A comparative study of magnetic and dielectric behaviors for La1−xBixMn1−yFeyO3 series

(with x = 0.5, 0.7 and y = 0.3, 0.7)

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2012 J. Phys.: Condens. Matter 24 255902

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IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 24 (2012) 255902 (6pp) doi:10.1088/0953-8984/24/25/255902

A comparative study of magnetic anddielectric behaviors forLa1−xBixMn1−yFeyO3 series (with x = 0.5,0.7 and y = 0.3, 0.7)

Asish K Kundu1,2, V K Jha1, Md Motin Seikh3, R Chatterjee4 andR Mahendiran2

1 Discipline of Physics, Indian Institute of Information Technology, Design and Manufacturing,Dumna Airport Road, Jabalpur-482005, India2 Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3,Singapore-117542, Singapore3 Department of Chemistry, Visva-Bharti University, Shantiniketan-731235, India4 Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi-11016, India

E-mail: asish.k@gmail.com and asish.kundu@iiitdmj.ac.in

Received 20 March 2012, in final form 2 May 2012Published 28 May 2012Online at stacks.iop.org/JPhysCM/24/255902

AbstractWe have carried out an extensive investigation into the effect of doping on both the A- andB-sites for the multiferroic La0.5Bi0.5Mn0.5Fe0.5O3 in relation to its physical properties. Thetemperature dependent magnetization and dielectric response are determined for differentpercentages of Bi- and Fe-substitutions. For La0.5Bi0.5Mn0.7Fe0.3O3, there is a prominentferromagnetic transition TC around 110 K, whereas the other La0.5Bi0.5Mn0.3Fe0.7O3 andLa0.3Bi0.7Mn0.3Fe0.7O3 phases fail to exhibit any clear transition. On the other hand, for theFe-rich phases, the coercive field increases to 2450 Oe compared to 1720 Oe (for the Mn-richphase). All the compositions exhibit coexistence of ferromagnetic and antiferromagneticphases at low temperatures. The temperature dependent dielectric constant of the investigatedsamples varies from 32 000 to 500 at room temperature and the data has been analyzed usingthe universal dielectric response model.

(Some figures may appear in colour only in the online journal)

1. Introduction

Bismuth based perovskite manganites have attracted muchattention owing to their wide range of interesting physicalproperties, e.g. multiferroics, magneto-dielectric, caloric,resistance etc, and some of them are promising candidatesfor useful technological applications [1, 2]. Despite thenumerous investigations on these materials in the past fewyears very few perovskite manganites have been known asferromagnetic-insulators (FMIs) [1–9]. A good knowledge ofincorporation of dopants is important in condensed matterphysics, since most of the multiferroics reported recentlyrequire doping of the initial pure ABO3 perovskite at the A-

and/or B-site [5, 6, 9]. We have considered the following twomajor points to design our new materials. Firstly, in ABO3perovskite, the strong ferromagnetic (FM) interaction betweenthe B elements and, secondly, an A-site cation containing alone pair electron which could induce ferroelectricity throughlattice distortion. With such strategies we have been able tosuccessfully synthesize a number of FMIs which are derivedfrom perovskite manganites using different dopants like Fe,Co, Ni and/or Bi [7–9].

The present investigation is intended to find out the com-positional effect on the physical properties of the manganitebased FMI La1−xBixMn1−yFeyO3, where x = 0.5, 0.7 and y =0.3, 0.7. Here we report comprehensive studies on magnetic,

10953-8984/12/255902+06$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

J. Phys.: Condens. Matter 24 (2012) 255902 A K Kundu et al

Table 1. Lattice parameters, magnetic and electrical data for La1−xBixMn1−yFeyO3. Where a, b, c, are the lattice parameters, θp is theCurie–Weiss temperature, µeff is the effective paramagnetic moment, HC is the coercive field (at 10 K), ρ300 is the electrical resistivity (at300 K) and ε′ and tan δ are the real parts of dielectric constant and loss respectively at 300 K.

