TMR) ratio, which is important for the development of advanced nonvolatile spintronics memories and logics.[4] Therefore, field-free magnetization switching in AFM/CoFeB/MgO structures has been drawing intense attention.[5] In these structures, however, the PMA was not strong enough to guarantee the long-term data storage for magnetoresistive random-access memory and was reported to be deteriorated upon annealing at temperatures above ~200 °C due to the diffusion of Mn.[6] Besides, the spin-orbit torque of these AFM/CoFeB/MgO systems was not strong enough to implement low-power consumption for magnetization control. All the mentioned bottlenecks hinder the development of these structures in SOT devices. In this work[7], we investigated the modulation of thermal stability and SOT in IrMn/CoFeB/MgO structures through atom thick W insertion. The films consisting of Ta(2 nm)/IrMn(5 nm)/W(tW)/Co40Fe40B20(1 nm)/MgO(2.5 nm)/Ta(3 nm) were deposited on thermally oxidized Si substrate by magnetron sputtering, followed by annealing at 300°C for 30 minutes under an in-plane magnetic field of 1.5T. Subsequently, the films were patterned into Hall bar devices with the longitudinal channel and transverse channel width of 5 µm and 3 µm, respectively. Figure 1(b) presents magneto optical Kerr effect (MOKE) measurements, which shows that samples with W insertion exhibit sharp PMA signals while the magnetic signal of reference sample without W insertion vanished. This W layer serves as a thermal barrier to prevent the diffusion of Mn during annealing, which is confirmed by the results of spherical aberration corrected transmission electron microscope (ACTEM) and atomic-resolution electron energy-loss spectroscopy (EELS). Figure 1(d) shows field-dependent Hall resistance (RHall) measurements with in-plane applied magnetic fields μ0Hx. The magnetization could not be fully pulled into the in-plane direction with magnetic field even up to 1000 mT, indicating the strong PMA of the samples with W insertion which is much larger than that in our previous report without W insertion, the PMA field of which is only ~300 mT.[6] Figure 2(b) presents W layer thickness dependence of SOT effective fields. Both damping-like effective field △μ0Hx and field-like effective field △μ0Hy are tuned by W layer thickness. Focusing on the variation trend of effective fields with W layer thickness, we find that the changes in SOT fields and PMA fields (Fig. 2a) track each other fairly closely. In order to investigate visually this relation, we plot SOT fields as a function of PMA fields in Fig. 2c. SOT fields scale roughly linearly with PMA fields, which is in consistence with theoretical calculations and experimental demonstration in other systems.[8] That is, the existence of interfacial spin-orbit coupling (ISOC) at heavy metal/FM interfaces quantified by PMA fields would reduce, in a linear manner, the effective SOT exerted on the FM layer. Finally, field-free magnetization switching was implemented in the sample with W insertion, and the obtained exchange bias field of 1.5 mT is comparable to that shown in our previous work without W insertion [6]. In summary, we have demonstrated that the thermal stability and PMA fields in IrMn/CoFeB/MgO heterostructures can be effectively enhanced by incorporating a thin W layer at the IrMn/CoFeB interface via suppressing the diffusion of Mn during annealing. Moreover, SOT fields scaling linearly with ISOC in IrMn/W/CoFeB quantified by PMA fields indicates that efficient SOT could be achieved by minimizing the ISOC via the tuning of insertion W layer thickness. Finally, field-free switching of perpendicular magnetization is achieved when a thin W layer is inserted. This work demonstrates an efficient scheme for thermally robust technological application based on AFM spintronics.[7]

