Materials Letters 63 (2009) 2198–2200

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Materials Letters

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Magnetoelectric properties of microwave sintered particulate composites

Shashank Agrawal a, Jiping Cheng a, Ruyan Guo a, Amar S. Bhalla a, Rashed A. Islam b, Shashank Priya b,⁎a Materials Research Institute, Pennsylvania State University, University Park, PA 16802, United Statesb Materials Science and Engineering, University of Texas Arlington, TX 76019, United States

⁎ Corresponding author. Tel.: +1 540 231 0745.E-mail address: spriya@vt.edu (S. Priya).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.07.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 March 2009Accepted 14 July 2009Available online 20 July 2009

Keywords:MagnetoelectricMicrowaveCompositeDielectricPiezoelectricMagnetostrictiveSintering

This study reports the results on three different particulate composites synthesized using conventionalsintering, microwave sintering under magnetic field, and microwave sintering under electric field. The resultsshow that magnetoelectric coefficient in the case of samples synthesized using microwave approach yieldsthe same magnitudes as that of conventional sintering. The process of microwave sintering is rapid andsignificantly reduces the time and energy required for synthesizing composites as compared to conventionalsintering.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Magnetoelectric (ME) materials become magnetized when placedin an electricfield, and electrically polarizedwhenplaced in amagneticfield. Dielectric polarization of a material under the magnetic field oran induced magnetization under an electric field requires the simul-taneous presence of long-range ordering of magnetic moments andelectric dipoles [1]. The conditions for the occurrence of ferroelectricityand magnetic order in the same material, often accompanied byferroelasticity, implies (a) the presence of adequate structural buildingblocks permitting ferroelectric-type ionic movements, (b) magnetic-interaction pathways, usually of the superexchange type, and (c) thefulfillment of symmetry conditions. It is well known now that therealizable magnitude of magnetoelectric voltage coefficient in singlephase materials is about 1–20 mV/cm·Oe, which is not sufficient forapplications [2,3]. Further, most of the single phase materials can beused only at low temperature (with the exception of BiFeO3), involveexpensive materials and processing technique, and suffer fromdegradation under cyclic conditions [4]. Hence, ME composites basedupon the product property of materials have been developed [5–7].TheME effect can be realized using composites consisting of individualpiezomagnetic andpiezoelectric phases or individualmagnetostrictiveand piezoelectric phases.

Thermodynamic analysis of themagnetoelectric composites showsthat free energy of the system is strongly dependent upon the elasticcoupling between the phases. The elastic coupling can be maximizedby having coherent response from the magnetostrictive phase under

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D.C. bias such that the stress on the piezoelectric lattice across thegrains is in phase with each other. This is only possible if there isuniform distribution of the magnetostrictive phase within the piezo-electric matrix with textured domain structure. In case of any artificialcomposites, made either by using single crystals or powder proces-sing, the coupling effect between two phases is limited. There aremainly three defects which limit the response: (i) connectivity of themagnetic phases that reduces the resistivity and hence the polingability of the ceramic, (ii) structural porosity due to the sinteringdefects throughmismatched crystal structure, and (iii) diffusion of theions across the magnetic and piezoelectric grains reducing theindividual magnetostriction and piezoelectric constants. Thus, whatis required is a processing technique that can lead to the uniformphase distribution with homogeneous microstructure and reducedcross-diffusion. Homogeneous distribution can be obtained by usingthe nanoscale particles as the starting powder while the cross-diffusion can be reduced by using the microwave sintering methodthat allows rapid densification in a significantly shorter time period.The rapid nucleation and grain growth process at low temperaturesreduce the chance of diffusion thus maintaining the original piezo-electric and magnetic constants [8,9].

Theaimof thismanuscript is to synthesize thecomposites of the samecomposition under the conventional and microwave processing condi-tions and demonstrate the ability of themicrowave process to reproducehigh ME coefficients at low synthesis temperatures and shorter times.

2. Experimental

In this study 0.69 Pb(Mg1/3Nb2/3)O3–0.31 PbTiO3 (PMN–PT)/NiMn0.1Fe1.9O4 (NFM) composites were synthesized using both

Fig.1. SEMmicrographs of PMN–PT(31%):NiMn0.1Fe1.9O4 (64:36) composite (a) sinteredin H-field at 1075 °C, and (b) sintered via conventional sintering technique at 1200 °C.

Fig. 2. (a): Dielectric response of conventionally sintered PMN–PT/NFM sample, and(b): dielectric response of microwave sintered PMN–PT/NFM sample.