Compositions La0.5Bi0.5Mn0.7Fe0.3O3 La0.5Bi0.5Mn0.3Fe0.7O3 La0.3Bi0.7Mn0.3Fe0.7O3

Space group Pnma Pnma Pnma

a (A) 5.441(5) 5.501(8) 5.493(7)b (A) 7.787(3) 7.823(5) 7.844(2)c (A) 5.492(5) 5.411(6) 5.562(6)θp (K) 100 −788 −435µeff (µB/f.u.) 3.1 4.1 3.4HC (Oe) 1720 1980 2450ρ300 (� cm) 9 2837 22ε′ 32 000 2900 500tan δ 6.1 2.9 2.4

electrical and dielectric properties of La0.5Bi0.5Mn0.7Fe0.3O3,La0.5Bi0.5Mn0.3Fe0.7O3 and La0.3Bi0.7Mn0.3Fe0.7O3. We ob-served that though the Mn-rich composition exhibits a FM TC,it lacks of long-range FM ordering and shows glassy nature.However, the Fe-rich phases do not exhibit any clear signatureof FM ordering. Interestingly, none of these compositionsescape out of phase separation, i.e. the coexistence of phases.The dielectric behavior of these insulators is significantlydifferent in Mn- and Fe-rich compositions.

2. Experimental procedure

Polycrystalline La0.5Bi0.5Mn0.7Fe0.3O3, La0.5Bi0.5Mn0.3Fe0.7O3 and La0.3Bi0.7Mn0.3Fe0.7O3 samples were prepared by theconventional sol–gel method [7]. Finally, the powder sampleswere pressed into rectangular bars and sintered at 1223 Kin a platinum crucible for 24 h to obtain the single-phasecompounds. Phase purity was checked by x-ray powderdiffraction (XRPD) measurements and oxygen stoichiometrywas determined by chemical titration [7], for all samples thevalues are 3 ± 0.02. The structural parameters fitted with theorthorhombic structure (space group Pnma) similar to thoseof our previous studies [8, 9] are presented in table 1.

The magnetization and resistivity were carried outwith a Quantum Design physical properties measurementsystem (PPMS; SQUID) and the dielectric measurementsby an Agilent 4292A impedance analyser. The details ofthe magnetization and electrical measurements procedure ismentioned in our previous work [7, 8].

3. Results and discussion

Figure 1 shows the temperature dependent zero fieldcooled (ZFC) and field cooled (FC) magnetization, M(T),for La0.5Bi0.5Mn0.7Fe0.3O3, La0.5Bi0.5Mn0.3Fe0.7O3 andLa0.3Bi0.7Mn0.3Fe0.7O3, in an applied field of 1000 Oe.Unlike the multiferroic phase La0.5Bi0.5Mn0.5Fe0.5O3having weak FM behavior [9], the Mn-rich phaseLa0.5Bi0.5Mn0.7Fe0.3O3 (x = 0.5 and y = 0.3) shows(figure 1(a)) a paramagnetic (PM) to FM transition atTC ∼ 110 K. In contrast, heavily Fe-substituted phasesLa0.5Bi0.5Mn0.3Fe0.7O3 (x = 0.5 and y = 0.7) even with the

Figure 1. Temperature dependent ZFC (open symbol) and FC(solid symbol) magnetization, M(T), at H = 1000 Oe. and fielddependent isothermal magnetic hysteresis, M(H), curves at differenttemperatures for (a) La0.5Bi0.5Mn0.7Fe0.3O3,(b) La0.5Bi0.5Mn0.3Fe0.7O3 and (c) La0.3Bi0.7Mn0.3Fe0.7O3.

modification of A-site composition, i.e. La0.3Bi0.7Mn0.3Fe0.7O3 (x = 0.7 and y = 0.7) leads to significant changein FM behavior and becomes non-ferromagnetic typeeven at lower temperatures (figures 1(b) and (c)). ForLa0.5Bi0.5Mn0.7Fe0.3O3, the ZFC and FC magnetization dataexhibit considerably larger divergence below ∼50 K, evenat a field as high as 5000 Oe. This indicates that themagnetic interactions in this system are different from thelong-range ferromagnetism. On the other hand, for the Fe-richsystems, a gradual increase in the M(T) is observed below100 K, which could be attributed to the weak FM or cantedantiferromagnetic (AFM) ordering similar to BiFeO3 [10].

