Electric-Field-Controlled Nonvolatile Magnetization Rotation and

Jun 6, 2018 - The purely electric-field-controlled nonvolatile and reversible ... but also the electric-field-tuned magnetoresistance effects were obt...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Electric-Field-Controlled Nonvolatile Magnetization Rotation and Magnetoresistance Effect in Co/Cu/Ni Spin Valves on Piezoelectric Substrates Wenbo Zhao,† Weichuan Huang,† Chuanchuan Liu,† Chuangming Hou,† Zhiwei Chen,† Yuewei Yin,*,† and Xiaoguang Li*,†,‡ †

ACS Appl. Mater. Interfaces 2018.10:21390-21397. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/10/19. For personal use only.

Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, and CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei 230026, China ‡ Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China S Supporting Information *

ABSTRACT: Electric-field control of magnetism is a key issue for the future development of low-power spintronic devices. By utilizing the opposite strain responses of the magnetic anisotropies in Co and Ni films, a Co/Cu/Ni/ 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) spin-valve/piezoelectric heterostructure with ∼7 nm Cu spacer layer was properly designed and fabricated. The purely electric-fieldcontrolled nonvolatile and reversible magnetization rotations in the Co free layer were achieved, whereas the magnetization of the Ni fixed layer was almost unchanged. Accordingly, not only the electroresistance but also the electric-field-tuned magnetoresistance effects were obtained, and more importantly at least six nonvolatile magnetoresistance states in the strain-tuned spin valve were achieved by setting the PMN-PT into different nonvolatile piezo-strain states. These findings highlight potential strategies for designing electric-field-driven multistate spintronic devices. KEYWORDS: multiferroic heterostructures, piezo-strain effect, nonvolatile, magnetization rotation, electric-field-controlled spintronics memory functions can be realized in “straintronic” devices by integrating strain effect in spintronic devices, including spin valves or magnetic tunnel junctions,26 and several straininduced or assisted magnetization and magnetoresistance (MR) switchings have been experimentally confirmed.16,28−30 For instance, the electric-field manipulation of the coercive field of the free layer in a spin valve grown on PbZr0.5Ti0.5O3 ferroelectric film has been demonstrated with the assistance of magnetic fields.30 Purely electric-field-controlled magnetization rotations and the corresponding MR switchings were also achieved in the free layers of CoFeB-based magnetic tunnel junctions grown on PMN-PT substrates.16,28,29 However, the electric-field-induced magnetization and MR switchings in these “straintronic” devices are volatile or require the assistance of magnetic fields, limiting their application potentials. The difficulty to integrate a complex spintronic device into electrically controlled multiferroic heterostructures has precluded the demonstration of nonvolatile manipulation of magnetization, and thus, purely electric-field-controlled nonvolatile magnetization rotations and MR states have not been

1. INTRODUCTION Nonvolatile magnetization manipulation in multiferroic and magnetoelectric materials using purely electric fields instead of magnetic fields or large currents is highly desired in ultralowpower spintronic devices, such as field sensors, magnetic random access memories, and spin logics.1−6 Recently, there have been a lot of efforts aiming at electric-field control of magnetizations in multiferroic heterostructures through different mechanisms, including field effect,7−9 exchange coupling,10,11 exchange bias,12 and strain effect.13−15 Among various mechanisms, the strain-mediated coupling in magnetostrictive/piezoelectric structures has been proved to be a very energy efficient way to achieve nonvolatile magnetization manipulation.16−18 Most importantly, the strain effect can manipulate not only the amplitude but also the orientation of the magnetization in multiferroic heterostructures,19−22 which is extremely promising for the design of spintronic devices.23,24 For example, benefiting from the electrically controlled lattice strain in 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT)25−27 and BaTiO322 ferroelectric single crystals with large piezoelectric coefficients, nonvolatile magnetization rotations in ferromagnetic (FM) films have been demonstrated in La0.6Sr0.4MnO3/PMN-PT,19 Co/PMN-PT,20,21 Ni/BaTiO3,22 etc. multiferroic heterostructures. On this basis, it has been predicted by theory that high densities and low-power logic or © 2018 American Chemical Society

