Author name used in this publication: H. L. W. Chand2002-2003 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Choy, CL

Liu, J

Nan, CW

Wan, JG

Wang, GH

Journal of Applied Physics

PolyU Institutional Repository

JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 12 15 JUNE 2003Giant magnetoelectric effect of a hybrid of magnetostrictiveand piezoelectric compositesJ. G. Wana) and J.-M. LiuDepartment of Physics and National Laboratory of Solid State Microstructures, Nanjing University,Nanjing 210093, China and Department of Applied Physics and Materials Research Centre,The Hong Kong Polytechnic University, Kowloon, Hong Kong, ChinaH. L. W. Chand and C. L. ChoyDepartment of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University,Kowloon, Hong Kong, ChinaG. H. WangDepartment of Physics and National Laboratory of Solid State Microstructures, Nanjing University,Nanjing 210093, ChinaC. W. NanDepartment of Materials Science and Engineering, Tsinghua University, Beijing 100084, China~Received 18 December 2002; accepted 2 April 2003!A magnetoelectric ~ME! hybrid structure is constructed by an efficient coupling betweenmagnetostrictive Terfenol-D (Tb0.27-0.30Dy0.73-0.70Fe1.90-1.95)/epoxy and piezoelectric lead–zirconate–titanate @Pb(Zr0.52Ti0.48)O3#/epoxy composites. Significant ME effect produced by thepiezoelectric k31 coupling with the longitudinal vibration of the magnetostrictive component over awide range of frequency is observed. It is revealed that the ME effect can be enhanced by applyingan optimized magnetic bias. A magnetoelectric voltage coefficient as high as 8700 mV/cm Oe isrecorded at the resonance frequency of 59.2 kHz for the structure with an optimized magnetic biasof 0.7 kOe. Our measurement confirms that the eddy current loss remains negligibly small at anoperating frequency as high as ;200 kHz, predicting very promising applications of the present MEstructure. © 2003 American Institute of Physics. @DOI: 10.1063/1.1577404#I. INTRODUCTIONMagnetoelectric ~ME! effect is defined as a variation ofdielectric polarization in a system as a response to an appliedmagnetic field, or an induced magnetization by an externalelectric field.1 With this effect, an effective conversion be-tween electric energy and magnetic energy becomes pos-sible. The ME materials thus represent one kind of smartstructures which have received continuous attentions as po-tential sensors for magnetic field measurements and trans-ducers for magnetoelectric conversion.2,3 Since the ME ef-fect was observed in antiferromagnetic Cr2O3 compound,4quite a lot of ME materials and structures have been discov-ered and developed. Unfortunately, for almost all ME com-pounds ever discovered, the ME effect, characterized by themagnetoelectric voltage coefficient, e.g., aE5(]E/]H)Twith E the probed electric field and H the applied magneticfield, and T the temperature, is too small (aE;20 mV/cm Oe! to be utilized for practical purposes.4–6However, a much higher ME effect has been identified inspecially designed composites in which the magnetostrictivephase is combined with the piezoelectric one so that an effi-cient magnetomechanical–piezoelectric coupling betweenthe two phases is achieved.7 Since then, much effort has beenmade along this line. For example, Van den Boomgaarda!Author to whom correspondence should be addressed; electronic mail:wanjg@nju.edu.cn9910021-8979/2003/93(12)/9916/4/$20.00et al.8,9 synthesized a bulk composite of CFO ~cobalt ferrite!with BaTiO3, but the ME yield was ;50 times lower thanthat predicted theoretically. The CFO–lead–zirconate–titanate ~PZT! multilayered structures prepared by Harsheet al.10 showed an aE;75 mV/cm Oe, which was still muchlower than the predicted value. Recently, strong ME effectwas demonstrated in a nickel ferrite ~NFO!