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Structural, dielectric and magnetic properties of cobalt ferrite prepared using auto combustion and ceramic route
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Structural, dielectric and magnetic properties of cobalt ferrite prepared using auto combustion and ceramic route C. Murugesan n, M. Perumal, G. Chandrasekaran Department of Physics, School of Physical, Chemical and Applied Sciences, Pondicherry University, Puducherry 605 014, India
art ic l e i nf o
Keywords: Ferrite Auto combustion Ceramic method Dielectric constant Ac electrical conductivity Saturation magnetization
a b s t r a c t Cobalt ferrite is synthesized by using low temperature auto combustion and high temperature ceramic methods. The prepared samples have values of lattice constant equal to 8.40 Å and 8.38 Å for auto combustion and ceramic methods respectively. The FTIR spectrum of samples of the auto combustion method shows a high frequency vibrational band at 580 cm 1 assigned to tetrahedral site and a low frequency vibrational band at 409 cm 1 assigned to octahedral site which are shifted to 590 cm 1 and 412 cm 1 for the ceramic method sample. SEM micrographs of samples show a substantial difference in surface morphology and size of the grains between the two methods. The frequency dependent dielectric constant and ac conductivity of the samples measured from 1 Hz to 2 MHz at room temperature are reported. The room temperature magnetic hysteresis parameters of the samples are measured using VSM. The measured values of saturation magnetization, coercivity and remanent magnetization are 42 emu/g, 1553 Oe, 18.5 emu/g for the auto combustion method, 66.7 emu/g, 379.6 Oe, and 17.3 emu/g for the ceramic method, respectively. The difference in preparation methods and size of the grains causes interesting changes in electrical and magnetic properties. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Spinel ferrites with general formula MFe2O4 (M¼ Co, Ni, Zn, or other metals) are well known as they continue to be one of the most important scientific and industrial materials by virtue of their electrical and magnetic properties [1]. Among them, cobalt ferrite is a well-known inverse spinel structured hard magnetic material with high coercivity, moderate magnetization and highest magnetocrystalline anisotropy [2,3]. It has Co2 þ ions in B site, Fe3 þ ions distributed in A and B sites equally [4]. It has been reported the properties of ferrites are sensitive to method of preparation and post sintering process [5,6]. Ferrites are prepared using several methods such as co-precipitation, flash combustion, citrate precursor, sol–gel and ceramic techniques. Conventionally ceramic method has been a well approved method for the synthesis of ferrites. Recently researchers focus on the auto combustion method. This method is preferred as it yields ferrite samples with good chemical homogeneity, high purity and crystallinity [7]. An improvement is made over the conventional auto combustion method without using external water in the precursor stage which is reported as novel combustion route [8,9]. It is claimed in the novel combustion method that the ferrite samples are free from water born impurities and have fine particle size of narrow distribution. In the present work, the
n
Corresponding author. Tel.: þ 91 9600564874. E-mail address: [email protected] (C. Murugesan).
improved novel combustion route is employed for the synthesis of cobalt ferrite. To have a direct comparison of impact of nano size of particles, cobalt ferrite is also synthesized using the ceramic method and studied parallely for the structural, dielectric and magnetic properties of the samples with respect to the preparation methods.
2. Experimental 2.1. Synthesis of cobalt ferrite using auto combustion route Required amount of cobalt nitrate, ferric nitrate and citric acid are taken in a beaker as starting materials and mixed magnetically to obtain a homogeneous mixture. The precursor mixer in the form of gel is heated on a hot plate under constant stirring at temperature of about (80–90) 1C to remove inherent water present. The dried gel is heated on a hot plate until self ignited. The entire preparation processes completed within 30 min. The final product is annealed at 300 1C for 1 h to remove the volatile compound present in the final product. The procedure of synthesis reported here involves low temperature, short duration and less cost over the similar methods reported earlier [10]. 2.2. Synthesis of cobalt ferrite using ceramic route Oxides of cobalt and iron are taken in stoichiometric ratio and ground in a mortar and pestle for 1 h. The homogeneous mixture
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is subjected to pre sintering in air at 1100 1C for 15 h and furnace cooled at a rate of 100 1C per hour. The product is ground, pelletized and post sintered at 1150 1C for 8 h and furnace cooled. Hereafter, the synthesized CoFe2O4 particles using auto combustion and ceramic routes are termed as CoFe2O4-A and CoFe2O4-B respectively. 2.3. Characterization Structure of CoFe2O4 powder is identified using X-Ray Diffraction (XRD) study in PANalytical X'pert PRO diffractometer. The diffraction pattern recorded using Cu-Kα1 radiation with the wavelength of 1.5406 Å. Fourier Transform Infrared (FTIR) spectra is observed using a Shimadzu-8700 FTIR spectrometer over the range of 1200–400 cm 1 at room temperature. Surface morphology of the samples is analyzed using Scanning Electron Microscope (SEM) – Hitachi S-3400 N. Dielectric measurements are carried out over the frequency range of 1 Hz to 2 MHz using Broadband Dielectric Spectrometer (BDS) – Novocontrol TechnologiesConcept-80. Magnetic properties are investigated using Vibrating Sample Magnetometer (VSM) Lakeshore VSM-7410. All the measurements were performed at room temperature.
