Pt)x Perpendicular Magnetic Anisotropy

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Low-Voltage Control of (Co/Pt) Perpendicular Magnetic Anisotropy Heterostructure for Flexible Spintronics Shishun Zhao, Ziyao Zhou, Chunlei Li, Bin Peng, Zhongqiang Hu, and Ming Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03097 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Low-Voltage Control of (Co/Pt)x Perpendicular Magnetic Anisotropy Heterostructure for Flexible Spintronics Shishun Zhao, Ziyao Zhou,* Chunlei Li, Bin Peng, Zhongqiang Hu, Ming Liu*

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China E-mail: [email protected], [email protected]

KEYWORDS: voltage control of magnetism, flexible magnetoelectric devices, ionic gel gating, ferromagnetic resonance, perpendicular magnetic anisotropy

ABSTRACT The trend of mobile internet yells for portable and wearable as bio-device interface. E field control of magnetism is a promising approach to achieve compact, light-weight and energy efficient wearable devices. Within a flexible sandwich heterostructure, the perpendicular magnetic anisotropy switching was achieved via low-voltage gating control of ionic gel in mica/Ta/(Pt/Co)x/Pt/ionic gel/Pt, where (Pt/Co)x plays as a functional layer. By conducting in situ VSM, EPR and MOKE measurements, a 1098 Oe magnetic anisotropy field change was determined at bending state with tensile strain, corresponding to a magnetic 1

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anisotropy energy change of 3.16 × 10 / and a giant voltage tunability coefficient of 0.79 × 10 / ∙ . The low voltage and strain dual control of magnetism on mica substrates enables tunable flexible spintronics devices with promoted degree of manipulation.

The arrival of the wireless mobile internet era yields a significant challenge for wearable devices as human-computer interaction interfaces. Flexible devices, thereby, have expanded dramatically for real-time health detection, monitors, indicators, and even for data storages and wireless communications.1,

2

State-of-the-art technologies have realized flexible,

stretchable, curvilinear devices with flexible structures design or flexible materials as substrates or functional layers, ranging from stretchable electronics with stretchable silicon circuits,3 stretchable optics based on organic LED linked with stretchable interconnects,4 novel pressure sensors based on organic field effect transistors (OFET) fabricated with 3D printer, layers

6

5

high sensitive flexible anisotropic magnetic sensors based conformal magnetic

and so on. However, few studies have been done in flexible spintronics, especially

voltage tunable flexible spintronics devices. Electric field (E-field) modulates magnetism is one of the most energy efficient pathways toward fast, compact, light-weight devices. Conventional magnetoelectric (ME) devices based on rigid, planar chips achieve voltage control magnetism by the virtue of strain induced inverse magnetostriction effect or interface charge accumulation. 7-17 For flexible ME devices, the materials will be bent, stretched or even twisted,1 and such variable strain conditions are fatal for strain mediated ME coupling.2 In contrast, the charge mediated ME coupling 2


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mechanism became a practical approach. Considering the fragile nature of the thick inorganic dielectric layer and the leakage problem of the thin flexible inorganic dielectric layer, gel form dielectric layer emerged as the promising candidate, which is easy to synthesis and conformal to various flexible substrates under almost any strain circumstances.18 Ionic gel(IG) film made from ionic liquid and dielectric polymer has been utilized to control metal-insulator transition,19,

20

exchange bias in antiferromagnetic system,

18

spin-orbital

torques,21 and so on. The applied E-field creates an electric double layers (EDL) across the object/gel interface, and the considerable charge density, usually up to 1015/cm2, yields a large E-field at the interface.

22-25

For oxide magnetic films, the polarized ionic liquid may

induce ion injection into oxides causing a property modulation via structure change.26-29 Moreover, the ionic gel based low-voltage (< 5 V) tunable flexible magnetoelectric device is a safe pathway to sophisticated embodiments. In this work, mica/(Co/Pt)x/IG/Pt flexible heterostructure was fabricated and worked as voltage tunable flexible ME devices. Perpendicular magnetic anisotropy (PMA) heterostructure – (Co/Pt)x is selected as an ideal magnetic multilayer heterostructure for ultrahigh density data storage on flexible substrates.30 With a small gating voltage of 4 V, the flexible ME device displays an explicit spin reorientation transition (SRT) behavior, where the magnetic easy axis tends to rotate form the out-of-plane to the in–plane direction reversibly for both flat state film and bent state film, evidenced via in situ vibrating sample magnetometer (VSM) measurement. The gating induced magnetic anisotropy energy change was 2.49 × 10 / for the flat state and 3.16 × 10 / for the bent state, comparable 3