Received: March 6, 2018 Accepted: June 6, 2018 Published: June 6, 2018 21390

DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Research Article

ACS Applied Materials & Interfaces

field is turned off, a nonvolatile piezo-strain state (P+4 r ) with ) ∼ 0.09% at zero electric field can be obtained. While ε(P+4 r after a −1.5 kV/cm electric field is applied and turned off, another nonvolatile piezo-strain state (P−1.5 ) with ε(P−1.5 )∼ r r 0.02% at zero electric field is achieved. Thus, two electric-fieldcontrolled reversible and nonvolatile piezo-strain states (P−1.5 r and P+4 r ) can be obtained at zero electric field by cycling the electric fields between +4 and −1.5 kV/cm (see the red line in Figure 1b). Here, the nonvolatile piezo-strain may be related to different aspects, such as the electric-field-induced rhombohedral to orthorhombic phase transformation,32 different ferroelectric domain configurations,33,34 crystal miscut,35 and the existence of defects in the near-surface region36 (see Section 1, Supporting Information for the detailed discussion). To investigate the magnetic anisotropies of Co (Figure 1c,d) and Ni (Figure 1e,f) films manipulated via the in situ piezostrain controlled by electric fields, the detailed piezo-strain status dependencies of MEA (confirmed by the angle dependencies of the remanent magnetization ratio) were investigated using a rotating sample magneto-optic Kerr effect (rot-MOKE) technique.31 Representative normalized magnetic hysteresis loops at different strain statuses are shown in Figure S2, Supporting Information. Usually, the uniaxial anisotropy in ferromagnetic metals or alloys could be initialized by applying an in-plane magnetic field higher than the saturation field during film deposition.37 For our samples, the saturation fields of Co and Ni at room temperature are about 50 and 40 Oe, respectively (see Figure S2b,d, Supporting Information). Therefore, magnetic fields of 200 Oe in the [100] or [010] directions, higher than the saturation fields of Co and Ni, were applied during the film growth, which was sufficient to locate the MEA of both Co and Ni along the [100] (Figure 1c,e) or [010] (Figure 1d,f) directions, respectively. For Co/PMN-PT heterostructure, when the initial MEA direction is located along [100], the MEA remains in the [100] direction at P−1.5 state, and the MEA is nonvolatilely rotated r +4 90° to the [010] direction at P+4 r and Pon states, as shown in Figure 1c. This is because the positive magnetostriction effect of Co tends to align its MEA along the direction with relatively +4 strong tensile strain, which is the [010] direction at P+4 r and Pon 20 states. On the other hand, if the as-grown MEA direction of Co is along [010], the MEA directions are still along the [010] +4 −1.5 states, as direction with the tensile strains at P+4 on , Pr , and Pr shown in Figure 1d. Although for Ni/PMN-PT heterostructure, the negative magnetostriction effect of Ni tends to align its MEA perpendicular to the direction with relatively strong tensile strain.22 Thus, the MEA of Ni stays in the as+4 grown [100] MEA direction at the Pi, P−1.5 , P+4 r r , and Pon states, as shown in Figure 1e. For the Ni film with the as-grown MEA direction along [010] as shown in Figure 1f, the piezo-strain of P+4 on state is large enough to rotate the MEA of Ni to the [100] direction. Although at P+4 r state without strong enough tensile strain, the MEA of Ni rotates back to the [010] direction resulting in a volatile MEA rotation. On the basis of the different MEA rotation behaviors of Co and Ni films (Figure 1c,e), an electric-field-controlled Co/Cu/ Ni spin valve could be designed with Co as a free layer and Ni as a fixed layer. Thus, Co(15 nm)/Cu/Ni(30 nm)/PMN-PT heterostructures with Cu thicknesses of 3 and 7 nm were constructed, and the magnetic anisotropies of Co and Ni films were investigated by rot-MOKE at room temperature. The measurement setup is schematically shown in Figure 2a, with the electric field applied along the [010] direction of the PMN-

reported yet in any spin valves or magnetic tunnel junctions grown on piezoelectric materials. In this study, we designed and fabricated a purely electricfield-controlled prototype memory based on a Co/Cu/Ni/ PMN-PT spin-valve/piezoelectric heterostructure. We found that the magnetic easy axis (MEA) of the top Co free layer went through a nonvolatile 90° rotation, whereas the MEA of the bottom Ni fixed layer remained still, resulting in a variation of the MR effect in the spin valve. Interestingly, more nonvolatile MR or electroresistance (ER) states could be obtained by setting the PMN-PT at different nonvolatile piezostrain states, and multistate memories were obtained accordingly.

2. RESULTS AND DISCUSSION Figure 1 shows the magnetic properties of Co and Ni films in FM/PMN-PT (001) heterostructures, measured by a rotating

Figure 1. (a) Schematic of the rot-MOKE measurement setup on the FM films grown on PMN-PT substrates. (b) Piezo-strain (ε) vs electric-field (E) loops for PMN-PT substrate measured along the [010] direction at room temperature, from the initial state (black dashed line), between −4 and +4 kV/cm (gray line), and between −1.5 and +4 kV/cm (red line). Angular-dependent remanent magnetization ratio at room temperature at different piezo-strain states for Co (c, d) and Ni (e, f) films, with initial MEAs located along the [100] and [010] directions, respectively.

sample magneto-optic Kerr effect (rot-MOKE) technique.31 With an electric field applied along the pseudocubic [010] direction of the PMN-PT substrate, a nonvolatile tensile strain in the [010] direction is generated on the FM layer. The piezostrain (ε) versus electric-field loops for PMN-PT substrate measured along the [010] direction are shown in Figure 1b. The initial piezo-strain state (Pi) of PMN-PT substrate is defined as zero strain state (ε(Pi) = 0), and the piezo-strain with an applied electric field of +4 kV/cm (defined as P+4 on state) is about ε(P+4 on ) = 0.13%. After the +4 kV/cm electric 21391

DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Schematic of the rot-MOKE measurement setup on the spin-valve/PMN-PT heterostructure. Angular-dependent remanent magnetization ratio of Co and Ni layers in Co/Cu/Ni/PMN-PT heterostructures with 3 and 7 nm Cu layer, measured at room temperature at P−1.5 r (b, c) and P+4 r (d, e) states, respectively. The insets schematically show the MEA directions in Co and Ni layers.

