Multilayers of Ba0.5Sr0.5TiO3 (BSTO) and BaFe12O19 (BaM), with tunable permeability and permittivity are attractive systems for radio frequency and microwave applications. We have grown multilayers of BSTO and BaM using magnetron sputtering on Al2O3 substrates. Film growth conditions such as sputtering parameters were optimized to obtain high quality multilayers. X-ray diffraction established that both BSTO and BaM were formed and cross-sectional SEM studies showed sharp interfaces between BSTO and BaM layers. Magnetization showed a large coercivity (similar to 2000 Oe) consistent with the hexaferrite component. The hysteresis loops also revealed the distinct influence of magnetocrystalline and shape anisotropies at different temperatures

Coffey, K. R.

Dudney, N. J.

Frey, N. A.

Heindl, R.

Srikanth, H.

Srinath, S.

English

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2005 Growth and characterization of sputtered BSTO/BaM multilayers S. Srinath N. A. Frey R. Heindl H. Srikanth K. R. Coffey University of Central Florida See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Article; Proceedings Paper is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Srinath, S.; Frey, N. A.; Heindl, R.; Srikanth, H.; Coffey, K. R.; and Dudney, N. J., "Growth and characterization of sputtered BSTO/BaM multilayers" (2005). Faculty Bibliography 2000s. 5694. https://stars.library.ucf.edu/facultybib2000/5694 Authors S. Srinath, N. A. Frey, R. Heindl, H. Srikanth, K. R. Coffey, and N. J. Dudney This article; proceedings paper is available at STARS: https://stars.library.ucf.edu/facultybib2000/5694 J. Appl. Phys. 97, 10J115 (2005); https://doi.org/10.1063/1.1853874 97, 10J115© 2005 American Institute of Physics.Growth and characterization of sputteredBSTO∕BaM multilayersCite as: J. Appl. Phys. 97, 10J115 (2005); https://doi.org/10.1063/1.1853874Published Online: 11 May 2005S. Srinath, N. A. Frey, R. Heindl, H. Srikanth, K. R. Coffey, and N. J. DudneyARTICLES YOU MAY BE INTERESTED INMultiferroic magnetoelectric composites: Historical perspective, status, and future directionsJournal of Applied Physics 103, 031101 (2008); https://doi.org/10.1063/1.2836410Growth and characterization of BSTO/hexaferrite composite thin filmsJournal of Applied Physics 93, 7999 (2003); https://doi.org/10.1063/1.1543129Growth and characterization of sputtered BSTO/BaM multilayersS. Srinath, N. A. Frey, R. Heindl, and H. Srikantha!Department of Physics, University of South Florida, Tampa, Florida 33620K. R. CoffeyAMPAC, University of Central Florida, Orlando, Florida 33620N. J. DudneyCondensed Matter Sciences Division, Oak Ridge National Lab, Oak Ridge, Tennessee 37831sPresented on 9 November 2004; published online 11 May 2005dMultilayers of Ba0.5Sr0.5TiO3 sBSTOd and BaFe12O19 sBaMd, with tunable permeability andpermittivity are attractive systems for radio frequency and microwave applications. We have grownmultilayers of BSTO and BaM using magnetron sputtering on Al2O3 substrates. Film growthconditions such as sputtering parameters were optimized to obtain high quality multilayers. X-raydiffraction established that both BSTO and BaM were formed and cross-sectional SEM studiesshowed sharp interfaces between BSTO and BaM layers. Magnetization showed a large coercivitys,2000 Oed consistent with the hexaferrite component. The hysteresis loops also revealed thedistinct influence of magnetocrystalline and shape anisotropies at different temperatures. © 2005American Institute of Physics. fDOI: 10.1063/1.1853874gINTRODUCTIONMagneto-dielectric composites and multilayered materi-als have been studied over the years as attractive systems fortunable applications where the individual dielectric constantsand permeabilities of the components could be tuned by ex-ternally applied electrical and magnetic bias. Recently, re-newed interest in systems classified as ferroelectromagnetsor multiferroics has opened new avenues to investigate thesubtle coupling between the ferroelectric and magnetic com-ponents in these materials.1,2 The fundamental interest inmultiferroics is due to the coexistence of both ferroelectricand