Imprint Control of Nonvolatile Shape Memory with AsymmetricFerroelectric MultilayersWoo-Hee Kim,† Jong Yeog Son,*,‡ Young-Han Shin,*,§ and Hyun Myung Jang∥

†Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94305, United States‡Department of Applied Physics, Kyung Hee University, 1732 Deogyeong-daero, Yongin 446-701, Republic of Korea§Department of Physics and EHSRC, University of Ulsan, Ulsan 680-749, Republic of Korea∥Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro,Nam-Gu, Pohang 790-784, Republic of Korea

*S Supporting Information

Ferroelectric materials exhibit multifunctional propertiessuch as piezoelectricity, nonvolatile charge, and electro-

optic functions.1−5 Accordingly, ferroelectric materials have beenan interesting subject of intensive scientific investigation,especially for ferroelectric nonvolatile random access memories(FeRAM) and actuators.5−10 Basically, ferroelectric memoriesutilize the remanent polarization originating from the employedferroelectric materials themselves, enabling suitable operation bycharge detection.11,12 Once electric polarization is changed by anexternal electric field, piezoelectric strain is generated under thebiased states.13,14 Even though the applied electric field isswitched off, two different polarized states still remain to beobserved and thereby can be utilized for nonvolatile digitalinformation. In contrast, piezoelectric strains cannot possessthese nonvolatile characteristics with the two observable differentstates because the generated piezoelectric strains are returned tothe same positions without an applied electric field.5 In otherwords, a symmetric piezoelectric butterfly curve is expected withregard to the piezoelectric strain axis, indicating that no shapememory window is available.Meanwhile, an imprint, which is a defect (e.g., fatigue) in a

ferroelectric material, is known to generate a shift in theferroelectric hysteresis loop, leading to a failure of storedferroelectric information.15 Therefore, the symmetric piezo-electric butterfly curve can be shifted to be asymmetric inpiezoelectric thin films by virtue of an electrical imprintfield.16−18 It is interesting to note that the shifted hysteresisloop with an asymmetric piezoelectric strain provides a potentialoperating source for nonvolatile piezoelectric shape memory.5

The origin of this imprint effect in ferroelectric materials is theinternal electric field induced by electrode configurations,trapped charge, and external processes such as optical andthermal processes.18−22 In terms of realizing the use ofpiezoelectric strains in ferroelectric materials, Morita et al.demonstrated that control of the imprint electric field, which isinduced by treatment with a high electrical field of 3.5 kV/mm at150 °C, could be used to produce viable piezoelectric shapememory devices.5 Motivated by these intriguing possibilities ininnovative memory applications, we believe that the artificialmanipulation of imprint effects is essential to achieve highlyefficient nonvolatile shape memories.23

Herein, we propose that the ferroelectric multilayer structurecomposed of two different ferroelectric thin films enables

artificial control of the imprint effect in ferroelectric hysteresis.More specifically, we show that the shift in ferroelectric hysteresiscould be effectively modulated by adjusting the thickness of eachferroelectric layer or the thickness ratio in the ferroelectricmultilayered schemes. On the basis of a theoretical first-principles calculation, we estimated characteristics of thedepolarization field arising from an internal electric field in aPt/PbTiO3 (PTO) BaTiO3 (BTO)/Pt multilayer structure. Toexperimentally achieve adjustable piezoelectric shape memories,we fabricated well-defined and epitaxially multilayered PTO/BTO capacitors with the different thickness ratios, and weobserved the asymmetric ferroelectric and piezoelectric hyste-resis behaviors with different ratios of PTO and BTO layers.We start by schematically describing our desired shape

memory scheme. To begin with, ferroelectric multilayeredcapacitors consisting of PTO and BTO are selected becauseboth the ferroelectric materials possess the simple crystalstructure of ABO3-type perovskite made up of the same B siteatom of Ti and two different A site atoms of Pb and Ba with thesame atomic valences, as shown in Figure 1a. In the multilayeredferroelectric structures, we speculate that internal electric fieldsoriginating from their asymmetric structures also exist. As Figure1b shows, the induced internal electrical fields, however, candecrease with an increase in the ratio of PTO to BTO.Accordingly, the imprinted ferroelectric hysteresis loops switchback and forth from zero electric field to the magnitudes of theinternal electric fields in accordance with the ratio of multilayersdescribed in Figure 1c. The imprinted ferroelectric hysteresisloops then respond to the external strains of the ferroelectricmultilayers. Figure 1d shows the strain curves of the imprintedferroelectric multilayers as a function of the external electricfields, where the nonvolatile shape memory window emergesbased on a shift in the strain curve. This result suggests that thecontrol and optimization of the internal electric fields, which areassociated with the thickness ratio of the multilayer ferroelectrics,is essential to obtain the maximized nonvolatile shape memorywindow. Besides that, it is worth noting that the PTO and BTOmultilayer structures currently under investigation can bereplaced by another pair of ferroelectric materials such as

