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Large magnetodielectric effect in nickel zinc ferrite–lithium niobate nanocomposite
Chemical Physics Letters 541 (2012) 96–100
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Large magnetodielectric effect in nickel zinc ferrite–lithium niobate nanocomposite Shilpi Banerjee a,b, Partha Hajra a, Asim Bhaumik b, Sri Bandyopadhyay c, Dipankar Chakravorty a,⇑ a
MLS Professor’s Unit, Indian Association for the Cultivation of Science, Kolkata 700 032, India Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India c School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney 2052, Australia b
a r t i c l e
i n f o
Article history: Received 3 February 2012 In final form 16 May 2012 Available online 30 May 2012
a b s t r a c t Nanocomoposites of Ni0.5Zn0.5Fe2O4 (NZF) and LiNbO3 were synthesized. The average diameter of nanoparticles was 9.5 nm and lithium niobate had a thickness of 2.5 nm. The sample showed room temperature (RT) ferromagnetism. A magneto-dielectric coefficient of 20% was observed at a magnetic field of 0.5 T. This was explained using inhomogeneous conductor model. It arose due to NZF resistance decrease by about 35% for the applied magnetic field. Nanoindentation studies on the sample showed a creep strain rate (103 s1) at RT. This was ascribed to a large grain boundary diffusion of Fe3+ ions in NZF phase. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Materials showing both ferroelectric and ferromagnetic order (usually referred to as multiferroic) are rare in nature [1–5]. Interest in these systems has grown in recent times because of potential applications in device fabrication e.g., sensors and actuators, multiple state memories, magnetic field sensors, smart sensors, signal processing systems like electric field tunable filters, phase shifters etc. Magnetodielectric coupling on which the above mentioned applications are based is rather small in single phase multiferroic materials [6–8]. Hence research has been focused on synthesizing composites of ferroelectric and ferromagnetic phases in nanodimensions such that the magnetodielectric coupling coefficient shows an order of magnitude increase as compared to single phase multiferroic materials. Some of the examples are CoFe2O4–BaTiO3 core–shell nanostructures prepared by a chemical method [9– 13], Pb(Zr0.53Ti0.047)O3/CoFe2O4 multilayers grown on niobiumdoped SrTiO3 substrates by pulsed-laser deposition [14], thin films comprising of CoFe2O4 nanoparticles embedded in Bi3.15Nd0.85Ti3O4 matrix by solution deposition technique [15], Ni0.5Zn0.5Fe2O4 nanoparticles embedded into the ferroelectric copolymer polyvinylidene-fluoride [16]. All these nanocomposites showed reasonably large magnetodielectric effect. In recent years mesoporous oxides like SiO2, TiO2, SrTiO3, BaTiO3 and ZrO2 have been synthesized by chemical routes [17–23]. Mean pore diameters in the range 2–30 nm were obtained in these studies. We have prepared nanocomposites of ferroelectric and ferromagnetic phases by first
⇑ Corresponding author. Address: Indian Association for the Cultivation of Science, 2A& 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India. Fax: +91 3324732805. E-mail address: [email protected] (D. Chakravorty). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.05.056
synthesizing mesoporous Ni0.5Zn0.5Fe2O4 nanoparticles of diameters 20 nm and then impregnating them with a precursor solution for LiNbO3, the final product being obtained after suitable heat treatment of this mixture. A large magnetodielectric (MD) effect with a MD parameter of 20% was obtained. The data analysis led to the conclusion that a large magnetoresistance of the Ni0.5Zn0.5Fe2O4 phase was responsible for this behavior. We also carried out nanoindentation studies on the nanocomposites. They exhibited substantial creep strain rate at room temperature. The details are reported in this Letter. 2. Experimental At first nanoporous Ni0.5Zn0.5Fe2O4 nanoparticles were prepared. Nitrate salts of iron, nickel and zinc, respectively (molar ratio Fe:Ni:Zn = 4:1:1) and citric acid monohydrate (molar ratio of total nitrate salt: citric acid monohydrate = 1:1.4) were first separately milled and then mixed together. The mixture was put in an autoclave at 453 K for 24 h. Resultant product was washed with distilled water and then dried at 333 K for a few hours. The formation of nickel zinc ferrite was confirmed by taking X-ray diffraction pattern using a model Bruker D8 diffractometer with CuKa radiation and from adsorption isotherms measured by a Quantachrome Surface Area Analyzer Autosorb-1C at 77 K. The pore surface area was found to be 80 m2 g1 and the average pore diameter estimated to be 2.5 nm. The next step was to prepare lithium niobate solution. At first niobium ethoxide was mixed with 2-propanol. Then acetylacetone was added to the above mixture to reduce the condensation rate of the niobium ethoxide. Lithium nitrate was dissolved in distilled water to make a solution. Both the above solutions were mixed together to obtain a homogeneous solution of lithium niobate. After stirring for some time the as prepared por-