**References:**

[1] S. Peng, D. Zhu, and J. Zhou, Adv. Electron. Mater., Vol. 5, p. 1900134 (2019). [2] S. Fukami, C. Zhang, and S. Duttagupta, Nat. Mater., Vol. 15, p. 535 (2016). [3] S. Peng, D. Zhu, and W. Li, Nat. Electron., Vol. 3, p. 757 (2020). [4] L. M. Loong, W. Lee, and X. Qiu, Adv. Mater., Vol. 28, p. 4983 (2016). [5] S. Peng, J. Lu, and W. Li, in International Electron Devices Meeting (IEDM 2019), p. 661 (2019). [6] J. Jeong, Y. Ferrante, and S. V. Faleev, Nat. Commun., Vol. 7, p. 10276 (2016). [7] D. Xiong, S. Peng, and J. Lu, Appl. Phys. Lett., Vol. 117, p. 212401 (2020). [8] L. Zhu, D. C. Ralph, and R. A. Buhrman, Phys. Rev. Lett., Vol. 122, p. 077201 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/63f395f651fd514c6f6ed9a6ea32c25b.jpg)
FIG. 1. (a) Schematic of deposited IrMn/W/CoFeB stacks. (b) MOKE measurements of film samples with and without W insertion under perpendicular magnetic fields. (c) Micrograph of the fabricated Hall bar device and illustration of the setup for Hall bar measurements. (d) Hall resistance (RHall) of samples annealed at 300 °C with different W layer thickness under in-plane magnetic fields.

![](https://assets.underline.io/uploads/markdown_image/1/image/7b1c7f9360e3bf05a096e68429572ad2.jpg)
FIG. 2. (a) W layer thickness dependence of effective PMA fields. (b) W layer thickness dependence of damping-like fields (purple square) and field-like fields (blue circle). (c) Scaling of damping-like fields (purple square) and field-like fields (blue circle) with PMA fields. (d) SOT-driven magnetization switching under various in-plane magnetic fields for the sample with 0.5 nm W insertion layer.

 Modulation of thermal stability and spin-orbit torque in IrMn/CoFeB/MgO structures through atom thick W insertion

