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FEAT

Pedro Martins and Senentxu Lanceros-Mndez *

1.

Theelecmaof athehasin amewav

Iof tperwhima

Drialibiltionpiezoelectric and magnetostrictive components produces an ME response several orders of magnitude higher than those in single-phase ME materials. [ 10 ]

The ME effect in such composites is a product tensor prop-erty which results from the cross interaction between the pie-zoelectric and magnetostrictive phases in the multiferroic ME composite, whereas the sum and scaling properties denote the

cign

m

atc

t

e have been proposed

Since the fi eld of researchas a complex taxonomy anferent fi elds, [ 3 ] the basic conlisted and defi ned in Table 1

This Feature Article will ical developments and a disin the ME research fi eld dumainly focus on polymer-baof polymer-based ME matelaminated composites (Figucomposites (Figure 1 c) will

Then, the obtained ME materials will be ordered bmain possible applications ture Article will end with some concluding remarks and a sum-

polymer-based ME tic fi eld.

Polymer-Based Magnetoelectric Materials

Polymer-based magnetoelectric (ME) materials are an interesting, chaland innovative research fi eld, that will bridge the gap between fundamresearch and applications in the near future. Here, the current state ofart on the different materials, the used confi gurations for the developmof sobt rsom obasrem

DOI:

Prof. S. Lanceros-MendezCentro/Departamento de FsicaUniversidade do Minho4710-057, Braga, PortugalE-mail: lanceros@fi sica.uminho.pt Dr. P. Martins, Prof. S. Lanceros-MendezINL-International Iberian Nanotechnology Laboratory4715-330 Braga, Portugal

Adv. Fu 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

mary of the distribution of the maximum coeffi cient by reference, type, and DC magne 10.1002/adfm.201202780

nct. Mater. 2013, 23, 33713385Introduction

magnetoelectric (ME) effect, defi ned as the variation of the trical polarization of a material in the presence of an applied

gnetic fi eld or as the induced magnetization in the presence n applied electric fi eld, [ 13 ] can be seen as the bridge between electric and magnetic properties of matter. [ 4 ] The ME effect drawn increasing interest due to its potential applications reas such as information storage, spintronics, multiple-state mories, sensors, actuators, transformers, gyrators, micro-e devices, optical waves, diodes, among others. [ 48 ] n order to positively match the technological requirements hese and other applications, a strong ME effect at room tem-ature has been obtained from multiferroic (MF) composites, ch is generally obtained by combining piezoelectric and gnetostrictive components. [ 9 ]

ifferent from what happens with the single-phase ME mate-s so far available at room temperature, the larger design fl ex-ity of MF composites allows the introduction of multifunc-al properties in which the coupling interaction between the

to the which a chanmecha

MEH=

or,

MEE=

Equorders in MF positesmagne

Baseroom tdevices

ensors and actuators, as well as the main values of the ME couplinained for the different polymer-based systems are summarized. Fue of the specifi c applications that are being developed for those p

ed ME materials are addressed as well as the main advantages andaining challenges in this research fi eld. . [ 13 ] h described in this Feature Article d typically involves terms from dif-cepts of the ME research fi eld are . provide an overview on the histor-cussion on the main achievements ring the last decades, and will then sed ME materials. Three main types rials, nanocomposites ( Figure 1 a), re 1 b), and polymer as a binder

be then discussed. coeffi cients for polymer-based ME y composite type and some of the will be discussed. Finally, this Fea-URE A

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average or the enhancement of effects which are already present in the con-stituent phases. [ 5 ] In this way, composites can be used to generate an ME response from the combination of materials which themselves do not allow the ME phenomenon. [ 4 ]

Once a magnetic fi eld is applied to the composite, strain in the magnetostrictive phase is induced. This is transmitted to the piezoelectric constituent, which unde-goes a change in electrical polarization. In an analogous way, the reverse effect can occur: when an electric fi eld is applied

omposite, strain is induced in the piezoelectric phase s transmitted to the magnetostrictive phase, leading to e in the magnetization. The above mentioned coupling ism can be written as: [ 11 ]

magnetic

mechanical mechanical

electric (1a)

electric

echanical mechanical

magnetic (1b)

tion 1 shows the coupling of the electric and magnetic rough an elastic interaction. Therefore, the ME response omposites is an extrinsic effect, dependent on the com- microstructure and the coupling interaction across the ostrictive and piezoelectric interfaces. [ 12 ] d on this property and the large ME effect obtained at mperature for some composites, some prototype ME

lenging ental the ent

g ther, lymer-

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FEATU

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CLE 1.1. History

Fouremer

Indiscoelecttricaldiscodiscometrnon-the tcessfeffecDzyain thand Sat thof M

M

=

wereof ne

Inratiotechnthe nical imprME s

Inagainpositshiftecomppositthe tmagn

Thbeenneticpiezonatedc) pa

Toplentto reby a large

1.2. NAppl

The singl

Julius-Maximilians-Universitt Wrzburg, Germany. He was Research Scholar at Montana State University, Bozeman, USA (19961998) and visiting scientist at the A. F. Ioffe Physico-Technical Institute (1995), Pennsylvania State

ersity (2007) and University of Potsdam (2008). He sociate Professor at the Physics Department of the ersity of Minho, Portugal and since 2012 he is Associate archer at the INL (International Iberian Nanotechnology ratory). His work is focused on the development of

mer-based smart materials and their applications.

Pedro Martins graduated in

d

s considered insuffi cient for most of the proposed prac-plications. [ 43 ] In addition, there is a wide variation of the on temperatures (paraelectric to ferroelectric, paramag- antiferromagnetic, antiferromagnetic to ferromagnetic) single-phase ME materials and a limited number of ls that exhibits ME behavior at room temperature. [ 44 ] In y, most of these MEs can be only used at low tempera- 10 K), which severely hinders the design and applica- devices. ite ME coeffi cients obtained in ceramic MF composites hree orders of magnitude higher than in single phase ls, [ 45 ] such composites may become fragile and are lim- deleterious reactions at the interface regions, leading electrical resistivity and high dielectric losses > 0.1, hin-in this way the incorporation into devices. [ 11 ] Apart from rementioned disadvantages, ceramic composites still her problems such as being expensive, dense and brittle, can lead to failure during operation. [ 46,47 ] In this way, -based ME materials are not attractive from a techno-

point of view.

Adv. Funct. Mater. 2013, 23, 33713385nlinelibrary.com 2013 WILEY-VCH Verlag GmbH & Co

years (1888, 1894, 1905, and 1926) were pivotal in the gence of the ME research fi eld ( Figure 2 ): 1888, Rntgen, before winning the Nobel Prize due to the very of the X-rays, observed that a dielectric moving in a

ric fi eld became magnetized. [32] The reverse effect, the elec- polarization of a dielectric moving in a magnetic fi eld was vered by Wilson in 1905. [33] Between the fi rst and second veries indicted above, Pierre Curie, in 1894, based on sym-y considerations enunciated the possibility of ME effect in moving crystals. [4,34] In 1926, Debye introduced and coined erm magnetoelectricity for the effect that was not suc-ully proved experimentally at that time. [35] n 1959, the ME t was predicted to occur in chromium oxide (Cr 2 O 3 ) by loshinskii, [36] prediction that was experimentally confi rmed e following year by Astrov. [37] In 1966, the group of Ascher chmidt at the Battelle Institute in Geneva and Newnham

e Pennsylvania State University discovered a high number E boracites [38] and phosphates. [39] E coupling coeffi cients, defi ned as

P

H (2)

not as high as necessary for applications, but the number w ME materials increased signifi cantly. 1973, scientifi c work on ME materials reached a satu-n point since it was felt that single-phase MEs were not ologically applicable due to the weak ME coupling and eed of very low temperatures. Furthermore, the theoret-

considerations gave no indication or hope of signifi cant ovements. [ 40 ] As a result of this dead-end, the intensity of cientifi c activity strongly declined for almost 20 years. the 1990s, interest in ME materials strongly increased due to the relationship between the ME and MF com-es: the main object of scientifi c investigations into the ME d from single phase ME materials, to the search for MF ounds with higher ME coupling. [ 41 ] In those novel com-

es, the ME response is due to elastic coupling between wo constituent phases, one piezoelectric and the other etostrictive. [ 5 ] ree main types of bulk magnetoelectric composites have

investigated both experimentally and theoretically: a) mag- metals/alloys e.g., laminated Terfenol-D or Metglas and electric ceramics such as lead zirconate titanate; b) lami- Terfenol-D and Metglas and piezoelectric polymers; and

rticulate composites of ferrite and piezoelectric ceramics. [ 42 ] day, ME research is a strong research area, showing still y of mysteries, promises, and challenges. One of them is place the ceramic in bimorphs or superlattice composites polymer or polymer-based piezoelectric matrix to achieve r areas or non-planar structures. [ 13 ]

on Polymeric ME Materials: Problems Regarding ication Developments

magnitude of the ME coupling coeffi cient in most of the e phase MF materials is in the range of 120 mV/(cm Oe)

Univis AsUnivReseLabopoly

basetions

which itical aptransitinetic tofor themateriathis watures ( tions of

Despbeing tmateriaited byto low dering the afohave otwhich ceramiclogical Senentxu Lanceros-Mendez graduated in physics at the University of the Basque Country, Spain (1991), obtaining his Ph.D. (1996) at the Institute of Physics of the

Physics and Chemistry in 2006 and received the PhD degree in Physics in 2012, from the University of Minho, Braga, Portugal. Part of his thesis work was in collaboration with the Basque Country University, Spain and Cambridge University, United Kingdom, concerning the development of polymer-based magnetoelectric materials. His work is focused on polymer-

magnetoelectric materials for technological applica-and electroactive polymers.