Figure 3. (a) Schematic of the resistance measurement structure on a Co/Cu/Ni/PMN-PT heterostructure with a 7 nm Cu layer. (b) Crosssectional HRTEM image of the Co/Cu/Ni/PMN-PT heterostructure, the inset shows the SAED pattern. (c) Magnetic hysteresis loops for Co and Ni layers at Pi state, measured along [100] at 80 K. Magnetic field-dependent resistance and MR at 80 K of an as-grown Co/Cu/Ni spin valve on PMN-PT with magnetic field along the [100] (d) and [010] (e) directions. (f) Temperature-dependent MRAP ratio measured with magnetic field along the [100] direction.

PT substrate, and the as-grown MEAs of the Co and Ni films are located along the [100] direction, as shown in Figure S3, Supporting Information. At P−1.5 state, no matter the Cu r

thickness is thin (3 nm, Figure 2b) or thick (7 nm, Figure 2c), the MEAs of Co and Ni films are located along the [100] direction, consistent with the results of Co and Ni single layers 21392

DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Research Article

ACS Applied Materials & Interfaces

Figure 4. Magnetic hysteresis loops for Co and Ni layers at P−1.5 (a) and P+4 r r (b) states, respectively, measured along [100] at 80 K. Magnetic field(c, d) and P+4 dependent resistance and MR of the spin valve along the [100] and [010] directions, measured at 80 K at P−1.5 r r (e, f) states, respectively. The olive and blue lines refer to magnetic fields sweeping from −1000 to +1000 Oe and vice versa, respectively, whereas the black square spot lines refer to the minor loops. The insets schematically show the magnetization directions of Co and Ni layers. Magnetic fielddependent ER and ΔMR of the spin valve along the [100] (g) and [010] (h) directions.

the as-grown Co and Ni films were aligned to be along the [100] direction of PMN-PT, and a current-perpendicular-toplane (CPP) geometry was used, as shown in Figure 3a. Figure 3b shows the cross-sectional high-resolution transmission electron microscopy (HRTEM) image and the selected area electron diffraction (SAED) pattern of an as-grown heterostructure device. The interfaces between different layers are smooth. In the SAED pattern, the (200) and (110) reflections of the PMN-PT substrate are marked by white arrows, and the ring corresponds to the reflections of the polycrystalline Ni, Cu, and Co films. The normalized magnetic hysteresis loops along the [100] direction for the Co and Ni films in Pi state at 80 K are shown in Figure 3c. Here, the remanent strain states were induced at room temperature, and then the samples were cooled to 80 K for magnetic and transport measurements. Consistent with the results at room temperature (see Figure S3, Supporting Information), the MEA directions of the Co and Ni films at Pi state stay in the [100] direction at 80 K, and the coercive fields of Co and Ni films are about ±250 and ±510 Oe, respectively. It is noted that the resistance of our spin valve is higher than that of the previous reported all-metal spin valves (10−8 to >10 Ω),39−44 which may be owing to the polycrystalline nature of our sample,45 the contribution from

in Figure 1c,e. At P+4 r state, for the spin valve with 3 nm Cu layer, the MEA of Ni film stays in the [100] direction (Figure 2d), consistent with the result shown in Figure 1e; whereas the MEA of Co film also stays in the [100] direction, different from the MEA rotation in single Co film in Figure 1c. This may be attributed to the strong coupling between the Co and Ni layers, which aligns the MEA of Co to be parallel to that of Ni.38 On the contrary, as shown in Figure 2e, for the spin valve with a thicker (7 nm) Cu layer at P+4 r state, the MEA of Ni stays in [100] but that of Co rotates to [010], consistent with the results of Co and Ni single layers in Figure 1c,e. The different responses of the spin valve with 7 nm Cu layer should be owing to the relatively weak magnetic coupling between Co and Ni layers. It implies that the strain effect dominates the magnetic anisotropies for the spin valve with 7 nm Cu layer. Therefore, the Co/Cu/Ni/PMN-PT heterostructure with 7 nm Cu could be adopted as an electric-field-controlled magnetoresistive prototype device. More importantly, the rotation of the MEA of the Co free layer is nonvolatile and reversible. To demonstrate the functionality of this straintronic device, the electric-field-controlled MR properties of a cross-strippatterned Co(15 nm)/Cu(7 nm)/Ni(30 nm) spin valve grown on PMN-PT heterostructure were studied. Here, the MEAs of 21393

DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a) ε vs E loops for PMN-PT substrate measured at room temperature, along the [010] orientation, between +4 kV/cm and different negative electric fields. Normalized magnetic hysteresis loops along [100] for Co (b) and Ni (c) at 80 K. (d) ε-Dependent ER and ΔMR of the spin valve at zero magnetic field. (e) Magnetic field-dependent resistance of the spin valve along the [100] direction, measured at 80 K at various nonvolatile piezo-strain states. 48 from 500 Oe at P−1.5 state to 320 Oe at P+4 At P−1.5 r r state. r state, when the applied magnetic fields are parallel to the MEAs of Co and Ni, i.e., along the [100] direction, the magnetic field-dependent resistance loops show typical spinvalve MR curvature, as shown in Figure 4c. Sharp resistance switchings are obtained when the magnetization of Co (at around ±220 Oe) and Ni (at around ±500 Oe) FM layers flips in magnetic fields, consistent with the coercive field values (around ±230 Oe for Co and ±500 Oe for Ni) in Figure 4a. Although for the applied magnetic fields (along the [010] direction, Figure 4d) perpendicular to the MEAs of Co and Ni at P−1.5 state, the R−H curves are smooth, and the minor R−H r loop exhibits volatile resistance changes. The MR behaviors observed at P−1.5 state (Figure 4c,d) are similar to those at Pi r state (Figure 3d,e). On the other hand, at P+4 state, for r magnetic fields (along the [100] direction, Figure 4e) perpendicular to the MEA of Co and parallel to the MEA of Ni, the MR experiences a steep switching near the coercive field of Ni at approximately ±340 Oe, consistent with the magnetization behaviors of Co and Ni in Figure 4b. On the contrary, for magnetic fields (along the [010] direction, Figure 4f) parallel to the MEA of Co and perpendicular to the MEA of Ni, the R−H curves at P+4 r state show sharp resistance switchings around the coercive field of Co (approximately ±210 Oe). Comparing the resistances at different piezo-strain states, for example, Figure 4c,e, it is found that the resistances of the spin valve at the magnetic P state in higher magnetic fields are different, which is due to a magnetization-rotation-irrelevant resistance change induced by pure strain variation,49 namely strain-resistance (SR) effect. To distinguish the contributions of the SR and the magnetization rotation on the resistance switchings, we calculated the resistance changes from P−1.5 to r P+4 r states controlled by electric fields as

current-in-plane resistance,44 and/or the possible weak oxidization of the 3d metals in the spin valve. Defining θ as the relative angle between the magnetization directions of Co and Ni layers in the Co/Cu/Ni spin valve, the θ-dependent CPP resistance Rθ is given by46 Rθ = 1 − a × cos2(θ /2) RAP

(1)

where RAP is the CPP resistance of the spin valve when the two FM layers are magnetically antiparallel (defined as AP state) and a is a constant. The CPP resistance of a spin valve increases as θ increases. Typical spin-valve MR effect was obtained when applying magnetic fields parallel to the MEAs of the Co and Ni layers as shown in Figure 3d at 80 K. Sharp resistance switchings from magnetically parallel (P) state to AP state and from AP to P state are obtained when the magnetization of Co (at around ±240 Oe) and Ni (at around ±520 Oe) FM layers flips in magnetic fields, consistent with the coercive field values in Figure 3c. The MR effect is mainly related to the spin-valve effect, and the AMR contribution is small (see Section 4, Supporting Information). The minor R− H curve was also displayed in Figure 3d, indicating the nonvolatility of P and AP states in zero magnetic field. For magnetic fields (along [010]) perpendicular to the MEAs of Co and Ni, the magnetization orientations of both layers rotate continuously, leading to the continuous changes of θ and R. Thus, the R−H curves are smooth, as shown in Figure 3e. Figure 3f shows the temperature-dependent MR ratio (MR = (R(H) − R(0))/R(0)) at AP state (MRAP) of an as-grown spin valve, measured along the [100] direction. As expected, the MRAP increases with decreasing temperature.47 Figure 4a,b shows the normalized magnetic hysteresis loops at P−1.5 and P+4 r r states for the magnetic fields along the [100] direction at 80 K, respectively. Similar to the results at Pi state (Figure 3c), the MEA directions of Co and Ni films at P−1.5 r state stay in the [100] direction at 80 K, whereas at P+4 r state, the MEA of Co rotates to the [010] direction and the MEA of Ni stays in the [100] direction with its coercive field decreasing

ER(H ) = (R+(H ) − R−(H ))/RP−(0)

(2)

where R−(H) and R+(H) denote the resistance at P−1.5 and P+4 r r − states, respectively, and Rp (0) denotes the resistance at 21394

DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Research Article

ACS Applied Materials & Interfaces magnetic P state in zero magnetic field at P−1.5 state, as shown r in Figure 4g,h. In higher magnetic fields (region I of Figure 4g and region IV of Figure 4h), because the magnetizations of the Co and Ni layers are both parallel to the magnetic field direction, the MR effect could be eliminated, and the corresponding ER (approximately −7.35%) should represent the pure strain effect-induced resistance switching. In principle, the strain-resistance SR ratio is basically magnetic field independent, as marked by the horizontal solid line in Figure 4g,h. Thus, the resistance switching induced by magnetization rotations can be calculated as ΔMR(H ) = ER(H ) − SR

In addition, at least six MR or ER states at different nonvolatile piezo-strain states have been achieved, demonstrating its potential in multistate memories.

