Free Field Electric Switching of Perpendicularly Magnetized Thin Film

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Surfaces, Interfaces, and Applications

Free field electric switching of perpendicularly magnetized thin film by spin current gradient Shaohai Chen, Jihang Yu, Qidong Xie, Xiangli Zhang, Weinan Lin, Liang Liu, Jing Zhou, Xinyu Shu, Rui Guo, Zongzhi Zhang, and Jingsheng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09146 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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ACS Applied Materials & Interfaces

a.

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Free field electric switching of perpendicularly magnetized thin film by spin current gradient Shaohai Chen,† Jihang Yu,† Qidong Xie,† Xiangli Zhang,‡ Weinan Lin,† Liang Liu,† Jing Zhou,† Xinyu Shu,† Rui Guo,† Zongzhi Zhang,‡ Jingsheng Chen*,† †

Department of Materials Science and Engineering, National University of Singapore, 117576,

Singapore ‡

Key Lab of Micro and Nano Photonic Structures (Ministry of Education), Department of

Optical Science and Engineering, Fudan University, Shanghai, 200433, China ABSTRACT: To realize high speed non-volatile magnetic memory with low energy consumption, electric switching of perpendicular magnetization by spin-orbit torque (SOT) in the heavy metal/ferromagnetic (HM/FM) structure has recently attracted intensive attention. Conventionally, an external in-plane magnetic field for breaking the symmetry is required in achieving electric switching of perpendicular magnetization. However, electric switching without external field is the prerequisite for integration of magnetic functionality into the integrated circuit devices. Here we propose a new method of utilizing a W wedge in the Pt/W/FM structure to induce a spin current gradient, which can result in an in-plane equivalent field along the wedge thickness gradient direction. We experimentally demonstrate the deterministic magnetization switching of perpendicular Co/Ni multilayers without external magnetic field when the electric current is along the wedge thickness gradient direction. Our findings shed light on free field electric switching of magnetization by a new physical parameter - an asymmetric spin current induced by a bilayer wedge structure. KEYWORDS: spintronics, spin−orbit torques, free field electric switching, spin-hall effect, spin current gradient.


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1. INTRODUCTION Electric current induced magnetization switching by spin orbital torque (SOT) in a heavy metal(HM)/ ferromagnetic(FM)/ oxide insulator(I) heterostructure has attracted great interest due to the application for ultra-low power and fast spintronic devices.1-6 The SOT arises from the spin Hall effect (SHE) in the HM layer and/or the Rashba effect at the interfaces. 7-10 According to the scenario of SHE, when an in-plane electric current is applied in the HM/FM structure, a spin current is generated in the HM layer due to spin orbit coupling and transferred to FM layer, exerting a spin transfer torque on the magnetization of FM layer.11-12 However, an external magnetic field along the current direction is usually required for the deterministic switching of a perpendicularly magnetized ferromagnet.13 Free magnetic field electric switching of magnetization is of paramount importance for the integration of spintronic devices with CMOS. In order to achieve the deterministic zero magnetic field electric current induced magnetization switching (ZFS) in perpendicularly magnetized HM/FM structure, different methods have been utilized to induce the asymmetry, e.g. gradient change of magnetic anisotropy of FM layer along current direction by gradient oxidation of top insulate layer,14 induce a unidirectional in-plane magnetic moment by changing the geometric symmetry of FM layer,15-17 or by antiferromagnetic exchange coupling,18-21 or induce an effective magnetic field along the current direction by a spin current gradient.22-23 In current work, we present a new method for SOT induced ZFS. We introduce an asymmetry of the spin current along the electric current direction, as shown in Figure 1a, which is realized by Pt/W bilayer with a wedge structure for W layer. Since Pt and W possess opposite spin Hall angles θSH ,24-25 the spin current produced by the Pt layer and W layer possess opposite directions. By controlling the wedged W layer thickness smaller than its spin diffusion length of 1.2nm, 25