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Table 1 Comparison of dhkl values obtained from electron diffraction rings from Figure 2b with ASTM data. Observed dhkl (nm)
ASTM data of Ni0.5Zn0.5Fe2O4 (JCPDS No. 08-0234) (nm)
ASTM data of LiNbO3 (JCPDS No. 85-2456) (nm)
0.371 0.304 0.279 0.250 0.213 0.187 0.149
0.2966 (2 2 0)
0.3749 (0 1 2)
0.2533 (3 1 1)
0.2735 (1 0 4)
0.2110 (4 0 0) 0.1874 (0 2 4) 0.1485 (4 4 0)
Miller indices of lattice planes are shown within parentheses.
3. Results and discussion Figure 1. X-ray diffractograms of porous NZF–LN nanocomposite.
ous nickel zinc ferrite powder was dispersed in the above solution and stirred for 1 day. The lithium niobate soaked porous nickel zinc ferrite powder was collected after filtration and washed with water and 2-propanol. The powder was dried in an oven at 333 K and then heat treated at 873 K for 4 h. To confirm the phases present in the material, X-ray diffraction pattern was taken using model Bruker D8 diffractometer using CuKa radiation. The microstructure of the nanocomposite was studied by a JEOL 2010 Transmission Electron Microscope operated at 200 kV. Magnetization of the sample was measured over the temperature range 2–300 K in a Quantum Design MPMS system. For measurement of magneto-dielectric parameter the powder was compacted in a die of 1 cm diameter by applying a load of 5 t. Both the surfaces were electroded by silver paint supplied by M/S Acheson Colloiden, Netherland. The pellet sample was placed between the pole pieces of an electromagnet supplied by M/S Control Systems & Devices, Mumbai, India. The capacitance change as a function of applied magnetic field was measured using an Agilent E4890A precision LCR meter. Nanoindentation measurements on the composite samples were carried out by Hysitron Triboindenter instrument (Model No.: TI900) using a Berkovich diamond indenting tip of radius of curvature 50 nm. From the load–displacement curve hardness, elastic modulus and creep-strain rates were calculated.
Figure 1 gives the X-ray diffractogram for the composite powder soaked with the lithium niobate precursor solution and subsequently heat treated at 873 K for 4 h. It can be seen that the peaks of both the phases nickel zinc ferrite (NZF) and lithium niobate (LN), respectively are present in the XRD pattern. It indicates that the nanopores were filled with LN. Figure 2a is the transmission electron micrograph for the composite specimen. The dimensions of the composite particles are found to be 15–20 nm. Figure 2b is the electron diffraction pattern obtained from Figure 2a. The interplanar spacings were calculated from the positions of the diffraction spots. The results are summarized in Table 1. We have compared the dhkl values as obtained from the X-ray and electron diffraction data with those of JCPDS File No. 08-0234 and 85-2456 values for Ni0.5Zn0.5Fe2O4 and LiNbO3, respectively. It should be evident that both the phases are present in our nanocomposites. We also give the high resolution transmission electron micrograph in Figure 2c. The lattice images of some of the planes are indicated by arrows and their interplanar spacings are also shown in FFT image of Figure 2c which is indicated in the inset. The presence of lithium niobate phase within nickel zinc ferrite is evident. In Figure 3a is shown the variation of magnetization as a function of temperature under both zero-field cooled (ZFC) and field cooled (FC) conditions. The nature of variation is the characteristic feature of superparamagnetic behavior. Figure 3b gives the magne-
Figure 2. (a) Transmission electron micrograph of porous NZF–LN nanocomposite. (b) Selected area electron diffraction patterns obtained from Figure 2a. (c) High resolution transmission electron micrograph of Figure 2a. Inset shows FFT image of Figure 2c.