As core device of spin-transfer torque magnetic random-access memory (STT-MRAM) and some other spintronics devices [1], the magnetic tunnel junction (MTJ) has been widely studied to improve performances such as endurance, scalability, read reliability and power consumption. The interfacial perpendicular magnetic anisotropy (PMA) in the CoFeB/MgO/CoFeB structure with a tunnel magnetoresistance (TMR) ratio of over 120% [2] proves that PMA can be used to build high-performance out-of-plane MTJ for STT-MRAM cells. Latter research found that the heavy metal (HM)/ferromagnet interface made great contribution to PMA [3]. First-principles investigations agree that both the MgO/CoFe interface and the CoFe/HM interface attribute to magnetic anisotropy energy (MAE) in the MgO/CoFe/HM structures [4, 5]. Moreover, the MgO/CoFeB/Ta/CoFeB/MgO double-interface structure was demonstrated to improve thermal stability by a factor of 1.9 while keeping comparable critical switching current with the single-interface structure Ta/CoFeB/MgO [6]. The MgO/CoFeB/W/CoFeB/MgO structure was demonstrated to possess strong PMA and large TMR ratio (249%) after annealing at a high temperature [7]. To investigate the MAE in the W-based structure, it is necessary to research the interaction within CoFeB/W/CoFeB layers on MAE. CoFeB/W/CoFeB sandwich may form ferromagnetic or antiferromagnetic coupling (FMC or AFMC), which is expressed as interlayer exchange coupling (IEC). However, it remains unclear about the interactions between MAE and IEC in the perpendicularly magnetized CoFeB/W/CoFeB structure, making the way to enhance PMA in double-interface structure not straight forward. In this work, we first investigated the MAE and IEC of the MgO/CoFe/W/CoFe/MgO structures as a function of W thickness with first-principles calculations as shown in Fig. 1 [8]. Clear oscillation of IEC can be observed in Fig. 1(a). The long and short period of IEC oscillations are 4.8 monolayers (MLs) and 3.2 MLs, respectively, according to simulation. Figure 1(b) shows the MAEs of MgO/CoFe/W/CoFe/MgO structures for FMC and for the arrangement corresponding to IEC are calculated for all thicknesses of W layers. Moreover, for the structure with W thickness of 7 MLs, a strong FMC as well as a strong PMA can be obtained simultaneously, which is promising for high-density STT-MRAM applications with strong thermal stability. Also, we observe a clear oscillation of MAE in ferromagnetically coupled MgO/CoFe/W/CoFe/MgO structure related to the thickness of W layer. To further study the effect of interface interactions on MAE, we calculate the k-space-resolved MAE according to force theorem [8]. Figure 1(c-f) shows the k-resolved MAEs of the ferromagnetic coupled CoFe/W/CoFe structures with the thickness of W layer varies from 1 ML to 4 MLs. The spin-orbit coupling (SOC) breaks the symmetry of 2D-Brillouin-zone slightly making MAE at the points around [kx,ky]= [0.457, 0.257] possessing the largest PMA in Fig. 2(c-f). These points that make huge contributions to MAE ferromagnetically coupled are called critical points. The electron states near the Fermi surface have a large influence on MAE. Therefore, we consider spin-resolved bands at critical points. The bands near Fermi energy for spin-up and spin-down states are different, which can be explained by exchange splitting. To explore the properties of the specific electron states, the band-decomposed charge densities of these specific electron states near Fermi energy are shown in Fig. 2(a-d). We can see that only spin-up electron states occupy W layers and that the alternating oscillation of the charge densities at center of W layers: antinode at 1 ML and 3 MLs W layer, while node at 2 MLs and 4 MLs W layers. It is reported that the electron wavefunction confined in thin film is modulated by an envelope function in reference [8]. For a given energy, as the thickness of the W layer in CoFe/W/CoFe structure varies, the center of film layers within the quantum well shows the alternation of node and antinode, shown in Fig. 2(a-d), indicating the presence of QWSs in W layers. The oscillation of IEC in CoFe/W/CoFe originates from QWSs [8]. Thus, these results clearly show that the QWSs correlate the oscillation of IEC and MAE in W-based double-interface structures. In summary, we investigated the MAE and IEC of the MgO/CoFe/W/CoFe/MgO structures as a function of W thickness with first-principles calculations. The oscillations of both IEC and MAE with the change of W thickness were observed in this structure. QWSs in the W layer are demonstrated to be critical for the oscillations of MAE, leading to significant enhancement of PMA by properly tuning W thickness.

**References:**

[1] S. Peng et al., Nature Electronics, vol. 3, no. 12, pp. 757-764, 2020/12/01 2020, doi: 10.1038/s41928-020-00504-6. [2] S. Ikeda et al., Nature materials, vol. 9, no. 9, p. 721, 2010. [3] S. Peng et al., Advanced Electronic Materials, vol. 5, no. 8, p. 1900134, 2019. [4] S. Peng et al., Appl. Phys. Lett., vol. 110, no. 7, p. 072403, 2017. [5] C.-H. Chang, K.-P. Dou, G.-Y. Guo, and C.-C. Kaun, NPG Asia Materials, vol. 9, no. 8, p. e424, 2017. [6] H. Sato, M. Yamanouchi, S. Ikeda, and H. Ohno, IEEE Trans. Magn., vol. 49, no. 7, pp. 4437-4440, 2013. [7] M. Wang et al., Nature communications, vol. 9, no. 1, p. 671, 2018. [8] J. Chen et al., J. Phys. D: Appl. Phys., 2020.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/128137b585eeffa691bd5dc9bde797c5.jpg)
Fig. 1. (a) Interlayer exchange coupling (IEC) of the CoFe/W/CoFe structures as a function of the thickness of W monolayers (MLs). (b) Calculated magnetic anisotropy energy (MAE) of the MgO/CoFe/W/CoFe/MgO structures as a function of the thickness of W MLs for the ferromagnetic coupling (FMC) and for the arrangement corresponding to calculated IEC. The k-resolved MAE of (c)CoFe/W 1ML/CoFe, (d)CoFe/W 2MLs/CoFe, (e)CoFe/W 3MLs/CoFe, (f)CoFe-/W 4MLs/CoFe structures in 2D-Brillouin-zone. Critical k points are labeled by black rectangles.