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the net spin current density 𝐽𝑠 = 𝐽𝑐,𝐻𝑀 𝜃𝑆𝐻,𝐻𝑀 , where Jc,HM is the current density within the HM layer, injected in the FM layer is contributed and adjusted by both Pt and wedged W layer. When an electron current 𝑱𝑒 flows along the x direction, the spin current generated from Pt layer decays in different degrees by the wedges W layer due to the spin relaxation effect.26 Combining the opposite spin current generated by the W layer, the 𝑱𝒔 intensity changes dramatically in the xdirection. In a determined region where the 𝑱𝑠 is mainly contributed by Pt or W layer, as shown in Figure 1b, the 𝑱𝑠 induced damping-like field HDL varies with 𝑱𝑒 (∝ 𝑱𝑐,𝐻𝑀 ), whereas the net spin current gradient along the x-direction 𝜕𝑱𝒔 /𝜕𝑥 does not vary with 𝑱𝑒 . The deterministic ZFS of the perpendicular magnetization in HM/FM/I heterostructure is achieved when the electric current flows along the wedge thickness gradient direction. By comparing with the experimental results under ZFS circumstance, we believe it is the spin current gradient 𝑱𝑆′ results in an equivalent field Hequi along the gradient direction and thus achieve the ZFS. On the other hand, from the application perspective, both the Ni/Co multilayer structure and CoFe based CoFeX amorphous alloys can be the FM layer in our design, because both of them could provide good perpendicular magnetic anisotropy (PMA). In order to avoid the postannealing treatment and provide the system more controllable parameters, such as the FM layer thickness and the PMA energy,27 the Ni/Co multilayer structure was utilized in the experiment. 2. EXPERIMENTAL SECTION Two sets of samples were deposited on thermally oxidized Si substrate by a Lesker CMS-18 Model magnetron sputtering system with the base pressure better than 110-8 Torr. The film structure of samples with a wedged W layer is substrate/Ta (1)/Pt (5)/W (0~0.7)/[Ni (0.56)/Co (0.28)]4/AlOx (3), and the other one without the wedge structure is substrate/Ta (1)/Pt (5)/W (tW,uni=0, 0.28, 0.55, 0.83)/[Ni (0.56)/Co (0.28)]3/Ni (0.56)/SiO2 (4), where the number in the


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parenthesis is the thickness in nm. The AlOx capping layer is obtained by natural oxidation of the Al layer. For the non-wedge structure layers, the deposition rates for the Ta, Pt, W, Ni, Co, Al and SiO2 layers are 0.043, 0.077, 0.092, 0.040, 0.031, 0.030 and 0.020 nm/s, respectively, and all these layers are fabricated with the substrate rotated at 30 rad/min for guaranteeing the homogeneity. For the wedged W layer, the deposition rate is 0.070 nm/s and the wedge structure is realized by linearly moving a shutter, which is between the target and the substrate and controlled by a step motor. After deposition, all samples are patterned into Hall bar structure by a standard laser lithography and ion milling processes. For current induced magnetization switching measurements, a dc pulsed current Ipulse with pulse width of 1ms is applied along the long side of the devices. After each pulse (at least 4 seconds), an ac excitation current Iac ( 0.2 nm where the ZFS phenomenon starts to be observed from device DIII. Based on the abscissa of the cross points, the Hequi in devices DIII to DVI are extracted as -4.2, -5.3, -8.8 and -16.8 Oe, respectively. The negative sign indicates the direction of Hequi is along the –x direction. The Hequi of the devices DIII to DVI is displayed in Figure 3j, where the device location on the wedge is represented by the W middle thickness tw,mid. Since different mechanisms can assist to realize the ZFS,14-23 we carry out the following experiments. Figure 4c to 4d show the normalized AHE loops of device DIII and DV, respectively. Compared with device DIII, it is noticed the remnant Hall resistance of device DV decreases


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from 100% to 99.5%. This indicates that the magnetic moment of device DV is not completely along the normal z direction when the external field is 0 Oe, which suggests the easy axis of Co/Ni multilayer may not be in the direction of film normal and is likely along the normal direction of the wedge, as shown in Figure 4a and 4b, due to the origin of the PMA of Co/Ni from interfacial anisotropy