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Figure 3. (a) Magnetization as a function of temperature under ZFC and FC conditions. (b) Magnetic hysteresis loops for different temperature.
Figure 4. (a) Variation of dielectric constant (er) as a function of magnetic field at room temperature measured at a frequency 10 kHz. (b) The variation of extracted value of magnetoresistance with magnetic field at 10 kHz. (c) Variation of dielectric constant as a function of frequency for different magnetic fields.
tization vs. magnetic field curves at temperatures in the range 2– 300 K. The inset shows a magnified view of the curves near low magnetic field. It is evident that the material exhibits ferromagnetic hysteresis loop even at room temperature. Figure 4a shows the variation of dielectric constant e0 as a function of applied magnetic field at room temperature measured at a frequency 10 kHz. It is found that the dielectric constant decreases
with applied magnetic field. For a magnetic field 0.5 T the value of the magneto-dielectric coefficient (M.D.) defined by (e(H) e(0))/ e(0) is estimated to be 20%, where e(H) and e(0) refer to the dielectric constants for magnetic fields H and zero, respectively. The dielectric loss (tan d) for the nanocomposite was found to be 0.01. The M.D. effect of the system is analyzed using the model of a heterogeneous system made up of two phases with a large dif-
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Figure 5. (a) Creep curve for porous NZF and (b) Creep curve for porous NZF–LN nanocomposite for 900 lN load. Inset shows their steady state creep, respectively.
Table 2 Nanoindentation studies on porous NZF and porous NZF–lithium niobate for load = 900 lN. Sample
Young’s Modulus E (GPa)
Hardness H (GPa)
Hc (nm)
Creep strain rate (s1)
Porous NZF Porous NZF–lithium niobate
4.8 17.38
0.22048 0.37261
220.48 372.61
1.3 103 8.95 104
ference in electrical conductivities viz., lithium niobate and nickel zinc ferrite, respectively with interfaces between the two phases. For such a system the impedance can be written as a series combination of two capacitors with different dielectric losses. The effective dielectric constant e0 is given by [24]
e0 ðxÞ ¼
1 C 0 ðRi þ Rb Þ
si þ sb s þ x2 si sb s 1 þ x2 s2
ð1Þ
where, Rb is the resistance of lithium niobate, Ri is the resistance of the interfacial layer formed by nickel zinc ferrite phase, si = CiRi, Ci is the capacitance of the nickel zinc ferrite layer; sb = CbRb, Cb being the capacitance of the niobate phase; s = (siRb + sbRi)/(Ri + Rb) and C0 = e0A/t, A being the area of the specimen capacitance, t its thickness and e0 the free space dielectric permittivity. Though the experimental data could be fitted to Eq. (1) by assuming the presence of a positive magnetoresistance in the nanodimensional nick el zinc ferrite phase of the form RðHÞ ¼ R0 þ R1 exp HHS where R0, R1 and HS are the fitting parameters, such as possibility was ruled out in view of negative magnetoresistance effect reported in the literature for nickel zinc ferrite [25]. The results were therefore fitted by assuming a resistance change as given by RðHÞ ¼ R0 þ R1 exp HHS . The values of Cb, Ci, Rb and Ri were estimated from the measurements carried out on pellets comprising of nanoparticles of lithium niobate and nickel zinc ferrite, respectively. The values used for the present analysis are Cb = 10 pf, Ci = 100 pf, Rb = 108 X and Ri = 105 X. The theoretically fitted curve is shown by the solid line in Figure 4a which shows that the theoretical results are in satisfactory agreement with the experimental data. The extracted nature of magnetoresistance variation in the nickel zinc ferrite nanophase is shown in Figure 4b. It is evident that a resistance decrease of about 35% for an applied magnetic field of 0.5 T is shown by this phase. This is ascribed to spin-dependent scattering as discussed previously [25]. In Figure 4c is shown the variation of dielectric constant as a function of frequency for different magnetic fields. It is evident that the dielectric constant decreases as a function of frequency as well as the applied magnetic field. Such behavior is consistent with that