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J. Phys.: Condens. Matter 24 (2012) 255902 A K Kundu et al

Figure 2. Temperature dependent magnetic ac-susceptibility forLa0.5Bi0.5Mn0.7Fe0.3O3 (a) in-phase component, χ ′ and (b) out ofphase χ ′′ components at different frequencies (hac = 10 Oe).

The low temperature magnetic phase has been in-vestigated in detail to characterize the nature of FMinteractions. The isothermal magnetic hysteresis loops, M(H),have been recorded at different temperatures for all thecompositions (see corresponding insets of figure 1). Forthe La0.5Bi0.5Mn0.7Fe0.3O3, one indeed observes (insetof figure 1(a)) a prominent hysteresis loop at 10 K,with a remanent magnetization (Mr) of ∼0.2 µB/f.u.and a coercive field (HC) of ∼1720 Oe. However,above TC the M(H) behavior becomes linear, corre-sponding to a PM state. In contrast, the coercive fieldsfor La0.5Bi0.5Mn0.3Fe0.7O3 and La0.3Bi0.7Mn0.3Fe0.7O3 are1980 and 2450 Oe, respectively, at 10 K (inset offigures 1(b) and (c)), which are higher than that of theMn-rich phase. For none of these compositions, does theM(H) curve depict saturation even at 50 kOe. The highestvalue of magnetic moment, ∼1.2 µB/f.u. is obtained forLa0.5Bi0.5Mn0.7Fe0.3O3, whereas for La0.5Bi0.5Mn0.3Fe0.7O3and La0.3Bi0.7Mn0.3Fe0.7O3 the corresponding values aremuch lower (for 10 K and 50 kOe). Such a lowermoment is counterintuitive, since substitution of Fe forMn changes the d4 (S = 2) cation to d5 (S = 2.5). Thusthe smaller values of moment for La0.5Bi0.5Mn0.3Fe0.7O3and La0.3Bi0.7Mn0.3Fe0.7O3, may be related to the changein mode of magnetic interaction in the Fe-rich phases,i.e. the FM interaction between Fe3+–O–Mn3+ ions weakenby the strong Fe3+–O–Fe3+ AFM interaction. Though theMn-rich phase shows a clear FM TC, the obtained M(H)behavior is unexpected for the FM system, rather it is acharacteristic feature of glassy ferromagnets [7–9]. Therefore,we have carried out frequency dependent ac-magneticmeasurements (figure 2) at four different frequencies forLa0.5Bi0.5Mn0.7Fe0.3O3 to confirm the glassy behavior.

Figure 3. Temperature dependent inverse magnetic susceptibility,χ−1, plot for (a) La0.5Bi0.5Mn0.7Fe0.3O3,(b) La0.5Bi0.5Mn0.3Fe0.7O3 and (c) La0.3Bi0.7Mn0.3Fe0.7O3. Solidsymbols and solid lines represent the experimental data andapparent fit to the Curie–Weiss behavior.

These results are similar to those previously observed formultiferroic La0.5Bi0.5Mn0.5Fe0.5O3 [9] suggesting that atlow temperature the La0.5Bi0.5Mn0.7Fe0.3O3 phase representsa state that is very close to the spin-glass type phase.Therefore, at low temperatures there must be a subtle balancebetween the FM (Fe3+–O–Mn3+) and AFM (Fe3+–O–Fe3+

and/or Mn3+–O–Mn3+) interactions or in other words thesystem is electronically phase separated into FM and AFMclusters and there will always be a competition between themgiving rise to a glassy FM state akin to earlier studies [7–9].