30-31

. In order to accurately evaluate the easy axis direction, we

measured the anisotropic magnetoresistance (AMR) of the devices DIII to DVI under the inplane magnetic field Hinp varying from 50 to 2000 Oe. As shown in Figure 4e and 4f, the angle dependences of the resistivity ρ(θ) for device DIII are symmetric around θ = 90o for both the inplane unsaturated and saturated magnetic field, where θ is the angle between the current and Hinp. This suggests that the magnetic easy axis in device DIII is in the film normal direction or the tilting of the easy axis is too small to be detected.15 Whereas for the devices DIV to DVI as shown in Figure 4g and Figure S2 (see S2 in Supporting information), all ρ(θ) show asymmetric behavior around θ = 90o when the magnetic field is not high enough to align the magnetization in field direction. When the magnetic field is strong enough to align the magnetization along the field direction, ρ(θ) becomes symmetric around θ = 90o and is well fitted by cos2θ, as shown in Figure 4h. These results suggest that the magnetic easy axis tilts away from the film normal in the x-z plane.15 We roughly estimated the tilting angle of the easy axis away from the film normal by γ = cos(-1) Rrem where 𝛾 is the tilting angle and Rrem is the remnant Hall resistance. The tilting angles for devices DIV to DVI are very close and around 5.5 o away from the film normal. It has been reported by You et al.15 and Vineeth et al.17 when the magnetic easy axis of the heterostructure tilted in x-z plane and the electric current flows along ±y direction, the spin current induced torque would drive the magnetization of FM layer along x direction and ZFS occurs. As shown in Figure 5a and 5b, when the charge current is applied along the ±y direction,


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it will result in the ±x-direction spin angular momentum. When the spin angular momentum is large enough, the magnetization of the FM layer will be switched to +x or –x direction. After withdrawing the charge current, the magnetization of the FM layer will switch to the nearest direction along the magnetic easy axis due to the system magnetic anisotropy. In contrast, this mechanism does not result in ZFS when the electric current is applied along the ±x direction. As a result, it can be known that the achieved ZFS in devices DIV to DVI with current flowing in the x-direction is not derived from the tilted magnetization. We perform similar experimental measurement for our devices DIII to DVI with applying electric current along the ±y direction. For device DIII, no ZFS is observed because its magnetization easy axis is along the film normal direction. For devices DIV to DVI, similar deterministic magnetic switching is observed when with Hy=0 Oe. Figure 5c and 5d show the experimental results of current induced magnetization switching of device DV when the charge current is applied along the ±y direction. Under the same 𝐽𝑐,𝐻𝑀 , it can be noticed that the current induced magnetization switching ratio with Hy=0 Oe is much smaller that with Hy=±30 Oe and the ZFS case with current flowing in x direction. The partial magnetization switching in the ZFS case can be attributed to the insufficient applied charge current, which cannot produce enough spin current to switch all magnetic moments to one direction. Corresponding to the Figure 1a and 1b, along the x direction, it can be noticed that there should exist a critical thickness for the W layer where the net 𝐽𝑠 is zero. Under the critical thickness, (or within the Pt-dominant region,) the 𝑱′ 𝑺 is always pointing to –x direction, which is same with Hequi of devices DIII to DVI. Moreover, due to the spin relaxation effect,26 it can be understandable that the non-linearly variation of 𝑱𝑆 results in the intensity variation of 𝑱′ 𝑆 is also non-constant. As a result, we attribute the origination of Hequi of the devices DIII to DIV to the