predicted by the Catalan model used here [24]. It must be mentioned that the magnetodielectric effect in the present system is not related to any magnetoelectric coupling but rather is purely a magnetoresistance driven phenomenon. The mechanical property of the nanocomposites was studied by using nanoindentation technique. Figure. 5a and b indicate the variation of creep strain rate with time of the mesoporous NZF and lithium niobate–NZF nanocomposite, respectively at 900 lN load. The inset of figure shows the steady state creep. The value of Young’s Modulus, hardness and creep strain rate of porous NZF and lithium niobate porous NZF nanocomposite for 900 lN load are given in Table 2. It can be seen that in both the porous NZF and LiNbO3–porous NZF nanocomposites the observed creep strain rate at room temperature is 103 s1. This is indicative of a superplastic behavior for the nanodimensional oxide systems concerned. It is interesting to note that porous NZF shows a higher creep strain rate than the nanocomposite comprising of lithium niobate and porous nickel zinc ferrite. It is believed that the high creep strain rate is due to a high grain boundary diffusion of Fe+3 ions in the NZF phase. When the pores are filled up with lithium niobate the above diffusion is inhibited resulting in a lower creep strain rate of the composite. This behavior is reflected in the observation of higher values of Young’s Modulus and Hardness in the case of the nanocomposite than those of the NZF phase. 4. Conclusions Nanoporous NZF with an average pore diameter of 2.5 nm was prepared by a solution route. LiNbO3 was impregnated into NZF by soaking the latter in a solution containing the salt and subsequently subjecting it to a suitable heat treatment. A large magnetodielectric parameter of 20% was measured in this nanocomposite system. The results were fitted to Catalan’s model of inhomogeneous conductor. The extracted values of electrical resistivities showed a 35% decrease for an applied magnetic field of 0.5 T in the case of NZF. Nanoindentation studies on the samples gave a creep strain rate of 103 s1 which indicated a superplastic behav-
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ior for the nanodimensional oxide system. This was ascribed to a high grain boundary diffusion of Fe+3 ions in the NZF phase. Acknowledgements The Letter was supported by a Grant from Department of Science and Technology, Govt. of India, New Delhi under an Indo-Australian Project on Nanocomposites. Shilpi Banerjee thanks Council of Scientific and Industrial Research, New Delhi for the award of a Senior Research Fellowship. The help of Dhriti Ranjan Saha in carrying out some calculations is gratefully acknowledged. D. Chakravorty thanks Indian National Science Academy, New Delhi for giving him an Honorary Scientists position. References [1] [2] [3] [4]
N.A. Spaldin, M. Fiebig, Science 309 (2005) 391. M. Fiebig, M.K. Kneip, M. Bayer, J. Appl. Phys. 99 (2006) 08E302. A. Filippetti, N.A. Hill, J. Magn. Magn. Mater. 236 (2001) 176. M. Fiebig, T. Lottermoser, D. Frohlich, A.V. Goltsev, R.V. Pisarev, Nature (London) 419 (2002) 818. [5] J. Strempfer et al., Phys. Rev. B 75 (2007) 212402. [6] J. Wang et al., Science 299 (2003) 1719. [7] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694.