ferromagnetic ordering in the same system. This presentsa new property of reverse tunability. In other words, the per-meability can respond to electrical bias and the dielectricconstant tuned by a magnetic field. This discovery has fur-ther highlighted the promise of magnetoelectric materials ingeneral for frequency agile, tunable applications in rf andmicrowave devices.Most of the multiferroics known at present areferroelectric–antiferromagnetic systems. This results in aconspicuous absence of response to magnetic fields withoutspontaneous magnetization. In contrast, ferromagnetic mate-rials mostly are centrosymmetric without any observablespontaneous polarization rendering them ineffective for mul-tiferroics applications. As an alternative, it is prudent to fab-ricate composite multilayer structures of ferroelectric–ferromagnetic compounds using different growth techniqueswith growth parameters optimized based on thickness, mi-crostructure, grain size and shape. While ferrite–ferroelectricmaterials have been investigated mainly with inverse spinelclass of ferrites,3 there has been relatively little effort in de-velopment of composites in the technologically importantclass of hexagonal ferrites. Hexaferrites exhibit better mag-netic response in the GHz region4 compared to spinel ferritesbecause of their higher anisotropy and larger ferromagneticresonance sFMRd frequencies and are thus useful in micro-wave applications. Moreover, the sizable coercivitys,2000 Oed in hexaferrites can also be utilized to self-biasnonreciprocal rf devices that require magnetic fields such asphase shifters, circulators, etc.5With the above desirable characteristics in mind, wehave chosen to grow multilayers of BaFe12O19 sBaMd andBaSr0.5Ti0.5O3 sBSTOd, and optimize the microstructure andmagnetic properties. BaFe12O19 typically has a high perme-ability s.100d and BaSr0.5Ti0.5O3 has a large dielectric con-stant s.100d. These values can be controlled somewhat bythe growth conditions like annealing, texturing, grain size,etc. The application potential of both the above materials iswell known. BSTO has the potential to be used in thin filmcapacitors, tunable filters,6 and antennas. Even though, indi-vidually the properties of BSTO and BaM materials are quitewell known,7–13 not many reports14–16 are available on com-posite layer growth of these important materials. In this pa-per we report our successfully optimized growth conditionsneeded for the growth of quality multilayers of BaM andBSTO. To our knowledge, this is the first such report onBSTO/BaM multilayered films.In the past we reported on the growth conditions andproperties of cosputtered BSTO/BAM films s2 micronsthickd.14 Here the effect of growth and microstructure on themagnetic properties is studied for the multilayers grown onalumina substrates. A key result is that the coupling of theferroelectric and magnetic layers induces a change in thebulk magnetization as compared to a pure BaM.EXPERIMENTTrilayers of BSTO and BaM are deposited from 3-in.ceramic sputter targets sSTMC, Ohiod on polycrystalline AladAuthor to whom correspondence should be addressed. Electronic mail:sharihar@cas.usf.eduJOURNAL OF APPLIED PHYSICS 97, 10J115 s2005d0021-8979/2005/97~10!/10J115/3/$22.50 © 2005 American Institute of Physics97, 10J115-1substrates using a magnetron sputtering system in the BaM/BSTO/BaM structure. To study the effect of magnetoelasticcoupling in multilayers, we have also grown the pure films ofBSTO and BaM for comparative analysis under identicaldeposition conditions. The films were rf sputter deposited atroom temperature in an argon pressure of 4 mT with argongas flow of 20 sccm. The rf power used was 70 W. Theinduced target bias voltage for BaM and BSTO was302±3 V and 196±2 V, respectively. The as-deposited filmssmultilayers and also the pure filmsd were amorphous andwere subsequently annealed at 1000 °C in the presence of2–3 psi oxygen pressure to get the desired crystallinephases. Films were also deposited in the microstrip structureso as to measure the rf properties. The films were structurallycharacterized by XRD, SEM, AFM and