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spin current gradient 𝑱𝑆′ .22 Based on macromagnetic model and by adding the term of 𝐻𝑒𝑞𝑢𝑖 in Landau-Lifshitz-Gilbert equation, we calculate the magnetization switching states within the Ptdominant region. (see S3 in Supporting Information) The simulation results show that the polarity of magnetic switching loops are consistent with our experimental results, which switch from –Mz to +Mz (or from +Mz to –Mz) direction when the applied Jc,HM is positive (or negative). To further understand the effect of the W layer at different positions along the wedge thickness gradient direction, we investigate the magnetic properties and the current induced magnetization switching of the non-wedge structure samples with tW,uni = 0, 0.28 and 0.55 nm, respectively. As shown in Figure 6a to 6c, the remnant Hall resistances of all AHE loops are equal to 1, which indicates that all devices have their easy axis along the film normal. The magnetic anisotropy fields Hk estimated from hard-axis magnetic field dependence of RHall curves (Figure 6d to 6f) decreases from 3.3 kOe to 2.8 kOe with increasing tW,uni to 0.55 nm. This trend is consistent with the VSM results shown in Figure 2c. Corresponding to the wedge structure devices DIII to DVI, it can be expected that there exists an anisotropy gradient in the Ni/Co multilayers along the x direction, 𝑑𝐻𝑘 /𝑑𝑥 ≠ 0. Based on the results from Yu et al.14, it can be known that the anisotropy gradient induced z-direction effective magnetic field can assist to achieve the ZFS. However, in device DIII, the ZFS was observed only when the electric current was applied along the ±x direction instead of the ±y direction. Therefore, it can be indicated that the anisotropy gradient is also not the main reason for achieving the ZFS in our devices. The current induced magnetization switching results of the three devices with Hx = 0 and ±150 Oe are shown in Figure 6g to 6i, respectively. With Hx = ±150 Oe, deterministic magnetic switching is obtained in the three devices and the critical switching current density Jc,HM with


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tW,uni=0, 0.28 and 0.55 nm are 3.4×107, 3.9×107and 4.2×107 A/cm2, respectively. The Jc,HM values are comparable with the reported Jc,Pt= 3×107 A/cm2 in Pt/[Co/Ni]3 structure32 and Jc= 3 ~ 4.5×107 A/cm2 in Pt/W/CoFeB structure33. Moreover, in corresponding to the AHE results, it can be seen that with a negative Hx and a positive Jc,HM, the magnetizations of all the three devices are switched from -M to +M direction. This suggests that the net spin current produced by Pt/W bilayer with tW,uni= 0.55 nm is still dominated by Pt layer. With Hx = 0 Oe, no ZFS is observed regardless of tW,uni. This is different from the results reported by Ma et al.34, who utilized oblique sputtering technique to obtain the W layer in the Pt/W/CoFeB structures and observed the ZFS phenomenon. However, with the same Pt/W/CoFeB structure and non-oblique sputtering technique, no ZFS was observed by Liu et al.33. Since the oblique sputtering would induce a shallow wedge structure, we believe the observed ZFS phenomenon by Ma et al. is also attributed to the spin current gradient. In the HM/FM/I heterostructure, Mann et al.35 show that the required effective in-plane magnetic field, which is related to interfacial Dzyaloshinskii-Moriya interaction (DMI), to achieve deterministic switching can be adjusted by the interfacial material and the inserting layer thickness. By considering the wedged W layer as an inserting layer, it can be expected that the DMI induced in-plane field possesses a liner intensity gradient along the x direction.32 Corresponding to our design and experimental results, we think the wedge structure induced DMI variation may provide part contribution to the ZFS phenomenon, and this needs to be further studied. 4. CONCLUSION To conclude, we have achieved zero magnetic field electric current induced magnetization switching in (Ni/Co)4 multilayers deposited on Pt/W bilayer where the W layer has a wedge structure and the charge current flows along the wedge thickness gradient direction. We believe


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that the observed equivalent magnetic field Hequi along the charge current direction is derived from the spin current gradient. The magnitude of Hequi is dependent on the location of wedge structure and should be tunable by changing the slope of the wedge structure.