[8] S.R. Shannigrahi, A. Huang, N. Chandrasekhar, D. Tripathy, A.D. Adeyeye, Appl. Phys. Lett. 90 (2007) 022901. [9] G.V. Duong, R. Groessinger, R.S. Turtelli, IEEE Trans. Magn. 42 (2006) 3611. [10] V. Corral-Flores, D. Bueno-Baques, D. Carrillo-Flores, J.A. Matutes-Aquino, J. Appl. Phys. 99 (2006) 08J503. [11] K. Raidongia, A. Nag, A. Sundaresan, C.N.R. Rao, Appl. Phys. Lett. 97 (2010) 062904. [12] V. Corral-Flores, D. Bueno-Baques, R.F. Ziolo, Acta Mater. 58 (2010) 764. [13] Y. Deng et al., Chem. Phys. Lett. 496 (2010) 301. [14] J.X. Zhang, G.Y. Dai, H.L.W. Chan, J. Appl. Phys. 107 (2010) 104105. [15] X.L. Zhong, G.B. Wang, M. Liao, G.J. Huang, S.H. Xie, Y.C. Zhou, Appl. Phys. Lett. 90 (2007) 152903. [16] Y. Guo, Y. Liu, G. Wang, R.L. Withers, H. Chen, L. Jin, P. Smith, J. Phys. Chem. C 114 (2010) 13861. [17] Y. Wang, H. Xu, X. Wang, X. Zhang, H. Jia, L. Zhang, J. Qiu, J. Phys. Chem. B 110 (2006) 13835. [18] S.G. Liu, H. Wang, J.P. Li, N. Zhao, W. Wei, Y.H. Sun, Mater. Res. Bull. 42 (2007) 171. [19] R.Z. Hou, P. Ferreira, P.M. Vilarinho, Microporous. Mesoporous. Mater. 110 (2008) 392. [20] R.Z. Hou, P. Ferreira, P.M. Vilarinho, Chem. Mater. 21 (2009) 3536. [21] S. Banerjee, A. Datta, A. Bhaumik, D. Chakravorty, J. Appl. Phys. 110 (2011) 064316. [22] Y. Li, N.H. Lee, E.G. Lee, J.S. Song, S.J. Kim, Chem. Phys. Lett. 389 (2004) 124. [23] C. Bouvy, W. Marine, B.L. Su, Chem. Phys. Lett. 438 (2007) 67. [24] G. Catalan, Appl. Phys. Lett. 88 (2006) 102902. [25] A.K.M. Akther Hossain, M. Seki, T. Kawai, H. Tabata, J. Appl. Phys. 96 (2004) 1273.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Large magnetodielectric effect in nickel zinc ferrite–lithium niobate nanocomposite Shilpi Banerjee a,b, Partha Hajra a, Asim Bhaumik b, Sri Bandyopadhyay c, Dipankar Chakravorty a,⇑ a
MLS Professor’s Unit, Indian Association for the Cultivation of Science, Kolkata 700 032, India Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India c School of Materials Science and Engineering, University of New South Wales, Kensington, Sydney 2052, Australia b
a r t i c l e
i n f o
Article history: Received 3 February 2012 In final form 16 May 2012 Available online 30 May 2012
a b s t r a c t Nanocomoposites of Ni0.5Zn0.5Fe2O4 (NZF) and LiNbO3 were synthesized. The average diameter of nanoparticles was 9.5 nm and lithium niobate had a thickness of 2.5 nm. The sample showed room temperature (RT) ferromagnetism. A magneto-dielectric coefficient of 20% was observed at a magnetic field of 0.5 T. This was explained using inhomogeneous conductor model. It arose due to NZF resistance decrease by about 35% for the applied magnetic field. Nanoindentation studies on the sample showed a creep strain rate (103 s1) at RT. This was ascribed to a large grain boundary diffusion of Fe3+ ions in NZF phase. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Materials showing both ferroelectric and ferromagnetic order (usually referred to as multiferroic) are rare in nature [1–5]. Interest in these systems has grown in recent times because of potential applications in device fabrication e.g., sensors and actuators, multiple state memories, magnetic field sensors, smart sensors, signal processing systems like electric field tunable filters, phase shifters etc. Magnetodielectric coupling on which the above mentioned applications are based is rather small in single phase multiferroic materials [6–8]. Hence research has been focused on synthesizing composites of ferroelectric and ferromagnetic phases in nanodimensions such that the magnetodielectric coupling coefficient shows an order of magnitude increase as compared to single phase multiferroic materials. Some of the examples are CoFe2O4–BaTiO3 core–shell nanostructures prepared by a chemical method [9– 13], Pb(Zr0.53Ti0.047)O3/CoFe2O4 multilayers grown on niobiumdoped SrTiO3 substrates by pulsed-laser deposition [14], thin films comprising of CoFe2O4 nanoparticles embedded in Bi3.15Nd0.85Ti3O4 matrix by solution deposition technique [15], Ni0.5Zn0.5Fe2O4 nanoparticles embedded into the ferroelectric copolymer polyvinylidene-fluoride [16]. All these nanocomposites showed reasonably large magnetodielectric effect. In recent years mesoporous oxides like SiO2, TiO2, SrTiO3, BaTiO3 and ZrO2 have been synthesized by chemical routes [17–23]. Mean pore diameters in the range 2–30 nm were obtained in these studies. We have prepared nanocomposites of ferroelectric and ferromagnetic phases by first