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FEA

noprocomMEtemshepro

2.

Astricnapomadis

Figand

Table 1. ME basic concepts.

Con

Mu le

Fer b

Fer b

Fer ca

Pie d

Pie c fi

es

Elec nc

Ma ct

Pie

coe

d t

m

s

Pie

coe

all

en

ss

Ma

coe

Ma

coe

Advcept Defi nition

tiferroic Material that possesses two or all three ferroic properties (ferroe

roelectric Material that possesses a spontaneous and stable polarization that can

romagnetic Material that possesses a spontaneous and stable magnetization that can

roelastic Material that possesses a spontaneous and stable deformation that

zoelectricity Variation of the strain of a material as a linear function of an applied electric fi el

of applied stress.

zomagnetism Variation of the strain of a material as a linear function of an applied magneti

function of applied str

trostriction Variation of the strain of a material as a quadratic fu

gnetostriction Variation of the strain of a material as a quadratic fun

zoelectric

ffi cient

Relates the mechanical strains produced by an applied electric fi eld and is calle

components d ij , where i indicates the direction of polarization generated in the

applied fi eld), and j is the direction of the applied

zomagnetic

ffi cient

Relates the mechanical strains produced by an applied magnetic fi eld and is c

where i indicates the direction of magnetization generated in the material wh

fi eld), and j is the direction of the applied stre 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2.1. PrepME Mat

Regardinymer-bapolymerand pylene) (P

MF Psolution

granules are fi rst dissolvedformamide (DMF)) at 75 of magnetic fi llers is admechanical stirring processolution is poured onto a eliminate voids and dried

Concerning the fabricatwo different methods havthe method presented by Mof magnetostrictive nanoobtained mixture is then pthat nanoparticles are weto avoid loose aggregates. [

is added. Further, the obtmechanical stirrer with ultrof the polymer. Flexible fi lmtion on a clean glass substrcrystallization are perfor

A more recent approach to obtain highly fl exible and nbrittle ME composites and to solve all the aforementioned blems is to use polymer-based nanocomposites. [ 13 , 48 ] In parison with the ceramic ME composites, polymer-based

materials can be easily fabricated by conventional low-perature processing into a variety of forms, such as thin ets or molded shapes, and can exhibit improved mechanical perties. [ 13 ]

Polymer-Based Magnetoelectric Materials

previously mentioned, three main types of magnetoelec- polymer-based composites can be found in the literature:

nocomposites, polymer as a binder, and laminated com-sites. In the following section, the main characteristics, terials, achievements, and limitations of each type will be cussed.

ure 1 . Types of polymer-based ME materials: a) nanocomposites, b) laminated composites, c) polymer as a binder composites.

gnetostrictive

ffi cientRelates the mechanical strains produced by an applied magnetic fi eld and is called the coeffi cient.

where i indicates the direction of magnetization generated in the material when the magnetic fi eld is zer

fi eld), and j is the direction of the applied stress (or the induced strain

gnetoelectric

ffi cientRelates the polarization/voltage produced by an applied magnetic fi eld and is called the coeffi cient.

where i indicates the direction of polarization/voltage generated in the material when the electric fi eld is

electric fi eld), and j is the direction of the applied magnetic fi eld (or the induced m

. Funct. Mater. 2013, 23, 33713385aration of Polymer-Based

is a tensor, with components ij , o (or the direction of the magnetic

).

[ 27,28 ]

is a tensor, with components ij ,

zero (or the direction of the applied

agnetization).

[ 30,31 ] TURE A

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References

ctricity, ferromagnetism and ferroelasticity). [ 14,15 ]

e hysteretically switched by an applied electric fi eld. [ 16,17 ]

e hysteretically switched by an applied magnetic fi eld. [ 18,19 ]

n be hysteretically switched by an applied stress. [ 20,21 ]

or a change in the material polarization as a linear function [ 21,22 ]

eld or a change in the material magnetization as a linear

s.

[ 23,24 ]

tion of an applied electric fi eld. [ 25,26 ]

ion of an applied magnetic fi eld. [ 27,28 ]

he strain constant, or the d coeffi cient. d is a tensor, with

aterial when the electric fi eld is zero (or the direction of the

tress (or the induced strain).

[ 25,26 ]

ed the d m coeffi cient. d is a tensor, with components d ij ,

the magnetic fi eld is zero (or the direction of the applied

(or the induced strain).

[ 27 , 29 ] 3373wileyonlinelibrary.com

erials and Interface Effects

g the preparation of particulate pol-sed ME nanocomposites, two distinct s have been used, polyurethane (PU) oly(vinylidene fl uoride-trifl uoroeth-(VDF-TrFE)), a copolymer of PVDF. U fi lms are usually obtained by the cast method. In such method, PU in a solvent (typically N,N-dimethyl- C for 1 h. Then, the desired amount

ded to the polymer solution and a s is performed. Finally, the obtained

clean glass substrate and degassed to at 70 C. [ 49 ] tion of P(VDF-TrFE based MF fi lms, e been reported in refs [ 42 , 50,51 ] . In artins et al., [ 42 , 50 ] the desired amount

particles is added to DMF and the laced in an ultrasound bath to ensure ll dispersed in the solution and also 23 ] Afterwards, P(VDF-TrFE) powder ained mixture is placed in a Tefl on asound bath for complete dissolution s are obtained by spreading the solu-

ate. Solvent evaporation and polymer med inside an oven at controlled

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tempsamp

Alpartican uprepaketoncrystais esselectr(900

Thpolymin thbetweonly charain dethe Mratio

Inoped nanointerfites cadvanthe istate

both the matrix and the inclusion and leads to changes in the displacement fi eld and

Figureerature and crystallization is achieved by cooling down the

2 . Highlights in the investigation of the ME effect. linelibrary.com 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

les to room temperature. ternatively, as reported in ref [ 51 ] , magnetostrictive nano-les are dispersed in the P(VDF-TrFE) solution matrix in

ltrasound bath. The P(VDF-TrFE) solution is previously red by dissolving P(VDF-TrFE) pallets into methyl-ethyl-e (MEK). After vacuum and thermal treatments, the fi nal llized MF fi lms are obtained. Since electrical polarization ential to obtain piezoelectric responses, the MF fi lms are ically poled by submitting the fi lms to high electric fi elds kV/cm as a maximum strength). e synthesis of the magnetostrictive nanoparticles used in er-based ME particulate nanocomposites is well discussed

e literature. [ 5257 ] On the other hand, the interface effects en nanoparticles and the polymer matrix has received little attention. In this kind of MF nanocomposite, the cteristics of the interface becomes a prominent factor [ 58 ] termining the magnetoelectroelastic properties as well as E effect of the MF nanocomposite, due to the increasing

of the interfacial area to volume. recent years, surface elasticity theory [ 59 ] has been devel-to account for the effects of surfaces and interfaces at the meter-scale. These studies show that depending upon the ace design, the effective properties of the nanocompos-an be either enhanced or reduced. [ 60 ] In this way, taking tage of the theory of linear elasticity, it was found that

nterfacial stress displays short-range effect on the stress in nanocomposites, which introduces internal stresses in

reduction of the piezoelecinterface is even more harposites. A proper Terfenol-the perfect coupling betwestrains without appreciableroughly decrease the displeading to a decrease in th

Continuing their study ME response of Terfenol-Nan et al. [ 63 ] discussed thecomposite. This coupling formulation by using the Ginterfaces between Terfendecrease the displacementa decrease in the ME respwhen the particle sizes weof the imperfect interface large amount of interfacestechnologically important pnetostrictive composites ththe quantitative correlatifor composites and their iexplored.