Fig. 2. FTIR spectrum of CoFe2O4 prepared using auto combustion and ceramic route.
3. Results and discussion 3.1. Structural and morphological analysis 3.1.1. X-ray diffraction study The room temperature XRD patterns of CoFe2O4 samples obtained from both the synthesis routes are shown in Fig. 1. The XRD patterns of the samples conform with JCPDS no. 22-1086 for cobalt ferrite. The crystallite size is calculated using Scherrer's formula [11] for CoFe2O4-A. D ¼ 0:9λ=β cos θ
ð1Þ
here, D is crystallite size, λ is X-ray wavelength, β is full width at half maximum and θ is Bragg angle. The size of the crystallite is 28 nm for CoFe2O4-A and that of CoFe2O4-B is 301 nm samples. Lattice constant calculated using the following relation: "
a¼
λ h2 þ k2 þl2 2 sin 2 θ
#1=2 ð2Þ
here, a is lattice constant, λ is wavelength, and hkl are Miller indices. The calculated value of lattice constant is 8.40 Å for CoFe2O4-A and 8.38 Å for CoFe2O4-B. The variation in the lattice constant is attributed to the change in the kinetics of the reaction methods which may alter substantially site occupancy of cations in A and B sites of cobalt ferrite and nature of cations [5].
Fig. 1. XRD pattern of CoFe2O4 prepared using auto combustion and ceramic route.
3.1.2. FTIR analysis The infrared spectra of samples observed at room temperature are shown in Fig. 2. Two persistent and characteristic absorption bands are found in the range of frequency from 590 to 409 cm 1. The frequencies of IR bands are in agreement with the report of Waldron [12] for similar ferrites. In ferrites the metal ions are situated in two different sub-lattices designated as tetrahedral (A) and octahedral (B) sites. The stretching of these geometrical configurations is attributed to the band at around 600 cm 1 for tetrahedral group and 450 cm 1 for octahedral group. The information about the presence of tetrahedral and octahedral coordination confirms the formation of spinel structure. The FTIR spectrum of samples of the auto combustion method shows a high frequency vibrational band at 580 cm 1 assigned to tetrahedral site and a low frequency vibrational band at 409 cm 1 assigned to octahedral site which are shifted to 590 cm 1 and 412 cm 1 for the ceramic method sample. It is noticed that the vibrational frequencies are shifted to the low frequency region for CoFe2O4-A when compared with CoFe2O4-B. The shift in frequency occurs due to change in bond length between the ions in their coordinated structures [13].
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Fig. 4. Frequency dependent dielectric constant of CoFe2O4 prepared using auto combustion and ceramic route.
Fig. 3. Typical SEM image of CoFe2O4 prepared using, (a) auto combustion route and (b) ceramic route.
3.1.3. SEM analysis It is clear from room temperature SEM micrographs shown in Fig. 3(a) and (b) that the CoFe2O4-A shows agglomerated and spherical shaped grains but CoFe2O4-B shows agglomerated and irregular shaped grains. The grains of CoFe2O4-B are larger as they are prepared through a prolonged and high temperature heat treatment compared with CoFe2O4-A. The explanation is in agreement with XRD analysis, that is, the crystallite size are smaller for CoFe2O4-A than CoFe2O4-B. 3.2. Electrical analysis 3.2.1. Dielectric constant Dielectric constant as a function of frequency is shown in Fig. 4. It is observed that the dielectric constant decreases rapidly at low frequency and attains a frequency independent nature at higher frequencies, which is a normal behavior in the ferrite system. This is due to space charge polarization or Maxwell–Wagner type interfacial polarization resulting from the inhomogeneous nature of dielectrics [14,15]. This model suggests that the dielectric medium is made up of well conducting grains separated by poorly conducting grain boundaries. In cobalt ferrite, hopping of electron between Fe2 þ and Fe3 þ ; hole hopping between Co2 þ and Co3 þ contribute to the polarization [16]. Dielectric constant of CoFe2O4-A is one order of magnitude higher than that of CoFe2O4-B.
Fig. 5. Frequency dependent ac conductivity of CoFe2O4 prepared using auto combustion and ceramic route.