⇑ Corresponding author. Address: Indian Association for the Cultivation of Science, 2A& 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700 032, India. Fax: +91 3324732805. E-mail address: [email protected] (D. Chakravorty). 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.05.056
synthesizing mesoporous Ni0.5Zn0.5Fe2O4 nanoparticles of diameters 20 nm and then impregnating them with a precursor solution for LiNbO3, the final product being obtained after suitable heat treatment of this mixture. A large magnetodielectric (MD) effect with a MD parameter of 20% was obtained. The data analysis led to the conclusion that a large magnetoresistance of the Ni0.5Zn0.5Fe2O4 phase was responsible for this behavior. We also carried out nanoindentation studies on the nanocomposites. They exhibited substantial creep strain rate at room temperature. The details are reported in this Letter. 2. Experimental At first nanoporous Ni0.5Zn0.5Fe2O4 nanoparticles were prepared. Nitrate salts of iron, nickel and zinc, respectively (molar ratio Fe:Ni:Zn = 4:1:1) and citric acid monohydrate (molar ratio of total nitrate salt: citric acid monohydrate = 1:1.4) were first separately milled and then mixed together. The mixture was put in an autoclave at 453 K for 24 h. Resultant product was washed with distilled water and then dried at 333 K for a few hours. The formation of nickel zinc ferrite was confirmed by taking X-ray diffraction pattern using a model Bruker D8 diffractometer with CuKa radiation and from adsorption isotherms measured by a Quantachrome Surface Area Analyzer Autosorb-1C at 77 K. The pore surface area was found to be 80 m2 g1 and the average pore diameter estimated to be 2.5 nm. The next step was to prepare lithium niobate solution. At first niobium ethoxide was mixed with 2-propanol. Then acetylacetone was added to the above mixture to reduce the condensation rate of the niobium ethoxide. Lithium nitrate was dissolved in distilled water to make a solution. Both the above solutions were mixed together to obtain a homogeneous solution of lithium niobate. After stirring for some time the as prepared por-
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Table 1 Comparison of dhkl values obtained from electron diffraction rings from Figure 2b with ASTM data. Observed dhkl (nm)
ASTM data of Ni0.5Zn0.5Fe2O4 (JCPDS No. 08-0234) (nm)
ASTM data of LiNbO3 (JCPDS No. 85-2456) (nm)
0.371 0.304 0.279 0.250 0.213 0.187 0.149
0.2966 (2 2 0)
0.3749 (0 1 2)
0.2533 (3 1 1)
0.2735 (1 0 4)
0.2110 (4 0 0) 0.1874 (0 2 4) 0.1485 (4 4 0)
Miller indices of lattice planes are shown within parentheses.
3. Results and discussion Figure 1. X-ray diffractograms of porous NZF–LN nanocomposite.
ous nickel zinc ferrite powder was dispersed in the above solution and stirred for 1 day. The lithium niobate soaked porous nickel zinc ferrite powder was collected after filtration and washed with water and 2-propanol. The powder was dried in an oven at 333 K and then heat treated at 873 K for 4 h. To confirm the phases present in the material, X-ray diffraction pattern was taken using model Bruker D8 diffractometer using CuKa radiation. The microstructure of the nanocomposite was studied by a JEOL 2010 Transmission Electron Microscope operated at 200 kV. Magnetization of the sample was measured over the temperature range 2–300 K in a Quantum Design MPMS system. For measurement of magneto-dielectric parameter the powder was compacted in a die of 1 cm diameter by applying a load of 5 t. Both the surfaces were electroded by silver paint supplied by M/S Acheson Colloiden, Netherland. The pellet sample was placed between the pole pieces of an electromagnet supplied by M/S Control Systems & Devices, Mumbai, India. The capacitance change as a function of applied magnetic field was measured using an Agilent E4890A precision LCR meter. Nanoindentation measurements on the composite samples were carried out by Hysitron Triboindenter instrument (Model No.: TI900) using a Berkovich diamond indenting tip of radius of curvature 50 nm. From the load–displacement curve hardness, elastic modulus and creep-strain rates were calculated.
