Signature of ferroelectricity in magnetically ordered Mo-doped CoFe2O4
G. D. Dwivedi,1 K. F. Tseng,2 C. L. Chan,2 P. Shahi,3 J. Lourembam,3 B. Chatterjee,1
A. K. Ghosh,1 H. D. Yang,2 and Sandip Chatterjee3,*1Department of Physics, Banaras Hindu University, Varanasi 221 005, India
2Department of Physics, National Sun Yat Sen University, Kaohsiung 80424, Taiwan, Republic of China3Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India
�Received 28 April 2010; revised manuscript received 18 August 2010; published 18 October 2010�
Coexistence of both magnetic ordering and ferroelectricity �with giant dielectric constant� have been ob-served for the first time in Co�Fe1−xMox�2O4. The magnetization of Co�Fe1−xMox�2O4 �x ranges from 0 to 0.1�was found to increase with doping concentration of Mo. The magnetic properties indicate that Mo goes into thetetrahedral site. The giant dielectric constant may be attributed to the Maxwell-Wagner relaxation mechanism.
Ferrimagnetic Spinel ferrites constitute an important classof magnetic materials. The magnetic and electrical propertiesdepend on the nature and distribution of their cations in thetetrahedral �A� and octahedral �B� sublattices of a cubicstructure. The CoFe2O4 is a very important magnetic mate-rial which has covered a wide range of applications includingelectronic devices, ferro-fluids, magnetic delivery microwavedevices, and high-density information storage due to itswealth of magnetic and electronic properties, such as cubicmagnetocrystalline anisotropy, high coercivity, moderatesaturation magnetization, high Curie temperature Tc, magne-tostriction, high-chemical stability, and electrical insulation,etc.1–7
The recent investigations of the substitutions at the Fesites of CoFe2O4 by various cations �Ga, Mn, etc.� haveshown great potentiality of the doping for inducing magneticproperties.8–10 On the other hand, the magnetoelectric �ME�effect, which is defined as the variation in dielectric polar-ization in response to the magnetic field or vice versa, iswidely investigated recently.11,12 Materials exhibiting simul-taneous magnetic and ferroelectric properties have potentialapplications such as magnetic field sensors, transducers, andinformation storage. The ME effect is found to be accompa-nied by high-dielectric constant in both single-phasemultiferroics13 and magnetoelectric composites.14–16 In thepresent investigation we have partially substituted Fe site bymolybdenum under the impression that it will be highly in-teresting due to its high valence state �+6� and its d0 elec-tronic configuration. The idea is that a partial substitution ofFe3+ by Mo6+ in CoFe2O4 should induce electron doping,and consequently, concentration of Fe2+ ions should increasein the tetrahedral sites. Furthermore, ions with d0 electronsare responsible for ferroelectricity. Hence Mo doping inCoFe2O4 may induce multiferroicity in these compositions.
The Molybdenum substituted samples with compositionsCo�Fe1−xMox�2O4 �x ranges from 0 to 0.1� were synthesizedby the conventional ceramic method.17 Stoichiometricamounts of Co3O4, Fe2O3, and MoO3 were mixed andground for several hours. The resulting mixture was calcinedat 1000 °C for 12 h and further heated at the same tempera-ture after an intermediate grinding. The powder was thenpelletized which was followed by sintering �cooling rate5 °C /min.� the pellets at 1150 °C for 48 h with intermediategrindings. The obtained samples were characterized by x-ray
powder diffraction using the Cu K� radiation. The patternindicates that all the compositions are of single phase. Themagnetization measurement was carried out using a commer-cial superconducting quantum interference device �MagneticProperties Measurement System �MPMS� XL-7, QuantumDesign Inc.� magnetometer. The dielectric constant was mea-sured using an LCR meter �Agilent 4980A� and the PE loopwas measured using the 609E-6 �Radiant Technologies�.