4. EXPERIMENTAL SECTION 4.1. Device Fabrication. The Co and Ni films and Co/Cu/Ni multilayer films were deposited on (001)-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single-crystal substrates (5 mm × 5 mm × 0.5 mm) using an ion beam sputtering technique at room temperature, and a 200 Oe magnetic field along the [100] or [010] direction was adopted during the films preparation. The spin valve with a size of top Co electrode of 10 μm × 10 μm was patterned into a cross-strip geometry by a three-step UV photolithography and Ar ion milling process, whereas the size of the bottom Ni electrode is 100 μm × 500 μm. The top and bottom leads were separated by a 100 nm thick SiO2 film with a resistance about 1010 Ω. Here, four different spin-valve samples were patterned on one PMN-PT (001) substrate, and then the PMN-PT was cut into 1.2 mm × 1.2 mm × 0.5 mm piece with one spin valve. The side polarization electrodes for PMNPT polarization were then grown with a separation of 1.2 mm (as shown in Figure 3a), and the maximum applied voltage was 480 V (4 kV/cm). 4.2. Characterization. The transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern of the as-grown spin-valve/PMN-PT heterostructure cross section were characterized with a high-resolution transmission electron microscope (HRTEM, JEOL JEM-ARM200F microscope operating at 200 keV, equipped with a spherical aberration corrector on the condenser lens system). The in-plane piezo-strain was monitored by a Radiant Technologies Precision Premier II tester with a laser interferometric vibrometer. The temperature-dependent dielectric constant of PMN-PT crystal was measured at 1 kHz using an LCR meter (Agilent 4294A). rot-MOKE measurements were performed with a NanoMOKE II system (Durham Magneto Optics Ltd., U.K.). The electrical transport properties were examined in a physical property measurement system (PPMS, EverCool II, Quantum Design). The out-of-plane resistance of the spin valve was measured using a four-point probe method with a bias current of 100 μA, and the voltage drop on the spin valve ranged from about 20 to 40 mV, depending on the resistance of the spin valve.

(3)

The ΔMR value in zero magnetic field (ΔMR(0)) represents the resistance change caused by magnetization rotation controlled via purely electric fields without external magnetic fields. From the black square spot line in Figure 4g calculated from the minor R−H loops at P−1.5 and P+4 r r states, we can see that the ΔMR(0) values are approximately +0.03 and −0.03% for the θ changes from 0 to 90° and from 180 to 90°, respectively. This clearly indicates that the purely electric-fieldinduced nonvolatile 90° magnetization rotation of Co layer causes approximately 0.03% resistance change in the spin valve, and accordingly the magnetization rotation could be read out by resistance measurement. In region II of Figure 4g, the ΔMR value in the green line is approximately −0.07%, denoting the θ variation from 180 to 0°; whereas in regions III (Figure 4g) and V (Figure 4h), the ΔMR is about +0.07%, corresponding to the θ switching from 0 to 180°. We could even set the PMN-PT into more different nonvolatile piezo-strain states and achieve multiple electricfield-controlled MR effects. Figure 5a shows the ε−E loops for PMN-PT substrate by cycling electric fields from +4 kV/cm to various electric fields (−1.5, −1.4, −1.3, −1.2, −1.1, and 0 kV/ cm) along the [010] direction, and six nonvolatile piezo-strain states (marked as A−F) at zero electric field were obtained accordingly. With increasing tensile strain in the [010] direction, the MEA of Co gradually rotates from the [100] (the as-grown MEA) to [010] direction, as shown in Figure 5b, whereas the MEA of Ni layer remains along [100], but its coercive field decreases with increasing piezo-strain, as depicted in Figure 5c. The corresponding R−H curves measured at different nonvolatile piezo-strain states for another spin-valve sample with the same structure are shown in Figure 5e, and the calculated ER (the resistance changes from A state to different nonvolatile piezo-strain states) and ΔMR in zero magnetic field at different nonvolatile piezo-strain states are shown in Figure 5d. It is clearly shown that the amplitudes of both ER and ΔMR increase monotonously with increasing piezo-strain, and six different MR or ER states were obtained, corresponding to the six nonvolatile piezo-strain states A−F and demonstrating the potential of the straintronics in multistate memories.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03761. Discussions of the possible origins for the nonvolatile piezo-strain states in PMN-PT, normalized magnetic hysteresis loops of Co and Ni films, as-grown MEAs of the Co and Ni films in a spin valve, and the CPP resistance of Co and Ni films are included (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.G.L.). *E-mail: [email protected] (Y.W.Y.). ORCID

Yuewei Yin: 0000-0003-0965-4951 Xiaoguang Li: 0000-0003-4016-4483

3. CONCLUSIONS In summary, we have designed and fabricated Co/Cu/Ni/ PMN-PT spin-valve/piezoelectric straintronic devices and obtained the electric-field-controlled nonvolatile magnetization rotations in Co free layer and the corresponding MR changes in the spin valve. The nonvolatile and reversible magnetization rotation and MR manipulation by purely electric fields without the assistance of magnetic fields show the significance of straintronics in the electric-field-controlled spintronic devices.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China and National Key Research and Development Program of China (2016YFA0300103 and 2015CB921201), 21395

DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Research Article

ACS Applied Materials & Interfaces

(17) Taniyama, T. Electric-field control of magnetism via strain transfer across ferromagnetic/ferroelectric interfaces. J. Phys. Condens. Matter. 2015, 27, No. 504001. (18) Heidler, J.; Piamonteze, C.; Chopdekar, R. V.; Uribe-Laverde, M. A.; Alberca, A.; Buzzi, M.; Uldry, A.; Delley, B.; Bernhard, C.; Nolting, F. Manipulating magnetism in La0.7Sr0.3MnO3 via piezostrain. Phys. Rev. B 2015, 91, No. 024406. (19) Zhao, W. B.; Zhang, D. L.; Meng, D. C.; Huang, W. C.; Feng, L.; Hou, C. M.; Lu, Y. L.; Yin, Y. W.; Li, X. G. Electric-field-controlled nonvolatile magnetic switching and resistive change in La0.6Sr0.4MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) heterostructure at room temperature. Appl. Phys. Lett. 2016, 109, No. 263502. (20) Yang, S. W.; Peng, R. C.; Jiang, T.; Liu, Y. K.; Feng, L.; Wang, J. J.; Chen, L. Q.; Li, X. G.; Nan, C. W. Non-volatile 180 degrees magnetization reversal by an electric field in multiferroic heterostructures. Adv. Mater. 2014, 26, 7091−7095. (21) Biswas, A. K.; Ahmad, H.; Atulasimha, J.; Bandyopadhyay, S. Experimental Demonstration of Complete 180 degrees Reversal of Magnetization in Isolated Co Nanomagnets on a PMN-PT Substrate with Voltage Generated Strain. Nano Lett. 2017, 17, 3478−3484. (22) Ghidini, M.; Pellicelli, R.; Prieto, J. L.; Moya, X.; Soussi, J.; Briscoe, J.; Dunn, S.; Mathur, N. D. Non-volatile electrically-driven repeatable magnetization reversal with no applied magnetic field. Nat. Commun. 2013, 4, No. 1453. (23) Hu, J. M.; Li, Z.; Chen, L. Q.; Nan, C. W. High-density magnetoresistive random access memory operating at ultralow voltage at room temperature. Nat. Commun. 2011, 2, No. 553. (24) Hu, J. M.; Chen, L. Q.; Nan, C. W. Multiferroic Heterostructures Integrating Ferroelectric and Magnetic Materials. Adv. Mater. 2016, 28, 15−39. (25) Shrout, T. R.; Chang, Z. P.; Kim, N.; Markgraf, S. Dielectric behavior of single crystals near the (1−x)Pb(Mg1/3Nb2/3)O3-(x)PbTiO3 morphotropic phase boundary. Ferroelectr., Lett. Sect. 1990, 12, 63−69. (26) Pertsev, N. A.; Kohlstedt, H. Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers on ferroelectric substrates. Nanotechnology 2010, 21, No. 475202. (27) Zhou, W.; Xiong, Y.; Zhang, Z.; Wang, D.; Tan, W.; Cao, Q.; Qian, Z.; Du, Y. Multilevel Resistance Switching Memory in La2/3Ba1/3MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) Heterostructure by Combined Straintronics-Spintronics. ACS Appl. Mater. Interfaces 2016, 8, 5424−5431. (28) Li, P. S.; Chen, A. T.; Li, D.; Zhao, Y. G.; Zhang, S.; Yang, L. F.; Liu, Y.; Zhu, M. H.; Zhang, H. Y.; Han, X. F. Electric field manipulation of magnetization rotation and tunneling magnetoresistance of magnetic tunnel junctions at room temperature. Adv. Mater. 2014, 26, 4320−4325. (29) Pertsev, N. A. Magnetic tunnel junction on a magnetostrictive substrate: An ultrasensitive magnetic-field sensor. Appl. Phys. Lett. 2016, 109, No. 132406. (30) Lei, N.; Devolder, T.; Agnus, G.; Aubert, P.; Daniel, L.; Kim, J. V.; Zhao, W.; Trypiniotis, T.; Cowburn, R. P.; Chappert, C.; Ravelosona, D.; Lecoeur, P. Strain-controlled magnetic domain wall propagation in hybrid piezoelectric/ferromagnetic structures. Nat. Commun. 2013, 4, No. 1378. (31) Mattheis, R.; Quednau, G. Determination of the anisotropy field strength in ultra-thin magnetic films using Longitudinal MOKE and a rotating field: the ROTMOKE method. J. Magn. Magn. Mater. 1999, 205, 143−150. (32) Wang, Z.; Wang, Y.; Ge, W.; Li, J.; Viehland, D. Volatile and nonvolatile magnetic easy-axis rotation in epitaxial ferromagnetic thin films on ferroelectric single crystal substrates. Appl. Phys. Lett. 2013, 103, No. 132909. (33) Zhang, S.; Zhao, Y.; Xiao, X.; Wu, Y.; Rizwan, S.; Yang, L.; Li, P.; Wang, J.; Zhu, M.; Zhang, H.; Jin, X.; Han, X. Giant electrical modulation of magnetization in Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3(011) heterostructure. Sci. Rep. 2014, 4, No. 3727.

and this work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.