Received: August 12, 2014Revised: November 19, 2014

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BiFeO3 and BiAlO3 for the purpose of shape memoryapplications.24,25

Next, we carried out first-principles calculations to betterunderstand the feasibility of the proposed imprint characteristicsat the atomic level in these multilayer structures of Pt/PTO/BTO/Pt. Specifically, the estimations were performed by a local-density approximation with a plane wave basis set and projectoraugmented wave method using the Vienna Ab-initio SimulationPackage.26−28 The projector augmented wave (PAW) methodwas utilized with a cutoff energy of 500 eV.29 The exchangecorrelation functional was given by a generalized gradientapproximation (GGA) of the Perdew−Burke−Ernzerhof versionrevised for solids (PBEsol).30 The Brillouin zone was divided by a8× 8× 1Monkhorst−Packmesh.31 A vacuum region of about 15Å was placed with the periodic boundary condition along zdirection. First, we confirmed the macroscopic-averaged electro-static potentials of two symmetric Pt5/TiO2-(PbO-TiO2)5/Pt5and Pt5/TiO2-(BaO-TiO2)5/Pt5 capacitors as a reference inFigure 2a.32,33 For both the PTO system and the BTO system, 5unit cells were utilized along the normal direction, and the topand bottom were terminated with 2 unit cells of Pt. The in-planelattice parameters were set to 3.92 Å in a tetragonal structure, andthe atoms were fully relaxed until the force on each atom was lessthan 0.001 eV/Å. Additionally, the ratio of the asymmetricferroelectric capacitor was set to 1:1 with Pt5/TiO2-(BaO-TiO2)5-(PbO-TiO2)5/Pt5 in Figure 2b. For the PTO−BTOmixed system, each 5 unit cells of BTO and PTO were employed

in conjunction with the same number of Pt layers. Its in-planelattice parameter was also set to 3.92 Å. Here, the depolarizationfields were calculated from the average slope of the macroscopic-averaged electrostatic potential. In the case of symmetriccapacitors, the depolarization fields of PTO and BTO are −5.0and −4.7 MV/cm, respectively, and their depolarization fieldsshould be symmetric with respect to the polarization direction(Figure 2a,b). Considering the symmetry of depolarization fieldsin the single layer capacitors, the imprint effect does not seem tooccur therein. On the contrary, depolarization fields in theasymmetric two-layer Pt5/TiO2-(BaO-TiO2)5-(PbO-TiO2)5/Pt5capacitor are found to be asymmetric with respect to itspolarization direction and thereby induce an internal electric fieldresulting in a shift in the hysteresis loop (Figure 2c,d).We fabricated ferroelectric multilayer capacitors similar to the

ferroelectric multilayer capacitors employed for the first-principles calculations. These capacitors consisted of ferro-electric PTO and BTO layers with top and bottom SrRuO3(SRO) electrodes on SrTiO3 (STO) substrates. Although Ptelectrodes were utilized as a simple electrode in first-principlesdensity-functional calculations, Pt cannot provide epitaxialferroelectric thin films. In addition, the SRO electrode has

Figure 1. Schematic illustrations of ferroelectric multilayer capacitors,internal electric fields, imprinted ferroelectric hysteresis, and shapememory window. (a) Multilayered capacitors composed of PTO andBTO ferroelectrics with ratios of 1:1 and m:1, respectively. (b) Internalelectric field plots versus ratio of PTO and BTO ferroelectric layers (1:1,2:1, and 3:1). (c) Polarization curves versus electric field for the PTOand BTO ferroelectric layers with ratios of 1:1, 2:1 and 3:1, whereimprinted ferroelectric hysteresis loops are generated by internal electricfields. (d) Strain curves versus electric field for the PTO and BTOferroelectric layers with ratios of 1:1, 2:1, and 3:1. Once the externalelectric fields are switched off, a nonvolatile piezoelectric shape memorywindow arises from the shifts in strain curves.

