Appl Phys ADOI 10.1007/s00339-010-5726-9

Dynamic response of converse magnetoelectric effectin a PMN-PT-based multiferroic heterostructure

Yajie Chen · Trifon Fitchorov · Anton L. Geiler ·Jinsheng Gao · Carmine Vittoria · Vincent G. Harris

Received: 21 December 2009 / Accepted: 22 April 2010© Springer-Verlag 2010

Abstract A multiferroic heterostructure, consisting of a25 µm thick Metglas® ribbon affixed to a lead magne-sium niobate–lead titanate (PMN-PT) crystal, was system-ically studied to investigate the time response of conversemagnetoelectric coupling under the application of electricfields at low frequencies (0.05 < f < 10 Hz). This multi-ferroic heterostructure exhibits a considerably strong con-verse magnetoelectric effect, CME = −80%, where CME =[M(E) − M(0)]/M(0), and a converse ME coupling con-stant, A = 22.5 Oe-cm/kV, at frequencies below 1 Hz andnear saturation electric polarization. A switching time (ts),representing the response time of the CME coupling, is mea-sured to be 0.6 seconds for this heterostructure under the ap-plication of instantaneous electric fields. The switching timeresults in significant influences on the magnetoelectric effectespecially at frequencies higher than 2 Hz. The dynamic re-sponse of CME coupling is predominantly determined byferroelectric relaxation within the PMN-PT crystal, as op-posed to the magnetic relaxation of the Metglas® ribbon.A model was used to describe the dynamic behavior of CMEcoupling in disordered systems such as PMN-PT.

1 Introduction

Continued miniaturization of conventional solid-state elec-tronic devices is largely achieved by reducing the physicalsize of active elements (i.e., memory elements, logic gates,

Y. Chen (�) · T. Fitchorov · A.L. Geiler · J. Gao · C. Vittoria ·V.G. HarrisCenter for Microwave Magnetic Materials and IntegratedCircuits, and the Department of Electrical and ComputerEngineering, Northeastern University, Boston, MA 02115, USAe-mail: y.chen@neu.edu

transistors, etc.) or by engineering devices based upon newmaterials possessing multifunctional properties capable ofperforming more than one operation or function within thesame or smaller active volume. Among systems experienc-ing cooperative phenomena, i.e., magnetic ordering, elec-tric ordering, piezoelectricity, etc., multiferroic (MF) ma-terials offer unique potential for innovation in new elec-tronic applications and technologies [1, 2]. For clarificationpurposes, here we refer to materials and constructs havingferromagnetic and ferroelectric components as multiferroic(MF), while the effect derived from the coupling of electricto magnetic fields (vice versa) or associated effects on func-tional properties will be referred to as the magnetoelectric(ME) effect.

Multiferroic materials simultaneously exhibit ferroelec-tricity and ferromagnetism (or antiferromagnetism) and cantypically be realized by two materials design paths, that is, as“natural” or single phase multiferroic compounds, or as “ar-tificial” multiferroic composites or heterostructures. How-ever, most single phase multiferroic materials exhibit a mag-netoelectric coupling response at low temperatures [3] thatseverely hinder their use in practical devices. In contrast,the artificially structured materials, typically constructed asmultilayered heterostructures or as granular composites, of-ten exhibit large magnetoelectric coupling at or above roomtemperature [4–7]. In comparison to the single phase multi-ferroics, these artificial MF composites are particularly at-tractive as a pathway to realizing multifunctional devices[8–10]. Furthermore, the artificial MF structures are rela-tively simple and cost-effective to design and fabricate. Asa result there has been a great amount of interest in under-standing both the fundamental physics as well as the engi-neering potential of these materials and structures [11–13].