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REFERENCES

(1) Eerenstein, W.; Mathur, N. D.; Scott, J. F. Multiferroic and magnetoelectric materials. Nature 2006, 442, 759−765. (2) Hu, J.-M.; Duan, C.-G.; Nan, C.-W.; Chen, L.-Q. Understanding and designing magnetoelectric heterostructures guided by computation: progresses, remaining questions, and perspectives. npj Comput. Mater. 2017, 3, No. 18. (3) Huang, W. C.; Yang, S. W.; Li, X. G. Multiferroic heterostructures and tunneling junctions. J. Materiomics 2015, 1, 263−284. (4) Ramesh, R.; Spaldin, N. A. Multiferroics: progress and prospects in thin films. Nat. Mater. 2007, 6, 21−29. (5) Lu, C. L.; Hu, W. J.; Tian, Y. F.; Wu, T. Multiferroic oxide thin films and heterostructures. Appl. Phys. Rev. 2015, 2, No. 021304. (6) Huang, W. C.; Fang, Y.-W.; Yin, Y.; Tian, B. B.; Zhao, W. B.; Hou, C. M.; Ma, C.; Li, Q.; Tsymbal, E. Y.; Duan, C.-G.; Li, X. G. Solid-State Synapse Based on Magnetoelectrically Coupled Memristor. ACS Appl. Mater. Interfaces 2018, 10, 5649−5656. (7) Cherifi, R. O.; Ivanovskaya, V.; Phillips, L. C.; Zobelli, A.; Infante, I. C.; Jacquet, E.; Garcia, V.; Fusil, S.; Briddon, P. R.; Guiblin, N.; Mougin, A.; Unal, A. A.; Kronast, F.; Valencia, S.; Dkhil, B.; Barthelemy, A.; Bibes, M. Electric-field control of magnetic order above room temperature. Nat. Mater. 2014, 13, 345−351. (8) Zhu, Q. X.; Yang, M. M.; Zheng, M.; Zheng, R. K.; Guo, L. J.; Wang, Y.; Zhang, J. X.; Li, X. M.; Luo, H. S.; Li, X. G. Ultrahigh Tunability of Room Temperature Electronic Transport and Ferromagnetism in Dilute Magnetic Semiconductor and PMN-PT Single-Crystal-Based Field Effect Transistors via Electric Charge Mediation. Adv. Funct. Mater. 2015, 25, 1111−1119. (9) Lourembam, J.; Wu, J. C.; Ding, J. F.; Lin, W. N.; Wu, T. Electric field tuning of phase separation in manganite thin films. Phys. Rev. B 2014, 89, No. 014425. (10) Heron, J. T.; Bosse, J. L.; He, Q.; Gao, Y.; Trassin, M.; Ye, L.; Clarkson, J. D.; Wang, C.; Liu, J.; Salahuddin, S.; Ralph, D. C.; Schlom, D. G.; Iniguez, J.; Huey, B. D.; Ramesh, R. Deterministic switching of ferromagnetism at room temperature using an electric field. Nature 2014, 516, 370−373. (11) Saenrang, W.; Davidson, B. A.; Maccherozzi, F.; Podkaminer, J. P.; Irwin, J.; Johnson, R. D.; Freeland, J. W.; Iniguez, J.; Schad, J. L.; Reierson, K.; Frederick, J. C.; Vaz, C. A. F.; Howald, L.; Kim, T. H.; Ryu, S.; Veenendaal, M. V.; Radaelli, P. G.; Dhesi, S. S.; Rzchowski, M. S.; Eom, C. B. Deterministic and robust room-temperature exchange coupling in monodomain multiferroic BiFeO3 heterostructures. Nat. Commun. 2017, 8, No. 1583. (12) Wu, S. M.; Cybart, S. A.; Yu, P.; Rossell, M. D.; Zhang, J. X.; Ramesh, R.; Dynes, R. C. Reversible electric control of exchange bias in a multiferroic field-effect device. Nat. Mater. 2010, 9, 756−761. (13) Buzzi, M.; Chopdekar, R. V.; Hockel, J. L.; Bur, A.; Wu, T.; Pilet, N.; Warnicke, P.; Carman, G. P.; Heyderman, L. J.; Nolting, F. Single domain spin manipulation by electric fields in strain coupled artificial multiferroic nanostructures. Phys. Rev. Lett. 2013, 111, No. 027204. (14) Amrillah, T.; Bitla, Y.; Shin, K.; Yang, T. N.; Hsieh, Y.-H.; Chiou, Y.-Y.; Liu, H.-J.; Do, T. H.; Su, D.; Chen, Y.-C.; et al. Flexible multiferroic bulk heterojunction with giant magnetoelectric coupling via van der Waals epitaxy. ACS Nano 2017, 11, 6122−6130. (15) Liu, Y.; Zhao, Y. G.; Li, P. S.; Zhang, S.; Li, D. L.; Wu, H.; Chen, A. T.; Xu, Y.; Han, X. F.; Li, S. Y.; Lin, D.; Luo, H. S. ElectricField Control of Magnetism in Co40Fe40B20/(1-x)Pb(Mg1/3Nb2/3)O3xPbTiO3 Multiferroic Heterostructures with Different Ferroelectric Phases. ACS Appl. Mater. Interfaces 2016, 8, 3784−3791. (16) Xiong, R. X.; Fang, B.; Li, G.; Xiao, Y. G.; Tang, M. H.; Li, Z. Electric-field tuning of ferromagnetic resonance in CoFeB/MgO magnetic tunnel junction on a piezoelectric PMN-PT substrate. Appl. Phys. Lett. 2017, 111, No. 062401. 21396

DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397

Research Article

ACS Applied Materials & Interfaces (34) Zheng, M.; Zheng, R. K. Electric-Field-Tunable Ferroelastic Control of Nonvolatile Resistivity and Ferromagnetic Switching in Multiferroic La0.67Ca0.33MnO3/[PbMg1/3Nb2/3O3]0.7[PbTiO3]0.3 Heterostructures. Phys. Rev. Appl. 2016, 5, No. 044002. (35) Bilani-Zeneli, O.; Rata, A. D.; Herklotz, A.; Mieth, O.; Eng, L. M.; Schultz, L.; Biegalski, M. D.; Christen, H. M.; Dörr, K. SrTiO3 on piezoelectric PMN-PT(001) for application of variable strain. J. Appl. Phys. 2008, 104, No. 054108. (36) Levin, A. A.; Pommrich, A. I.; Weißbach, T.; Meyer, D. C.; Bilani-Zeneli, O. Reversible tuning of lattice strain in epitaxial SrTiO3/La0.7Sr0.3MnO3 thin films by converse piezoelectric effect of 0.72Pb(Mg1/3Nb2/3)O3−0.28PbTiO3 substrate. J. Appl. Phys. 2008, 103, No. 054102. (37) O’handley, R. C. Modern Magnetic Materials: Principles and Applications; Wiley: New York, 2000. (38) Unguris, J.; Celotta, R. J.; Pierce, D. T. Observation of two different oscillation periods in the exchange coupling of Fe/Cr/ Fe(100). Phys. Rev. Lett. 1991, 67, 140−143. (39) Jedema, F. J.; Filip, A. T.; Van Wees, B. J. Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve. Nature 2001, 410, 345. (40) Patel, M.; Fujimoto, T.; Gu, E.; Daboo, C.; Bland, J. A. C. Novel magnetoresistance behavior in single trilayer spin valves. J. Appl. Phys. 1994, 75, 6528−6530. (41) Urazhdin, S.; Loloee, R.; Pratt, W. P. Noncollinear spin transport in magnetic multilayers. Phys. Rev. B 2005, 71, No. 100401. (42) Childress, J. R.; Carey, M. J.; Maat, S.; Smith, N.; Fontana, R. E.; Druist, D.; Carey, K.; Katine, J. A.; Robertson, N.; Boone, T. D.; Alex, M.; Moore, J.; Tsang, C. H. All-Metal Current-Perpendicular-toPlane Giant Magnetoresistance Sensors for Narrow-Track Magnetic Recording. IEEE Trans. Magn. 2008, 44, 90−94. (43) Luo, L. Q.; Kabir, M.; Dao, N.; Kittiwatanakul, S.; Cyberey, M.; Wolf, S. A.; Stan, M.; Lu, J. Spin-torque oscillation in large size nanomagnet with perpendicular magnetic fields. J. Magn. Magn. Mater. 2017, 432, 356−361. (44) Lee, S.-F.; Yang, Q.; Holody, P.; Loloee, R.; Hetherington, J. H.; Mahmood, S.; Ikegami, B.; Vigen, K.; Henry, L. L.; Schroeder, P. A.; Pratt, W. P.; Bass, J. Current-perpendicular and current-parallel giant magnetoresistances in Co/Ag multilayers. Phy. Rev. B 1995, 52, 15426−15441. (45) Reiss, G.; Vancea, J.; Hoffmann, H. Grain-boundary resistance in polycrystalline metals. Phys. Rev. Lett. 1986, 56, 2100−2103. (46) Dauguet, P.; Gandit, P.; Chaussy, J.; Lee, S. F.; Fert, A.; Holody, P. Angular dependence of the perpendicular giant magnetoresistance of multilayers. Phys. Rev. B 1996, 54, 1083−1087. (47) Kimura, T.; Sato, T.; Otani, Y. Temperature evolution of spin relaxation in a NiFe/Cu lateral spin valve. Phys. Rev. Lett. 2008, 100, No. 066602. (48) Xiang, J. S.; Ye, J.; Yang, Y. L.; Xie, Y.; Li, W.; Chen, Z. Y. The manipulation of magnetic coercive field and orientation of magnetic anisotropy via electric fields. J. Phys. D: Appl. Phys. 2016, 49, No. 325002. (49) Kuczynski, G. C. Effect of elastic Strain on the electrical resistance of metals. Phys. Rev. 1954, 94, 61−64.

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DOI: 10.1021/acsami.8b03761 ACS Appl. Mater. Interfaces 2018, 10, 21390−21397