Figure 2. Macroscopic-averaged electrostatic potentials (black curves)and average depolarization fields (blue arrows) obtained from first-principles calculations for (a) positively polarized PTO, (b) positivelypolarized BTO, (c) positively polarized BTO/PTO, and (d) negativelypolarized BTO/PTO capacitors with Pt electrodes. Depolarizationfields of multilayer TiO2-(BaO-TiO2)5-(PbO-TiO2)5 are not symmetricwith respect to the polarization direction, which induces a negativeinternal field to shift its hysteresis loop in the positive field direction.

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been known to exhibit a high endurance against polarizationfatigue, allowing for the improved electrode−ferroelectricinterfaces in comparison with the elemental metal electrodessuch as Pt.34,35 For these reasons, we adopted epitaxial SROelectrodes grown by pulsed laser deposition (PLD) on the STOsubstrate, which is the base material for the epitaxial growth ofupper PTO and BTO thin films. The same SRO is chosen as thetop electrode material as well; the other purpose of this adoptionis to remove the imprint effect that can originate from differenttop and bottom electrode configurations.36,37 First, the detailedstructural properties were characterized by X-ray diffraction(XRD) and transmission electron microscope (TEM) analysesfor multilayer structures of PTO/BTO/SRO/STO.Figure 3a,b shows the theta−two theta (θ−2θ) XRD patterns

for the multilayered structure of PTO (50 nm)/BTO (50 nm)/

SRO (50 nm)/STO. These data indicate that the PTO/BTO/SRO films are highly (001)-oriented without any secondaryphases along the (001) orientation of the STO substrate. The in-plane heteroepitaxial relationship between the STO substrateand the multilayer PTO/BTO/SRO was further investigated viaa phi (Φ)-scan shown in Figure 3c. From the relative positions ofall in-plane peaks in the Φ-scan, it is clear that the threeconsecutive thin films of PTO, BTO, and SRO were grownepitaxially on the STO substrate, and the unit cells of PTO, BTO,and SRO were placed in parallel with the unit cell of the STOsubstrate along the in-plane direction. Figure 3d shows the cross-sectional TEM image of the multilayer structure of PTO (65nm)/BTO (5 nm)/SRO (50 nm)/STO, also indicating theepitaxial growth relationship constrained on the STO substrate.We then prepared epitaxially multilayered SRO/PTO/BTO/

SRO capacitors with three different thickness ratios of PTO toBTO (5:1, 9:1, and 13:1). Figure 4a shows the ferroelectric

polarization versus electric field (P−E) curves of the three SRO/PTO/BTO/SRO capacitors at a measurement frequency of 10kHz. All the P−E hysteresis loops of the films exhibit a well-defined saturation with remnant polarizations (2Pr) over 33 μC/cm2 and asymmetry in the direction of the positive electric fieldsdue to the imprint effect, as expected from the theoreticalcalculations. These shifts in hysteresis loops indicate theexistence of internal electric fields in the PTO/BTO multilayerstructures. Estimations of the internal electric fields in themultilayered structures are shown in Figure 4b. Here, we can seea linear decrease in the internal fields with respect to theincreasing thickness ratio. The highest value of the internalelectric field was observed for the 5:1 sample.To further confirm the feasibility of these devices for

nonvolatile shape memory applications, we investigated appliedelectric field dependence of the shape memory displacementbased on piezoelectric force microscopy (PFM) measurements.Here, the shape memory displacement is defined as thedifference between the displacements for the two stable statesat the point of zero voltage. Figure 4c shows the asymmetric

Figure 3. Structural properties of the PTO/BTO/SRO/STOmultilayerby (a−c) XRD and (d) TEM analyses. (a, b) out of plane θ−2θ patternsand (c) in-plane Φ-scan patterns of the PTO (50 nm)/BTO (50 nm)/SRO (50 nm)/STO structure. Herein, the spectra of Φ-scans wereobtained by keeping the Bragg angle at (102), (101), (111), and (110)for PTO, BTO, SRO, and STO, respectively. (d) Cross-sectional TEMimage of the PTO (65 nm)/BTO (5 nm)/SRO (50 nm)/STOstructure.