Conventional MF heterostructures, in their most com-mon constructs, consist of ferromagnetic magnetostrictive

Y. Chen et al.

elements mechanically coupled to piezoelectric elements.Magnetostrictive materials play an important role in provid-ing a medium to couple magnetic fields to strain, whereasthe piezoelectric element serves to convert strain to voltage.Such elements function in a bidirectional fashion in whichmagnetic fields are generated in response to applied elec-tric fields, as well as electric fields generated in responseto magnetic fields. Due to the availability of high quality,inexpensive, single crystal piezoelectric materials, such aslead magnesium niobate–lead titanate (PMN-PT) and leadzinc niobate–lead titanate (PZN-PT), as well as lead zir-conate titanate (PZT) ceramics, the supporting substrates ofMF heterostructures have evolved to be piezoelectric ele-ments. The magnetostrictive element is typically bonded, orin some cases deposited directly on the surface of the piezo-electric substrate [14]. Ideal magnetic material candidatesfor such applications possess a large magnetostrictive coef-ficient but, importantly, low magnetic field magnetostrictiveresponse and small hysteresis loss (i.e., low coercive fields).

During the past decade, a number of important ap-plications based on the magnetoelectric effect have beenproposed or demonstrated, mostly relying upon MF het-erostructures. Among these applications are ac and dc mag-netic field sensors, transformers and gyrators, actively tun-able microwave devices, including filters, phase shifters, anddelay lines, as well as hybrid spintronic–MF devices as po-tential MRAM elements [15–18].

In recent years, researchers have begun to acknowledgethe importance of the dynamic response of the magnetoelec-tric effect [19–22]. Most studies have focused on the directmagnetoelectric effect (not converse magnetoelectric effect)and PZT-based MF constructs [23, 24]. It is now clear thatthe transition from research to practical applications will re-quire a complete understanding of the dynamic behavior ofsuch systems and in particular domain dynamics, polariza-tion switching, and temporal response of the CME effect toapplied electric or magnetic fields. This work aims to buildthe fundamental knowledge base necessary to accelerate thedevelopment of MF structures towards practical engineer-ing applications. Here, we present results of studies of thelow frequency time domain response of the converse mag-netoelectric effect of a MF heterostructure consisting of aMetglas® ribbon affixed to a lead magnesium niobate–leadtitanate (PMN-PT) single crystal. In contrast to conventionalMF constructs, the present work addresses the dynamic re-sponse of converse magnetoelectric (CME) coupling in thelaminated composite structure.

2 Experiment

The present work focuses on the dynamic properties ofa layered multiferroic structure consisting of a ferromag-netic magnetostrictive Metglas® ribbon affixed to piezo-electric lead magnesium niobate–lead titanate (PMN-PT)

single crystal substrate. The amorphous Metglas® ribbon,Metglas Inc. 2605C0, has a thickness of 23∼30 µm, satu-ration magnetization 4πMs = 18 kG, and saturation mag-netostriction coefficient λs = 35 ppm. It is noteworthy thatthe (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 single crystal, withx = 28–32% PT, not only has a large d31 of ∼−1000 pC/Nfor 〈001〉 poling and ∼−1500 pC/N for 〈011〉 poling, butalso features anisotropic in-plane piezoelectric coefficientsd31 and d32, i.e., d31 = −1500 pC/N and d32 = 900 pC/N,when poled in the 〈011〉 direction [25]. In comparison,the more widely used lead zirconate titanate (PZT) ce-ramics have a piezoelectric coefficient, d31, far smaller at∼−200 pC/N. As a result, PMN-PT crystals have attractedmuch interest in due to the relatively large electromechan-ical coefficients. In particular, the anisotropic 〈011〉-typePMN-PT crystals have naturally become favored candidatesfor use in ultrasensitive magnetoelectric devices [26, 27].Here, we employed the 〈011〉-type PMN-PT crystal withdimensions of L10×W5×T0.5 mm, coated with Au elec-trodes as a means of applying electric fields. This multi-ferroic heterostructure was designed to operate in the L-TME coupling mode (i.e., longitudinal magnetized/transversepolarized) and consisted of a laminated structure of a Met-glas® ribbon and 0.7PMN-0.3PT single crystal poled alongthe 〈011〉 direction. The two components were bonded withquick curing ethyl cyanoacrylate-based adhesive. Measure-ment of the converse magnetoelectric coupling was per-formed in the field and geometry configurations depicted inFig. 1. An external magnetic field (H) was applied alongthe 〈100〉 (d31) direction of the PMN-PT crystal, whilethe 〈011〉 (d33) direction aligned perpendicular to the het-erostructure plane. It is noteworthy that an optimized thick-ness ratio of the magnetic and ferroelectric components is aprerequisite for obtaining large converse magnetoelectric ef-fect (CME) [28]. For the heterostructure of the present study,a thickness ratio of 0.05, where t = 25 µm for the Metglas®

ribbon and t = 500 µm for the PMN-PT crystal, was used.