![](https://assets.underline.io/uploads/markdown_image/1/image/9ab5158b814c60523c1152f11e0ead35.jpg)
Fig. 2. Charge densities of bands near Fermi energy of (a)CoFe/W 1ML/CoFe, (b)CoFe/W 2MLs/CoFe, (c)CoFe/W 3MLs/CoFe, (d)CoFe/W 4MLs/CoFe. The charge density of spin-up electrons of these bands at the center of structures switch within node and antinode as a function of the number of W layers.

 Correlation of interfacial perpendicular magnetic anisotropy and interlayer exchange coupling in CoFe/W/CoFe structures

Magnetic random-access memory (MRAM) has recently emerged as a promising way for next generation memory due to its nonvolatility, low power consumption and high density. How to achieve energy-efficient magnetization switching is one of the main challenges for MRAM. Recent studies propose spin orbit torque (SOT) as a promising method for data writing [1]. However, an external magnetic field along current direction is usually needed to realize deterministic switching. It can be overcome by using antiferromagnet/ferromagnet structures in which exchange bias can replace external field [2]. Voltage controll of magnetic anisotropy (VCMA) is another candidate method for lowering down the energy height by inducing electron accumulation at the ferromagnet/oxide interface[3]. Combination of VCMA and SOT effect may lead to decrease of power consumption by a gate voltage lowering the energy barrier height. For example, Yoda et.al propose a new spintronics-based memory employing the VCMA effect as a bit selecting principle and the SOT effect as a writing principle. [4]. Back et al reported that a gate voltage can be used to manipulate both perpendicular magnetic anisotropy and spin orbit torque[5]. Hence, it is essential to make it clear whether the voltage-controlled spin-orbit effective torques (VCSOT) or voltage controlled magnetic anisotropy causes the decrease of critical current, or both. In this work, we investigate VCSOT effect and VCMA effect in the IrMn/CoFeB/MgO structures. The film structures were thermally oxidized Si substrate/Ta(2)/IrMn(5)/CoFeB(0.94)/MgO(2.5)Al2O3(5), (with film thickness in nm in the parentheses). The films were patterned into Hall bar by ion beam etching (IBE) and lithography as shown in Fig. 1(a). Fig. 1(b) shows the micrograph of the Hall bar and the measurement setup. Furthermore, to explore voltage-gated SOT switching, a gate voltage is applied at the top electrode and the switching loops are obtained with no external field applied as shown in Fig. 1(d). Figure 1(f) presents critical current densities with 5mT field under different gate voltages. The critical current density Jc shows a decreasing tendency with increasing VMgO. These results indicate that a gate voltage can be used to effectively modulate SOT-driven perpendicular magnetization switching and this is a feasible writing method for low energy consumption. Next, spin-orbit effective fields at the different voltage were measured by using a second harmonic method as shown in Fig. 2 (a). The damping-like torque efficiency decreases with positive voltage increasing, whereas the field-like torque efficiency shows weaker voltage dependence. These results demonstrate that VMgO can modulate spin torques in IrMn/CoFeB/MgO system. The damping-like torque efficiency under VMgO of -0.4 V is 1.5 times larger than that under VMgO of 0.4 V. A larger JC is required for the magnetization switching under positive voltage due to the reduction of θSHE. Through VCSOT effect, positive voltage tends to increase Jc, which is reverse to the experimental results in Fig. 1(f). In-plane hysteresis loops at different gate voltages are obtained as shown in Fig. 2(b). The dependence of anisotropy energy Keff(VMgO) on gate voltage is shown in Fig.2 (c). By linear fitting, Keff decreases by 55% when VMgO changes from -0.6V to 0.6V. According to theoretical research from Ref.[6], the Jc for SOT switching is proportional to Keff. Therefore, the decrease of Keff will lead to significant reduction of Jc. Figure 2(d) shows the interfacial anisotropy coefficient Ki. We can see that the Ki changes linearly with VMgO due to VCMA effect. From the slope of the linear fitting, the VCMA coefficient ξ of 34.3 fJ/Vm is obtained for IrMn/CoFeB/MgO structure. Therefore, VCSOT effect is not the reason for the reduction of Jc in the voltage-gated SOT switching. The decrease of Jc in the voltage-gate SOT switching is mainly caused by the reduction of PMA due to VCMA effect. In summary, we demonstrate field-free switching in the IrMn/CoFeB/MgO structures and show the critical switching current can be modulated by the control voltage. Moreover, VCSOT effect and VCMA effect are explored. We measured spin-orbit effective torques under different voltage. We concluded that through VCSOT effect, positive voltage tends to increase Jc, which is reverse to the experimental results. Next, we measured anomalous Hall resistance at different voltage to explore VCMA effect. From quantitative calculation, the critical switching current at different voltage is consistent with experiments. Therefore, we concluded VCSOT effect hindered the reduction of Jc, while the VCMA effect made a main contribution to the Jc reduction. This work provides comprehensive understanding of voltage-gated spin torque switching.