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conventional and microwave sintering process. Sintered specimenswere poled under a D.C. field of 3 kV/mm for 20–30 min in a siliconeoil bath at 40–50 °C. The magnetoelectric coefficient (dE/dH) wasmeasured by an A.C. magnetic field at 1 kHz. The charge (Q) generatedfrom the composite was measured in terms of voltage (V) using anautomated charge amplifier and lock-in amplifier. The output signalfrom the charge amplifier was monitored using a digital oscilloscopeHP 54601A (Hewlett Packard Co. USA). The peak electric field gener-ated in response to the appliedmagnetic field was computed using therelation:

E =Q

2Ae0eð1Þ

where QP–P=C·VP–P, ɛ is the dielectric constant and A is the area ofthe sample. The nomenclature of the samples is as follows: P standsfor 0.69PMN–0.31PT, N — NFM, CS — conventional sintering, HS —

microwave sintering in magnetic field, and ES — microwave sinteringin electric field. Thus, PN82CS corresponds to 0.82 (PMN–PT)–0.18NFM sintered using conventional technique.

The sample sintered in microwave field was processed at lowertemperature compared to sample sintered in a conventional furnace.The samples of both 64:36 and 82:18 compositions were sintered in E-field environment at 1150 °C with the holding time of 10 min com-pared to 1200 °C in conventional furnacewith the holding time of 2 h.In H-field, the samples of 64:36 compositionwere sintered at 1075 °C

and 82:18 composition were sintered at 1150 °C. This relates to thefact that due to the rapid coupling of microwaves in magnetic field,64:36 compositions had faster sintering due to the high percentage offerrite present in the composites. Fig. 1 compares the fracture surfacemicrostructure of microwave and conventionally sintered sample. Itcan be seen from this figure that a dense microstructure and similargrain size can be obtained at much lower temperatures using themicrowave sintering.

3. Results and discussion

Fig. 2(a) and (b) compares the dielectric response of the PN82series samples synthesized using conventional and microwaveprocessing conditions. Several differences can be observed betweenthe two samples, namely (a) magnitude of the dielectric constant issignificantly higher for the microwave sintered samples, (b) magni-tude of dielectric loss is significantly lower at higher frequencies forthe microwave sintered samples, (c) dielectric spectra show the pro-nounced peak at the transition temperature for microwave sinteredsamples, and (d) space charge relaxation at the high temperature isreduced for the microwave sintered samples. These differences indi-cate the significant changes occurring in themicrostructure during theprocessing. The results can be explained if it is assumed that duringmicrowave sintering the H-component of the field interacts with themagnetostrictive phase in the composite and enhances the homo-geneous grain growth.

Fig. 3(a)–(c) compares the ME response of composite samplessynthesized under two different conditions. As expected the higher

Fig. 3.Magnetoelectric coefficient of composites in (a) conventional sintering, (b)microwavesintering under H-field, and (c) microwave sintering under E-field.

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NFM (Mn-modified NF) content samples exhibit higher ME coeffi-cient. Further, it can be seen in this figure that ME coefficient of themicrowave sintered samples is of the same order as conventionalsintered samples. For example — ME coefficient of the PN64 seriescomposite sintered in microwave E-field is 125 mV/cm Oe at 100 Oebias field as compared to 140 mV/cm Oe for the samples sinteredconventionally. Further, the response of the microwave sintered

samples is much flatter as a function of the bias field. The response ofthe PN82 series samples is much higher in the microwave sintering ascompared to conventional sintering. This result indicates that thepresence of microwave absorbing components such as Fe2O3 results inbetter sintered bodies.

TheseME responses of the particulate composites can be explainedusing the Van Den Boomgaard et al. model and making the followingassumptions (1) the dielectric constant of piezoelectric phase is muchgreater than the dielectric constant of the ferrite, (2) Young's modulifor both the phases are equal, and (3) there is perfect couplingbetween the piezoelectric and magnetostrictive phases [10]. Simply,the ME voltage coefficient of the composite can be expressed as:

dE=dHð Þcomp = dx=dHð Þcomp dE=dxð Þcomp

= mv dx=dHð Þferrite dE=dxð Þpiezoelectric

ð2Þ

where (dx/dH) is the change in dimension per unit magnetic field and(dE/dx) is the inverse of the quantity, change in dimension per unitelectric field; the subscript comp stands for composite; mv is thevolume fraction of the ferrite. It can be inferred from Eq. (2) that forthe same ferrite fraction, higher ME response will be obtained for thesamples with higher deformation under applied magnetic field and ifthis deformation is directional than even higher coefficients can beobtained. Higher deformation can be achieved in the sample withhomogeneous microstructure and uniform magnitude of the magne-tostriction. Thus, the results of Fig. 3 for PN64 and PN82 series can beinterpreted in terms of microstructure where microwave sinteringenhances the densification and homogenization for lower ferrite con-tent samples while similar microstructure is obtained for the higherferrite content.

4. Summary

We found that composites synthesized under microwave proces-sing conditions were able to reproduce high ME coefficients at lowsynthesis temperatures and shorter times as compared to thatobtained in conventional sintering conditions.

Acknowledgements

This work is supported by NSF and DARPA grants.

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