Figure 1 gives the X-ray diffractogram for the composite powder soaked with the lithium niobate precursor solution and subsequently heat treated at 873 K for 4 h. It can be seen that the peaks of both the phases nickel zinc ferrite (NZF) and lithium niobate (LN), respectively are present in the XRD pattern. It indicates that the nanopores were filled with LN. Figure 2a is the transmission electron micrograph for the composite specimen. The dimensions of the composite particles are found to be 15–20 nm. Figure 2b is the electron diffraction pattern obtained from Figure 2a. The interplanar spacings were calculated from the positions of the diffraction spots. The results are summarized in Table 1. We have compared the dhkl values as obtained from the X-ray and electron diffraction data with those of JCPDS File No. 08-0234 and 85-2456 values for Ni0.5Zn0.5Fe2O4 and LiNbO3, respectively. It should be evident that both the phases are present in our nanocomposites. We also give the high resolution transmission electron micrograph in Figure 2c. The lattice images of some of the planes are indicated by arrows and their interplanar spacings are also shown in FFT image of Figure 2c which is indicated in the inset. The presence of lithium niobate phase within nickel zinc ferrite is evident. In Figure 3a is shown the variation of magnetization as a function of temperature under both zero-field cooled (ZFC) and field cooled (FC) conditions. The nature of variation is the characteristic feature of superparamagnetic behavior. Figure 3b gives the magne-
Figure 2. (a) Transmission electron micrograph of porous NZF–LN nanocomposite. (b) Selected area electron diffraction patterns obtained from Figure 2a. (c) High resolution transmission electron micrograph of Figure 2a. Inset shows FFT image of Figure 2c.
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Figure 3. (a) Magnetization as a function of temperature under ZFC and FC conditions. (b) Magnetic hysteresis loops for different temperature.
Figure 4. (a) Variation of dielectric constant (er) as a function of magnetic field at room temperature measured at a frequency 10 kHz. (b) The variation of extracted value of magnetoresistance with magnetic field at 10 kHz. (c) Variation of dielectric constant as a function of frequency for different magnetic fields.
tization vs. magnetic field curves at temperatures in the range 2– 300 K. The inset shows a magnified view of the curves near low magnetic field. It is evident that the material exhibits ferromagnetic hysteresis loop even at room temperature. Figure 4a shows the variation of dielectric constant e0 as a function of applied magnetic field at room temperature measured at a frequency 10 kHz. It is found that the dielectric constant decreases
with applied magnetic field. For a magnetic field 0.5 T the value of the magneto-dielectric coefficient (M.D.) defined by (e(H) e(0))/ e(0) is estimated to be 20%, where e(H) and e(0) refer to the dielectric constants for magnetic fields H and zero, respectively. The dielectric loss (tan d) for the nanocomposite was found to be 0.01. The M.D. effect of the system is analyzed using the model of a heterogeneous system made up of two phases with a large dif-
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Figure 5. (a) Creep curve for porous NZF and (b) Creep curve for porous NZF–LN nanocomposite for 900 lN load. Inset shows their steady state creep, respectively.
