DSpace at VNU: Geomagnetic sensors based on Metglas PZT laminates

Duc Department of Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, E3

Geomagnetic sensors based on Metglas/PZT laminates

D.T Huong Giang∗, P.A Duc, N.T Ngoc, N.H Duc

Department of Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, E3 Building, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam

Article history:

Received 21 November 2011

Received in revised form 19 March 2012

Accepted 19 March 2012

Available online 28 March 2012

Keywords:

Magnetic sensors

Geomagnetic sensors

Magnetoelectric effects

Multiferroics

Apotentialgeomagnetic-fieldsensorisproposedonthebasisofanoptimal2Dconfigurationof magne-toelectricNi-basedMetglas/PZTlaminates.Thissensorcanperfectlyservetomeasureboththestrength andtheorientationoftheearth’smagneticfield.AnincrediblyhighME-voltageresponseof0.871V/Oeto thegeomagneticfieldwitharesolutionof3×10−4Oehasbeenachievedforcompositelaminateswitha sizeof15mm×1mm.Withrespecttothefieldinclination,anangularsensitivityof3.86×10−3V/degree andanangularresolutionof10−1degreehavebeendetermined.Thissimpleandlow-costmagnetic-field sensorispromisingforapplicationsnotonlyasnovelsmartcompassesandinglobalpositioningdevices, butalsoasmagneticbiosensors

© 2012 Elsevier B.V All rights reserved

1 Introduction

Theprincipleofglobalpositioningisbasedonthefactthatboth

thestrengthandtheinclinationofthegeomagneticfieldisa

well-definedfunctionofthegeographicposition.Theweakgeomagnetic

fields,however,canonlybedetectedwithsensingdevicesofvery

highsensitivity.Besidethetraditionaltypesofmagneticsensorson

thebasisoffluxgate,Halleffect,superconductingquantum

interfer-enceandgiantmagnetoresistancespinvalves,suchasensorcould

recentlyberealizedthankstothemagnetoelectric(ME)effect[1–3]

Thissimple,low-costsensor,furthermore,isfeaturedby

function-ingattheroomtemperature

The ME effect has been observed in multiferroics and/or

ferromagnetic-ferroelectriccomposites(hereafterdenotedasME

materials) In these materials,an electric polarization P in the

materialshallrespondtotheappliedmagneticfieldH,whereasa

magnetizationMwillrespondtotheappliedelectricfieldE.The

polarizationprocessin anMEsampleasresponse tothe

exter-nalappliedmagneticfieldshallcreatesanelectricfieldofE=˛E·H,

where˛E(=dE/dH)denotesthemagnetoelectricvoltagecoefficient

Asaresult,avoltageVME=t·E(=˛E·t·H)appearsbetweenthe

sur-facesofthesampleofthethicknesst.Largemagnetoelectricvoltage

coefficientsofferpotentialdeviceapplicationsashighlysensitive

magnetic-fieldsensors,microwavefilters,transformers,and

gyra-tors[4]

∗ Corresponding author Tel.: +84 4 3754 9665; fax: +84 4 3754 7460.

E-mail address: giangdth@vnu.edu.vn (D.T.H Giang).

Regardingthehighmagnetoelectricvoltagecoefficients, mul-tiferroiccompositesonthebasisofmagnetostrictiveferritesand rareearth-transitionintermetallicshavebeenstudiedintensively sincethebeginningofthiscentury[2,3,5–10].Inparticular, opera-tionprinciple,designandfunctioningcharacteristicsofthesenew

MEsensorshavealsobeendescribed[2,3].Valuesofmagneticfield response (dVME/dH)as highas 0.06×10−3V/Oe, 56×10−3V/Oe and13×10−3V/OewerereportedforMEsensorsusing magne-tostrictive Ni0.5Zn0.5Fe2O3 ferrites [5], Terfenol-Dlaminates [6] andTerfecohanthinfilms[7],respectively.Inanapproachtouse

