PHYSICAL REVIEW B 82, 134428 共2010兲
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 of Physics, National Sun Yat Sen University, Kaohsiung 80424, Taiwan, Republic of China 3Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India 共Received 28 April 2010; revised manuscript received 18 August 2010; published 18 October 2010兲 2Department
Coexistence of both magnetic ordering and ferroelectricity 共with giant dielectric constant兲 have been observed for the first time in Co共Fe1−xMox兲2O4. The magnetization of Co共Fe1−xMox兲2O4 共x ranges from 0 to 0.1兲 was found to increase with doping concentration of Mo. The magnetic properties indicate that Mo goes into the tetrahedral site. The giant dielectric constant may be attributed to the Maxwell-Wagner relaxation mechanism. DOI: 10.1103/PhysRevB.82.134428
PACS number共s兲: 75.85.⫹t, 77.80.⫺e, 75.30.Cr
Ferrimagnetic Spinel ferrites constitute an important class of magnetic materials. The magnetic and electrical properties depend on the nature and distribution of their cations in the tetrahedral 共A兲 and octahedral 共B兲 sublattices of a cubic structure. The CoFe2O4 is a very important magnetic material which has covered a wide range of applications including electronic devices, ferro-fluids, magnetic delivery microwave devices, and high-density information storage due to its wealth of magnetic and electronic properties, such as cubic magnetocrystalline anisotropy, high coercivity, moderate saturation magnetization, high Curie temperature Tc, magnetostriction, high-chemical stability, and electrical insulation, etc.1–7 The recent investigations of the substitutions at the Fe sites of CoFe2O4 by various cations 共Ga, Mn, etc.兲 have shown great potentiality of the doping for inducing magnetic properties.8–10 On the other hand, the magnetoelectric 共ME兲 effect, which is defined as the variation in dielectric polarization in response to the magnetic field or vice versa, is widely investigated recently.11,12 Materials exhibiting simultaneous magnetic and ferroelectric properties have potential applications such as magnetic field sensors, transducers, and information storage. The ME effect is found to be accompanied by high-dielectric constant in both single-phase multiferroics13 and magnetoelectric composites.14–16 In the present investigation we have partially substituted Fe site by molybdenum under the impression that it will be highly interesting due to its high valence state 共+6兲 and its d0 electronic configuration. The idea is that a partial substitution of Fe3+ by Mo6+ in CoFe2O4 should induce electron doping, and consequently, concentration of Fe2+ ions should increase in the tetrahedral sites. Furthermore, ions with d0 electrons are responsible for ferroelectricity. Hence Mo doping in CoFe2O4 may induce multiferroicity in these compositions. The Molybdenum substituted samples with compositions Co共Fe1−xMox兲2O4 共x ranges from 0 to 0.1兲 were synthesized by the conventional ceramic method.17 Stoichiometric amounts of Co3O4, Fe2O3, and MoO3 were mixed and ground for several hours. The resulting mixture was calcined at 1000 ° C for 12 h and further heated at the same temperature after an intermediate grinding. The powder was then pelletized which was followed by sintering 共cooling rate 5 ° C / min.兲 the pellets at 1150 ° C for 48 h with intermediate grindings. The obtained samples were characterized by x-ray 1098-0121/2010/82共13兲/134428共5兲
powder diffraction using the Cu K␣ radiation. The pattern indicates that all the compositions are of single phase. The magnetization measurement was carried out using a commercial superconducting quantum interference device 关Magnetic Properties Measurement System 共MPMS兲 XL-7, Quantum Design Inc.兴 magnetometer. The dielectric constant was measured using an LCR meter 共Agilent 4980A兲 and the PE loop was measured using the 609E-6 共Radiant Technologies兲. Figure 1 shows the x-ray diffraction pattern of Mo-doped CoFe2O4 samples. The lattice parameters obtained from Reitveld refinement obey the Vegards’ law indicating that the samples are of single phase. It has been observed that the structure is cubic with Fd3m space group. The parameter “a” can completely be determined through two independent cation-anion distances, designated Rtetra and Rocta.18 2 2 a = 2.0995Rtetra + 关5.8182Rocta − 1.4107Rtetra 兴1/2