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with the strain mediated ME tunable device in rigid planar chips form,8-11, 13, 31, 32 and the ionic liquid gating induce voltage control magnetic anisotropy (VCMA) on planar ferromagnetic layers.33 The ionic gel gating induced PMA switching comes from the giant E-field induced asymmetry at the multilayer interfaces, which generates an in-plane Rashba field resulting in an in-plane easy axis orientation. Comparing with our previous work of ionic gel gating of RKKY interaction on flexible substrates,34 the in-situ electron paramagnetic resonance (EPR) quantitatively measurement further ensured the 90-degree easy axis rotation phenomenon, and confirmed a 1098.63 Oe magnetic anisotropy field change, corresponding to a large 3.16 × 10 / magnetic anisotropy energy change. Basing on the principle of the ferromagnetic resonance, a concept demo of frequency range tunable microwave device was demonstrated in this all flexible heterostructure. In addition, the in situ magneto optical Kerr effect (MOKE) microscopy revealed the domain revolution of the ionic gel gating control device at bent state, which also gives out a potential path for future voltage tunable flexible memories.

Results and Discussions Device Fabrication and Strain Effect of the Flexible Substrate Figure 1(a) demonstrates the schematic of the flexible mica/Ta (4 nm)/(Pt(0.9 nm)/Co(1 nm))2/Pt(0.8 nm)/IG(1 mm)/Pt((10 nm)) heterostructure. The top Pt electrode was deposited on to flexible 80 µm thick Kapton substrate. The ionic gel, as shown in an inset in the right of Figure

1(a),

consisted

of

ionic

liquid,

1-ethyl-3-methylmidazolium 4


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bia(trifluoromethylsulfonyl) imide, ([EMIM][TFSI]), and poly(vinylidene fluoride) (PVDF). The ionic liquid ions operated as the functional charge carrier and the PVDF gel, like a sponge, acted as the flexible and stretchable support. Based on the requirements, the ionic gel film could be attenuated with decreasing the thickness of the mold or spin coating. The (Co/Pt)2 films modulated in this work were deposited onto 20 µm thick flexible mica substrates with DC sputtering, and the growth rate was meticulously calibrated with quartz crystal microbalance. More device fabrication detail was shown in the experiment section. The PMA film behaves a great conformal property with the mica substrate, as shown in the Figure 1(b), which is also evidenced by EPR spectrum after 200 times bending as shown in Figure S1. These three flexible layers attract each other like the sandwich structure. The strain dependent magnetic anisotropy of the mica/Ta/(Pt/Co)2/Pt was investigated. The strain variation condition was realized via different curvature states (curvature k=0.04 mm-1, 0.067 mm-1 and 0.2 mm-1) and the flat state. Magnetic hysteresis loops were measured respectively for both the in-plane and the out-of-plane direction as shown in Figure 1(c) and (d). As we applied a larger strain (or bending the film with a smaller curvature), the magnetization was enhanced, evidencing by the magnetic hysteresis loop of the bent film of out-of-plane direction as shown in Figure 1(c). More clearly, the saturated magnetization was plotted as a function of the curvature in Figure 1(c) inset. To compare the magnetic anisotropy change, the in-plane magnetic hysteresis loop was normalized as illustrated in Figure 1(d). By bending the film harder, the loops of the in-plane direction indicated an enhanced PMA property of the sample. These phenomena could be explained by the inverse 5