Theoretical calculationsMF particulate composite (PZT), and polymer has Greens function techniquthe ME voltage coeffi ciene surfactant modifi cation leads to a tric response. Thus, such an inactive mful to the ME response of the com-D particles/polymer interface ensures en these phases, transferring elastic losses. Any imperfect interfaces will lacement transfer capability, thereby e ME response of the composites. on the effects of the interface on the D/P(VDF-TrFE) layered composites, effective coupling properties of such was expressed in a convenient matrix reens function technique. Imperfect

ol-D and P(VDF-TrFE) phases would transfer capability, thereby leading to onse of the composites. In particular, re at the nanometer scale, the effect would be more pronounced due to a in the composites. However, in the iezoelectric ceramic/epoxy and mag-

at have been extensively investigated, on between the coupling behavior nterface imperfection remains to be

of the ME properties in a three-phase of Terfenol-D, lead-zirconate-titanate been also reported [ 64 ] based on the e. It was shown that the values of ts are very sensitive to mechanical

Adv. Funct. Mater. 2013, 23, 33713385the stress fi eld created by a external loading. Shrinkage of the inclusion was also observed. Unlike the classical results, in the theory of linear elasticity, the effective bulk modulus is a function of the interfacial stress and the size of the inclusion. [ 61 ]

The nonclassical interface condition has also been studied by Pan et al. [ 60 ] and it was observed that such interface condition exerts a signifi cant infl uence on the local and overall magnetoelectroelastic responses of MF com-posites, in particular when the fi llers are at the nanometer-scale. It was also demonstrated that it was possible to enhance the ME coeffi -cient of a MF composite consisting of magne-tostrictive fi llers reinforced in a piezoelectric matrix by designing an electrically highly con-ducting interface.

Keeping the interface of nanocomposites as subject of study, but in an more experi-mental approach, Nan et al. [ 62 ] have inves-tigated the role of the Terfenol-D/polymer interface layer, induced by surfactant modifi -cation of the Terfenol-D particle surfaces, in the ME properties of the composites. It was reported that by adding a silane surfactant to the surface of the nanoparticles, the piezo-electricity of the composite is diminished since th

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K

FEAmagnetostrictive components of a MF layered composite. The ezM c)a re nenr srgd

bh th

aeliiiahfs

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boundary conditions of the composite sample, and that the polarfacialon thof thME rarticlganiceffectpositwas opositlargelargeinterfThe than dynacomp

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Adv. Fu 2013 WILEY-VCH Verlag GmbH & Co.

increasthe piein the are MEthe thi( < 0.001It was epoxy fiby decusing abetweebecomweak itransfenent toin the in a laexpecteepoxy bility, tlayered

Anomentedeffect ccase, thspin-podielectrdielectrto applthan th

In tmost ocomposimilarinorganeffect clinear electriceffects reporterecentlobservafaces o

Conthe Nia novemined the intoscillatmore, to the dtype of

ChaFe, denet al., [ 8

BaTiO 3interfaanisotr

ization orientation in PZT particles and the inactive inter- layer surrounding PZT particles have a signifi cant effect e ME response of the composite. The quantitative analysis e effect of such mechanical boundary conditions in the esponse of the composites will be later discussed in this e. Interface effects have been already addressed in inor- ME composites. In particular, additional to the interface s in the ME response of BaTiO 3 /NiFe 1.98 O 4 nanocom-

es, Sreenivasulu et al. [ 65 ] also reported the size effects. It bserved that the particulate BaTiO 3 /NiFe 1.98 O 4 nanocom-

es exhibit large ME coeffi cient values (about fi ve times r) than the BaTiO 3 /NiFe 1.98 O 4 microcomposites due to the piezoelastic dynamic strain coeffi cients and an adequate ace contact between both BaTiO 3 and NiFe 1.98 O 4 phases. magnetostriction was higher in nanograined NiFe 1.98 O 4 micrograined NiFe 1.98 O 4 and thus, the magnetostrictive

mic coeffi cient was proposed to have large values for nano-osites than the microcomposites of similar compositions. garding the experimental study of the interface and size s on the ME properties of polymer-based ME nanocom-

es and polymer as a binder composites it is therefore ved a clear need of further investigation. A systematic and stive study of the effect of the PVDF/ferrite interface has

nevertheless reported concerning the nucleation of the oactive -phase of PVDF, and consequently in their fer-ctric, piezoelectric and ME properties. [ 6670 ] interesting outcome of this investigation on PVDF/fer-omposites is the fact that the ferrite nanoparticles used e magnetostrictive phase in the ME nanocomposites have bility to nucleate the electroactive -phase of PVDF, [ 66 ] due e electrical interactions between the negative nanoparticle ces and the positively charged polymeric CH 2 groups [ 69 , 71 ] . fact will allow to reduce the production costs of the poly-ased ME nanocomposites, [ 72 ] leading to the develop- of applications of such as didactics, toys and disposable es. [ 13 ] Focusing on layered or laminate ME structures, confi gurations are usually obtained by gluing together lectroactive polymer and the magnetostrictive phase using oxy binder [ 9 , 7376 ] by the direct deposition of one phase on f the other, [ 77,78 ] or by hot-molding techniques. [ 79,80 ] such ME layered composites, the quality of the interface s determined by the interface coupling parameter is only dent on surface inhomogeneities and interactions between

ituents during the preparation as well as misfi t strains. a parameter is a measure of differential deformation en the piezoelectric and magnetostrictive layers and k = 1 ideal interface and k = 0 for the case without coupling. [ 5 ]

sed on modifi ed constitutive equations and the fi nite ent method, the quality of the MF interface has been ed. [ 81 ] It was verifi ed that the ME effect of laminated mag-trictive/piezoelectric MF nanocomposites was remarkably dent on the thickness and characteristics of binder layer. on the constitutive equations, results show that, when

nterfacial layer is somewhat stiff and the binder layer is a large ME effect will be produced. order to treat such an interfacial bonding effect, Nan [ 82 ] changed the shear modulus and the relative thickness e conductive epoxy used for bonding the piezoelectric and

nct. Mater. 2013, 23, 33713385 3375wileyonlinelibrary.comGaA, Weinheim

TURE A

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in the thickness of the conductive epoxy fi lms between oelectric and magnetostrictive phase leads to a decrease E voltage coeffi cients, since the conductive epoxy fi lms

inert. It was also reported that a very low ratio between kness of epoxy and thickness of the MF composite is good enough for producing a giant ME response. lso discovered that the elastic modulus of the ME inert lms has a signifi cant effect on the ME response values, asing the shear modulus of the thin epoxy fi lms, i.e.,

very fl exible epoxy as the binder, the interfacial bonding the piezoeletric and magnetostrictive components s weak due to the formation of a sliding interface. The terfacial contact would lead to appreciable losses of ring elastic strain/stress from the piezoelectric compo-the magnetostrictive component, and thus the decrease hear modulus of the thin interfacial epoxy fi lms results e decrease in the ME response of the composites. As , any imperfect interfacial bonding produced by the

inders would decrease the displacement transfer capa-ereby leading to a decrease in the ME response of the composites. er type of interface ME coupling has been imple-

by Rondinelli et al. [ 83 ] It was demonstrated that the ME n arise from a carrier-mediated mechanism. In such magnetic response is mediated by the accumulation of arized carriers at the interface between a nonmagnetic c and a ferromagnetic metal. For the ferromagnetic/c interface, this kind of ME effect is linear with respect ed electric fi eld and the magnitude is two orders lower t for the interface bonding mechanism. e case of ferromagnetic/ferroelectric heterointerfaces, the studies have been reported in all the inorganic ME ites, though it is expected that the main behaviors are for polymer-based ME materials. For example, in the ic ME composite SrRuO 3 /BaTiO 3 interface, [ 84 ] the ME n be further enhanced owing to fi eld effect and non-ariation of the ferroelectric polarization with applied fi eld. Experimental manifestation of the predicted ME riven by these purely electronic mechanisms were later for the La 0.8 Sr 0.2 MnO 3 /PbZr 0.2 Ti 0.8 O 3 bilayers. [ 85 ] More

, the interface bonding ME effect was verifi ed by the tion of room-temperature multiferroicity at the inter- BaTiO 3 ultrathin fi lm with Fe or Co. [ 86,87 ] ary to the case of Fe/BaTiO 3 system mentioned above,