**References:**

[1] M. Miron, K.Garello and G. Gaudin., Nature, vol. 476, no. 7359, pp. 189–193. (2011). [2] S.Z. Peng, D. Q. Zhu and W S Zhao., Adv. Electron. Mater. Vol. 5 pp. 1100134. (2019). [3] W.G. Wang, M. Li and C.L.Chien., Nat.Mater, vol. 11, no. 1, pp. 64–68. (2012). [4] H. Yoda, N. Shimomura and A. Kurobe., IEEE Electron Devices Meet. (IEDM). pp. 27.6.1-27.6.1. (2016). [5] L. Chen, M.Gmitra and C.H.Back., Nat. Electron., vol. 1, no. 6, pp. 350–355, (2018). [6] K.S. Lee, S.W. Lee and K.J. Lee., Appl. Phys. Lett. vol.102, pp. 112410. (2013).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/1ed57da223949489824ad933c4957076.jpg)
Fig.1 (a) Schematic of IrMn/CoFeB/MgO structures. (b) Micrograph of the Hall bar and the measurement setup. (c) SOT-driven magnetization switching with various in-plane external field. Voltage-gated SOT switching under different gate voltages with (d)no external field and (e)5 mT in-plane external field. (f) Critical current density Jc dependence on voltage applied to MgO (VMgO) with 5 mT field applied.

![](https://assets.underline.io/uploads/markdown_image/1/image/177e5ff0550d0f2846103ad2879572c8.jpg)
Fig.2 (a) The dependence of spin-orbit effective torques on voltage in MgO layer (VMgO). The error bars are standard deviations. (b) Anomalous Hall resistance (RHall) under different gate voltages (VMgO) (c) Keff and (d) Ki dependence on VMgO.

 Experimental demonstration of voltage-gated spin-orbit torque switching in antiferromagnet/ferromagnet structure

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 (&lt;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.

