This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/apl/91/8/10.1063/1.2767174.Lead zirconium titanate/nickel ferrite (PZT/NFO) composites have been produced by crystallizing and spinodally decomposing a gel in a magnetic field below the Curie temperature of NFO. The gel had been formed by spinning a sol onto a silicon substrate. The ensuing microstructure, characterized by atomic force microscopy, magnetic force microscopy, (Lorentz) transmission electron microscopy, and scanning electron microscopy, is nanoscopically periodic and, determined by the direction of magnetic annealing field, anisotropic. The wavelength of the PZT/NFO alternation, 25nm, agrees within a factor of 2 with the theoretically estimated value. The macroscopic ferromagnetic and magnetoelectric responses correspond qualitatively and semiquantitatively to the features of the nanostructure. The maximum of the field dependent magnetoelectric susceptibility equals 1.8V∕cmOe

Calzada M. L.

Manfred Wuttig

Shenqiang Ren

Van Suchetelen J.

Applied Physics Letters

English

Ren, Shenqiang

Wuttig, Manfred

KU ScholarWorks

Spinodally synthesized magnetoelectricShenqiang Ren and Manfred WuttigaDepartment of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742Received 24 May 2007; accepted 7 July 2007; published online 20 August 2007Lead zirconium titanate/nickel ferrite PZT/NFO composites have been produced by crystallizingand spinodally decomposing a gel in a magnetic field below the Curie temperature of NFO. The gelhad been formed by spinning a sol onto a silicon substrate. The ensuing microstructure,characterized by atomic force microscopy, magnetic force microscopy, Lorentz transmissionelectron microscopy, and scanning electron microscopy, is nanoscopically periodic and, determinedby the direction of magnetic annealing field, anisotropic. The wavelength of the PZT/NFOalternation, 25 nm, agrees within a factor of 2 with the theoretically estimated value. Themacroscopic ferromagnetic and magnetoelectric responses correspond qualitatively andsemiquantitatively to the features of the nanostructure. The maximum of the field dependentmagnetoelectric susceptibility equals 1.8 V/cm Oe. © 2007 American Institute of Physics.DOI: 10.1063/1.2767174Landau et al. predicted the existence of solids displayinga magnetoelectric ME response in 1960.1 In composite ma-terials, this ME effect is realized by using the concept ofproduct properties introduced by Van Suchetelen in 1972.2The recent revival of research on MEs has resulted in im-pressive progress in both natural and artificial, i.e., compos-ite, MEs.3–7 Composite MEs have been synthesized “topdown” by macroscopically combining the ferromagneticand ferroelectric components and/or by applying thinfilm techniques to produce multilayers. Examples arebulk composites of Terfenol-D/PVDF PolyvinylideneDifluoride,8,9 Fe–Ga/PMN-PT Lead magnesium niobate-Lead titanate,10 CoFe2O4/BaTiO3,11 Terfenol-D/PMN-PT12LSMO/PZT,13,14 and Metglas/PVDF laminates.15,16 Pulsedlaser films from multiphase targets can self-organize underthe influence of epitaxial strains and form regular17,18two phase ME 1-3 CFO/BTO nanocom-posites.19–22 Here, we report on a ME composite formed byspinodal decomposition. In that case, only the innate thermo-dynamic driving force self-organizes a periodic ME nano-composite. Decomposition in a magnetic field should yieldan anisotropic laminate. This mode of synthesis was envi-sioned long ago2 but not pursued until now.Stoichiometric Pb1.1Zr0.53Ti0.47O350/ NiFe2O450lead zirconate titanate PZT/nickel femite NFO thinfilms were prepared by thermal decomposition of mixedmetal polymeric precursor gels followed by spin coatingonto Au-electroded silicon wafers and baking. The polymericprecursor solution was prepared using the Pechini method asdescribed by Calzada and Milne.23 The approximately200 nm thick films were patterned, magnetoannealed belowthe Curie temperature of NFO, and rapidly cooled to roomtemperature in zero field.Structural characterization was carried out by scanningx-ray microdiffraction using a D8 DISCOVER Bruker-AXS.Thermal properties were studied by differential thermal andthermogravimetric analyses. Transmission electron micros-copy TEM, including Lorentz TEM, images and selectedarea diffraction patterns were obtained with a JEOL 4000-FXand a JEOL 2100F emission TEM. Polarization values weredetermined with a RT 66A Radiant Technologies ferroelec-tric loop tracer. A superconducting quantum interference de-vice magnetometer, vibrating sample magnetometer, andmagnetic force microscopy were used to perform magneticcharacterization. Local compositions were determined withthe use of a JEOL JSM-6500F secondary electron emissionspectrometer.X-ray diffraction measurements confirmed that the com-posite consists of perovskite and spinel. The ferroelectric andferromagnetic Curie temperatures of the composite were20 °C below