the magnetic char-acterization was done using a commercial Physical PropertyMeasurement System sPPMSd from Quantum Design.RESULTS AND DISCUSSIONFigure 1 shows the representative XRD pattern for anannealed BSTO/BaM multilayer on alumina. The peaks cor-responding to Al2O3, BSTO, and BaM have been indexedand the analysis indicates presence of both BSTO and BaMphases. No evidence of impurity phases were found fromXRD measurements. However, some interdiffusion at the in-terfaces of the multilayer structure cannot be ruled out andXRD will not be able to detect impurities that are presentless than 5% by volume. In Fig. 2 we have presented thecross-sectional scanning electron microscope sSEMd imagefor one of the multilayer samples swith overall thickness of3 micronsd. A multilayer structure with well-defined and dis-tinctly visible layers from a coarse grain structure point ofview is evident even after annealing. SEM and AFM imagessnot presented hered show the elongated platelet grains typi-cal of BaM and the average grain size of the BaM particles isroughly of the order of 1.2 mm30.3 mm. The shape and sizeof the grains depends on the preparation conditions whichdetermines the magnetic properties of the films. It is reportedthat the addition of BSTO promotes the grain size and de-creases the coercivity.15 The grain size of the BaM particlesfor these multilayer films is very large for recording applica-tions as one needs to have smaller grains on the order of100 Å for maximum recording density.11 But for the micro-wave properties the dielectric constant and the permeabilityseem to increase with increase in the grain size.15Magnetization measurements were performed at varioustemperatures using a PPMS equipped with a 7 T supercon-ducting magnet. The top panel of Fig. 3 shows the magneti-zation measurements of the pure BaM films grown in thesame conditions. The saturation magnetization sMSd calcu-lated from the moment semud taking into account the volumeof the films is about 420 emu/cm3 and matches well withreported values.12,13 The accuracy of the value is influencedby the error in estimating the thickness of the films. Thecoercivity sHCd is observed to be around 2000 Oe. In thebottom panel of Fig. 3 we have shown the in-plane M –Hloops measured at 10 K and 300 K for a BaM/BSTOmultilayer. The presence of large coercivity sHC,1800 Oedat 10 K is consistent with the hard magnetic nature of theBaM phase. BSTO layers and the Al2O3 substrate contributeto a diamagnetic background, which is reflected in the nega-tive slopes of the M –H curves at higher fields in the roomtemperature data. This diamagnetic background has beensubtracted in the data shown. However, at 10 K there is aparamagnetic response at high fields which is not due to thesubstrate material. This type of behavior in the M –H loopsof pure BaM films has been observed and ascribed to theuniaxial anisotropy in the hexaferrite system.13,17 The mag-FIG. 1. X-ray diffraction scan of BSTO/BaM multilayer grown on an Al2O3substrate at RT and postannealed at 1000°.FIG. 2. Cross-sectional SEM image of the BSTO/BAM multilayer showingdistinctly visible individual layers.FIG. 3. M –H loops for sid pure BaM film; siid for the postannealed BSTO/BaM multilayer grown on a room temperature Al2O3 substrate at 10 K and300 K. A diamagnetic background due to the substrate has been subtractedfrom the M –H loop data at 300 K.10J115-2 Srinath et al. J. Appl. Phys. 97, 10J115 ~2005!netization hysteresis loops are almost the same for magneticfields applied parallel and perpendicular to the substrate at10 K.The saturation magnetization of the multilayer decreasesin comparison to the pure BaM films due to the presence ofBSTO. The important observation made from these magne-tization measurements also is the decrease in coercivity ofthese multilayers. The coercivity is known to be correlatedwith the grain size, and it has been reported that coercivitydecreases with an increase in grain size.11 A decrease in HCwith increase in BSTO concentration has been reported inbulk composite samples of Z-type hexaferritesBa3sCo0.4Zn0.6d2Fe23.4O41d sRef. 15d and