BaTiO 3 structure reported by Dai et al., [ 87 ] has shown type of interface bonding ME effect, which is deter-y the change of magnetic moments on Ni atoms near

rface, and that there existed an extraordinary intralayer on of magnetic moments within the Ni layer. Further- was demonstrated that the underlying physics was due ifferent interfacial electronic structure and the different

agnetic interaction. [ 87 ] ging the Ni component reported by Dai et al. to the ity-functional calculations were employed by Lukashev

] to investigate the effect of ferroelectric polarization of on the magnetocrystalline anisotropy of the Fe/BaTiO 3 e. It was found that the interface magnetocrystalline py energy decreased (from 1.33 to 1.02 erg cm 2 ) when

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CLE an optimal value of H DC and therefore a peak in the value vs H DC plot, [ 42 , 93 ] experiments show that remains more or

the ferroelectric polarization was reversed. This strong ME coup ling was explained in terms of the changing population of the Fe 3tion rev

It haas PVDto the fas inteusing PVDF electricline anwas shomagneby 50inducedwith Cface ma

In thME effa viableheterosthe intstructu

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less contion strthe matME comas the MmagnetFe 3 O 4 oits origithe lineand thepolymeites is mmatrix cfor the additioncomposthe bonpreventnanocomsuch na

RegaME nangiant Mtric polyreports CoFe 2 Omatrix osince thages beroelectrof PVDferrite nnetic anthe caseMF fi lmpolarizavalue oTrFE)/Napplied obtaine

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d orbitals at the Fe/BaTiO3 interface driven by polariza-ersal. s been also reported, [ 89 ] that organic ferroelectrics, such F, have an additional advantage of being weakly bonded erromagnet, thus minimizing undesirable effects such

rface chemical modifi cation and/or strain coupling. By fi rst-principles density functional calculations of Co/heterostructures it was demonstrated the effect of ferro- polarization of PVDF on the interface magnetocrystal-isotropy that controls the magnetization orientation. It wn that switching of the polarization direction alters the

tocrystalline anisotropy energy of the adjacent Co layer %, driven by the modifi cation of the screening charge by ferroelectric polarization. The effect was reduced

o oxidation at the interface due to quenching the inter-gnetization. is way, ref. [ 89 ] indicates that the electronically assisted

ects at the ferromagnetic/ferroelectric interfaces may be alternative to the strain mediated coupling in related tructures and the electric fi eld-induced effects on erface magnetic anisotropy in ferromagnet/dielectric res. infl uence of interfacial effects on the ME properties F based ME composites was also studied in the refer- ] Flexible SmFe/PVDF laminate composites were pre-y depositing of SmFe nanoclusters onto the PVDF fi lm luster beam deposition method. Since there was a small t of residual argon carrier gas molecules in the SmFe/fi lm, it was actually subjected to a residual compressive Accordingly, under the combination infl uence of nega-gnetostriction and compressive stress, the SmFe fi lm lso generate the magnetic anisotropy with an in-plane

tic easy axis, which allows a more effi cient magnetic-ical-electric coupling along the interface.

ticulate Nanocomposites

compared to their ceramic ME counterparts, a much variety of 03 type ME polymer-based nanocompos-e been reported in the last two decades. Polymers such oelectric PVDF and PU have been used in such ME mposites due to their good piezoelectric/electrostrictive ies. [ 90,91 ] e electrostrictive PU-based ME composites several inter-results have been obtained, including the extraction of ME current from the total output current response and xistence of both linear and quadratic ME responses in d PU fi lm. The obtained linear ME effect is of the same f magnitude as that of the Cr 2 O 3 single crystal (up to

cm 1 Oe 1 ) and a possible linear magnetoelastoelectric g between fi llers and polymer matrix not triggered by

tostriction has been also proposed. [ 92 ] linear voltage ME coeffi cients obtained in PU/Fe 3 O 4 and kel composites were 11.4 and 6.0 mV cm 1 Oe 1 , respec-t 7 Hz, 0 DC fi eld and 1 Oe AC fi eld. Even when it is ed that due to the magnetostriction it should be found . KGaA, Weinheim

stant with increasing H DC . This experimental observa-ongly suggested that the magnetostrictive properties of erial have no infl uence in the PU/Fe 3 O 4 and PU/Nickel

posites. This interesting fact has been confi rmed E response in PU composites is independent of the

ostrictive properties of the fi llers such as Terfenol-D, r Nickel. [ 49 ] In this way, the ME coupling does not have n in the magnetostriction of the particles but rather in ar elastic interaction between those particle aggregates highly polar microdomains of the semi-crystalline

r PU. [ 9395 ] Consequently, the coupling in PU compos-ainly due to the particular nature of the elastomer PU

omposed of both rubbery and polar domains. A support aforementioned mechanism is the fact that the simple of morphous carbon nanopowder into PU based ME ites enhances the quasistatic strain amplitude [ 96 ] since ding between the PU polymer and carbon nanopowder s slippage and effectively improves the strain in the

posite. [ 97 ] In any case, the origin of the ME coupling in nocomposites is not yet clearly established. [ 6 ] rding the use PVDF as the piezoelectric constituent of ocomposites and after the theoretical calculations of

E on ferromagnetic rare-earth-iron-alloys-fi lled ferroelec-mers in 2001 by Nan et al., [ 63 , 98 ] two main experimental can be found in the literature. Martins et al. introduced 4 and Ni 0.5 Zn 0.5 Fe 2 O 4 ferrite nanoparticles into a polymer f (P(VDF-TrFE). P(VDF-TrFE) was used instead of PVDF e copolymers of PVDF, containing VDF molar percent-tween 55 and 82, crystallizes from the melt in the fer-ic phase which is an essential factor for the preparation F based ME nanocomposites. [ 99100 ] The P(VDF-TrFE)/anocomposites exhibit ferroelectric, piezoelectric, mag-d direct ME effect dependent on the ferrite loading. In of P(VDF-TrFE)/CoFe 2 O 4 nanocomposite, the resultant s showed saturated hard magnetic properties, improved tion and piezoelectric response and a maximum 33 f 41.3 mV cm 1 Oe 1 . On the other hand, for P(VDF-i 0.5 Zn 0.5 Fe 2 O 4 composites, 33 increases linearly with H DC and a 1.35 mV cm 1 Oe 1 maximum value was

d for samples with 15 wt% ferrite. [ 50 ] ntrast to PU based composites, [ 49 ] the ME P(VDF-TrFE)/ 4 response is strongly infl uenced by the magnetostriction errite nanoparticles since an optimal value of the H DC ic fi eld was observed and consequently a peak on the H DC plot appears ( Figure 3 ). The observed difference the in plane and out-of-plane ME response of P(VDF-oFe 2 O 4 nanocomposites was fully attributed to the differ-the d 33 and d 31 P(VDF-TrFE) piezoelectric constants. [ 101 ] g et al. studied the effect of CoFe 2 O 4 nanoparticles on rphology, ferroelectric, magnetic and ME behaviors of TrFE)/CoFe 2 O 4 nanocomposites. Once again, the fer-ic and ME responses are strongly infl uenced by the ration of ferrite nanoparticles. [ 51 ] A signifi cant experi- 33 value around 40 mV cm 1 Oe 1 was obtained in this nanocomposites. Both experimental ME voltage coeffi -f Martins et al. and Zhang et al. were theoretical con-by a relatively simple model based on those of Wong

Adv. Funct. Mater. 2013, 23, 33713385

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FEA

oice-Drdrf icla0ppld

spatnit

dl- co

10

celd

and Srespon

33 =

where

L E =

dHpdH

=

Here, respectric cophaserespeczationnanoc

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2.3. Po

UnlikepositeME metrostr

Figure 3from re

. c.

Adv. Funstress cnetostr

Threand PV

In o f , of Temixturedielectra percothan 0.ME resME resthreshoonly refor 33 less thacomposmainlyTerfenothe MEto imprcles [ 106,

ferrite thresho

hin [ 102 ] and Zhou and Shin. [ 103 ] In this model, the ME se 33 can be expressed as:

(1 ) L E

(d31p

dYxpdHm

+ d32p dTypdHm

+ d33p dTzpdHM

)(dHmdH

)

(3)

L E and dHpdH

are given by:

[m + 2p ]

][(1 ) m + (2+ ) p

]

(4)

3p

(1 )(m + dMmdHm

)+ (2+ ) p

(5)

p and m indicate the polymer and magnetic phase tively; d 3n the piezoelectric coeffi cients; the dielec-nstant, the volume fraction of the magnetostrictive

; T and H are the stress and applied magnetic fi eld, tively; the magnetic permeability and M the magneti-. dMm is obtained from the magnetization curve of the

. Comparison between the responses of P(VDF-TrFE)/CoFe 2 O 4 (data f. [ 42 ] ) and PU/Fe 3 O 4 ME nanocomposites (data from ref. [ 49 ] ).