MEsensorsforthedeterminationofacmagnetic-fieldstrengths, Fetisovetal.[8]havesuccessfullydevelopedapromisingsensor withasensitivitybetterthan10−3Oeformilli-Hzfrequency mag-neticfields.Furthermore,strongeffortshavebeenundertakento enhancetheMEeffectsbyalteringtheshapeandthevolumeratioof thepiezoelectric/magnetostrictivelaminates[11]orbyimproving thelaminationprocess[12].Inthecaseofmicrofabrication,Greve

etal.[13]reportedagiantMEcoefficientashighas737V/cmOe for(Fe90Co10)78Si12B10-AlNthinfilmcomposites.Recently,anME sensorusingCo-basedMetglas/PZTlaminateswasdesigned, fab-ricatedandcharacterizedfordeterminingthestrengthsaswellas theorientationsofdc-andac-magneticfields[3],wherean ME-voltageresponse(dVME/dH)of2×10−3V/Oeatlowdcfieldsand,in particular,aresponse(dVME/dhac)ashighas17×10−3V/Oeatthe lowac-oneswasreported.Thesefindingsimplyagreatpotential forself-powereddetectionoflowac-magneticfields

For an optimal design of ME laminate sensors, modeling approaches have been undertaken by several research groups (e.g.[14–20]).While somemodels[15]havetakenintoaccount the effect of thethickness ratio betweenthe piezoelectric and

0924-4247/$ – see front matter © 2012 Elsevier B.V All rights reserved.

Fig 1.Schematic of the bilayer (a) and sandwich (b) Ni-based Metglas/PZT composite configuration Vector H dc , h ac and P shows the applied magnetic fields and the electrical polarization direction, respectively.

magnetostrictivephases,otherfinite-elementmagnetostatic

sim-ulations[18–20]haveconsideredtheroleofthemagnetostrictive

lengthonthemagneticfieldconcentrationinMEsensors.Although

moreappropriatedescriptionsofsomespecificaspectsofsensor’s

behaviorseem still necessaryin themodels,theseresultshave

demonstratedausefulapproachtosignificantlyenhancethe

sensi-tivityofmagnetostrictive/piezoelectriclaminatesasgeomagnetic

fieldsensors

Inthispaper,apotentialgeomagneticsensorispresentedby

optimizingthe2DconfigurationofthemagnetoelectricNi-based

Metglas/PZT laminates At low dc-magnetic fields, a huge

ME-voltageresponseashighas0.871V/Oewasobtainedforthesensor

with size of 15mm× 1mm The sensor is promising not only

forapplicationsinnovelsmartcompassesandglobalpositioning

devices,butalsoinmagneticbiosensors

2 Experimental

TheMEmagnetic-fieldsensor wasfabricatedbybonding an

out-of-planepolarizedpiezoelectricPZTplatewith

magnetostric-tive laminates For this purpose, the 200-␮m-thick PZT plate

(APCC-855)of American Piezoceramics Inc., PA, USA wasused

Themagnetostrictivelaminateswerecutfromthe18-␮m-thick

Fe76.8Ni1.2B13.2Si8.8melt-spunribbons(alsocalledNi-based

Met-glas) in different sizes according to the length-to-width ratio

(r=L/W),withr=1,1.5,3,7.5and15,andusedforthevarious

sam-plesinthis work.It isworthtonotethattheNi-basedMetglas

ribbonisasoftmagnetostrictivematerialwithamagnetostriction

coefficient()ofabout70×10−6andamagnetostrictive

suscep-tibility(␭=d/dH)of 1.5×10−6Oe−1.Thanks tothe mechanic

coupling between the components, the PZT plate undergoes a

forcedstraininducedbythemagnetostrictivelayersunderthe

in-plane(and/orout-of-plane)appliedmagnetic field.In this case,

theME-voltageVMEisinducedacrossthethicknessofthe

piezo-electricplate.Fig.1presentstheconfigurationofsuchfabricated

bilayerMetglas/PZTandsandwichMetglas/PZT/MetglasME

com-positelaminates

Inthesampleconfigurationsunderinvestigation,alinear

elec-tric polarization P is induced by a weak ac magnetic field hac

(=hosin(2fot))oscillatingattheresonantfrequencyinthe

pres-enceofadcbiasfieldHandtheMEvoltageVMEisdirectlymeasured

asaresponseoftheMEcompositetotheappliedmagneticfield.In

theexperimentalsetup,thebiasfieldHwasprovidedbyan

electro-magnet,andtheoscillatingfieldwithamplitudesofhac=10−2Oe

was generated by a Helmholtz coil The output voltage (VME),

inducedacrossthePZTlayeroftheMElaminatebytheacfield(hac)