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magnetostriction effect. For the (Co/Pt)2 PMA structure, the as-applied tensile strain will induce an enhanced PMA due the negative magnetostriction of cobalt.12 In situ Low-voltage Control Magnetic Anisotropy of Flexible PMA Structure As the previous report, with decreasing the cobalt thickness of the (Co/Pt)x multilayer heterostructure, the magnetic easy axis will rotate from the basal-plane direction toward the out-of-plane direction, which is defined as SRT.12, 35, 36 The (Co/Pt)x film in this work was near the SRT point and favored a perpendicular magnetic anisotropy, evidenced by the VSM measurement as illustrated in Figure 2(a) and (b) for basal plane and normal plane, respectively. At the flat state, a 4 V gating voltage applied on the ionic gel will shift the magnetic anisotropy field from ,, = 1339 Oe along the normal plane direction to ,, = 1301 Oe, indicating an easy axis switching behavior from the out-of-plane to the in-plane direction as shown in the schematic of Figure 2(a) and (b).12, 35, 36 Considering the PMA film is near the SRT point, the magnetic anisotropy energy change is significant to describe the VCMA during the gel gating process. From the normalized VSM data, the ionic gel gating process will induce a 2.49 × 10 / magnetic anisotropy energy change, corresponding to a 6.23 × 10 / ∙ ME coupling gating tunability coefficient calculated =

∆ ! ∆"

=

with

the

!,#$!%,&!%'(& ) !,#$!%,'('%'!$

∆"

=

formula

0 0 *2 1 +(-)/-)*2 1 +(-)/-

∆"

expressed

as

, where is the ME coupling

gating tunability coefficient, ∆3 is the magnetic anisotropy energy density change, ∆ is the voltage change, and 45 is the saturated magnetization. 37 The release of the Vg will shift the magnetic anisotropy field form along the basal plane direction to the out-of-plane 6


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direction and let the easy axis go backward to the initial state, the in-plane orientation, as demonstrated with the green-square-line, implying a reversible switching manner. With the ionic gel gating, an in-plane to out-of-plane easy axis switching behavior was observed. Meanwhile, by alternating the E-bias, a two logic states transition was achieved, from the practical point of view. When distorted the (Co/Pt)2 film at the curvature of 0.2 mm-1 with a homemade holder at 0 V, for the hard axis direction, the strain induced a clear anisotropy change as compared between the red-square line and the blue-round line in Figure 2(e), implying an enhanced perpendicular magnetic anisotropy. This is because at the bent state as demonstrated in the inset of the Figure 2(d)(e), the magnetic moment of (Co/Pt)s tend to increase under a tensile stress due to the negative magnetostriction coefficient of cobalt film.12 Just as the flat state, when the PMA film was bent to the curvature of 0.2 mm-1, a 4 V gating voltage applied on the ionic gel will induced an easy axis switching behavior from the out-of-plane to the in-plane direction, confirming the similar easy axis rotation behavior, as demonstrated in Figure 2 (d) and (e). The schematic of the in situ measurement was illustrated in the inset of Figure 2(a)(b) and (c)(d). The calculated gating tunability coefficients for the bent device 0.79 × 10 / ∙ , which was larger than the modification at the flat state and comparable with the strain mediated ME tunable device in rigid planar chips form,8-12, 16, 31, 32 and the ionic liquid gating induce voltage control magnetic anisotropy (VCMA) on planar ferromagnetic layers 33. Mechanism of the PMA Switching Behavior 7


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During the ionic gel gating process, the I-V property across the (Pt/Co)2/Pt/IG/Pt capacitor heterostructure was characterized in inert nitrogen gas as shown in the Figure S2. The electrochemical window was confirmed as the range from 0 V to 2.5 V. A a 0.8-nm thick Pt was used as a protecting layer from the electrochemical reaction. Compare with massive interface morphology in the process of ionic liquid gating control cobalt single layer,33 the solid/gel interface was much smoother as demonstrated in Figure S2, indicating a very low solid surface reaction rate and implying the gel gating modulation is a charge dominated process. Before applying the gating voltage, as confirmed via VSM, the moment is along the out-of-plane direction, and there is no Rashba field or net zero Rashba field in the PMA heterostructure due to the symmetric spin-orbital coupling states across multilayers, as illustrated in Figure 2(c). With the 4 V voltage bias, the polarization of the ions will give rise to the [EMIM]+ charge accumulation at the gel/PMA film interface, resulting in an extremely large E-field, which will modulate the spin-orbital coupling energy in the multilayer system with distance dependence. Accordingly, an asymmetric state at the Co/Pt interfaces, ferromagnetic metal film/normal metal film interfaces, will induce a nonzero in-plane Rashba field as illustrated in Figure 2(f), which alternates the easy axis direction to the in-plane.38, 39 The E-field at solid/gel interface will be released once the small 4 V gating voltage is withdrawn. Then, the restored symmetric spin-orbital coupling state vanish the net in-plane Rashba field and rotate the easy axis and the magnetic moment normal to the film. Quantitative Analysis of the Magnetic Anisotropy Change and the Demo of the Flexible RF/Microwave ME Device 8


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Furthermore, the E-field tunable flexible PMA gating process was also quantitatively examined with EPR. The initial magnetic anisotropy field of the ME device, or here the ferromagnetic resonance field (Hr), was characterized and plotted as the function of the angle between the external magnetic field and the film plane as depicted with the red-square line in Figure 3(a). The maximum Hr, max and the minimum Hr, min were along the in-plane direction and the normal direction, respectively, indicating an out-of-plane easy axis.