/PZT multilayeredstructure prepared by Srinivasan et al. using the tape castingtechnique.11,12 The reported aE value was ;1500 mV/cm Oe, which was in excellent agreement with the theoreticalprediction.These earlier studies allow us to emphasize the essentialrole of two issues in fabricating magnetoelectric compositeswith enhanced ME effect, on one hand, the two phaseschosen must have, respectively, large magnetostrictive andpiezoelectric effects. On the other hand, an effective cou-pling between the magnetomechanical transfer and piezo-electric transfer is the key factor for an enhanced MEeffect. In 2001, Ryu et al. developed a sandwich structurestacked by giant magnetostrictive material Terfenol-D(Tb0.27-0.30Dy0.73-0.70Fe1.90-1.95) and PZT disks. The measuredlargest value of aE reached 5900 mV/cm Oe.13,14 However,the wider application of this structure is seriously limited bythe presence of high eddy current loss in Terfenol-D above afew keloherz.15 To overcome this difficulty, Terfenol-D/polymer composite is an excellent alternative because of itslower eddy current loss in the region of high frequency.166 © 2003 American Institute of Physics9917J. Appl. Phys., Vol. 93, No. 12, 15 June 2003 Wan et al.Moreover, its tailorable properties enable it to meet the spe-cial ME coupling.16–18In this article, we report a ME composite structurebased on 0-3 Terfenol-D/Epoxy composite @magneto-strictive component ~MSCP!# combined with 0-3 Pb(Zr0.52Ti0.48)O3/epoxy composite @piezoelectric component~PECP!#. The structure operates on the longitudinal vibra-tional mode and a very strong ME yield at the resonancefrequency is observed. The PZT/epoxy composite is chosenas the PECP for two reasons. One is that an effective inter-face matching between MSCP and PECP can be easilyachieved by introducing the PZT/epoxy composite. The otheris that an efficient ME coupling between the two componentsis allowed, noting that the acoustic impedance of the twocomponents can be adjusted to optimization by changing thecontents of the polymer. Figure 1 is a schematic illustrationof the ME composite structure. The arrows P and M showthe poling direction of the PECP and magnetic domain ori-entation of the MSCP, respectively. Therefore, the MSCP isactivated in the longitudinal vibrational mode. When an ex-ternal ac magnetic field is applied, the mechanical deforma-tion in the MSCP is acoustically transferred to the PECP,thereby resulting in an induced electric field due to the trans-verse piezoelectric coupling k31 .II. EXPERIMENTThe ME composite structure was prepared by using acompression technique. Terfenol-D powder ~from BaotouRare-earth Institute, China! with an average size of 100 mmand PZT-502 ceramic powder ~from PKI, USA! with an av-erage size of 60 mm were separately mixed with epoxy resin~Spurr low-viscosity embedding media. Polyscience Inc.!and precompressed in the rectangular Plexiglass molds toform the Terfenol-D/Epoxy and PZT/epoxy composites withdimensions of 20 mm320 mm310 mm ~thickness!. The vol-ume fractions of Terfenol-D and PZT in the composites are0.7 and 0.6, respectively. Subsequently, the two precom-pressed composites were placed into a compaction die withcavity dimensions of 20 mm320 mm325 mm ~depth! inturn and laminated. The laminated composite was then com-pacted under a pressure of 50 MPa. In order to align theTerfenol-D powders, compaction was carried out under astatic magnetic field of 1.5 kOe, the direction of which isparallel to that of the pressure. After that, the laminated com-FIG. 1. Schematic of the magnetoelectric composite structure. The arrows Pand M show the poling direction of piezoelectric composite and magneticdomain direction of magnetostrictive composite, respectively.posite was cured at 70 °C for 8 h and then sliced into thesamples, as shown in Fig. 1. The dimensions of theTerfenol-D composite and PZT composite in the samplewere 6.8 mm ~length!36.2 mm30.7 mm and 9.1 mm~length!36.2 mm30.7 mm, respectively. Finally, the PZT/epoxy component was covered by air-dried silver electrodesand poled in silicone oil at 90 °C and 35 kV/cm for 25 min.The piezoelectric coefficient of the PZT composite asmeasured by a piezo d33 meter ~Model ZJ-3B! is d33550pm/V. The magnetostriction coefficient l for the Terfenol-Dcomposite was measured by using a MTI-2000 optical fiberdisplacement sensor. The ME coefficient aE5(]E/]H)T isdetermined by the induced electric field generated in thePECP under a small ac magnetic field less than 5 Oe ~gen-erated by a solenoid! superposed onto a dc magnetic biasHBias of up to 2.1 kOe ~generated by a pair of permanentNdFeB magnets!. Both the ac magnetic field and magneticbias are parallel to the longitudinal direction of the