wasmeasuredonacommercialDSPlock-inamplifier(Model7265

ofSignalRecovery),whichsimultaneouslycontrolledtheinput

cur-renttotheHelmholtzcoil.Thevalueofthe˛Ecoefficientisderived

thenfromtheequation:˛ =V /hac·t

3 Results and discussion

3.1 Shapeandsizedependenceoftheresonantfrequency Fig.2showstheac-magnetic-field-frequencydependenceofthe

MEcoefficient˛E measuredunderafixedbiasdc-magneticfield

of4Oefortheinvestigatedsquare-shaped(r=1)bilayer compos-itelaminatesofdifferentsizesof8mm×8mm,10mm×10mm,

12mm×12mmand15mm×15mm.Theresultsshowthatwith theincreasinglaminatesize,theresonanceisshiftedtowardlower frequencies(fr),whereas˛Esignificantlyincreases.Theobserved phenomenacanbedescribedin termofthevibratingplates,in whichoneofthenaturalfrequencies(fnm)ofthemodesisobtained fromthesolutiontothetwo-dimensionalwaveequationin Carte-siancoordinates[21]:

fnm= v2



n2

L2 + m2

W2, withvasthewavevelocityinthePZT,nandmasintegernumbers (1,2 )

Indeed,theexperimentalresultsarewellfittedwitha funda-mental frequency f11 (i.e withn=m=1)(see Fig 3).From this description,thephasevelocityturnsouttobeof2800m/sforPZT Thisfindingisconsistentwiththatreportedforthepiezoelectric bulkmaterial.ForsandwichMetglas/PZT/Metglasstructures,the

MEeffectcanberemarkablyincreased,while theresonant fre-quencyexhibitsnochange

As regards to the shape effect, in this paper, rectangular composite laminateswithdifferentlengthtowidth ratioswere investigated.For thefabricationoftheinvestigatedsamples,its longitudinaledge(thelengthL)waskeptfixedat 15mmwhile itstransversaledge(thewidthW)wasvariedfrom1to15mm,

Fig 2.ME coefficient as a function of the ac magnetic field frequency for

square-Fig 3.Resonant frequency vs.

(1/W 2 ) + (1/L 2 ) for square-shaped bilayer com-posites of different sizes L × W.

so that a series of rectangularlaminate samples was obtained

withtherespectivelength/widthratios(r=L/W)varyingfrom15

to1.Therespectivevaluesoftheresonantfrequencyfr,obtained

forthesesamplesareshowninFig.4forthecase themagnetic

fieldsareappliedalongthelengthofsamples.Itisinterestingthat,

exceptthesquare-shapedsample(withr=1),allcomposite

lami-natesexhibitaninvariantfrofabout100kHz,whichisnearly1.5

timeslowerthanthatobservedforthesamplewithr=1.Byusing

theaboveextractedwavevelocity,themeasuredresonant

frequen-ciesoftherectangularcompositelaminatesarewellfittedwiththe

fundamentalfrequenciesof theone-dimensionalwaveequation

f10=v/2L.Inthiscase,theresonantfrequencyisascribedasmainly

governedbythelongitudinallengthofthesample

3.2 ShapeandsizedependenceoftheMEcoefficient

Fig.5showsthebias-field dependenceof theMEcoefficient

for thedifferent investigated square-shaped samples measured

attheresonantfrequencies Ascanbeseen,for allsamplesthe

magnetoelectriccoefficientexhibitsasimilarbehavior:itinitially

increasesatlowappliedmagneticfields,reachesamaximumvalue

Fig 4. Resonant frequency vs the ratio of the length to the width r (= L/W) for bilayer

ME composites The fitted line f =v/2L is shown.

Fig 5.The magnetic field dependent of ME coefficient for 8 mm × 8 mm,

12 mm × 12 mm and 15 mm × 15 mm bilayer square-shaped samples.

ata certainmagneticfield(denotesastheoptimalfieldfor the maximalMEresponse)andthendecreaseswithfurther increas-ingmagneticfield.ItisapparentthatthevalueofMEcoefficient

isstronglyinfluencedbythesamplesize:thelargerthe interfa-cialarea (i.e.thesample size),thelower theoptimalmagnetic field,thehighertheMEvoltagecoefficientand,consequently,the highertheinitialslopeatlowmagneticfieldsisfoundforthe˛E(H) curves.Thisobservationcanbeunderstoodintermoftheso-called