12, 35, 36

The

bending of the device induced a tensile strain on the (Co/Pt)2 film as illustrated in the inset of Figure 3(b). And the comparison between the green-round line and the red-square line in Figure 3(a) gives out the special information of the strain induced small anisotropy change via an inverse magnetostriction effect.

8-11, 31, 32

This was evidenced by the 50.2 Oe left shift

of the Hr for the normal plane direction as shown in Figure 3(b), indicating an enhanced PMA property corresponding to the VSM results. At the bent state, the ionic gel gating generated an explicit 90-degree easy axis rotation confirmed by that the maximum Hr and the minimum Hr were along the normal direction and the in-plane direction, respectively, as demonstrated with the blue-triangle line in the Figure 3(a), converse with the initial state as illustrated with red-square line. The maximum magnetic anisotropy change is along the out-of-plane direction as illustrated in Figure 3(b) consistent with the VSM results, and a 1098 Oe Hr shift was achieved via ionic gel gating, corresponding to a large 274 Oe/V gating tunability coefficient. The EPR measurement is based on the principle of ferromagnetic resonance, which could be described by Kittel Equation and expressed as 6 = 789: + < =9: + < + 4>45 =, 9


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where 6 is the microwave frequency which is set here, 7 the gyromagnetic ratio, : the effective magnetic field and 45 the saturated magnetization. The ionic gel gating process will induce a : to shift the < .8-11 Then a concept model of voltage tunable flexible RF/microwave device is demonstrated as shown in Figure 3(c). The working principle is that the 6< will shift via ionic gel gating induced : as shown in Figure 3(d). For the device, the mica/Ta/(Pt/Co)2/Pt/IG/Pt sandwich heterostructure is transferred on to a flexible microwave wave guide made from low-loss fiber microwave guide materials reported before.40 And the waveguide is flexible based on the previous simulation report.41 The microwave response frequency will be tuned with circuit-voltage in flexible device. Compared with the previous reported tunable ME microwave devices, like FeCoB/PMN-PT (011) (320 Oe FMR field shift with 1.5 kV/cm)8 and FeGaB/PZN-PT (011) (1510 Oe FMR field shift with 6 kV/cm),11 (Pt/Co)x/PMN-PT (1100 Oe FMR field shift with 12 kV/cm at 100 K)12, LSMO/PMN-PT(011) (464 Oe FMR field shift with 12 kV/cm at 173 K)42 and IL/Co/Ta/SiO2 (88 Oe FMR field shift with 1.5 V at room temperature)31, this mica/Ta/(Pt/Co)x/Pt/IG/Pt laminates give a comparable or even higher voltage tunability for low voltage tunable wearable devices. Domain Revolution Behavior of the Flexible ME Device During the gel gating Process More directly, Magneto Optical Kerr Effect Microscopy (MOKE) was used to observe the domain dynamics. The geometry of the in-situ ionic gel gating control flexible PMA observation via MOKE is demonstrated as Figure 4(a). The incident polarized light through the mica substrate was focused at the interface between the PMA film and mica substrate, 10