the values of NFO and PZT. Hence, the chemi-cal compositions of the PZT and NFO components almostequal that of the “pure” components. This will justify the useof known properties of PZT and NFO in theoretical estima-tions. Results of microscopic characterizations are presentedin Figs. 1 and 2 displaying structural and compositional in-formation of a film annealed in fields of 0.6 T directed par-allel and perpendicularly to the plane of the film. The imagesshow how the microstructure evolved in the course of theannealing process. Initially, after a short annealing of 2 h at460 °C, the structure is dense but near amorphous, as can beseen from the electron diffraction pattern inset of Fig. 1a.At longer annealing times of 12 h at 480 °C, a striped pat-tern appears. The electron diffraction inset in Fig. 1b indi-cates that the composite is now polycrystalline, as can alsobe inferred from the ferroelectric and ferromagnetic charac-terizations to be discussed below. All microstructures are pe-riodic, as indicated by the Fourier transform insets. Thetransforms indicate the existence of decomposition waveswith a wavelength of approximately 25 nm. The same peri-odicity can be seen in the Fourier transform of a LorentzTEM pointing out that one component of the ME compositeis ferromagnetic, as also seen in magnetic force microscopyimages. Figure 1b also demonstrates that the wave vector kof the stationary decomposition wave is aligned parallel tothe annealing field Ha. The elemental composition analysesconfirm that the periodic stripes are composed of NFO andPZT, respectively see Fig. 2. This figure indicates that thechemical composition of the film alternates as it would in aspinodally decomposed solid. The power density in the upperaElectronic mail: wuttig@eng.umd.eduAPPLIED PHYSICS LETTERS 91, 083501 20070003-6951/2007/918/083501/3/$23.00 © 2007 American Institute of Physics91, 083501-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.237.46.100 On: Mon, 22 Sep 2014 16:48:08right points to two standing decomposition waves that aremutually shifted by 12, as expected.The images of the phase-separated film have the appear-ance of spinodal decomposition. Two kinds of compositionwaves appear as a solid decomposes in a magnetic field:24 1waves whose wavelength is affected by the ferromagneticnature of the solid, termed “parallel waves,” and 2 waveswhose wavelength is additionally affected by the magneticpoles that form upon decomposition, termed “perpendicularwaves.” The present experiment favors parallel waves whosewavelength is determined by parameters that are mostly un-available. For large undercooling, as is the case in thisexperiment,25 the wavelengths of the parallel and perpen-dicular waves approach each other. Hence, the wavelength ofperpendicular waves will be estimated since the magneticparameters controlling them are available. The ensuingwavelength t would thus read 2 /t22s /c2Ts−T+M /c2 /2K+A,24 where A is the exchange energy. Itcan be estimated by neglecting the derivative of the entropys and the gradient energy K. By approximating further tofinite differences and focusing on the observed structure,M2=0MT2 and c=1, MT being the saturation magnetiza-tion of NFO at the annealing temperature, it follows thatt28A /MT2. Inserting into this expression ANFO610−7 erg cm−1 and MT=1 emu/cm3, t12 nm. Thislength equals about one-half of the observed wavelength, 25 nm see Fig. 1b. In view of the approximationsmade, the difference between t and  is not unexpected.The macroscopic properties of the ME PZT/NFO com-posite agree qualitatively and quantitatively with the micro-structure shown in Fig. 1b. The PZT/NFO composite ismagnetically hard and soft when magnetized in the directionperpendicular and parallel to the stripe pattern. The differ-ence of the anisotropy energies in both directions, Kexp=0.2106 ergs/cm3, equals the difference of the demagne-tization energies in those directions, 12 Ms2N−N. WithMsNFO=265 emu/cm3 and N−N=2, KtMs2=0.22106 ergs/cm3=1.1 Kexp, signaling agreement.The ME coupling between the ferroelectric PZT and fer-romagnetic NFO components is elastically mediated, that is,the ME susceptibility H= PPZT/NFONFO/H.2Here, the magnetic field was applied perpendicular and par-allel to the PZT/NFO stripelike pattern structure depicted inFig. 