Rocta and Rtetra measure exactly the average value of the anion-cation distance on the octahedral and tetrahedral sites, 6+ ´兲 = 0.55 Å respectively. One can imagine that the Mo6+ 共rMo ions enter into the tetrahedral lattice sites 共A兲 of the spinel 3+ lattice. With the entering of Mo6+ some of the Fe3+ 共rFe ´ 兲 ions transform to Fe2+ 共r2+ = 0.77 Å ´ 兲 ions for = 0.645 Å Fe maintaining the charge neutrality. An increase in the population of Mo6+ cations and a decrease in the Fe3+ cations in the A sites contributes to the decrease in Rtetra while an increase in the population of Fe2+ cations in the A sites increases Rtetra. Moreover, to relax the strain some of the Fe2+ ions will be migrated to the octahedral site. Because of all these changes the change in the lattice parameter is very insignificant but the changes are able to effectively decrease the parameter a. Similar decrease in lattice parameter with Mo6+ doping in Fe site has been observed by Rezlescu et al.19 in spinel MgFe2O4. Figure 2 shows the temperature variation in magnetization of Co共Fe1−xMox兲2O4 共with x = 0.0– 0.10兲 for field cooled 共FC兲 and zero field cooled 共ZFC兲 at 100 Oe. The ZFC susceptibility decreases monotonically down to a certain temperature. The FC magnetization initially decreases with decreasing temperature and then saturates. On the other hand, with increase in Mo content the magnetization value also increases. One may expect the magnetization to increase with a decrease in the anisotropy constant of the Co2+. From
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©2010 The American Physical Society
PHYSICAL REVIEW B 82, 134428 共2010兲
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Intensity (arbitrary unit)
Lattice Parameter
8.374
Co(Fe1-xMox)2O4
8.373
8.372
8.371
0.00
0.02
0.04
0.06
0.08
0.10
Mo Concentration
x=0.10
x=0.08
x=0.04
x=0.00 20
40
60
80
FIG. 3. M-H hysteresis curve 共−7 T to +7 T兲 for x = 0.1 at temperature 10 and 300 K. Inset: M-H curve at 300 K in the extended scale 共−0.3 to +0.3 T兲.
2θ (deg.) FIG. 1. X-ray diffraction pattern of Co共Fe1−xMox兲2O4; with x = 0.00, 0.04, 0.08, and 0.10. Inset: variation in lattice parameter with Mo concentration.
the FC behavior one may predict that the anisotropy constant decreases with increase in Mo content which is nonmagnetic. This is because of the fact that for Mo-free sample the decrease in FC-M共T兲 with decrease in temperature is rapid. But as Mo content increases, the M共T兲 value increases very slowly. One may assume that for Mo-free sample the anisotropy is so high 共because of the Co2+ ions兲20 that the samples could not be magnetized sufficiently close to saturation. The M共H兲 for the Mo-substituted ferrite 共for x = 0.1兲 at 10 and 300 K have been presented in Fig. 3. Hysteresis loops are in agreement with Ferrimagnetic behavior. At high field, the magnetization increases almost linearly and tends to satura-
FIG. 2. Temperature variation in magnetization of Co共Fe1−xMox兲2O4 for both zero field cooled and field cooled at 100 Oe.
tion at a field as high as of 7 T. In the ideal situation, the magnetization per formula unit is represented by the net moment of that in tetrahedral 共A兲 and octahedral 共B兲 sites 共M = M B − M A兲. Therefore, in the present investigation it might be the case as we have already mentioned that Mo is entering into the tetrahedral site. Being the nonmagnetic Mo6+ ion in the tetrahedral site will increase the magnetization of the samples. Moreover, the transformation of Fe3+ ions into Fe2+ ions 共one Mo6+ ion will transfer three Fe3+ ions into Fe2+ ions to maintain the charge neutrality兲 will decrease the mag3+ 2+ ⬎ Fe 兲 in the A site. Effectively, the apprenetization 共Fe ciable increase in magnetization with Mo content can be observed, which is consistent with our result. Therefore, our speculation that Mo6+ is entering into the tetrahedral site can explain the structural and magnetization behavior satisfactorily. It is interesting to mention that in the spinel Mo-ferrite unlike the present investigation Mo is in the +3 and +4 states.21 We may predict that since the ionic radius of Mo6+ is smaller than that of Fe3+ it can enter into the Fe site whereas ionic radius of Mo3+/4+ is larger than that of Fe3+. Furthermore, Gillot et al.22 have reported that in Mo-ferrite trace of Mo6+ 共along with Mo3+/4+兲 state in tetrahedral site is observed only when it is oxidized ⬃500 ° C. In this respect it is not consistent with our present observation since in the present investigation the existence of only Mo6+ in the tetrahedral site is evident in unoxidized Co共Fe1−xMox兲2O4 samples. The evidence of Mo6+ has also been reported19 in Mo doped MgFe2O4 where similar to our case Mo6+ has been doped in Fe site. Figure 4 shows the temperature variation in the real part of the dielectric constant 共⬘兲 of Co共Fe1−xMox兲2O4 with x = 0 – 0.1 at 20 Hz. It is observed that with increasing Mo content ⬘ increases and for x = 0.1 ⬘ value is maximum. Above 200K, ⬘ increases sharply for all the compositions. Figure 5 shows the temperature variation in ⬘, ⬙ 共imaginary part of dielectric constant兲 and tan ␦ 共dielectric loss兲 for x = 0.1. Above a certain temperature 共e.g., 200 K兲, ⬘ in-
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SIGNATURE OF FERROELECTRICITY IN MAGNETICALLY… 4
50 Hz 100 Hz 200 Hz
3
2
P (µC/cm )
2 1 0 -1 -2 -3 -4
FIG. 4. Temperature variation in the real part of the dielectric constant 共⬘兲 for Co共Fe1−xMox兲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.