Figure 1 shows the x-ray diffraction pattern of Mo-dopedCoFe2O4 samples. The lattice parameters obtained from Re-itveld refinement obey the Vegards’ law indicating that thesamples are of single phase. It has been observed that thestructure is cubic with Fd3m space group. The parameter “a”can completely be determined through two independentcation-anion distances, designated Rtetra and Rocta.
18
a = 2.0995Rtetra + �5.8182Rocta2 − 1.4107Rtetra
2 �1/2
Rocta and Rtetra measure exactly the average value of theanion-cation distance on the octahedral and tetrahedral sites,
respectively. One can imagine that the Mo6+ �rMo6+ =0.55 �
ions enter into the tetrahedral lattice sites �A� of the spinellattice. With the entering of Mo6+ some of the Fe3+ �rFe
3+
=0.645 � ions transform to Fe2+ �rFe2+=0.77 � ions for
maintaining the charge neutrality. An increase in the popula-tion of Mo6+ cations and a decrease in the Fe3+ cations in theA sites contributes to the decrease in Rtetra while an increasein the population of Fe2+ cations in the A sites increasesRtetra. Moreover, to relax the strain some of the Fe2+ ions willbe migrated to the octahedral site. Because of all thesechanges the change in the lattice parameter is very insignifi-cant but the changes are able to effectively decrease the pa-rameter a. Similar decrease in lattice parameter with Mo6+
doping in Fe site has been observed by Rezlescu et al.19 inspinel MgFe2O4. Figure 2 shows the temperature variation inmagnetization of Co�Fe1−xMox�2O4 �with x=0.0–0.10� forfield cooled �FC� and zero field cooled �ZFC� at 100 Oe. TheZFC susceptibility decreases monotonically down to a cer-tain temperature. The FC magnetization initially decreaseswith decreasing temperature and then saturates. On the otherhand, with increase in Mo content the magnetization valuealso increases. One may expect the magnetization to increasewith a decrease in the anisotropy constant of the Co2+. From
the FC behavior one may predict that the anisotropy constantdecreases with increase in Mo content which is nonmagnetic.This is because of the fact that for Mo-free sample the de-crease in FC-M�T� with decrease in temperature is rapid. Butas Mo content increases, the M�T� value increases veryslowly. One may assume that for Mo-free sample the aniso-tropy is so high �because of the Co2+ ions�20 that the samplescould not be magnetized sufficiently close to saturation. TheM�H� for the Mo-substituted ferrite �for x=0.1� at 10 and300 K have been presented in Fig. 3. Hysteresis loops are inagreement with Ferrimagnetic behavior. At high field, themagnetization increases almost linearly and tends to satura-
tion at a field as high as of 7 T. In the ideal situation, themagnetization per formula unit is represented by the net mo-ment of that in tetrahedral �A� and octahedral �B� sites �M=MB−MA�. Therefore, in the present investigation it mightbe the case as we have already mentioned that Mo is enteringinto the tetrahedral site. Being the nonmagnetic Mo6+ ion inthe tetrahedral site will increase the magnetization of thesamples. Moreover, the transformation of Fe3+ ions into Fe2+
ions �one Mo6+ ion will transfer three Fe3+ ions into Fe2+
ions to maintain the charge neutrality� will decrease the mag-netization ��Fe
3+��Fe2+� in the A site. Effectively, the appre-
ciable increase in magnetization with Mo content can be ob-served, which is consistent with our result. Therefore, ourspeculation that Mo6+ is entering into the tetrahedral site canexplain the structural and magnetization behavior satisfacto-rily. It is interesting to mention that in the spinel Mo-ferriteunlike the present investigation Mo is in the +3 and +4states.21 We may predict that since the ionic radius of Mo6+ issmaller than that of Fe3+ it can enter into the Fe site whereasionic radius of Mo3+/4+ is larger than that of Fe3+. Further-more, Gillot et al.22 have reported that in Mo-ferrite trace ofMo6+ �along with Mo3+/4+� state in tetrahedral site is ob-served only when it is oxidized �500 °C. In this respect it isnot consistent with our present observation since in thepresent investigation the existence of only Mo6+ in the tetra-hedral site is evident in unoxidized Co�Fe1−xMox�2O4samples. The evidence of Mo6+ has also been reported19 inMo doped MgFe2O4 where similar to our case Mo6+ hasbeen doped in Fe site.