MSCP.The frequency f of the ac magnetic field varies in the rangeof 0–200 kHz. The solenoid was driven by a signal generatorand power amplifier. The induced electric field was measuredwith a high input impedance circuit and oscilloscope. The acmagnetic field and flux density were, respectively, measuredby using a pick-up coil and a search coil connecting with afluxmeter.III. RESULTS AND DISCUSSIONFigure 2 shows magnetostriction response of theTerfenol-D composite as function of applied magnetic field.One can observe the changing slope and saturation strain inthe butterfly-shaped curve of the double-sided strain versusapplied magnetic field. The typical magnetostriction coeffi-cient l for the Terfenol-D composite at 1.0 kOe is ;450ppm. Figure 3 presents the flux density of the MSCP in thesample versus magnetic field frequency f at various HBias .The measured response is quite similar to the relative perme-ability mr response reported earlier.16 Given a magnetic biasHBias , the fundamental longitudinal resonance appears at f;60 kHz and the peak at ;120 kHz represents the secondharmonic resonance.Figure 4 shows the dependence of aE on f, i.e., disper-FIG. 2. Mangeostriction response of the Terfenol-D composite as functionof applied magnetic field.9918 J. Appl. Phys., Vol. 93, No. 12, 15 June 2003 Wan et al.sion of the ME effect, under different HBias . In all cases, noremarkable frequency dispersion is observed except for theresonance ranges, indicating an excellent frequency stabilityof the ME effect. At HBias50, two peaks located at f559.8 and 29.9 kHz, respectively, are observed. The corre-sponding peak values for aE are 400 and 180 mV/cm Oe,which are comparable to the values reported.8–12 The peak atf 559.8 kHz is obviously attributed to the fundamental lon-gitudinal vibration of the sample, as confirmed by the mea-sured response of the flux density in Fig. 3, whereas theweaker peak at f 529.9 kHz is caused by the double-sidedstrain response of the Terfenol-D composite versus appliedmagnetic field. In more detail, the magnetostriction of theTerfenol-D composite is symmetric with magnetization andthe double-sided strain versus applied magnetic field or ‘‘but-terfly’’ results, as shown in Fig. 2. Hence the induced strainof the MSCP shows a doubling of the applied magnetic fieldfrequency f, which leads to the piezoelectric vibration of thePECP at 2f by the magnetomechanical–electric coupling.When the applied magnetic field frequency f is 29.9 kHz, thesample vibrates at a frequency of 2 f 559.8 kHz, whichagrees exactly with the fundamental longitudinal resonanceof the sample, thus a resonance peak of aE appears. Once anonzero magnetic bias HBias is applied, this so-called fre-quency doubling of the Terfenol-D composite will be gradu-FIG. 3. Flux density of MSCP in the sample as a function of applied mag-netic field frequency f.FIG. 4. Freuqency dependence of magnetoelectric voltage coefficient aEwith different magnetic bias HBias .ally suppressed,19 and the consequent ME effect will becomeweaker. In fact, the peak of aE at f 529.9 kHz almost disap-pears when the applied magnetic bias is above 0.2 kOe inthis work.Since the magnetostriction slope of the Terfenol-D com-posite around the zero magnetic bias point is so shallow,vibration of the MSCP under an ac magnetic field is veryinefficient, which causes the ME effect to be very weak inthis region. By applying a magnetic bias HBias , however, acoperation of MSCP can be moved to steeper regions of mag-netostriction curve, eventually getting into the middle of theburst region, where much larger strains are realized for agiven ac magnetic field, hence much stronger ME effects areachieved. As shown in Fig. 4, with HBias increasing to 0.25and 0.7 kOe, the peak intensities of aE at fundamental lon-gitudinal resonance are enhanced by 1 order of magnitude,reaching up to ;5100 and ;8700 mV/cm Oe, respectively.The detailed HBias dependence of aE is given in Fig. 5. Withincreasing HBias from zero, aE increase rapidly before reach-ing a maximum at HBias;0.7 kOe, and then drops slowly. Atthe same time, the resonance frequency shifts as a functionof HBias , decreases with increasing HBias before reaching aminimum at HBias;0.7 kOe, and then rises slowly. To under-stand