Figure 4. Experimental characterization of epitaxial multilayer SRO/PTO/BTO/SRO capacitors with thickness ratios of 5:1, 9:1, and 13:1,respectively. (a) Ferroelectric hysteresis loops of the three PTO/BTOcapacitors at a measurement frequency of 10 kHz. (b) Internal electricfields of the three PTO/BTO capacitors as a function of thickness ratio.(c) Piezoelectric butterfly curves of the three PTO/BTO capacitors as afunction of electric field.

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butterfly curves, and notable shifts in the piezoelectric strains areclearly observed therein, which reflect the influence of internalelectric fields in the ferroelectric multilayered structures. Theseferroelectric multilayer structures possess two different strainstates depending on the polarity of the applied electric fields priorto removal of the voltage. It is interesting to note that thewindows in nonvolatile shape memories indicated at zero voltagewere found to be about 1.7%, 0.6%, and 0.2% for thickness ratiosof 5:1, 9:1, and 13:1, respectively. These strain values aresignificantly higher than previous publications on piezoelectricstrains.38,39 Therefore, it is worth noting that the controllablestrain gaps formed by the effective modulation of thickness ratiosin PTO/BTO multilayer structures make these structuressuitable for practical applications of nonvolatile shape memory.In summary, we proposed controllable piezoelectric shape

memories composed of PTO/BTO multilayered structures,enabling artificial modulation for imprint characteristics inducedby internal electric fields. On the basis of our first-principlescalculations, we confirmed that the imprint characteristics couldbe triggered by the internal electric fields originating fromasymmetric depolarization fields in the Pt/PTO/BTO/Ptmultilayer structures. Then, PTO/BTO/SRO multilayers wereprepared on a base STO substrate by PLD, and structural XRDand TEM analyses indicated the epitaxial growth relationship ofthe multilayered films on the (001) STO. To experimentallyclarify the imprint effects, we fabricated SRO/PTO/BTO/SROcapacitors with three different thickness ratios of PTO to BTO(5:1, 9:1, and 13:1), and thereby shape memory windows withcontrollable strain gaps were demonstrated by the effectivemodulation of the thickness ratio in PTO/BTO multilayerstructures. This new insight potentially provides opportunitiesfor implementation of emerging piezoelectric shape memories.

■ ASSOCIATED CONTENT*S Supporting Information(1) Experimental methods, including the preparation of epitaxialmultilayer thin films and capacitors, characterization, and first-principles calculations. (2) Surface morphology of the epitaxiallymultilayered thin films composed of PTO(65 nm)/BTO(5nm)/SRO(50 nm) on a STO substrate. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: jyson@khu.ac.kr (J. Y. Son).*E-mail: hoponpop@ulsan.ac.kr (Y.-H. Shin).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korean government (Nos.2009-0093818, 2012-014007, and 2012R1A2A2A01046451)and by a grant from the Kyung Hee University in 2013 (KHU-20130691).

■ REFERENCES(1) Scott, J. F.; De Araujo, C. A. P. Science 1989, 246, 1400−1405.(2) Kathan-Galipeau, K.; Wu, P.; Li, Y.; Chen, L.-Q.; Soukiassian, A.;Xi, X.; Schlom, D. G.; Bonnell, D. A. ACS Nano 2010, 5, 640−646.(3) Bonnell, D. A. Science 2013, 339, 401−402.(4) Ramesh, R.; Aggarwal, S.; Auciello, O.Mater. Sci. Eng., R 2001, 32,191−236.