Fig. 1 Schematic diagram of a Metglas/PMN-PT multiferroic het-erostructure

Dynamic response of converse magnetoelectric effect in a PMN-PT-based multiferroic heterostructure

The magnetic properties were measured using a vibratingsample magnetometer (VSM, Lakeshore Model 7400), withthe magnetic field direction aligned parallel to the 〈100〉 di-rection (d31). Applied voltage ranged from –400 to 400 Vacross the PMN-PT crystal, corresponding to an electricfield strength (E) of −8 to +8 kV/cm. Low frequency mag-netoelectric response was measured by VSM while alternat-ing electric fields were applied across the PMN-PT crystalas a square wave, generated by a BK Precision 4011A Func-tional Generator and a High-Voltage Power Amplifier (TrekModel 609B-3). Electric field dependence of polarizationwas measured by a ferroelectric measurement system (Ra-diant Technologies, Inc.).

3 Experimental results and discussions

As sketched in Fig. 1, an induced magnetic easy axis istransverse to the external magnetic field [29]. It is worthnoting that the external magnetic field (sometimes calledthe magnetic bias field) strongly influences the strength ofthe converse magnetoelectric effect, defined here as CME =

Fig. 2 Dependence of magnetization (M) with time under an appli-cation of different electric fields with square waveform at f = 0.5 Hzand H = 20 Oe

[M(E) − M(0)]/M(0), where M(E) and M(0) representmagnetizations with and without the application of an elec-tric field (E), respectively. In this experiment, a 20 Oe bi-ased field is used to induce a maximum CME effect. De-tails of the static CME measurements have been publishedin [29].

In this paper, we focus on the electric field dependenceof the CME effect under low frequency square wave exci-tations. Figure 2 shows the time evolution of the magne-tization under square wave electric field having an ampli-tude of E = 8 kV/cm applied across the PMN-PT crystal.Due to a DC offset, the square wave consists of only pos-itive values of electric field, varying from 0 to +8 kV/cm.A time window of 300 seconds is captured for a drive fre-quency of 0.5 Hz. The magnetization reveals an obvioustime effect, especially for E = 5–6 kV/cm. Within sucha 300-second observation window, the magnetization de-cays by 10%, which corresponds to an enhancement of theCME effect in the present experiment. However, either high(E > 6 kV/cm) or low (E < 5 kV/cm) values of electricfield result in a negligible decay in magnetization. Further-more, time dependences of the magnetization for an elec-tric field drive frequency of 1, 0.05 and 0.1 Hz are shownin Figs. 3 and 4. In addition to a slight enhancement in theCME effect for a higher electric field, i.e., E = 8 kV/cm (seeFigs. 4(a) and 4(c)), there is an obvious decay of magnetiza-tion at E ∼ 6 kV/cm, as evident in Figs. 4(b) and 4(d). Anelectric field dependence of polarization (P vs. E) with dif-ferent frequencies can be referred to [29]. All of the data inFigs. 2–4 were measured at a bias magnetic field of 20 Oein order to maximize the CME effect. Possible mechanismresponsible for this behavior, including time evolution of themagnetization, electric polarization, and the CME effect arediscussed below in detail.

The variation of CME with frequency is of utmost im-portance to many practical applications. An important ques-tion is whether a large CME effect can be sustained over abroad range of frequencies. Figure 5 shows a remarkable de-pendence of the CME effect upon frequency in which CMEvalues drastically reduce above 2 Hz. A strong CME effectin excess of 80% can only be observed at very low frequen-cies. This outcome highlights the limitations of multiferroicheterostructures investigated here for practical applications,especially at high frequencies.