Figure 4polymerref. [ 64 ] ) 2013 WILEY-VCH Verlag GmbH & Co. KG

i) the min the

ii) a soft D nan

Althouallows hthe seconresponseFeature Athis MF taneouslyoptimizafact, theoboundarythe Greeical boun

dHmomposites. sible ME polymer-based nanocomposite structures were ynthesized using conducting polyaniline and nano-BiFeO 3 particles through in situ solgel polymerization malatha et al. [ 104 ] The morphology, crystalline structure, tic, and optical properties of polyaniline/BiFeO 3 compos-

ith various concentrations of nanofi ller were discussed e ME response of such nanocomposites has not yet been ed.

lymer as a Binder Composites

in the previous section, in the poly mer as a binder com-s the polymer is not used as the piezoelectric phase of the aterial but as a binder for the piezoelectric and magn-ictive particles that keep them together and provides the

ct. Mater. 2013, 23, 33713385TURE A

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upling between the piezoelectric matrix and the mag-tive fi llers. phase particulate composites of Tefenol-D alloy, PZT F [ 72 ] ( Figure 4 ) were the fi rst to be studied. er obtain the ME response, a small volume fraction, enol-D nanoparticles were dispersed in a PZT/PVDF by a simple blending technique and the obtained , piezoelectric and ME properties demonstrate that tion transition occurs at f 0.12. When f is lower

7 the MF composites exhibit good piezoelectric and onses but when 0.07 f 0.12 the piezoelectric and onse sharply drops and disappears at the percolation , above which the composite becomes a conductor and ond magnetostrictively. The maximum obtained value 2KOe was about 42 mV cm 1 Oe 1 at f = 0.06 which is half with those obtained for the PZT/ferrite ceramic e (115 mV cm 1 Oe 1 ). [ 105 ] Since this ME response is etermined by the f Terfenol-D , the pre-treatment of the D nanoparticles, by the use of surfactants can change oupling.. Surfactants are usually used in such a way ve dispersibility and dispersion stability of nanoparti-7 ] in different kind of matrices. In the case of the PZT/ramic composite, surfactants increase the percolation . This experimental change has two consequences:

aximum magnetostrictive fi ller concentration allowed ME nanocomposites is increased;

and inactive interfacial layer is induced in the Terfenol-oparticles.

Schematic representation of the particulate Terfenol-D/PZT/omposites (based on an experimental description reported in 3377wileyonlinelibrary.comaA, Weinheim

gh the fi rst consequence is extremely positive since it igher magnetostrictive content in the ME composite, d produces a negative effect on both the piezo and ME of the nanocomposites as already discussed in this rticle. [ 62 ] Further improvement in the ME response of

composite lies in increasing the f Terfenol-D and simul- ensuring good interfacial contact between phases by

tion the nanocomposite processing. In view of this retical calculations were performed on the mechanical conditions infl uence in the ME properties based on

ns function technique. [ 98 , 108 ] Three different mechan-dary conditions were considered:

3378

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wileyonl

FEATU

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iii) comin

Forare 11respecparticlhave apositeparticlcompoeffect.constaeffectieffect the PZ

Thesame Polymrate-dopoly(mTerfencance the saOe 1 , matrixtivity e

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to low ME performance. The ME properties are improved in the inelibrary.com 2013 WILEY-VCH Verlag GmbH & Co

pletely mechanical free boundary condition; pletely mechanical clamped in the zz direction and free

the transverse direction.

the composite with f = 0.06, the maximum 33 values 7, 362, and 62 mV cm 1 Oe 1 for situations i), ii) and iii), tively. The same calculations also revealed that the PZT es polarization and the inactive PZT/PVDF interface signifi cant effect on the ME properties of the nanocom-s. Random orientations of the polarization in the PZT es result in the disappearance of piezoelectricity in the sites, and thereby the disappearance of the extrinsic ME

Although the thin interfacial layer with the same elastic nts as the polymer matrix has only a slight effect on the ve magnetostriction of the composites, the piezoelectric is strongly infl uenced by the interfacial layer surrounding T particles. [ 64 ] infl uence of different polymers in the ME response of the kind of MF nanocomposites was recently investigated. [ 109 ] er electrolyte polyethylene (PEO) and lithium perchlo-ped PEO, lithium perchlorate-doped PEO (Li + -PEO) and ethyl methacrylate) (PMMA) were mixed separately with ol-D and PZT particles aiming to evaluate the signifi -of the polymer matrix conductivity in the ME response of mples. The obtained 31 were 1.3, 3.2, and 4.8 mV cm 1 respectively, for the Li + -PEO, PEO and PMMA polymer . Those results confi rm that samples with higher conduc-xhibit lower ME responses. [ 72 ] ough the fl exibility, structure, simple fabrication, and

haping of the polymer as a binder ME materials provide ive advantages in possible ME applications, these added es are limited since all of them are inferior to those of the late nanocomposites.

minated Composites

e three-phase Terfenol-D/PZT/PVDF particulate com-s of the previous section, the f Terfenol-D allowed in the omposites is quite low, which strongly limits the ME se of the MF nacomposites. To eliminate this limitation, rent class of ME material has been developed: laminated sites. minate bilayer or multilayer confi guration for ME com-

s has other advantages over bulk nanocomposites. In lar, the loss of polarization in bulk composites due to

e currents can be overcome in layered structures. The lectric phase can be poled to enhance the ME coupling is also possible to vary the poling and applied fi eld direc-o achieve maximum ME coupling. laminated composites consisting on one Terfenol-D/ particulate composite layer sandwiched between two VDF particulate layers prepared by hot-molding technique en reported. [ 80 ] The polymer phase PVDF is used just as er, with no infl uence on the ME properties of the lami-composite. Experiments show that with f PVDF 0.3, the ncentration of PVDF leads to low quality of the compos- the connection between the three phases is poor, leading

intermas f PVDinert PME actcompoobtainesensiviOe 1 atferencesitivity,the aninetostrin out-the PZ

Novsistingtwo Teinvestigsitivity below quencyites is thicknethe PZ( L ) equby incr( t p ) Thea 2/7 reffectiv t p / L , this due the lam

FinafabricaInc., Utigatedthe equ0.5 in tPZT coepoxy l504 OeOe 1 , rWhen incre0.6 andThe inincreasthat thdecreas

A simcouplintive mabi and one or modelehighesFe 3 O 4 /Oe 1 at. KGaA, Weinheim

ediate f PVDF concentration range (0.3 f PVDF 0.5) and F further increases ( f PVDF > 0.5), the high concentration of VDF causes weak dielectric, magnetostrictive, piezo and ivity of the three-phase laminated Terfenol-D/PZT/PVDF sites. A maximum value for 33 of 80 mV cm 1 Oe 1 was d at 1 kHz, 4 kOe, and f PVDF = 0.5. The maximum ME ty of such laminated composites can reach up to 3 V cm 1 a resonance frequency of around 100 kHz. [ 110 ] The dif- in the longitudinal ( 33 ) and transversal ( 31 ) ME sen- 3 and 3.8 V cm 1 Oe 1 , respectively, is fully attributed to sotropy of the laminated ME samples. At high bias, mag-iction becomes saturated faster under in-plane bias than of-plane bias producing a nearly constant electric fi eld in T, thereby decreasing 31 with increasing bias. el laminated conformations of the ME samples, con- on a PZT/PVDF particulate layer sandwiched between rfenol-D/PVDF particulate composite layers [ 111 ] were ated. With this conformation, the maximum ME sen- 33 was improved to 300 mV cm 1 Oe 1 at a frequency