is attributed to thedecrease in effective magnetocrystalline anisotropy. It is pos-sible that the decrease in saturation magnetization and coer-civity in our multilayer films are a result of a similar influ-ence of BSTO layers on the grain growth in neighboringBaM layers. It has also been reported that the increasein BSTO content in the Z-type hexaferritesBa3sCo0.4Zn0.6d2Fe23.4O41d resulted in improved complexpermeability and permittivity.15 However, it is to be notedthat in their case, the amount of BSTO doped is only 1.5%.With up to 50% BSTO/BaM mixture in our compositemultilayer films, we expect a broader range of tunability inthe complex impedance spectra which would be very usefulfor rf applications. We are in the process of depositing thesefilms in a capacitor structure with Au electrodes so as tomeasure the dielectric properties and also the electrical tun-ability. Our first attempt with a Pt underlayer and an Aucounter electrode on top of the multilayer, failed because ofPt diffusion across the interface during the postannealingstage. Further research is needed to optimize good metallicunderlayers and avoid interdiffusion.CONCLUSIONSMultilayers of BSTO/BaM have been successfullygrown for the first time and characterized structurally andmagnetically. Hysteresis loops revealed the influence of mag-netocrystalline and shape anisotropies associated with theBaM grain structure. Reductions in the saturation magnetiza-tion and coercivity are observed in the multilayer sampleswhen compared to pure BaM films and ascribed to the pos-sibility of the grain growth in the BaM phase being affectedby the presence of BSTO.ACKNOWLEDGMENTSWork at USF is supported by a grant from DARPAthrough ARO Grant No. DAAD19-03-1-0277. R.H. is a NSFIGERT fellow supported from Grant No. DGE-0221681.Work at Oak Ridge National Laboratory was sponsored bythe U.S. Department of Energy, Division of Materials Sci-ence. The authors thank Dr. Pankaj Poddar for helpful dis-cussions.1T. Kimura, T. Goto, H. Shintanl, K. Ishlzaka, T. Arima, and Y. Tokura,Nature sLondond 426, 55 s2003d.2N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha, and S. W. Cheong,Nature sLondond 429, 392 s2004d.3W. J. Kim, W. Chang, S. B. Qadri, H. D. Wu, J. M. Pond, S. W. Kirchoe-fer, H. S. Newman, D. B. Chrisey, and J. S. Horwitz, Appl. Phys. A:Mater. Sci. Process. 71, 7 s2000d.4P. Lubitz and F. J. Rachford, J. Appl. Phys. 91, 7613 s2002d; C. E. Patton,IEEE Trans. Magn. 24, 2024 s1988d.5T. Nakamura and K. Hatakeyama, IEEE Trans. Magn. 36, 3415 s2000d.6I. Vendik, O. Vendik, V. Pleskachev, and M. Nikol’ski, IEEE Trans. Appl.Supercond. 13, 716 s2003d.7P. Padmini, T. R. Taylor, M. J. Lefevre, A. S. Nagra, R. A. York, and J. S.Speck, Appl. Phys. Lett. 75, 3186 s1999d.8W. Chang, J. S. Horwitz, A. C. Carter, J. M. Pond, S. W. Kirchefer, C. M.Gilmore, and D. B. Chrisey, Appl. Phys. Lett. 74, 1033 s1999d.9J. Im, O. Auciello, P. K. Baumann, S. K. Streiffer, D. Y. Kaufman, and A.R. Kraus, Appl. Phys. Lett. 76, 625 s2000d.10M. Matsuoka, Y. Hoshi, M. Naoe, and S. Yamanaka, IEEE Trans. Magn.MAG-18, 1119 s1982d.11T. L. Hylton, M. A. Parker, M. Ullah, K. R. Coffey, R. Umphress, and J.K. Howard, J. Appl. Phys. 75, 5960 s1994d.12S. R. Shinde, R. Ramesh, S. E. Lofland, S. M. Bhagat, S. B. Ogale, R. P.Sharma, and T. Venkateshan, Appl. Phys. Lett. 72, 3443 s1998d.13S. Capraro, J. P. Chatelon, H. Joisten, M. Le Berre, B. Bayard, D. Barbier,and J. J. Rousseau, J. Appl. Phys. 93, 9898 s2003d.14R. Hajndl, J. Sanders, H. Srikanth, and N. J. Dudney, J. Appl. Phys. 93,7999 s2003d.15P. Huang, L. J. Deng, J. X. Xie, D. F. Liang, and L. Chen, J. Magn. Magn.Mater. 271, 97 s2004d.16M. S. Pinho, M. L. Gregori, A. S. Carvalho, R. C. R. Nunes, and B. G.Soares, Microwave Opt. Technol. Lett. 32, 326 s2002d.17Xiaoyu Sui and Mark H. Kryder, Appl. Phys. Lett. 63, 1582 s1993d.10J115-3 Srinath et al. J. Appl. Phys. 97, 10J115 ~2005!

Growth and characterization of sputtered BSTO/BaM multilayers

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Growth and characterization of sputtered BSTO/BaM multilayers

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