Figure 6(a) depicts the time response of magnetization toan instantaneous electric field excitation of amplitude E =8 kV/cm (see Fig. 6(b)) applied across the PMN-PT crystal.Clearly, the magnetization does not follow the abrupt transi-tion in electric field but rather experiences a delay of 0.6 sec-onds in magnetoelectric response. This 0.6 s time, definedhere as the switching time, ts, signals the near saturation ofCME coupling in each E-field cycle and we believe it isunique to each multiferroic heterostructure. The CME effect

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Fig. 3 Dependence of magnetization (M) with time under an applica-tion of different electric fields with square waveform at f = 1 Hz andH = 20 Oe

was calculated to be increasing within the time window of0.6 seconds. These data and best fit are provided in Fig. 6(c).Additionally, the frequency response of the ME transition isillustrated in Fig. 6(d), where the individual CME effect fre-quency components were derived from the calculated curvein Fig. 6(c). Within the critical time of ts = 0.6 s the appliedelectric fields at frequencies greater than 1 Hz are unableto induce the maximum CME effect. Only low frequencysignals of f ≤ 1 Hz result in a maximum CME effect. Theconclusions of the time domain analysis of the CME effectfor the present MF construct are fully consistent with aboveexperimental observations.

We next attempt to understand the underlying physicalmechanisms of the observed time domain behavior of theCME effect. The possible mechanisms include temporal re-laxation processes of the PMN-PT crystal, Metglas® ribbon,and the interface between the PMN-PT crystal and magneticmaterial. The Metglas® ribbon demonstrates a very shortmagnetic relaxation time, on the order of 10−6 seconds,making this a negligible contribution to the observed tran-sition time of 0.6 seconds. It is noteworthy that the PMN-PTcrystal exhibits a significant response time, up to 20 sec-onds, depending on the amplitude of the applied electricfield. A systematic investigation of the time response of po-

Fig. 4 Dependence of magnetization (M) with time under an appli-cation of different electric fields with square waveform at f = 0.1 and0.05 Hz, respectively (H = 20 Oe)

Fig. 5 Variation of the magnetoelectric effect (ME) with a frequencyunder a square wave electric field

larization under different magnitudes of electric field for thePMN-PT crystal as well as other ceramic materials was car-ried out by Jullian [30]. This work demonstrated an obvioustime dependence of the polarization when the applied elec-tric field (E) was either less than or close to the electric co-

Dynamic response of converse magnetoelectric effect in a PMN-PT-based multiferroic heterostructure

Fig. 6 (a) Time response of magnetization (M) under the abrupt ofan electric field, (b) a representative waveform for a jump of an elec-tric field (E = 8 kV/cm), (c) calculated magnetoelectric effect withindifferent time domains, and (d) calculated magnetoelectric effect at dif-ferent frequencies

ercive field (Ec = 5 kV/cm [29]). A broad range of chargingtimes, on the order of magnitude from 10−1 to 101 seconds,depending on the magnitude of applied electric field, was re-ported. Fast response of the polarization, within about 10−6

seconds, was observed only for high magnitudes of electricfield (E � Ec). The time dependence of the polarizationis simultaneously transformed to the time-dependence ofstrain in such a piezoelectric crystal. Therefore, these earlyexperimental results provide important insights into the timeevolution of the CME effect. Our experimental results indi-cate that an electric field of 8 kV/cm is insufficient to fullysaturate the polarization of the ferroelectric crystal, whichleads to a time delay of the polarization in response to anabrupt change in electric field. However, the contribution ofthe interface to time delay of magnetization, and in turn theCME effect in the multiferroic heterostructure, remains un-clear. Further work is needed to quantitatively evaluate theinfluence of the interface region on CME effects. Neverthe-less, we believe that the time dependence of the CME effectis predominately determined by the PMN-PT substrate inour experiments.

Fig. 7 Variation of the magnetoelectric effect (ME) with an appliedelectric field (E) at different frequencies (f = 0.05, 0.1, 0.5, and 1 Hz)

As the experimental data illustrates, the magnitude of theCME effect varies not only with frequency, but also withthe strength of applied electric field. In fact, the CME effectincreases rapidly for applied electric field values between5 and 7 kV/cm as shown in Fig. 7. With further increasesin electric field strength, only small gains are observed inthe CME effect. On the other hand, the CME effect exhibitsonly a slight dependence upon frequency in the range of0.05–1 Hz. Additionally, we noticed that the CME effectdoes not show a saturation state after one cycle of appliedelectric field. The time (or number of cycles) dependenceof the magnetization or CME effect is extremely sensitiveto frequency when the applied electric field is near the coer-cive field value of Ec = 5 kV/cm. A typical example is takenfrom the case of E = 5.6 kV/cm for f = 0.5 Hz as depictedin Fig. 2. We see a significant decay of magnetization withtime or cycles.