We have also studied the high temperature PM region,which shows that the magnetization curve is well fittedto the Curie–Weiss law (figure 3). From the fittingcurves, we have calculated the PM Weiss temperature(θp) and the effective magnetic moment (µeff); these arepresented in table 1. The obtained θp value is positivefor La0.5Bi0.5Mn0.7Fe0.3O3 whereas the values are negativefor La0.5Bi0.5Mn0.3Fe0.7O3 and La0.3Bi0.7Mn0.3Fe0.7O3. Thepositive θp value signifies the FM interactions in the hightemperature region. The positive magnetic interaction in theLa0.5Bi0.5Mn0.7Fe0.3O3 compound is due to the presence ofMn3+ and Fe3+ ions, which favor FM interaction. In contrast,for La0.5Bi0.5Mn0.3Fe0.7O3 and La0.3Bi0.7Mn0.3Fe0.7O3 thedominant interaction is AFM due to the Fe3+–O–Fe3+

interaction (weak FM contribution from Fe3+–O–Mn3+).Figure 4 shows the temperature dependent electrical

resistivity ρ(T) for three samples. The resistivity valueincreases gradually with decreasing temperature. It is noticed

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J. Phys.: Condens. Matter 24 (2012) 255902 A K Kundu et al

Figure 4. Temperature dependent electrical resistivity, ρ(T), for(a) La0.5Bi0.5Mn0.7Fe0.3O3 (square symbol),(b) La0.5Bi0.5Mn0.3Fe0.7O3 (circular symbol) and(c) La0.3Bi0.7Mn0.3Fe0.7O3 (triangular symbol).

that the samples are insulating throughout the measuredtemperature range (100 K ≤ T ≤ 400 K) and the resistivityvalue is very high at low temperatures, crossing instrumentlimitations. Thus, similar to the other Bi-based manganites [8]none of these samples show any insulator–metal transition inthe 100–400 K range. The ρ(T) curve confirms the insulatingbehavior and may also exhibit weak magnetoresistanceeffects [7, 8].

To further establish the ferroelectric behavior akinto multiferroic La0.5Bi0.5Mn0.5Fe0.5O3 [9], we have car-ried out the temperature dependent dielectric measure-ments for La0.5Bi0.5Mn0.7Fe0.3O3, La0.5Bi0.5Mn0.3Fe0.7O3and La0.3Bi0.7Mn0.3Fe0.7O3 in the temperature range of25–360 K. The corresponding dielectric constant and dielec-tric loss are presented in figures 5 and 6 respectively forall three samples, in the frequency range of 1 kHz–2 MHz.From the temperature dependent real and imaginary (datanot shown) dielectric plots it is observed that the dielectricvalue increases gradually with increasing temperature for allthree samples. In the case of La0.5Bi0.5Mn0.7Fe0.3O3, the realparts of the dielectric data exhibit plateaus (at low and hightemperatures) with a change in slope (figure 5(a)). Moreover, agradual increase of frequency dependent behavior is observedin the high temperature regions. The dielectric responseof this system demonstrates relaxor-like behavior, i.e. themagnitude of the dielectric constant decreases with increasingfrequency. A giant dielectric constant (up to 32 000 at100 kHz) is obtained near room temperature. With decreasingtemperature, the dielectric constant value rapidly decreasesto a lower value (<2000 at 100 kHz). A broad dielectricloss peak (figure 6(a)) is evident corresponding to the rapidchange in region of the dielectric constant. The loss peak shiftsto higher temperature as frequency increases (figure 6(a)),indicating a thermally activated relaxation. The dielectric lossincreases significantly with increasing temperature due to thecontribution of dc conductivity in the system.