“shearlagging”edgeeffect[22].Furthermore,ahugeME coeffi-cient of75.9V/cmOe isfoundatlowbiasfield ofonly10Oein thecomposite laminatewithr=1 By usingthesandwich Met-glas/PZT/Metglaslaminatestructures,theMEeffectcanincreaseup

to˛E=132.1V/cmOe.Althoughthisvalueisabout5timeslower thanthehighestMEcoefficientreportedfor(Fe90Co10)78Si12B10 -AlNthinfilmbyGreveetal.[13],thecompositelaminatesfabricated withasimpleandlow-costtechnologyinthisworksuggestavery promisingapplicationinpracticalsensors

Withthemotivationtofurtherenhancethelow-fieldMEvoltage coefficient,rectangular-shape compositelaminates withvarious length/width ratios have been prepared and investigated This motivation is basedon thefact that theenhancement of mag-netoelectric softness is related to the shape anisotropy Fig 6 showstheMEcoefficient˛Easafunctionofthedc-magneticfield strength forsandwichcomposite laminatesof differentsizesas

15mm×15mm,15mm×3mmand15mm×1mm, correspond-ingtotherespective length/widthratiosofr=1, 5and 15.The measurementswerecarriedoutwiththemagneticfieldsapplied

Fig 6.The ME coefficient as a function of bias magnetic field for rectangular-shaped sandwich Metglas/PZT/Metglas composites of different sizes 15 mm × 15 mm,

Fig 7.ME coefficient measured at H dc = 2 Oe (open square) and maximum ME

coef-ficient (close circle) as a function of the length to the width ratio (L/W).

alongthelengthofthesample.Theresultsshowthatthe

maxi-malMEcoefficientissignificantlyunchangedinsampleswithhigh

rvalues(remainingsignificantlyat131V/cmOe,obtainedforthe

samplewithr=15,seeFig.7).Theoptimalmagneticfieldforthe

maximalME response,however, strongly decreasesfrom21Oe

inthesamplewithr=1to7Oeinthesamplewithr=15

Conse-quently,themuchhigherinitialslopeatlow-magneticfieldsofthe

˛E(H)curvesisobserved.Thisimportantbehaviorisillustratedin

Fig.7withthedatameasuredinanappliedfieldof2Oe.Thehighest

MEcoefficientof62.61V/cmOehasbeenfoundinthesamplewith

lengthtowidthratior=15.Aswillbepresentedinthenextsection,

forpracticalgeomagneticsensorapplications,theoptimalsizeof

15mm×1mm(i.e.r=15)couldbechosenforsensorprototypes

3.3 Geomagneticsensorprototype

Fig.8showsphotographsoftheMEcompositelaminatesand

ageomagneticsensorprototypefabricatedusinganME

compos-itelaminate withoptimalrectangularsize of 15mm×1mm A

solenoidcoiliswrappedaroundtheMEcompositelaminateto

gen-eratetheac-magneticfieldattheresonantfrequency.Theeffective

fieldisbythiswayalignedin-planeandalongthelengthofthe

ribbons(i.e.perpendiculartotheelectricalpolarizationofthePZT

plate).Fortestingthesensoroperationintherangeofthe

geomag-neticfieldstrength,aHelmholtzcoil suppliedbyaKeithley230

Fig 9. The output ME voltage as a function of bias magnetic field for sensor proto-type The fitted curve is included.

currentsourcewasusedtogeneratethebiasmagneticfieldsinthe rangeupto1.5Oewiththeaccuracyof10−5Oe

Shown in Fig 9 is theME voltage response tothe external magnetic field As can be seen, a linear variation of the ME-voltagewiththeexternal magneticfield hasbeenfoundinthe field rangeupto1.0Oe.From thisresult,thesensor sensitivity couldbederivedashighas0.871V/Oe.Inamoredetailedanalysis, thefield resolution of 3×10−4Oe hasbeen estimated Surpris-ingly,thepresentME-basedsensorexhibitsa sensitivity,which

istwoordersofmagnitudehigherthanthatpreviouslyreported forsimilarmagnetic-sensordevicesandiscomparablewiththatof availablecommercialgeomagneticsensors[23].Thisconfiguration presentsagoodcombinationoftheexcellentmagneticsoftnessof Ni-basedMetglasribbonsandtheeffectsoftheshapeanisotropy Thissensorenablestodetectnotonlythegeomagneticfields,but alsothemagnetic fieldsof magnetic micro- and nano-beadsin biochipapplications