50 kHz and about 6 V cm 1 Oe 1 at the resonance fre- of around 80 kHz. The ME response of such compos-also strongly dependent on the applied bias and on the ss ratio ( t p / L ) between the Terfenol-D/PVDF layers and T/PVDF layer. Keeping the thickness of the composite al to 2.5 mm, the t p / L ratio was varied from 1/7 to 5/7 easing the thickness of the PZT/PVDF particulate layer values of the composites fi rst increase with t p / L until atio, which could be attributed to the increase in the e piezoelectric effect. However, with further increasing e ME sensitivity declines after a maximum value, which to the reduction in magnetostrictively induced strain of inated composites with increasing t p / L . [ 79 ] lly, three-phase Terfenol-D/PZT/binder composites were ted by substituting PVDF by Spurr epoxy (Polysciences SA). [ 112 ] The ME properties of such materials were inves- experimentally and theoretically confi rmed by the use of ivalent circuit approach. [ 113 ] Samples with a f Terfenol-D = he Terfenol-D/Spurr epoxy laminates with two different ncentrations ( f PZT = 0.6 and f PZT = 0.75) in the PZT/Spurr aminate were measured. At a frequency of 10 kHz and fi eld, the obtained value for 31 was 0.3 and 0.4 V cm 1 espectively for the f PZT = 0.6 and f PZT = 0.75 samples. the frequency was changed to the resonance ( 55 kHz), ases up to 10 V cm 1 Oe 1 in the case of the f PZT = 11 V cm 1 Oe 1 in the case of the f PZT = 0.75 composite. crease of with increasing f PZT is expected, due to the e of the piezoelectric phase. It is nevertheless to notice e improvement of the ME response is accompanied by a e of the fl exibility and strength of the composite. ilar ME composite concept uses PU to increase the ME

g between the piezoelectric PVDF and the magnetostric-terial (Fe 3 O 4 and Terfenol-D). [ 114 ] It was reported a ME in trilayered composites consisting in on layer PVDF and two layers of PE fi lled with Fe 3 O 4 or Terfenol-D particles, d by a driven damped oscillation system, [ 115,116 ] with a

t 33 obtained for the trilayered sample of PE + 2 wt% PVDF/PE + 2 wt% Fe 3 O 4 with a value of 753 mV cm 1 2000 Oe.

Adv. Funct. Mater. 2013, 23, 33713385

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FEAa PVDF/Metglas 2605SA1 laminate a composite at a non reso-nance frequency of 20 Hz and at 3 Oe DC magnetic fi eld. [ 74 ] k0i D

e

ti

e cfc

1

tof

6

d. [

acc bmeot

ot or

ao

te

. vsT

FurtTerfenoand a Pepoxy, [ 1

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i) larghigh

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Furthprone types oties of Pnates. [ 7

ally usepoling300 MVtemperlow eleuntil a utilizedparing vs DC attributmagnetrelated tion ofof the mzomagnimum the faccyclic pferent o 31 obtvalues laminaof the mtions inpoling)

A noites waP(VDF-

her, ME laminates of vinyl ester resin (VER)-bonded l-D magnetostrictive layer ( f Terfenol-D from 0.16 to 0.48) ZT piezoelectric layer glued together with a conductive

17 ] show 31 values increasing gradually with increasing

D in the MS layer reaching a saturation for f Terfenol-D > 0.4 the increasing elastic modulus and piezomagnetic coef-of the magnetostrictive phase. A maximum value of m 1 Oe 1 was obtained at 666 Oe DC fi eld with f Terfenol-D

agnetostrictive layer equal to 0.48. ayer disk prepared by bonding a PZT disk with Terfenol-iscosity epoxy disk [ 118 ] show at a bias of 3 kOe three res-

peaks with 33 values of 2.79 V cm 1 Oe 1 at 35 kHz, cm 1 Oe 1 at 100 kHz and 1.31 V cm 1 Oe 1 at 122 kHz ively. [ 119 ] The resonance peak at 122 kHz is attributed transversal resonance, [ 120,121 ] which is present in many ch laminated composites. [ 80 , 122 ] The observation of three onance peaks in laminated composites is for the fi rst ported in this work. , fl exible ME laminates ( Figure 5 a) composites were ed following similar approaches but with different

tostrictive layers, as for example, Metglas/PVDF uni- (Figure 5 b) and threelayer (Figure 5 c) sandwich rations. [ 73 ] e laminates required an applied H DC of only 8 Oe r to achieve a maximum ME response, 1/50th of that d for the previous ME laminates. These small mag-eld ME laminates have giant ME voltage coeffi cients ellent sensitivity to small variations in both AC and DC

tic fi elds. The Metglas layer and PVDF layers are glued r using an epoxy and both laminate types were found to strong ME enhancement: three-layer composites: 31 = m 1 Oe 1 ; unimorph composites: 31 = 310 V cm 1 Oe 1 , ar the longitudinal resonance frequency at 50 kHz. At

requencies, a maximum value of 7.2 V cm 1 Oe 1 was d for both geometries. Although the magnetostriction of s SA1 was only 42 ppm which is far smaller than the agnetostriction of Terfenol-D, the maximum value of its

e piezomagnetic coeffi cient d 33m = 4 10 6 Oe 1 is three times larger than the one for Terfenol-D d 33m = 1.2 e 1 due to the small saturation fi eld. [ 123 ] This extremely

C requirement is an important advantage of Metglas/

. a) Picture of a fl exible PVDF/Metglas unimorph laminate; orph confi guration, and c) three layer laminate. (Panels (b) and ased on the experimental description reported in ref. [ 73 ] ). 2013 WILEY-VCH Verlag GmbH & Co. K

ance imlinking amatrix wand conto those electric P(VDF-T

laminates over other previously reported types, offering al in practical applications. r the fi rst works on Metglas/PVDF laminate nanocom-, [ 73 ] several works were devoted to these promising terial. For example, taking advantage of the magnetic

ncentration effect of Metglas as a function of its sheet ratio values of 31 = 21.46 V cm 1 Oe 1 were obtained in

t. Mater. 2013, 23, 33713385 3379wileyonlinelibrary.comGaA, Weinheim

TURE A

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ing advantage of the anisotropy of PVDF/Metglas-SA1-XGDC laminates it was demonstrated the ability of such tes to be used as an ultra-sensitivity detection device of

C (1 10 9 Oe). [ 9 ] This high sensitivity is due to the:

piezoelectric voltage coeffi cient of PVDF that indicates a output voltage in response to a small variation of strain;

piezomagnetic coeffi cient of the Metglas-SA1-010XGDC alloy; vely small demagnetization factor of the Metglas-SA1-010XGDC alloy.

is proportional to the piezomagnetic and piezoelectric oeffi cients and inversely proportional to the demagnet-actor, a high sensitivity is characteristic of the Metglas/omposites. The maximum 31 value obtained in the etglas-SA1-MP4010XGDC laminate was 400 mV cm 1 kHz frequency and H DC = 3 Oe.

er, as it was found that the depolarization effect is occur in polymers such as PVDF, the effect of two

poling processes were investigated in the ME proper-VDF hexafl uoropropylene (PVDF-HPFP)/Metglas lami- ] After applying the so-called conventional poling, usu- in the poling of piezoelectric polymers [ 124 ] or cycling

125 ] In the fi rst, a D.C. electric fi eld ranging from 100 to /m was applied to the sample during 300 s at room ture. Regarding the second poling method, starting at tric fi elds, the sample is cycled through many loops onsistent behavior is indicated. Higher fi elds are then until the desired stable polarization is achieved. Com-oth methods, it was verifi ed a shift of the ME peak ( agnetic fi eld) of one method with respect to the other, d to the variation of the boundary conditions of the striction of the Metglas. Since the maximum peak is o the piezomagnetic coeffi cient of Metglas, the varia-magnetostrictive vibration will result in the variation agnetostrictive coeffi cient as well in a shift of the pie-

etic coeffi cient peak. In this case, variation in the max-f the values with different poling processes is due to that conventional poling uses DC electric fi eld, while ling employs an AC electric fi eld, which produces dif-ientation stresses in the dielectric polymer. The highest ined was 12 V cm 1 Oe 1 at 5 Oe and is lower than the btained for the previously discussed PVDF/Metglas s, however it has the advantage of allowing the change agnetic DC fi eld at witch is obtained through modifi ca-

the poling process (electric fi eld strength and type of

el approach to high performance ME polymer compos- presented with the chain-end cross-linked ferroelectric rFE)/Metglas 2605 SA1 composites. [ 126 ] The perform-

provement is due to the introduction of chain-end cross-nd polysilsesquioxane structures into the P(VDF-TrFE) hich leads to the formation of larger crystalline samples sequently better piezoelectric response in comparison of pristine P(VDF-TrFE) copolymers. With better piezo-properties a higher is expected. For the cross-linked rFE)/Metglas laminate an 31 value of 17.7 V cm 1 Oe 1

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was awherundevaluefurthof 65largeto a wtric la

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Table 2. Comparison of the main characteristics of the developed polymer-based magnetoelectric materials.