**References:**

[1] D.R. Xiong, S.Z. Peng and W.S. Zhao, Appl. Phys. Lett. Vol. 117, no. 21, pp. 212401 (2020). [2] P.H. Lin, B.Y. Yang and C.H. Lai, Nat. Mater. Vol. 18, pp. 335–341 (2019). [3] S.Z. Peng, D.Q. Zhu and W.S. Zhao, Adv. Electron. Mater. Vol. 5, pp.1900134 (2019). [4] Y.W. Oh, S.C. Baek and B.G. Park, Nat. Nanotechnol, Vol. 11, pp.878–884 (2016). [5] S.Z. Peng, J.Q. Lu and W.S. Zhao, IEEE Int. Electron Devices Meet. (IEDM), p. 28.6.1-28.6.4 (2019). [6] D. Wu, G.Q. Yu and K.L. Wang, Appl. Phys. Lett., Vol. 109, pp.222401 (2016). [7] Y.X. Ou, S.J. Shi and R.A. Buhrman, Phys. Rev. B, Vol. 93, pp. 220405(R) (2016). [8] S.Z. Peng, D.Q. Zhu and W.S. Zhao, Nat. Electron. Vol. 3, pp. 757–764 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/80a05ae9b4e976a9e973c73b4ec889d9.jpg)
Fig.1 Exchange bias and magnetization switching driven by current-induced SOT. a, Schematic configurations of four states for the CoFeB magnetization and interfacial IrMn spins and transitions among them. b-h, Concurrent switching of the magnetization and exchange bias field (b-d), independent switching of magnetization (e) and independent switching of exchange bias field (f-h) by a pulse current in the presence of an in-plane magnetic field.

![](https://assets.underline.io/uploads/markdown_image/1/image/2832a31a51b3b77bf9dccc9d8b4fd9ee.jpg)
Fig.2 Micromagnetic simulation results of exchange bias and its manipulation by SOT. a, Schematic of the AFM/FM exchange bias system. b,c, Hysteresis loops (b) before and (c) after application of SOT currents. d,e, Snapshots of the z component of the FM magnetization configuration (mz) at selected times. f, Switching of the exchange bias and magnetization under different SOT current densities.

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

Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have advantages of high packing density due to their high thermal stability, efficient current-induced magnetic switching, and unrestricted cell aspect ratio.[1] Therefore, PMA-based MTJs (p-MTJs) have broad application prospects on high-density and non-volatile spin-transfer torque magnetic random access memories (STT-MRAMs) and other spintronics devices.[2-4] In particular, p-MTJs with CoFeB/MgO structures show high tunnel magnetoresistance ratio.[4] In the W/CoFeB/MgO stacks, the interfacial magnetic anisotropy (Ki) of 1.98 mJ/m2 is acquired after annealing at 350 °C.[5] To further improve Ki, it is an effective way to use MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures, where the X represents the spacer material. [6] However, higher Ki is needed to scale down the MTJs devices and maintain high thermal stability.[7] In this work [8], we study the magnetic anisotropy of the double-interface free layer stacks with different annealing temperatures and different spacer materials. The samples of Ta(5)/MgO(2)/Co20Fe60B20(t)/X(0.4) /Co20Fe60B20(1.2)/MgO(2)/Ta(3) are deposited by sputtering system, where the numbers in parentheses indicate the thickness in nanometers, X including Ta, W, and Mo. The thickness of the bottom CoFeB layer t ranges from 0.4~3.2 nm. The films with Mo spacer layer exhibit the highest Ki of 4.06 mJ/m2 compared with the Ta- and W-based films. Furthermore, the Ki of Mo-based films remains at 2.93 mJ/m2 even after annealing at 500 °C for 1 h. Fig.1a and 1b show the structures with bottom CoFeB thickness of 0.8 nm and with spacer materials of Mo and W. As shown in the M-H (Magnetization M versus magnetic field H) curves, all the films show good PMA. Fig.1c and 1d show the corresponding high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDX) images. Saturation magnetization Ms and the total thickness of magnetic dead layer tDL can be extracted from the curves ( M/A versus tCoFeB curves), where A is the area of the sample and tCoFeB is the total thickness of CoFeB layers. The Ki can be extracted from the intercept of the Keff*teff versus teff curves, where Keff is effective magnetic anisotropy energy density and teff = tCoFeB- tDL. As shown in Fig.2a and 2b, Ki of 2.15±0.11 mJ/m2 and 2.90±0.34 mJ/m2 can be obtained for Ta- and W-based stacks, respectively. More importantly, the maximum of 4.06±0.14 mJ/m2 is acquired for the Mo-based structure after annealing at 350 °C as shown in Fig.2c. According to previous study, when the required thermal stability factor is 70, if the Ki reached 4 mJ/m2 , the device size can be reduced to 10 nm.[8] It can be shown in Fig.2d- 2f that the Ki decreases with the increase of annealing temperature. Note that the Ki reaches 2.93 mJ/m2 even after annealing at 500 °C for 1 h, which demonstrates the high thermal stability of this structure. Revealed by the HR-TEM and EDX images, the sharper interface, weaker interdiffusion, better interfacial uniformity and higher crystallinity can be seen in the Mo-based stack than the W-based stack. These results show that Mo plays an important role in improving the film quality. When the annealing temperature increases, the interfacial mixing intensifies, which leads to the decrease of Ki in Mo-based films. In summary, we study the dependence of magnetic anisotropy and film quality of the MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures on the spacer material and the annealing temperature. We find that, the Mo-based structure exhibits the strongest PMA (4.06 mJ/m2). Furthermore, Ki in the Mo-based stack is still as strong as 2.93 mJ/m2 even after annealing at 500 °C. The HR-TEM and EDX images of W- and Mo-based stacks show that the high thermal stability can be attributed to the high film quality in the Mo-based films. Our work provides a practical solution to scale down MRAM device while maintaining a high thermal stability.