creases sharply and shows high dielectric constant with weak-temperature dependence. As the frequency increases this temperature also increase. Around room temperature ⬘ ⬃ 104 for x = 0.1. At low temperature, electric dipoles freeze through relaxation process and there exists decay in polarization with respect to the applied electric field, which is confirmed by the sudden drop in ⬘. Peak observed in the temperature variation in loss curve corresponds to such sudden drop in ⬘ and shifting of the loss-peak toward lower temperature for lower frequency indicates thermally excited relaxation process. The frequency dependence of ⬘ for x = 0.1 sample is shown in the inset of Fig. 4. The Arrhenius plot23 共inset of Fig. 5兲 of f = f o exp共-Ea / KBT兲 for x = 0.1 sample, gives activation energy 共Ea兲 = 0.36 eV. The obtained activation energy falls in the range of giant dielectric
FIG. 5. ⬘, ⬙, and tan ␦ 共dielectric loss兲 as a function of temperature for Co共Fe1−xMox兲2O4 with x = 0.1 at different frequencies. Inset: Arrhenius plot for x = 0.1.
-3
-2
-1
0
1
2
3
E (kV/cm) FIG. 6. P-E loop at different frequencies for CoFe1.8Mo0.2O4 at 300 K.
materials.24–26 Rapid increase in f 共or decrease in the relaxation time, 兲 with increase in temperature indicates a faster polarization process with increasing dipole density. It has been observed that at 300 K, the value of ⬘ at lower frequency range is very high. As Mo is doped, defects near grain boundary increases. As a consequence, probability of interwell hopping increases which affects the dielectric relaxation largely at low frequency. As a matter of fact, giant ⬘ at low frequency is observed in the present Mo-doped samples. At higher frequency, intrawell hopping probability of charge carriers dominates. Thus ⬘ decreases largely at higher frequencies. The increase in Mo content leads to the heterogeneous distribution of the charge carriers. Such pileup charge carriers respond to the external alternating electric field and contribute to the dielectric polarization. The Arrhenius plot of f with temperature and shifting of steplike dispersion to higher frequency region both in real 共⬘兲 and imaginary part 共⬙兲 of the complex dielectric constant with increasing temperature indicates that probably the MaxwellWagner relaxation mechanism23,24,27–29 plays the dominant role in the dielectric behavior of Mo-doped CoFe2O4 samples. Along with Maxwell-Wagner relaxation, ferroelectric relaxor behavior might also be responsible for the large dielectric value as the loss peak shifts toward the higher temperature with increasing frequency. It deserves further study to reveal the real physical natures of the dielectric relaxation in the present compositions. The measured Polarization vs Electric field P-E loops at different frequencies are plotted in Fig. 6. The loop indicates weak ferroelectricity. Such relatively low-quality hysteresis might be due to the fact of relatively high-leakage current. It might be the case that the CoFe2O4 particles aggregate with each other due to the strong magnetization of them and subsequently channels are created. It is worthwhile to mention that there is a possibility of the increase in dielectric constant due to the space charge which is unrelated to ferroelectricity as has been observed by Catalan et al.30 But in the present investigation the signature of the existence of ferroelectricity 共weak兲 has been confirmed from the frequency dependence of the P-E curve 共Fig. 6兲 at
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FIG. 7. Resistivity of Co共Fe1−xMox兲2O4 with x = 0.1 as a function of T−1/4. The solid line represents the fitting curve with the VRH model.