Type

Nanoc

Polyme

LaminConstitution

omposite PE/Fe 3 O 4

PE/Nickel

P(VDF-TrFE)/Ni 0.5 Zn 0.5 Fe 2 O 4

P(VDF-TrFE)/CoFe 2 O 4

P(VDF-TrFE)/CoFe 2 O 4

r as a binder composites PVDF/Terfenol-D/PZT

PEO/Terfenol-D/PZT

Li + -PEO/Terfenol-D/PZT

PMMA/Terfenol-D/PZT

ate PVDF/Terfenol-D/PZT

PVDF/Terfenol-D/PZT

Spurr epoxy/Terfenol-D/PZT

PE/PVDF/Fe 3 O 4

VER/Terfenol-D/PZT

PZT/Terfenol-D/epoxy

Gd crystal/P(VDF-TrFE)/silver conductive epoxy

PVDF/Metglas unimorph

PVDF/Metglas three-layer

PVDF/Metglas

PVDF/Metglas

PVDF-HPFP/Metglaslinelibrary.com 2013 WILEY-VCH Verlag GmbH & Co

V cm 1

of 5.1 kThis inME lamfor reallaminakind ofi.e., a tcrystal the ferrpreparea silvercontactwas obamplituexploiti

As athe maordered

3. App

Based orials inare read

chieved under a DC magnetic fi eld of 3.79 Oe at 20 Hz, eas the value obtained for the pristine P(VDF-TrFE)/Metglas r the same conditions is 31 = 6.9 V cm 1 Oe 1 . The 31 s for cross-linked P(VDF-TrFE)/Metglas laminates can be er improved to 383 V cm 1 Oe 1 at a resonance frequency kHz. The later laminate composite not only shows the st value of in polymer-based ME materials but also points ay to improve the piezoelectric properties of the piezoelec-yer and hence the ME response. aving behind the ME PVDF based/Metglas composites, laminated polymeric ME materials section is concluded the large ME response from mechanically mediated etic fi eld-induced strain effect in a PVDF/Ni 50 Mn 29 Ga 21

e crystal. [ 75 ] Ni 50 Mn 29 Ga 21 single crystal shows giant DC etic fi eld induced strains of 610% in the tetragonal and rhombic martensitic phases, which has attracted interest E applications. [ 127 ] Showing obvious differences from

aditional magnetostrictive phases (Terfenol-D, ferrites or las), the mechanism of the giant magnetic fi eld-induced s is due to the reorientation of the martensitic twin vari-

under an applied magnetic fi eld as a result of magnetoc-line anisotropy. [ 128,129 ] DF/Ni 50 Mn 29 Ga 21 single crystal bilayered composites produced by adhering one layer of the ferromagnetic memory alloy to one layer of the piezoelectric polymer a conductive silver epoxy. The largest value 33 of 1.24

Cross-linked P(VDF-TrFE)/Metglas 2605

PVDF/Ni 50 Mn 29 Ga 21 H DC-Max. ME [Oe]

Ref. [mV cm 1 Oe 1 ]

resonance [mV cm 1 Oe 1 ]

0 [ 92 ] 11.4

0 [ 92 ] 6

5000 [ 50 ] 0.1 1.35

2500 [ 42 ] 4.1 41.3

2000 [ 51 ] 40

2000 [ 72 ] 42

1400 [ 109 ] 1.3

3.2

4.8

4000 [ 110 ] 80 3000

4000 [ 111 ] 300 6000

504 [ 113 ] 400 1100

2000 [ 114 ] 753

666 [ 117 ] 2700

3000 [ 119 ] 1310 2790

200 [ 131 ] 500

8 [ 73 ] 7200 238000

310000

8 [ 74 ] 21460

3 [ 9 ] 400

5 [ 76 ] 12000 . KGaA, Weinheim

Oe 1 obtained at 1 kHz and at an optimal magnetic fi eld Oe was experimentally and theoretically confi rmed. [ 28 , 130 ] vestigation not only reported a different constitution in inates but also created a distinct physical mechanism

izing such effect. An alternative concept in ME poly mer ted composites is based on thermal mediation. [ 131 ] This MF material uses the large magnetocaloric effect (MCE), emperature change induced in the ferromagnetic Gd by a magnetic fi eld and a large pyroelectric response in oelectric P(VDF-TrFE) (68/32 mol%). Composites were d by bonding a Gd crystal plate to the P(VDF-TrFE) with conductive adhesive epoxy to ensure a good thermal between the layers. An value of 0.5 V cm 1 Oe 1 tained at 293 K in an AC fi eld of 2.4 Hz and 120 Oe de. The was further enhanced to 0.9 V cm 1 Oe 1 by ng the magnetic fl ux concentration effect. [ 132 ] conclusion from this section, the results obtained for in polymer-based ME material are shown in Table 2 by composite type.

lications

n the previous sections it is concluded that ME mate- general and polymer-based ME materials in particular y for technological applications. Promising applications

4 [ 126 ] 17700 383000

5100 [ 75 ] 1240

Adv. Funct. Mater. 2013, 23, 33713385

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3.2. Ene

The ever decreasing power requirement of electronic sensors and devices [ 150 ] has attracted attention to energy harvesting tech-nologies. [ 151 ] In particular, there has been signifi cant interest in the area of the vibration energy based on piezoelectric and magnetic harvesters. [ 152155 ] After the fi rst hypothesis of ME materials as energy harvesting devices [ 156 ] some studies have been reported in this area. As described in the previous section, there have been signifi cant advances made in improving the

include magnetic fi eld sensors, transducers, fi lters, devices,others. [ 5

polymerpolymeribility, lin someto advanhighligh

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Adv. Fun 2013 WILEY-V

was deposited onto a commercial piezoelectric a Piezomechanik Gmbh.). As a result of the inversrictive effect, the effective anisotropy of the magnwas controlled by the applied voltage and used toetization from one state to the other. ositive voltage sets the magnetization in one of thns, whereas a negative voltage sets it in the othhe position is kept when no voltage is applied. Siation is stored magnetically, the readout can bmagnetoresistive techniques. Theoretical calculatonfi rmed that the properties of existing materia

ct. Mater. 2013, 23, 33713385compatscale. Wper laydevicesstrong increas

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Figure 6 . Representation of a

oscillators, phase shifters, memory and biomedical materials, among , 13 ] In some of these applications ic based ME materials, due to the s unique characteristics such as fl ex-ightweight, versatility, low cost and cases biocompatibility can be taken tage. Some of these applications are ted as follows.

r State-Memory

t the intense demand of multimedia storage many re being made to develop storage technologies with torage speed and density. [ 133,134 ] In the traditional two and 1) memories, the memory element is a magnetic unction that consists on an insulating tunnel barrier hed by two magnetic electrodes. [ 135 ] The resistance of nctions strongly depends on the relative orientation

agnetic moments, which is used to determine the state (0 or 1) from the two magnetic electrodes. [ 136 ] ed magnetic bits can then be read out nondestructively ting such resistance changes, however, in the writing the magnetic bits are usually encoded by the use of agnetic fi elds which is a process relatively slow and cally expensive. [ 3 ] These problems can be solved with ipulation of the magnetization direction by the use of ic fi eld, [ 137 ] taking advantage of the ME effect. [ 58 , 138 ] For

d of multi-state memory ( Figure 6 ) the multiferroicity sential factor for the information storage while the ME

agnetodielectric effect [ 139 ] is the mechanism for the and writing procedure. [ 140 ] r-state memory cell based on the ME PZT/Co bilayer

ite [ 140 ] has been already proposed. Co was used since it used in magnetic recording and PZT due to its ferro-

properties. The composite was obtained by gluing one PZT to one layer of Co with an epoxy. The polariza-

gnetization of such composite can be controlled by the on of magnetic and electric fi elds and the combination mnant ferroelectric polarization and magnetization in

/Co bilayer memory cell exhibits the desired four phys-s. Results gave clear four-state signals of 15.8, 4.4, 5.5, .3 V, which demonstrated the feasibility of the design. tinct room temperature ME memory was presented in ] in which a magnetoelastic nanostructured multilayer nm)/FeCo(5 nm)) with the required uni-axial charac-ATU

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ible with the realization of such a device at the nanometer ith the reduction of size, densities up to 40 Gbits cm 2 r can be expected for low energy, nonvolatile memory Given the very low expected power, such a device is a ontender for vertical integration of several layers, quickly ng the memory density. [ 141,142 ] e current electronic market demands are intimately

to the use of fl exible materials, [ 143 ] not only the mag-electric properties of materials will play a key role in re but also their mechanical properties. [ 144 ] In this way, stitution of PZT by a polymer in bilayer four-state ME ies will meet these new challenges. Contrary to what ed a few decades ago, when polymers were just used der in memory devices, [ 145,146 ] devices based on poly-aterials are now a interesting topic due to their sim-ood scability, low-cost, 3D stacking capability, and large for data-storage. [ 147 ] These electroactive polymers are deposited by ink-jet printing, spin-coating, or vacuum tion on a variety of substrates for the fabrication of ies. [ 148 ] In this way, polymers, may also acquire in the ture a more central status in the memory market due to meric four-state ME memory devices. static magnetic and electric fi elds are used for writing tion in the four-state ME memories and the ME coeffi -used for reading with the help of a small bias magnetic ch coeffi cient is determining in the practical applica-

this new kind of memories. It was already reported in ature that materials with ME coeffi cients in the order V cm 1 Oe 1 can be used as components of room tem- four-state memory prototypes. [ 149 ] Accordingly, and after d analysis of the ME coeffi cients obtained from the dif-pproaches for preparing polymer-based ME composites rized in Figure 9, it is possible to verify that almost all of n be used in the development of this kind of multi-state .

rgy Harvesting

four states memory based on ME materials.