Figure 4 shows the temperature variation in the real partof the dielectric constant ���� of Co�Fe1−xMox�2O4 with x=0–0.1 at 20 Hz. It is observed that with increasing Mocontent �� increases and for x=0.1 �� value is maximum.Above 200K, �� increases sharply for all the compositions.Figure 5 shows the temperature variation in ��, �� �imagi-nary part of dielectric constant� and tan � �dielectric loss� forx=0.1. Above a certain temperature �e.g., 200 K�, �� in-
20 40 60 80
0.00 0.02 0.04 0.06 0.08 0.10
8.371
8.372
8.373
8.374
x=0.10
x=0.08
x=0.04
x=0.00
Co(Fe1-xMox)2O4
Intensity(arbitraryunit)
2θ (deg.)
LatticeParameter
Mo Concentration
FIG. 1. X-ray diffraction pattern of Co�Fe1−xMox�2O4; with x=0.00, 0.04, 0.08, and 0.10. Inset: variation in lattice parameterwith Mo concentration.
FIG. 2. Temperature variation in magnetization ofCo�Fe1−xMox�2O4 for both zero field cooled and field cooled at 100Oe.
FIG. 3. M-H hysteresis curve �−7 T to +7 T� for x=0.1 attemperature 10 and 300 K. Inset: M-H curve at 300 K in the ex-tended scale �−0.3 to +0.3 T�.
DWIVEDI et al. PHYSICAL REVIEW B 82, 134428 �2010�
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creases sharply and shows high dielectric constant withweak-temperature dependence. As the frequency increasesthis temperature also increase. Around room temperature ���104 for x=0.1. At low temperature, electric dipoles freezethrough relaxation process and there exists decay in polariza-tion with respect to the applied electric field, which is con-firmed by the sudden drop in ��. Peak observed in the tem-perature variation in loss curve corresponds to such suddendrop in �� and shifting of the loss-peak toward lower tem-perature for lower frequency indicates thermally excited re-laxation process. The frequency dependence of �� for x=0.1 sample is shown in the inset of Fig. 4. The Arrheniusplot23 �inset of Fig. 5� of f = fo exp�-Ea /KBT� for x=0.1sample, gives activation energy �Ea�=0.36 eV. The obtainedactivation energy falls in the range of giant dielectric
materials.24–26 Rapid increase in f �or decrease in the relax-ation time, �� with increase in temperature indicates a fasterpolarization process with increasing dipole density.
It has been observed that at 300 K, the value of �� atlower frequency range is very high. As Mo is doped, defectsnear grain boundary increases. As a consequence, probabilityof interwell hopping increases which affects the dielectricrelaxation largely at low frequency. As a matter of fact, giant�� at low frequency is observed in the present Mo-dopedsamples. At higher frequency, intrawell hopping probabilityof charge carriers dominates. Thus �� decreases largely athigher frequencies. The increase in Mo content leads to theheterogeneous distribution of the charge carriers. Such pileupcharge carriers respond to the external alternating electricfield and contribute to the dielectric polarization. TheArrhenius plot of f with temperature and shifting of steplikedispersion to higher frequency region both in real ���� andimaginary part ���� of the complex dielectric constant withincreasing temperature indicates that probably the Maxwell-Wagner relaxation mechanism23,24,27–29 plays the dominantrole in the dielectric behavior of Mo-doped CoFe2O4samples. Along with Maxwell-Wagner relaxation, ferroelec-tric relaxor behavior might also be responsible for the largedielectric value as the loss peak shifts toward the higher tem-perature with increasing frequency. It deserves further studyto reveal the real physical natures of the dielectric relaxationin the present compositions. The measured Polarization vsElectric field P-E loops at different frequencies are plotted inFig. 6. The loop indicates weak ferroelectricity. Such rela-tively low-quality hysteresis might be due to the fact of rela-tively high-leakage current. It might be the case that theCoFe2O4 particles aggregate with each other due to thestrong magnetization of them and subsequently channels arecreated. It is worthwhile to mention that there is a possibilityof the increase in dielectric constant due to the space chargewhich is unrelated to ferroelectricity as has been observed byCatalan et al.30 But in the present investigation the signatureof the existence of ferroelectricity �weak� has been confirmedfrom the frequency dependence of the P-E curve �Fig. 6� at
FIG. 4. Temperature variation in the real part of the dielectricconstant ���� for Co�Fe1−xMox�2O4 with x=0–0.1 at 20Hz. Inset:real part of dielectric constant as a function of frequency for x=0.1 at 300 K.