However, the real parts of dielectric data for La0.5Bi0.5Mn0.3Fe0.7O3 (figure 5(b)) and La0.3Bi0.7Mn0.3Fe0.7O3(figure 5(c)) do not show any plateaus and the value increases

Figure 5. Temperature dependent dielectric constants for(a) La0.5Bi0.5Mn0.7Fe0.3O3, (b) La0.5Bi0.5Mn0.3Fe0.7O3 and(c) La0.3Bi0.7Mn0.3Fe0.7O3 at different frequencies.

rapidly with increasing temperature (above T ∼ 130 K).For La0.5Bi0.5Mn0.3Fe0.7O3 phase, we observe (figure 5(b))a dielectric maximum at around 345 K (at 10 kHz),which shift towards the high temperature (356 K) withincreasing frequency (200 kHz), but this feature is absent forLa0.3Bi0.7Mn0.3Fe0.7O3 (figure 5(c)). The obtained data (inthe temperature range <360 K) for the present systems revealthat the series could be described by dipolar type relaxationalong with variable type hopping conduction (as reportedearlier for a few of the earlier compounds [8]) of the chargecarriers. It is also well known that in semiconducting typematerials, localized charge carriers hopping between spatiallyfluctuating lattice potentials not only produce conductivity butalso give rise to a dipolar effect. For the Fe-rich samples, ataround 150–360 K a rapid increase of frequency dependentdielectric constant behavior is observed. A similar kindof sudden increase in the dielectric constant behavior isreported in the literature for Fe-doped systems [6, 9]. ForLa0.5Bi0.5Mn0.3Fe0.7O3 and La0.3Bi0.7Mn0.3Fe0.7O3 phasesthe rapid increase in the dielectric constant values above150 K (figures 5(b) and (c)) could arise from various factors,such as effects intrinsic to the system and/or extrinsic. Inthe case of intrinsic effects, the steep rise of the dielectricconstant over a short range of temperature could arise aroundthe onset of ferroelectric ordering in which the dipoles aresubjected to a double well energy barrier. In that case, thedielectric constant obeys the Curie–Weiss behavior abovethe transition temperature, which is not possible to crosscheck in the present study due to insufficient experimental

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J. Phys.: Condens. Matter 24 (2012) 255902 A K Kundu et al

Figure 6. Temperature dependence dielectric loss for(a) La0.5Bi0.5Mn0.7Fe0.3O3, (b) La0.5Bi0.5Mn0.3Fe0.7O3 and(c) La0.3Bi0.7Mn0.3Fe0.7O3 at different frequencies.

data points. In the present case, the effect observed couldbe due to the grain boundary and at temperatures above250 K a combined effect of the grain boundary and thespace charge effect could arise due to the semiconductingbehavior of the samples associated with the contact effects.Nevertheless, for La0.5Bi0.5Mn0.3Fe0.7O3 the transition peaksare prominent and remain well defined at high frequenciesi.e. the ferroelectricity in this system is intrinsic, as reportedfor multiferroics La0.8Bi0.2Fe1−xMnxO3 by Anjum et al[6]. Hence, observation of the frequency dispersion and itstemperature dependent studies could throw more light on theeffect observed.

Figure 7 shows the frequency dispersion (1 kHz–2 MHz)of the real (ε′) part of the dielectric constant for all threesamples at different temperatures. The frequency dependentε′ exhibits a large value at low frequency (<1 kHz) anda step-like decrease with increase in frequency (>1 kHz)at 75 K for La0.5Bi0.5Mn0.7Fe0.3O3 (figure 7(a)) andaround 250 K for La0.5Bi0.5Mn0.3Fe0.7O3 (figure 7(b))and La0.3Bi0.7Mn0.3Fe0.7O3 (figure 7(c)) respectively. Theobserved relaxation shifts towards higher frequencies withincreasing temperature similar to an activation behavior.Though the relaxation behavior looks like a Debye typerelaxation (a characteristic of the intrinsic dipoles presentin the system), the temperature dependent frequency shiftsuggests that the relaxation could be of Maxwell–Wagnertype, which arises due to the presence of regions with differentconductivities within the sample [11]. In the present samplesthe grain boundaries present and/or different magnetic clusters