ASSOCIATED CONTENT Supporting Information: S1. The Hall polarity of AHE loop in Co/Ni multilayer system S2. The AHE and AMR results of devices DIV, DV and DVI S3. Simulation results of the current induced magnetization switching in Pt-dominant regionThe following files are available free of charge.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected]. (J.S.C.) Author Contributions J.S.C. conceived and supervised the project. S.H.C. and Q.D.X. performed the sample fabrication. S.H.C. and J.H.Y. performed the experimental measurements and processed the data. X.L.Z. and S.H.C. performed the theoretical calculation. W.N.L., L.L., J.Z., X.Y.S., and R.G. assisted in the measurement. J.H.Y. and Z.Z.Z. commented on the manuscript. S.H.C. and J.S.C. wrote the manuscript and all authors contributed to its final version. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work is partially supported by Singapore National Research Foundation (NRF) under IIP Award No.NRF-IIP001-001, Singapore National Research Foundation (NRF) under CRP Award No. NRF-CRP10-2012-02, Singapore Ministry of Education MOE2018-T2-2-043, AMEIRG180022, A*STAR IAF-ICP 11801E0036 J.S.C. is the member of the Singapore Spintronic Consortium (SG-SPIN). Z.Z.Z. would like to thank the support from the National Natural Science Foundation of China (51671057 and 11874120). FIGURES CAPTION Figure 1: (a) The schematic of the Hall bar device with a wedged W layer in the film structure of Pt/W/[Ni/Co]4/AlOx. The red and blue arrows along z direction illustrate the injection of spin current in Pt and W layer, respectively. (b) The direction schematics of the applied electron current Je (white arrows), the damping-like field HDL (green arrows), the net spin current (red and blue arrows perpendicular to current direction) and the spin current gradient J’s (red and blue arrows parallel to current direction) in the Pt-dominant and W-dominant regions, respectively. The black arrow represents the magnetization M.

Figure 2: (a) The P-MOKE results of non-wedge structure thin film samples. (b) the AHE results of Hall bar devices with tW,uni = 0, 0.28, 0.55 and 0.83 nm.(c) The in-plane and out-of-plane M-H loops of non-wedge structure thin film samples.

Figure 3: (a) The cross-sectional view of the devices used for experimental measurements, named as DI to DVI from left to right, respectively. The thickness difference of the W wedge for each device is fixed as 0.2 nm. The middle thickness of W wedge layer from devices DI to DVI is gradually increased by 0.1 nm. (b) and (c) The current induced magnetization switching loops of the devices DIII and DV, respectively. The applied x-direction external magnetic field Hx


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various from -100 to +100 Oe. (d)-(i) The critical current density in heavy metal Jc,HM vs. Hx curves of the devices DI to DVI, respectively. The dashed lines in (f)-(i) indicate the value of the equivalent in-plane magnetic field Hequi of devices DIII to DVI, respectively. (j) The dependence of the Hequi values on the position of devices DIII to DVI.

Figure 4: (a) and (b) The schematics of the devices with the magnetic easy axis along the normal direction and with magnetic easy axis tiled into x-z plane, respectively. All devices are located in Pt-dominant region. (c) and (d) The AHE loops of devices DIII and DV, respectively. (e),(f) and (g),(h) The AMR results with different in-plane magnetic field Hinp of devices DIII and DV, respectively.

Figure 5: (a) and (b) The 3D view and cross-sectional view schematics (c) and (d) The experimental results of device DV when the electric current is applied along the +y direction and the external magnetic field Hy is ±30 and 0 Oe, respectively.

Figure 6: (a)-(c) The AHE loops of the non-wedge structure devices with tW,uni is 0, 0.28 and 0.55 nm, respectively. (d)-(f) The hard-axis magnetic field dependence of RHall curves of the three devices, respectively. (g)-(i) The current induced magnetization switching loops of the three devices with Hx=-150, 0 and +150 Oe, respectively. The dashed lines indicate the critical switching current density in heavy metal of the three devices.


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Figure1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Magnetization Reversal by Insertion of Au Spacer In Pt/Au/Co/Ni/Co/Ta. APL Mater. 2017, 5, 106104.


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Free Field Electric Switching of Perpendicularly Magnetized Thin Film

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