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which enables a real-time observation of the ionic gating induced domain revolution. Considering the short focus length of the object lens, the flexible device was bent with an inverse direction and the metal film surfers a compressive strain under this configuration. The domain switching behavior of the out-of-plane direction was recorded with camera as two videos showing in the supporting information for flat state and bent state, which both confirmed a normal plane easy axis. Well conformal property between the PMA film and the substrate in domain size was evidenced by both focused and unfocused domain structure shown in the same photo featured as Figure S3 (a). The MOKE hysteresis loop change certified the low-voltage ionic gel gating modulated reversible magnetic easy axis switching behavior as shown in Figure S3(b) and (c). For the bent ME device, more vividly, Figure 4 depicted the domain switching dynamics during the gating process, before which the device was magnetized with a -40 mT out-of-plane external magnetic field. Then the device was reset at 15.6 mT, there was not any domain reversal and this state was defined as the background as illustrated in Figure 4(b). The domain was switched or not depends on the gray level of the local area as described in the supporting information. With sequentially increased gating voltage, the domain reversal will be quantized as shown in Figure 4 from 0% at 0 V, to 0.63% at 2.5, to 14.04% at 3 V, to 79.32% at 3.3 V, to 94.85% at 3.5 V and 98.82% at 4 V. For the easy axis 90-degree switching behavior of the mica/Ta/(Pt/Co)2/Pt/IG/Pt, corresponding to a two logic state switching, researchers may achieve voltage driven flexible memories. It is worthy to mention that the electrical control of magnetism is even more 11


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importance due to the fact that magnetic head can hardly operate and tune the local magnetism on flexible substrate. Thereby, our ionic modulation of flexible spintronics provides an effective, energy efficient way of controlling magnetism. An alternative low DC bias still switch the moment direction in a reversible manner. Conclusion In summary, in this work we achieved a tunable 90-degree easy-axis switching on a flexible mica/Ta/(Pt/Co)x/Pt/IG/Pt sandwich heterostructure with low-voltage ionic gel gating approach. The PMA switching phenomenon was characterized with in situ VSM, EPR and MOKE observation. The ionic gel gating generates an up to 3.16 × 10 / magnetic anisotropy energy change in a reversible manner, corresponding to a 0.79 × 10 / ∙ or 274 Oe/V gating tunability coefficient. This giant tunability came from E field modification of the spin-orbital coupling, which yields an in-plane Rashba field to orientate the moment into the film plane. Basing on the principle of ferromagnetic resonance, a concept demo of low-voltage tunable microwave response devices with flexible heterostructures was demonstrated.

Experiment section Device Fabrication: 10 nm Pt was deposited on to the 0.08 mm thick kapton paper as top gate electrode with 20 W DC sputtering, and the background pressure is lower than3 × 10)? mT. The ionic liquid, 1-ethyl-3-methylmidazolium bia(trifluoromethylsulfonyl) imide, (abbreviated as [EMIM][TFSI], purchased from Chengjie Chemicals Co. Ltd., Shanghai, 12


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China), and poly(vinylidene fluoride) (PVDF, purchased from SIGMA-ALDRICH Co.) were dissolved in dimethyl-formamide (DMF, for HPLC purchased from SIGMA-ALDRICH Co.) and acetone (Kelong Chemicals. Co. Ltd., Chengdu, China). The mass ratio of the ionic liquid, the polymer, DMF and acetone was 1:4:5:5. The mixture was stirred at 400 rpm 45 ℃ overnight. Then the thick sol was transformed onto a 4-inch surface polished SiO2 substrate and confined with 1 mm thick glass slides in a square shape. Base on the requirements, the ionic gel film could be attenuated with decreasing the thickness of the glass slides or spin coating. After this, the mold and the sol was moved into the vacuum oven and dried at 45 ℃ for at least 24 hours to form the ionic gel. The gel film was cut with blade before use. The Ta (4 nm)/(Pt(0.9 nm)/Co(1 nm))2/Pt(0.8~0.9 nm) heterostructure film was deposited onto 0.02 mm thick mica substrate with 20 W DC sputtering with a low background pressure under 1.5 × 10)? mT. The 20 D thick mica substrate was bought form Taiyuan mica Co. Jilin, China. By the virtue of Van der Waals' force, these three flexible layers attract each other like the sandwich structure. And two Au wires were used to connect the top gate and the PMA film, respectively. In situ Magnetic Property Measurement: The in situ VSM measurement was conducted with a LakeShore 7404 VSM system. The device and the holder was sealed with parafilm (BEMIS, BEMIS FLEXIBLE PACKING) to isolate oxygen in the air. The DC voltage was applied with Keysight 2901A. The in situ EPR measurement was conducted with JEOL FA200 EPR system. The device was characterized in a TE 011 mode microwave cavity filled with inert nitrogen gas. The domain revolution was 13


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observed with an out-of-plane external H field via MOKE Microscope (Evico Magnetic Co.). The in situ gel gating test with different curvature was realized with homemade holders adapted for VSM, EPR and MOKE measurement.