**References:**

[1] Ikeda, S., Miura, K., Ohno, H. Nature materials, 9(9), 721-724 (2010). [2] Wang, M., Cai, W., Zhao, W. Nature electronics, 1(11), 582-588 (2018). [3] Peng, S., Zhu, D., Zhao, W. Nature Electronics, 1-8 (2020). [4] Wang, M., Cai, W., Zhao, W. Nature communications, 9(1), 1-7 (2018). [5] An, G. G., Lee, J. B., Hong, J. P. Acta Materialia, 87, 259-265 (2015). [6] Hwang, C. S. Advanced Electronic Materials, 1(6), 1400056 (2015). [7] Peng, S., Kang, W., Zhao, W. IEEE Magnetics Letters, 8, 1-5 (2017). [8] Cheng, H., Chen, J., Zhao, W. Advanced Electronic Materials, 6(8), 2000271 (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c74384cddd3ce3d115a89232da765fba.jpg)
Fig.1. (a,b) The schematic of Ta(5)/MgO(2)/Co20Fe60B20(0.8)/X(0.4) /Co20Fe60B20(1.2)/MgO(2)/Ta(3) with spacer material of (a) X=Mo and (b) X=W. (c,d) The corresponding HR-TEM images and EDX line profiles with spacer material of (c) X=Mo and (d) X=W. (e,f) The corresponding hysteresis loops with the in-plane (IP) field (red line) and out-of-plane (OP) field (black line) for different spacer materials of (e) X=Mo and (f) X=W. In the inset of (e) and (f), a M-H image under the small OP field with spacer material of (e) X=Mo and (f) X=W.

![](https://assets.underline.io/uploads/markdown_image/1/image/75ebcd2b300cee56aef34e4a13c0417a.jpg)
Fig.2. (a-c) The Ki fittings for MgO (2)/Co20Fe60B20(t )/X(0.4)/ Co20Fe60B20(1.2)/MgO(2) stacks under annealing at 350 °C with spacer layer X of (a) Ta, (b) W, and (c) Mo. (d-f) The Ki fittings for MgO(2)/Co20Fe60B20(t ) /Mo(0.4)/Co20Fe60B20(1.2)/MgO(2) stacks with the annealing temperature of (d) 400 °C, (e) 450 °C, and (f) 500 °C.

Shouzhong Peng

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