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magnwill A comay methmicr

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power consumption of 75 mW for a duration of 620 ms.

Despite all these developments, the next generation of energy-harvesting applications, such as wearable energy-harvesting systems, may require the piezoelectric materials to be fl exible, lightweight, and even biocompat-ible. [ 163 ] In this way, ME materials based on piezoelectric polymers may be an interesting approach to meeting these requirements due to their fl exibility, versatility, and low cost. [ 164 ] Some of the above reported ME coef-fi cients on polymer-based ME materials are of the same order of magnitude as the best

Figuritude of the ME coeffi cient of laminate composites, which improve the ME energy harvesting effi ciency ( Figure 7 ). mbined magnetic and vibration energy harvesting device be implemented on silicon using the thin fi lm deposition ods and fabrication process fl ow and combination with the o-machining technique. [ 157 ] e energy harvesting in the Terfenol-D/PZT/Terfenol-D ate composites has been reported to provide an energy

ity of 2.0 mW per cubic inch with vibrations of 21 Hz and mg. [ 158 ] Furthermore, a windmill based on this approach

der development. [ 159 ] Ceramic based laminates energy sting materials constituted by PZT/CoFe 2 O 4 and PZT/nol-D have been also reported. [ 160162 ] In the case of the CoFe 2 O 4 energy harvester, magnetically forced extensional tions of laminated plates with piezoelectric and piezomag- layers were theoretically analyzed. It was shown that such cture can be used to harvest magnetic energy and convert

electric energy. The theoretical ME coeffi cient reported for ZT/CoFe 2 O 4 energy harvester was 2.5 V Oe 1 . more experimental work, ME PZT/Terfenol-D laminates placed between an oscillating spherical steel bearing and e-earth magnet (NdFeB) to produce a peak rms power of W from an rms host acceleration of 61 mG at 9.8 Hz. [ 162 ] approach may be useful in the future for kinetic energy sting for applications where the host accelerations are

iaxial. electromagnetic energy harvesting scheme by using the Terfenol-D transducer and a power management circuit presented in ref. [ 161 ] . In such a transducer, the vibrating induced from the magnetostrictive Terfenol-D in the mic magnetic fi eld converges using a Bebronze ultra- horn. Consequently, more vibrating energy can be con-d into electricity by the PZT. A switching capacitor net- for storing electricity was also reported. The output of

ones obinvestigemergenharvesti

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e 7 . Representation of the ME energy harvesting mechanism. nlinelibrary.com 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ransducer charged the storage capacitors in parallel until oltage across the capacitors reached a threshold, and then apacitors were automatically switched to being in series. is way, more capacitors can be employed in the capacitor ork to further raise the output voltage in discharging. For eak magnetic fi eld environment, an active magnetic gen-

r and a magnetic underground coil antenna were used roducing an ac magnetic fi eld of 0.21 Oe at a distance of 0 m. In combination with the supply management circuit, lectromagnetic energy harvester under an AC magnetic of 1 Oe can supply power for wireless sensor nodes with Figure 8 . Representation of the ME magnetic fi eld sensing mechanism.

Adv. Funct. Mater. 2013, 23, 33713385tained in the ME materials that are already being used/ated as energy harvesters, and this will encourage the ce of the next generation of polymer-based ME energy-

ng materials.

netic Field Sensors

ic sensors have been in use for well over 2000 years. plications were for direction fi nding in navigation. [ 165 ] agnetic sensors are also used in navigation but many

ses have evolved. The technology for sensing mag-lds has also evolved driven by the need for improved

ity, smaller size, and compatibility with electronic . [ 166 ] Traditional magnetic sensors like Hall or magne-ve sensors need power supply, which raises some limi-In this context, self-powered magnetic fi eld sensors ctly transfer magnetic energy into electric signals are

interest and can be realized thanks to the ME effect 8 ). [ 167 ]

ing the suggestion by Nan et al. [ 63 ] to use ferroelectric s/rare-earthiron alloys composites, such as magnetic

sors in radioelectronics, optoelectronics, and microwave ics and transducers, magnetoelectric bulk composites ng 95 wt% of yttrium-iron garnet and 5 wt% of lead d multilayer composite material consisting of PZT and

.5 Fe 2 O 4 were used for sensor applications. [ 168 ] The ME omprised a disk or plate from the magnetoelectric mate- two electrodes for connecting to the voltage meter. This owed by numerous reports about PZT based magnetic sors; [ 167172 ] vortex magnetic fi eld sensor based ring-type fenol-D sensors, [ 170 ] PZT/(Fe 80 Co 20 ) 78 Si 12 B 10 laminates sor for microtesla sensitivity [ 167 ] and the effect of the

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FEA Acknowledgements The aut(FCT) foNMed-SThe aut2010 Euthe suppmodifi edentries i

mutual inductance on the magnetic fi eld sensitivity of the ME PZT/Metglas laminate, [ 172 ] respectively.

The low fl exibility, cost, and fragility of PZTs [ 173,174 ] do not meet the challenges of future sensor applications, [ 175,176 ] there-fore multiferroic and ME polymer-based composites are pos-sible successful alternatives for the more traditional ceramic based ME magnetic sensors. [ 42 , 44 , 50 ] In this way, the ME lami-nates of Metglas/PVDF magnetic fi eld sensors were experimen-tally studied in ref. [ 177 ] and the performance was compared to the prediction from a theoretical analysis. The fi eld sensi-tivity and signal-to-noise ratio (SNR) of ME laminates were also investigated. The results indicate that increasing the electrode area (number of layers) of PVDF can enhanced the fi eld sen-sitivity and SNR. This work has introduced a fi gure of merit to characterize the overall infl uence of the piezolayer properties on the SNR and has shown that newly developed piezoelectric single crystals of PMN-PT and PZN-PT have potential to reach very high SNR for ME magnetic sensors. The results also show that the ME coeffi cients which are presently used to compare the ME materials developed may not be relevant when using thes

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[ 1 ] L. Pe

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Adv. 2013 WILEY-VCH Verlag GmbH & Co.

e ME materials for magnetic sensors. his new kind of ME magnetic sensors also have enormous ntial as by-products related to magnetic sensors: electric cur- sensors, speed sensors, angular sensors, electronic steering, ttle control, battery management, vehicle transmission, dig-compasses, and GPS devices [ 165 ] are just some examples and y of them are already being studied. [ 170 , 178,179 ]

onclusions

onclusion, polymer-based ME materials are a promising arch fi eld with large interest for applications that cer-ly will appear soon. The results obtained from the different roaches for preparing such composites are presented in le 2 and summarized in Figure 9 . The obtained values of magnetoelectric coeffi cients as well as the broad range of magnetic fi eld at which they respond, together with the

ibility, robustness and ease of fabrication related to the mer-based materials, allow a large range of applications, in icular in the fi elds of sensors and actuators.

[ 6 ] J. M [ 7 ] H.

05 [ 8 ] Y.-

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[ 10 ] J. v [ 11 ] C. [ 12 ] M.

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sit [ 15 ] L.

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Mare 9 . Distribution of the maximum polymer-based magnetoelectric fi cient ( ) by reference, type and DC magnetic Field at maximum.

Funct. Mater. 2013, 23, 33713385 3383wileyonlinelibrary.comKGaA, Weinheim

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hors acknowledge the Foundation for Science and Technology r fi nancial support through PTDC/CTM/69316/2006, NANO/D/0156/2007 and PTDC/CTM-NAN/112574/2009 projects. hors also thank the support from the COST Action MP1003, ropean Scientifi c Network for Artifi cial Muscles. P.M. thanks ort of the FCT (grant SFRH/BD/45265/2008). This article was after online publication. Article numbers were added to the

n the reference section.

Received: September 25, 2012 Revised: November 28, 2012

Published online: March 5, 2013

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