(5) Morita, T.; Kadota, Y.; Hosaka, H. Appl. Phys. Lett. 2007, 90,082909.(6) Scott, J. F. Science 2007, 315, 954−959.(7) Son, J. Y.; Lee, G.; Jo, M.-H.; Kim, H.; Jang, H. M.; Shin, Y.-H. J.Am. Chem. Soc. 2009, 131, 8386−8387.(8) Son, J. Y.; Shin, Y.-H.; Ryu, S.; Kim, H.; Jang, H. M. J. Am. Chem.Soc. 2009, 131, 14676−14678.(9) Son, J. Y.; Ryu, S.; Park, Y.-C.; Lim, Y.-T.; Shin, Y.-S.; Shin, Y.-H.;Jang, H. M. ACS Nano 2010, 4, 7315−7320.(10) Son, J. Y.; Lee, J.-H.; Song, S.; Shin, Y.-H.; Jang, H. M. ACS Nano2013, 7, 5522−5529.(11) Cohen, R. E. Nature 1992, 358, 136−138.(12) Catalan, G.; Seidel, J.; Ramesh, R.; Scott, J. F. Rev. Mod. Phys.2012, 84, 119−156.(13) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.;Homma, T.; Nagaya, T.; Nakamura, M. Nature 2004, 432, 84−87.(14) Lee, J.-W.; Park, C.-S.; Jo, J.-H.; Kim, H.-E. Appl. Phys. Lett. 2007,91, 072903.(15) Warren, W. L.; Dimos, D.; Pike, G. E.; Tuttle, B. A.; Raymond, M.V.; Ramesh, R.; Evans, J. T., Jr. Appl. Phys. Lett. 1995, 67, 866−868.(16) Kim, S.-H.; Lee, D.-S.; Hwang, C. S.; Kim, D.-J.; Kingon, A. I.Appl. Phys. Lett. 2000, 77, 3036−3038.(17) Schorn, P.; Ellerkmann, U.; Bolten, D.; Boettger, U.; Waser, R.Integr. Ferroelectr. 2003, 53, 361−369.(18) Morita, T.; Cho, Y. Appl. Phys. Lett. 2006, 88, 112908.(19) Lee, E. G.; Wouters, D. J.; Willems, G.; Maes, H. E. Appl. Phys.Lett. 1996, 69, 1223−1225.(20) Kim, S.-H.; Lee, D.-S.; Hwang, C. S.; Kim, D.-J.; Kingon, A. I.Appl. Phys. Lett. 2000, 77, 3036−3038.(21) Ye, Z.; Tang,M. H.; Zhou, Y. C.; Zheng, X. J.; Cheng, C. P.; Hu, Z.S.; Hu, H. P. Appl. Phys. Lett. 2007, 90, 042902.(22) Mokry, P.; Tagantsev, A. K.; Fousek, J. Phys. Rev. B: Condens.Matter Mater. Phys. 2007, 75, 094110.(23) Kadota, Y.; Morita, T. Smart Mater. Struct. 2012, 21, 045002.(24)Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu,B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.;Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Science 2003, 299,1719−1722.(25) Zylberberg, J.; A. Belik, A.; Takayama-Muromachi, E.; Ye, Z.-G.Chem. Mater. 2007, 19, 6385−6390.(26) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 48, 13115.(27) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169.(28) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758.(29) Blochl, P. E. Phys. Rev. B 1994, 50, 17953.(30) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.;Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Phys. Rev. Lett.2008, 100, 136406.(31) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188−5192.(32) Balderschi, A.; Baroni, S.; Resta, R. Phys. Rev. Lett. 1988, 61, 734.(33) Sai, N.; Kolpak, A. M.; Rappe, A. M. Phys. Rev. B: Condens. MatterMater. Phys. 2005, 72, 020101.(34) Eom, C. B.; Van Dover, R. B.; Phillips, J. M.; Werder, D. J.;Marshall, J. H.; Chen, C. H.; Cava, R. J.; Fleming, R. M.; Fork, D. K.Appl. Phys. Lett. 1993, 63, 2570−2572.(35) Do, D.-H.; Evans, P. G.; Isaacs, E. D.; Kim, D. M.; Eom, C. B.;Dufresne, E. M. Nat. Mater. 2004, 3, 365−369.(36) Friessnegg, T.; Aggarwal, S.; Ramesh, R.; Nielsen, B.; Poindexter,E. H.; Keeble, D. J. Appl. Phys. Lett. 2000, 77, 127−129.(37) Wenbin, W.; Wang, Y.; Pang, G. K. H.; Wong, K. H.; Choy, C. L.Appl. Phys. Lett. 2004, 85, 1583−1585.(38) Shan-Tao, Z.; Alain Brice, K.; Emil, A.; Helmut, E.; Jurgen, R.Appl. Phys. Lett. 2007, 91, 112906.(39) Zhou, Z. H.; Gao, X. S.; John, W.; Fujihara, K.; Ramakrishna, S.;Nagarajan, V. Appl. Phys. Lett. 2007, 90, 052902.

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