Table 2 Nanoindentation studies on porous NZF and porous NZF–lithium niobate for load = 900 lN. Sample
Young’s Modulus E (GPa)
Hardness H (GPa)
Hc (nm)
Creep strain rate (s1)
Porous NZF Porous NZF–lithium niobate
4.8 17.38
0.22048 0.37261
220.48 372.61
1.3 103 8.95 104
ference in electrical conductivities viz., lithium niobate and nickel zinc ferrite, respectively with interfaces between the two phases. For such a system the impedance can be written as a series combination of two capacitors with different dielectric losses. The effective dielectric constant e0 is given by [24]
e0 ðxÞ ¼
1 C 0 ðRi þ Rb Þ
si þ sb s þ x2 si sb s 1 þ x2 s2
ð1Þ
where, Rb is the resistance of lithium niobate, Ri is the resistance of the interfacial layer formed by nickel zinc ferrite phase, si = CiRi, Ci is the capacitance of the nickel zinc ferrite layer; sb = CbRb, Cb being the capacitance of the niobate phase; s = (siRb + sbRi)/(Ri + Rb) and C0 = e0A/t, A being the area of the specimen capacitance, t its thickness and e0 the free space dielectric permittivity. Though the experimental data could be fitted to Eq. (1) by assuming the presence of a positive magnetoresistance in the nanodimensional nick el zinc ferrite phase of the form RðHÞ ¼ R0 þ R1 exp HHS where R0, R1 and HS are the fitting parameters, such as possibility was ruled out in view of negative magnetoresistance effect reported in the literature for nickel zinc ferrite [25]. The results were therefore fitted by assuming a resistance change as given by RðHÞ ¼ R0 þ R1 exp HHS . The values of Cb, Ci, Rb and Ri were estimated from the measurements carried out on pellets comprising of nanoparticles of lithium niobate and nickel zinc ferrite, respectively. The values used for the present analysis are Cb = 10 pf, Ci = 100 pf, Rb = 108 X and Ri = 105 X. The theoretically fitted curve is shown by the solid line in Figure 4a which shows that the theoretical results are in satisfactory agreement with the experimental data. The extracted nature of magnetoresistance variation in the nickel zinc ferrite nanophase is shown in Figure 4b. It is evident that a resistance decrease of about 35% for an applied magnetic field of 0.5 T is shown by this phase. This is ascribed to spin-dependent scattering as discussed previously [25]. In Figure 4c is shown the variation of dielectric constant as a function of frequency for different magnetic fields. It is evident that the dielectric constant decreases as a function of frequency as well as the applied magnetic field. Such behavior is consistent with that
predicted by the Catalan model used here [24]. It must be mentioned that the magnetodielectric effect in the present system is not related to any magnetoelectric coupling but rather is purely a magnetoresistance driven phenomenon. The mechanical property of the nanocomposites was studied by using nanoindentation technique. Figure. 5a and b indicate the variation of creep strain rate with time of the mesoporous NZF and lithium niobate–NZF nanocomposite, respectively at 900 lN load. The inset of figure shows the steady state creep. The value of Young’s Modulus, hardness and creep strain rate of porous NZF and lithium niobate porous NZF nanocomposite for 900 lN load are given in Table 2. It can be seen that in both the porous NZF and LiNbO3–porous NZF nanocomposites the observed creep strain rate at room temperature is 103 s1. This is indicative of a superplastic behavior for the nanodimensional oxide systems concerned. It is interesting to note that porous NZF shows a higher creep strain rate than the nanocomposite comprising of lithium niobate and porous nickel zinc ferrite. It is believed that the high creep strain rate is due to a high grain boundary diffusion of Fe+3 ions in the NZF phase. When the pores are filled up with lithium niobate the above diffusion is inhibited resulting in a lower creep strain rate of the composite. This behavior is reflected in the observation of higher values of Young’s Modulus and Hardness in the case of the nanocomposite than those of the NZF phase. 4. Conclusions Nanoporous NZF with an average pore diameter of 2.5 nm was prepared by a solution route. LiNbO3 was impregnated into NZF by soaking the latter in a solution containing the salt and subsequently subjecting it to a suitable heat treatment. A large magnetodielectric parameter of 20% was measured in this nanocomposite system. The results were fitted to Catalan’s model of inhomogeneous conductor. The extracted values of electrical resistivities showed a 35% decrease for an applied magnetic field of 0.5 T in the case of NZF. Nanoindentation studies on the samples gave a creep strain rate of 103 s1 which indicated a superplastic behav-
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ior for the nanodimensional oxide system. This was ascribed to a high grain boundary diffusion of Fe+3 ions in the NZF phase. Acknowledgements The Letter was supported by a Grant from Department of Science and Technology, Govt. of India, New Delhi under an Indo-Australian Project on Nanocomposites. Shilpi Banerjee thanks Council of Scientific and Industrial Research, New Delhi for the award of a Senior Research Fellowship. The help of Dhriti Ranjan Saha in carrying out some calculations is gratefully acknowledged. D. Chakravorty thanks Indian National Science Academy, New Delhi for giving him an Honorary Scientists position. References [1] [2] [3] [4]
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