this magnetic bias dependence further, one notes thenegative DE effect for the MSCP,18,20 where DE5(EHBias2E0)/E0 with EHBias and E0 the Young’s modulus at a givenmagnetic bias HBias and zero bias, respectively. At the smallac magnetic field, with increasing HBias from zero, the mo-bility of the non-180° domain wall in the MSCP is enhancedand magnetically favored domains grow, which leads to thenegative DE effect ~the elastic moduli at constant magneticfield strength EH decreases!. At the so-called optimized mag-netic bias ~here it is HBias50.7 kOe!, the motion of the non-180° domain is maximum and EH reaches minimum. TheMSCP is softened enough that the magnetostrictive couplingfactor k33 is the highest, leading to the largest strain re-sponse. Assuming that the coupling between the MSCP andPECP has nothing to do with the magnetic bias, which seemsreasonable, the observed aE has a maximum at HBias50.7kOe. In fact, k33 can be calculated by17k335@12~ f rE/ f aE!2#1/2, ~1!FIG. 5. Magnetoelectric voltage coefficient aE measured at the resonanceand corresponding resonance frequency f r as functions of magnetic biasHBias .9919J. Appl. Phys., Vol. 93, No. 12, 15 June 2003 Wan et al.where f rE and f aE are the resonance frequency and antireso-nance frequency, respectively. By taking these characteristicfrequency from Fig. 3, the calculated maximum k33 at HBias50.7 kOe is 0.311, corresponding to a maximum aE58700 mV/cm Oe at 59.2 kHz, which is much higher thanthe values of previously reported magnetoelectric materialsand structures.4,10,11,13Moreover, the problem of high eddy current loss forTerfenol-D over the high frequency range has been overcomein the Terfenol-D/epoxy composite. The measurement dem-onstrates that the upper frequency limit for this ME compos-ite structure can be extended up to 100 kHz and higher, asshown in Fig. 4. According to the classical eddy currenttheory,21 the critical operating frequency f c at which seriouseddy current loss is expected can be calculated byf c52rpmrm0t2 , ~2!where r is the electric resistivity and t is the effective diam-eter of the MSCP. Taking r5390 mV m, t57 mm, and mr51.85 measured at HBias50.7 kOe, one has f c;2.2 MHz,which is ;1000 times higher than the critical frequency forbulk Terfenol-D. In fact, apart from the resonance near 60and 120 kHz, the measured relative permeability mr as afunction of f is flat enough until f ;200 kHz. This verifiesthat the eddy current loss is negligible over a wide range offrequency and the present ME composite structure can beused for high frequency applications at least up to 200 kHz.IV. CONCLUSIONIn conclusion, we have developed a ME compositestructure which combined the Terfenol-D/epoxy and PZT/epoxy composites as the MSCP and PECP, respectively. Itoperates by the longitudinal vibration of the MSCP drivingthe piezoelectric output of the PECP. Giant magnetoelectricvoltage coefficient at the fundamental longitudinal resonancemode of the MSCP with low eddy current loss has beenobserved, demonstrating a promising application potentialfor this ME composite structure. The frequency dependenceand magnetic bias dependence of the magnetoelectric coeffi-cient suggest that even higher conversion efficiency can beaccomplished if the coupling parameters, such as resonancefrequency, coupling coefficient, and acoustic impendenceetc. are further optimized.ACKNOWLEDGMENTSThis work was supported by Centre for Smart Materialsof the Hong Kong Polytechnic University, the NSFC ~GrantNo. 10021001!, and the National Key Projects for Basic Re-search of China ~Grant No. 2002CB613303! as well asLSSMS of Nanjing University.1 L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media~Pergamon, Oxford, 1960!, p. 119.2 T. H. O’Dell, Electron. Power 11, 266 ~1965!.3 L. P. M. Bracke and R. G. van Vliet, Int. J. Electron. 51, 255 ~1981!.4 V. J. Folen, G. T. Rado, and E. W. Stalder, Phys. Rev. Lett. 6, 607 ~1961!.5 S. Foner and M. Hanabusa, J. Appl. Phys. 34, 1246 ~1963!.6 H. Tsujino and K. Kohn, Solid State Commun. 83, 639 ~1992!.7 J. van Suchtelen, Philips Res. 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Giant magnetoelectric effect of a hybrid of magnetostrictive and piezoelectric composites

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