FIG. 5. ��, ��, and tan � �dielectric loss� as a function of tem-perature for Co�Fe1−xMox�2O4 with x=0.1 at different frequencies.Inset: Arrhenius plot for x=0.1.
-4
-3
-2
-1
0
1
2
3
4
-3 -2 -1 0 1 2 3
50 Hz100 Hz200 Hz
P(µC/cm
2 )
E (kV/cm)
FIG. 6. P-E loop at different frequencies for CoFe1.8Mo0.2O4 at300 K.
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300 K. It is found that with increasing frequency, width ofthe P-E loops decreases which supports the ferroelectric be-havior of this composition. We have also measured dc resis-tivity of the present system �shown in Fig. 7�. It has beenobserved below room temperature �300 K� the conductionmechanism is due to the variable range hopping of polaron31
which can be described by dc=0 exp��T0 /T�1/4�, where 0and T0 are constants and the hopping energy Eh is given byEh=0.25k�T0T3�1/4, where k is the Boltzman constant. TheEh calculated from the dc resistivity data is 0.38 eV at 290 K.The value is close to the activation energy Ea required fordielectric relaxation �shown in Fig. 5�. Therefore it can be
concluded that the behavior of charge carriers responsible fordielectric relaxation and dc conduction are almost same.This, in turn, also reveals that in the present system polariza-tion relaxation has close relation with the conductivity in thegrain interiors.
From the above discussion it is obvious that both ferro-electricity and magnetic ordering coexist in the Mo-dopedCoFe2O4. It is worthwhile to mention that this coexistenceoccurs around room temperature. It might be the case that d0
ness of the Mo6+ play major role for the origin of the ferro-electricity. The detail study of the actual mechanism of theorigin of the ferroelectricity in these compositions is underprogress and will be published elsewhere.
In summary, the present investigation shows the coexist-ence of both ferroelectricity and magnetic ordering in theMo-doped CoFe2O4 system at room temperature. The maxi-mum dielectric constant value has been observed for x=0.1sample. From the structural analysis and magnetic measure-ment it has been observed that Mo enters into the tetrahedralsite and in the tetrahedral site it transforms Fe3+ cations intoFe2+ cations to maintain the charge neutrality. The giant di-electric constant value in the present system might be due tothe Maxwell-Wagner relaxation process. The origin of ferro-electricity in the Mo-doped CoFe2O4 might be due to thepresence of d0—ness of Mo6+ ion. Detail study is underprogress to give the actual mechanism of the multiferroicityin the present Mo-doped CoFe2O4 system.
The work is supported by DST, India. S.C. is also gratefulto I.T., BHU. Authors are grateful to D. Kumar and OmPrakash, Ceramic Engineering Department, for providing fa-cilities in their Laboratory and for fruitful discussions. Thiswork was partially supported by National Science Council ofTaiwan under the Grant No. NSC97-2112-M-110-005-MY3.
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FIG. 7. Resistivity of Co�Fe1−xMox�2O4 with x=0.1 as a func-tion of T−1/4. The solid line represents the fitting curve with theVRH model.
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