Our experiments further indicate that the magnetizationdecay depends upon frequency, electric field strength, andmagnitude of the CME effect, as is illustrated in Fig. 8. Itis noted that the higher the electric field is for E > Ec, theshorter the after-effect is. This behavior could be related tothe intriguing and controversial aging effect observed in re-laxor ferroelectric materials, such as PMN-PT [31–33]. Pos-sible mechanisms of aging/fatigue effects for the PMN-PTcrystal have been identified. It may arise from ferroelectric-like polar nanoregions (PRNs) in this crystal [34]. In gen-eral, the time dependence of the polarization decay follow-ing an electric field step is well described by the stretchedpower law behavior of P(t) = P0 exp[−(t/τ )β ], with β < 1.In our experiment, since the increase in the ME effect withtime is attributed to the enhancement of strain under an ap-plied electric field, an unsaturated polarization is likely re-sponsible for the time-dependence of the magnetoelectric

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Fig. 8 Variation of aging strength of the magnetoelectric effect withan applied electric field (E) at different frequencies (f = 0.05, 0.1,0.5, and 1 Hz)

effect [35]. A polarization-independent aging behavior, aspostulated in the literature [36], is likely insignificant in thisMF heterostructure.

From Fig. 8, the time effect is more apparent when theelectric field is slightly higher than the coercive field (Ec).The field dependence of the CME effect is more sensitivethan the frequency dependence. Of course, time effect ofME coupling is measured by means of the magnetizationwith time under an electric field. As mentioned above, themagnetization achieves a constant value only after many cy-cles of the electric field excitation. The time dependence ofthe magnetization can be described by an exponential decayfunction: M(t) = M0 exp[−(t/τ )β ], where τ is a time con-stant. Figure 9 shows the time constant (τ) obtained from theabove exponential decay function fitting to the experimentaldata under applied electric field, E = 6 kV/cm. A linear in-crease of the time constant (τ ) with increasing frequency isobserved. The data suggest that the ME effect has a charac-teristic memory, i.e., it requires more cycles to stabilize theeffect at higher frequencies. This time window is apparentlydifferent from those at lower frequencies, which thereforesuggests that the after-effect is not simply determined by aconstant time domain. This phenomenon is quite similar topreviously reported observations in PMN-PT crystals andother ferroelectric materials [37], in which a loss of electricpolarization with time was determined to be proportional toN/f 2 (N and f denote the number of cycles and frequency,respectively). This infers that at high frequency excitationsthere is a small change in the polarization that is reflectedin the change in magnetization representing the ME effect.In comparison of the aging effects and decay time, a smallaging effect corresponds to a long decay time, as indicatedin Figs. 8 and 9.

Fig. 9 Frequency dependence of decay time constant (τ ) for themagnetoelectric effect at application of an applied electric field ofE = 6 kV/cm. Dashed line is a linear fit to the experimental data

4 Conclusions

In summary, the static and dynamic converse magnetoelec-tric effects in a multiferroic heterostructure (Metglas®/PMN-PT crystal) were investigated experimentally by measure-ments of the frequency and time response of the CME effectat different applied electric fields. The experiments indicatea sensitive dependence of the CME effect upon the magni-tude of the applied electric field, especially in the vicinity ofelectric coercive field (Ec), which is related to a critical be-havior near the coercive field. The multiferroic heterostruc-ture demonstrates a strong converse ME coupling, yieldinga coupling constant A = 22.5 Oe cm/kV. A CME effect of−80% was measured at E = 8 kV/cm. Importantly, the het-erostructure exhibits a switching time of 0.6 seconds underthe application of an instantaneous square wave form elec-tric field excitation. The time delay results in a reduction ofthe CME effect at frequencies above 1 Hz, which is assumedto arise from a relaxation effect of the PMN-PT crystal. Inaddition, aging behavior of the CME effect as a function ofcycle frequency, amplitude, and number, are observed anddiscussed.

Acknowledgement The authors acknowledge Dr. Ryusuke Haseg-awa for valuable discussions on Metglas® ribbon.

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