Regardingtheapplicationoftheproposedsensorindetermining theorientationoftheEarth’smagneticfield,anotherexperimental setupis illustratedinFig.10(a).ShowninFig.10(b) isthe sen-soroutputvoltageasafunctionoftheϕ-anglebetweenthesensor axis,i.e.theaxisintheplanealongthelengthofthelaminate.The zeroangle(ϕ=0◦)isdefinedwhenthesensor isinsucha posi-tionthatitsaxisisalignedparalleltotheEarth’sNorthmagnetic Pole.Itisclearlyseenfromthisfigurethatbyrotatingthesensorin horizontalplanefromϕ=0–360◦,thesensorsignalvaries periodi-callywithϕ,reachingamaximumvalueof356mVintheparallel

Fig 8.Sensor construction: Ni-based Metglas/PZT 15 mm × 1 mm laminates (a) and sensor prototype where the coil generating an ac field directly wraps around the ME

Fig 10.Experimental setup for measuring orientation of the Earth’s magnetic field

(a) and the output signal as a function of ϕ-angle between the sensor’s longitudinal

axis and the Earth’s North magnetic Pole (b).

alignments(i.e.ϕ=0and180◦)andvanishinginthe

perpendicu-laralignments(i.e.ϕ=90and270◦)ofthesensoraxiswithrespect

fromtheNorth-SouthdirectionoftheEarth’smagneticfield.This

findingsuggeststhatthefabricatedsensorcanbeusedfordetecting

boththestrengthandtheorientationofthegeomagneticfield

4 Concluding remarks

Thecompositelaminateconfigurationcombininghigh

perfor-manceNi-basedMetglasribbonsandpiezoelectricPZTplateshas

broughtbyanoptimalgiantmagnetoelectriceffectwitha

signif-icantMEcoefficientinthelowmagneticfieldrange.Apotential

geomagnetic-fieldsensorispreparedonthebasisoftheoptimal

laminateconfiguration.Thesensorcandetectpreciselynotonly

thestrength,butalsotheorientationoftheEarth’smagneticfield

Ahighsensibilityof0.871V/Oeandaresolutionintheorderof

10−4Oe withoutamplificationmake thisconfigurationa

poten-tialsensorforapplicationsinnovelsmartcompassesandglobal

positioningdevices

Acknowledgements

ThisworkwassupportedbyVietnamNationalUniversity,Hanoi

underthegrantedResearchProjectQG09.29,bytheNAFOSTEDof

VietnamundertheResearchProjectNumber103.02.86.09andby

theNationalProgramforSpaceTechnologyofVietnam.Theauthors

thankAssoc.Prof.Dr.N.T.HienfromtheVNUUniversityof

Engi-neeringandTechnologyforcriticalreadingofthemanuscript

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Biographies

France in 2005 In 2006, she joined the Faculty of Engineering Physics and Nano-technology at VNU University of Engineering and Technology, Vietnam National University, Hanoi, where she is currently an assistant professor Her research interests include magnetostrictive, magnetoresistance, magnetoelectric and mul-tiferroics materials, sensors and microsystems.

P.A Ducreceived his BSc and MSc degree in Physics from Hanoi National University

in 2004 and 2007, respectively He is currently working on his PhD dissertation in the area of magnetoelectric composites and applications.

N.T Ngochas studied at the University of Engineering and Technology, Vietnam National University, Hanoi and is finishing her master in Nanotechnology She is developing 3D-sensor for geomagnetic applications.

N.H Ducjoined the Cryogenic Laboratory, University of Hanoi as researcher after his graduation from the same group in 1980 He obtained his doctor degree in the same group in 1988 He has then received the French Habilitation in Physics at the Joseph Fourier University of Grenoble in 1997 and became a full professor of the Col-lege of Technology (now VNU University of Engineering and Technology), Vietnam National University, Hanoi in 2004 His extended research includes various aspects

of magnetism, such as: 4f–3d exchange interactions; giant magnetovolume, magne-tostrictive, magnetoresistive and magnetocaloric effects; magnetic phase transition; magnetic nanostructures; multiferroics; MERAM and biochips.