Figure 1 The structure of the flexible ME tunable material system. (a) The schematic of the flexible mica/Ta/(Pt/Co)2/Pt/IG/Pt sandwich heterostructure. And the insets shown in the right give out the detail configuration of the ME device. (b) The capture of flexible mica/Ta/(Pt/Co)2/Pt/IG/Pt sandwich heterostructure at bending state. (c) Strain induced magnetization change of the mica/(Co/Pt)2 of the out-of-plane, the inset on top left illustrates the strain dependence of the saturated magnetization of the mica/(Co/Pt)x, and the inset on the bottom right illustrates the geometry of the bending condition and the external magnetic field. (d) Strain induced magnetic anisotropy change of the mica/(Co/Pt)x for the in-plane direction, 14


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the inset on top left gives out a zoom in view to compare the hysteresis loops, and the inset on the bottom right illustrates the geometry of the bending condition and the external magnetic field.(x=2)

Figure 2 In situ low voltage control easy axis rotation test via VSM and ionic gel gating mechanism. (a)(b) The magnetic hysteresis loop evolution during the in situ ionic gel gating process along the in-plane direction of the flat film (a) and the out-of-plane direction (b), respectively as shown in the insets. And the magnetic hysteresis loop of initial state, 4-Volt gating state and the voltage released state were illustrated with the red-square line, blue-round line and green-triangle line, respectively. (d)(e) The magnetic hysteresis loop evolution during the in situ ionic gel gating process along the in-plane direction of the bent film by the curvature of 1/5 mm-1 (d) and the out-of-plane direction (f), respectively as shown with the insets. (c) and (f) illustrate the initial symmetric spin-orbital coupling state (c) and ionic gel gating induced asymmetric spin-orbital coupling interface state (f), respectively. The blue and yellow arrows stand for the Rashba field induced via the out-of-plane and in-plane 15


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spin orbital coupling change, respectively.

Figure 3 Ionic gel gating control of ferromagnetic resonance and concept demo of low-voltage tunable RF/microwave device based on flexible laminates. (a) Voltage control 90-degree magnetic easy axis rotation measured by in situ ESR measurement. The ferromagnetic resonance field was plotted as a function of the angle. The in-plane direction was defined as 0 degree. The anisotropy of the flat state, initial state and 4-Volt gating state at the curvature of 0.2 mm-1 were illustrated with the red-square line, blue-round line and green-triangle line, respectively. (b) Strain and ionic gel gating induced ferromagnetic anisotropy change evidenced by the ferromagnetic resonance field of the flat state, initial 16


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state and 4-Volt gating state at the curvature of 0.2 mm-1 as shown in the red, blue and green line, respectively. (c) Concept demo of low-voltage tunable RF/microwave device based on flexible laminates. (d) Ionic gel gating induced response frequency, or ferromagnetic resonance frequency shift.

Figure 4. Domain revolution during the ionic gel gating at bending state with MOKE observed via MOKE. (a) the configuration of the in situ MOKE observation; (b)~(g) The domain states at sequential gating voltages, for (b) at 0 V, (c) at 2.5 V, (d) at 3 V, (e) at 3.3 V, (f) at 3.5 V and (g) at 4 V.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website: http://pubs.acs.org. The

comparison

of

the

magnetic

anisotropy

of

mica/Ta/(Pt/Co)2/Pt and

the

mica/Ta/(Pt/Co)2/Pt after torture test (Figure S1); electrochemical property test and gel/solid interface morphology (Figure S2); In situ gating control ME flexible device via MOKE 17


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measure (Figure S3); the domain revolution in flat state (Video 1); and the domain revolution in bent state (Video 2); additional experimental details, equations, and references.

AUTHOR INFORMATION Corresponding Author Z.Z. Email: [email protected]; M.L. Email: [email protected] Present Addresses Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS The work was supported by the Natural Science Foundation of China (Grant Nos. 51472199, 11534015, 51602244), the National 111 Project of China (B14040), National Key R&D Program of China(2018YFB0407601). The authors acknowledge the support from the International

Joint

Laboratory

for

Micro/Nano

Manufacturing

and

Measurement

Technologies. Z.Z. and M.L were supported by the China Recruitment Program of Global 18


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Youth Experts.

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Pt)x Perpendicular Magnetic Anisotropy

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