3, lower inset. The negative magnetostriction of NFOshrinks the PZT, thereby decreasing its polarization by −8and −3 C/cm2 at 10 kOe for the two cases where H k andHk, k=2 /. The saturation is attributed to that of themagnetostriction of NFO. The expected magnetically in-duced polarization of PZT can be estimated asP,tPZT 4NFOd33PZT1/EPZT + 1/ENFO + 2/ESiO2,FIG. 1. Color online a TEM image of PZT/NFO sample annealed 2 h at460 °C. The inset shows the diffraction pattern of the as-spun amorphousstate of the composite. b TEM image of a PZT/NFO sample annealed 12 hat 480 °C, i.e., below the Curie temperature of NFO, in an in-plane mag-netic field Ha=0.6 T indicated by the arrow. The inset represents the dif-fraction pattern of the displayed structure. c Same as b but annealed in anout-of-plane magnetic field Ha=0.6 T.FIG. 2. Color online Standing compositional waves in the PZT/NFO com-posite. The noise of this compositional analysis does not permit statementsabout the exact composition of the PZT and NFO regions. However, it canbe seen that the concentration of the two groups of elements, Pb, Zr, Ti, andNi, Fe, forming the PZT and NFO components of the composite is maxi-mized in a periodic fashion. The power Fourier transform displayed at theupper right indicates the existence of two waves. The four maxima indicatetheir mutual phase shift of 12.083501-2 S. Ren and M. Wuttig Appl. Phys. Lett. 91, 083501 2007 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.237.46.100 On: Mon, 22 Sep 2014 16:48:08P,tPZT  14 EPZT + ENFO + 2ESiO2NFOd13PZT.NFO is the magnetostriction constant of NFO. Note that Pis independent of the wavelength. However, the magnitudeof the wavelength does affect the total charge that is accu-mulated at the electrodes. With EPZT=60 GPa, d33PZT=250 pC/N, d13PZT=125 pC/N, NFO=−33 ppm, ENFO=200 GPa,26 and ESiO2 =60 GPa,27 P,tPZT=−6 C/cm2 andP,tPZT=−3.9 C/cm2. Both expected polarizations agreewithin 30% with the experimentally observed values seeFig. 3. Using the maximum value of the derivativedNFO/dH,28 the theoretical estimates for the ME sus-ceptibilities become H,tPZT/NFO=2.37 V/cm Oe andH,tPZT/NFO=2.4 V/cm Oe. Differentiating theexperimentally observed PPZT= fH yields maximaHPZT/NFO=1.5 V/cm Oe and HPZT/NFO=1.8 V/cm Oe, see upper inset of Fig. 3, corresponding ap-proximately to the above estimates and substantially higherthan achieved by ceramic technologies.29 In general, the MEsusceptibility is a tensor.30 Only two of its components arepresented here.In summary, this letter demonstrates that MEs can besynthesized by spinodal decomposition. The observed wave-length of the decomposition wave agrees within a factor of 2with the estimated value and the experimentally determinedmacroscopic magnetic and ME characteristics of the PZT/NFO composite agree reasonably well with those estimatedtheoretically for the underlying nanostructure. It appears thatthe method described in this letter can serve to synthesize anumber of functional composites.The support of the National Science Foundation, GrantNo. DMR0354740, and the Office of Naval Research, Con-tract No. N000140110761 is acknowledged. The authors arealso grateful to I. Lloyd for her hospitality, S. Lofland, andthe UMD-MRSEC program, DMR-00-0520471 for permis-sion to use his/its equipment.1L. D. Landau, E. M. Lifshitz, and A. L. King, Am. J. Phys. 29, 6471961.2J. Van Suchetelen, Philips Res. Rep. 27, 28 1972.3R. Ramesh and N. A. Spaldin, Nat. Mater. 6, 21 2007.4M. Fiebig, J. Phys. D 38, R1 2005.5N. A. Spaldin and M. Fiebig, Science 309, 391 2005.6F. A. Smolenski and I. E. Chupis, Sov. Phys. Usp. 25, 475 1982.7W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature London 442, 7592006.8K. Mori and M. Wuttig, Appl. Phys. Lett. 81, 100 2002.9S. X. Dong, J.-F. Li, and D. Viehland, Appl. Phys. Lett. 83, 2265 2003.10S. X. Dong, J. Y. Zhai, N. G. Wang, F. M. Bai, J.-F. Li, D. Viehland, andT. A. Lograsso, Appl. Phys. Lett. 87, 222504 2005.11R. V. Chopdekar and Y. Suzuki, Appl. Phys. Lett. 89, 182506 2006.12S. Stein, M. Wuttig, D. Viehland, and E. Quandt, J. Appl. Phys. 97,10Q301 2005.13M. A. Zurbuchen, T. Wu, S. Saha, J. 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Wuttig, L. Salamanca-Riba,D. G. Schlom, and R. Ramesh, Appl. Phys. Lett. 85, 2035 2004.22J. van den Boomgaard and R. A. J. Born, J. Mater. Sci. 13, 1538 1978.23M. L. Calzada and S. J. Milne, J. Mater. Sci. Lett. 12, 1221 1993.24J. W. Cahn, J. Appl. Phys. 34, 358 1963.25M. Takahashi, J. R. Guimarae, and M. E. Fine, J. Am. Ceram. Soc. 54,291 1971.26A. H. Morrison and K. Haneda, J. Appl. Phys. 52, 2496 1981.27M. T. Kim, Thin Solid Films 283, 12 1996.28R. M. Bozorth, E. F. Tilden, and A. J. Williams, Phys. Rev. 99, 17881955.29G. Srinivasan, E. T. Rasmussen, and R. Hayes, Phys. Rev. B 67, 0144182003.30H. Grimmer, Acta Crystallogr., Sect. A: Found. Crystallogr. A48, 2661992.FIG. 3. Color online ME characteristic and ME susceptibilities upperinset of a PZT/NFO film composite annealed for 12 h at 480 °C in anin-plane magnetic field Ha=0.6 T. red line and stars: HaH k; black lineand open circles: HaH k. The inset on the lower right indicates the place-ment of the electrodes083501-3 S. Ren and M. Wuttig Appl. Phys. Lett. 91, 083501 2007 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:129.237.46.100 On: Mon, 22 Sep 2014 16:48:08

Spinodally synthesized magnetoelectric

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Spinodally synthesized magnetoelectric

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