300 K. It is found that with increasing frequency, width of the P-E loops decreases which supports the ferroelectric behavior of this composition. We have also measured dc resistivity of the present system 共shown in Fig. 7兲. It has been observed below room temperature 共⬍300 K兲 the conduction mechanism is due to the variable range hopping of polaron31 which can be described by dc = 0 exp关共T0 / T兲1/4兴, where 0 and T0 are constants and the hopping energy Eh is given by Eh = 0.25k共T0T3兲1/4, where k is the Boltzman constant. The Eh calculated from the dc resistivity data is 0.38 eV at 290 K. The value is close to the activation energy Ea required for dielectric relaxation 共shown in Fig. 5兲. Therefore it can be
*Corresponding author;
[email protected] C. Slonczewski, Phys. Rev. 110, 1341 共1958兲. 2 C. N. Chinnasamy, B. Jeyadevan, K. Shinoda, K. Tohji, D. J. Djayaprawira, M. Takahashi, R. J. Joseyphus, and A. Narayanasamy, Appl. Phys. Lett. 83, 2862 共2003兲. 3 C. N. Chinnasamy, B. Jeyadevan, O. Perales-Perez, K. Shinoda, K. Tohji, and A. Kasuya, IEEE Trans. Magn. 38, 2640 共2002兲. 4 A. K. Giri, E. M. Kirkpatrick, P. Moongkhamklang, S. A. Majetich, and V. G. Harris, Appl. Phys. Lett. 80, 2341 共2002兲. 5 H. Zheng, J. Wang, S. E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. Salamanca-Riba, S. R. Shinde, S. B. Ogale, F. Bai, D. Viehland, Y. Jia, D. G. Schlom, M. Wuttig, A. Roytburd, and R. Ramesh, Science 303, 661 共2004兲. 6 R. V. Chopdekar and Y. Suzuki, Appl. Phys. Lett. 89, 182506 共2006兲. 7 H. Zheng, F. Straub, Q. Zhan, P.-L. Yang, W.-K. Hsieh, F. Zavaliche, Y.-H. Chu, U. Dahmen, and R. Ramesh, Adv. Mater. 18, 2747 共2006兲. 8 B. Zhou, Y. Zheng, C. Liao, F. Cheng, and C. Yan, Appl. Phys. Lett. 79, 1849 共2001兲. 9 Y. Melikhov, J. E. Snyder, D. C. Jiles, A. P. Ring, J. A. Paulsen, C. C. H. Lo, and K. W. Dennis, J. Appl. Phys. 99, 08R102 1 J.
concluded that the behavior of charge carriers responsible for dielectric relaxation and dc conduction are almost same. This, in turn, also reveals that in the present system polarization relaxation has close relation with the conductivity in the grain interiors. From the above discussion it is obvious that both ferroelectricity and magnetic ordering coexist in the Mo-doped CoFe2O4. It is worthwhile to mention that this coexistence occurs around room temperature. It might be the case that d0 ness of the Mo6+ play major role for the origin of the ferroelectricity. The detail study of the actual mechanism of the origin of the ferroelectricity in these compositions is under progress and will be published elsewhere. In summary, the present investigation shows the coexistence of both ferroelectricity and magnetic ordering in the Mo-doped CoFe2O4 system at room temperature. The maximum dielectric constant value has been observed for x = 0.1 sample. From the structural analysis and magnetic measurement it has been observed that Mo enters into the tetrahedral site and in the tetrahedral site it transforms Fe3+ cations into Fe2+ cations to maintain the charge neutrality. The giant dielectric constant value in the present system might be due to the Maxwell-Wagner relaxation process. The origin of ferroelectricity in the Mo-doped CoFe2O4 might be due to the presence of d0—ness of Mo6+ ion. Detail study is under progress to give the actual mechanism of the multiferroicity in the present Mo-doped CoFe2O4 system. The work is supported by DST, India. S.C. is also grateful to I.T., BHU. Authors are grateful to D. Kumar and Om Prakash, Ceramic Engineering Department, for providing facilities in their Laboratory and for fruitful discussions. This work was partially supported by National Science Council of Taiwan under the Grant No. NSC97-2112-M-110-005-MY3.
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