Figure 7. Frequency dependence of dielectric constants at differenttemperatures for (a) La0.5Bi0.5Mn0.7Fe0.3O3,(b) La0.5Bi0.5Mn0.3Fe0.7O3 and (c) La0.3Bi0.7Mn0.3Fe0.7O3.

could act as a region of different conductivity as observedin the case of magnetic studies. The large values in the lowfrequency ε′ imply that the grain boundary capacitance islarger than the bulk grain capacitance of the sample. Hence,the charge accumulated at the grain boundaries could give riseto a Maxwell–Wagner type relaxation phenomenon [11]. Thegradual increase of the ε′ values for La0.5Bi0.5Mn0.3Fe0.7O3

and La0.3Bi0.7Mn0.3Fe0.7O3 at low frequency (for T > 250 K)could arise due to the space charge (due to dc conduction)and other contact based effects (figures 7(b) and (c)). Thedetailed analyses of the influence of microstructure over therelaxation and the influence of different electrode materialswith different sample thicknesses are beyond the scope ofthis paper. Finally, to verify whether the dielectric response ofthe samples is due to their FE nature or some other artifacts,we have analyzed the room temperature frequency dependentdata by using the universal dielectric response (UDR)model [12]. Figure 8 shows the log f versus log f ε′ plots forall three samples. From the fitting curve it is noticed thatLa0.5Bi0.5Mn0.7Fe0.3O3 follows a linear behavior throughoutthe frequency range, whereas for La0.5Bi0.5Mn0.3Fe0.7O3 andLa0.3Bi0.7Mn0.3Fe0.7O3 the linear regions are limited to acertain frequency regime. Therefore, according to the UDRmodel, the dielectric response of the La0.5Bi0.5Mn0.7Fe0.3O3

sample is intrinsic and for the other two Fe-rich systems theresponses are combined effects.

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J. Phys.: Condens. Matter 24 (2012) 255902 A K Kundu et al

Figure 8. Logarithm of frequency versus log (f ε′) plots at 300 Kfor (a) La0.5Bi0.5Mn0.7Fe0.3O3 (square symbol),(b) La0.5Bi0.5Mn0.3Fe0.7O3 (circular symbol) and(c) La0.3Bi0.7Mn0.3Fe0.7O3 (triangular symbol). Open symbols andthe solid line represent the experimental data and apparent fit to theUDR model as described in the text.

4. Conclusions

We have investigated the relevant physical properties ofthe multiferroic La0.5Bi0.5Mn0.5Fe0.5O3 playing with thesubstitution on both the A- and B-sites. We observed thatthe increased Mn content (La0.5Bi0.5Mn0.7Fe0.3O3) leadsto the prominent TC at 110 K as well as FM correlationin the high temperature paramagnetic phase. However, itfails to exhibit long-range magnetic ordering and showsspin-glass behavior arising out of a delicate balance betweenthe FM and AFM interactions. On the other hand, theFe-rich compositions, irrespective of the A-site change, do notshow any FM TC and the dominant magnetic interaction isAFM due to dominant Fe3+–O–Fe3+ interaction. However,the coexistence of the FM cluster at low temperature isreflected in the isothermal magnetization for Fe-rich samples.All the samples are highly insulating and the resistivityincreases with the decreasing temperature as expected. Thedielectric constant and dielectric loss measurements revealeda Maxwell–Wagner type relaxation phenomenon related tograin boundary charge accumulation rather than Debye typerelaxation of intrinsic dipoles. Such grain boundaries arerational from the view point of magnetic clusters as well.

However, application of the universal dielectric responsemodel at room temperature revealed that the dielectricresponse of La0.5Bi0.5Mn0.7Fe0.3O3 is intrinsic, whereas it isa combined effect for the Fe-rich phases.

Acknowledgments

We gratefully acknowledge Mr R V K Maheshwar andDr S K Barik for their help during the impedancemeasurements. AKK thanks DST, Govt. of India forthe BOYSCAST research fellowship (SR/BY/E-10 dated08.06.2011).

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