3.2.2. Ac electrical conductivity The measurement of ac electrical conductivity of the samples with frequency at room temperature made and the plot of them is shown in Fig. 5. It is observed in Fig. 5 that the increase of conductivity of the sample is less at low frequency, but it is rapid at high frequency. This frequency dependent ac conductivity has been explained on the basis of Maxwell–Wagner double layer model for dielectrics. According to that, the resistive grain boundaries are more active at low frequencies. At higher frequencies the conductive grains become more active thereby increasing hopping of charge carriers [17]. It is explained that, in small polaron hopping ac conductivity increases with frequency whereas in large polaron hopping ac conductivity decreases with frequency [18]. In our investigation both samples show an increase in conductivity with frequency due to the small polaron conduction. It is further noticed that the ac electrical conductivity of CoFe2O4-A is higher than that of CoFe2O4-B. The obtained conductivity spectra obey the following Jonscher's power law:
sðωÞ ¼ sdc þ Aωs
ð3Þ
Here s(ω) is ac conductivity, sdc is the dc limit of s(ω), A is temperature dependent parameter and ω is frequency. A nonlinear
Please cite this article as: C. Murugesan, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.055i
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The FTIR spectrum shows high and low frequency absorption bands corresponding to stretching vibrations in tetrahedral and octahedral sites. Cobalt ferrite synthesized using the combustion method shows an increase in the frequency dependent dielectric constant and ac conductivity one order of magnitude when compared with the ceramic method. Cobalt ferrite prepared using the auto combustion method is having higher coercivity and the ceramic method having higher saturation magnetization. Comparison of the structural, morphological, electrical and magnetic properties is in support of cobalt ferrite synthesized by two routes satisfy that the novel combustion route can yield materials with less cost and short time for making memory devices. The present work concludes that the cobalt ferrite samples of the ceramic method yields bulk particles and the auto combustion method successfully provides nanoparticles as evidenced by their structural, electric and magnetic properties and may be exploited for appropriate applications. Fig. 6. Hysteresis curves of CoFe2O4 prepared using the auto combustion and ceramic method.
fitting was carried out on the basis of Jonscher's power law for all the conductivity curves. The calculated frequency exponents have values of 0.54 for CoFe2O4-A and 0.93 for CoFe2O4-B. If s o1, the hopping of the charges involves a translation motion with a sudden hopping [19]. 4. VSM study The hysteresis curves of cobalt ferrite are shown in Fig. 6. The measured values of saturation magnetization, coercivity and remanent magnetization are 42 emu/g, 1553 Oe, and 18.5 emu/g for CoFe2O4-A and 66.7 emu/g, 379.6 Oe, and 17.3 emu/g for CoFe2O4-B, respectively. The coercivity of the CoFe2O4-A is higher than CoFe2O4-B. It is reported that coercivity is closely related to the nature of packing of the grain. The higher coercivity of CoFe2O4-A is due to the increase of magneto crystalline anisotropy and surface anisotropy when compared with CoFe2O4-B [2,20]. The saturation magnetization of CoFe2O4-B is higher than CoFe2O4-A. It is due to the increase of spin disorder at the surface of the CoFe2O4-A [21]. The squareness ratio of CoFe2O4-A (0.44) is higher due to the pseudo-single domain nature. The squareness ratio of CoFe2O4-B (0.26) is low due to the multi domain nature of magnetic domains [22]. CoFe2O4 prepared using the auto combustion method has a reduction in size. Hence it has higher coercivity that will be more suitable for magnetic field memory applications as they can store more magnetic energy. 5. Conclusion The structural, dielectric and magnetic properties of cobalt ferrite, synthesized by two different methods such as auto combustion and ceramic methods are investigated. The value of lattice constant is higher for the sample prepared by the ceramic method.
Acknowledgment The authors thank the Central Instrumentation Facility, Pondicherry Universityand DST-FIST, Government of India for funding the facilities utilized in the present work. C. Murugesan thanks UGC for the financial assistance in the form of Rajiv Gandhi National Fellowship (RGNF) (Grant No. F. 14-2(SC)/2010 (SA-III)). References [1] A. Goldman, Modern Ferrite Technology, second ed., Springer, Pittsburgh, 2006. [2] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, J. Magn. Magn. Mater. 308 (2007) 289. [3] T. Niizeki, Y. Utsumi, R. Aoyama, H. Yanagihara, J. Inoue, Y. Yamasaki, H.N. Koike, E. Kita, Appl. Phys. Lett. 103 (2013) 162407. [4] K.K. Bharathi, R.J. Tackett, C.E. Botez, C.V. Ramana, J. Appl. Phys. 109 (2011) 07A510. [5] M.A. Ahmed, N. Okasha, S.I. El-Dek, Nanotechnology 19 (2008) 065603. [6] L. Ai, J. Jiang, Curr. Appl. Phys. 10 (2010) 284. [7] A. Sutka, G. Mezinskis, Front. Mater. Sci. 6 (2) (2012) 128. [8] P. Priyadharshni, A. Pradeep, P.S. Rao, G. Chandrasekaran, Mater. Chem. Phys. 116 (2009) 207. [9] Chyi-Ching Hwang, Tsung-Yung Wu, J.u.n. Wan, Jih-Sheng Tsai, Mater. Sci. Eng. B 111 (2004) 49. [10] A. Franco, E. Lima Jr., M.A. Novak, P.,R. Wells Jr, J. Magn. Magn. Mater. 308 (2007) 198. [11] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, Prentice-Hall Inc., Upper Saddle River, New Jersey, 2001. [12] R.D. Waldron, Phys. Rev. 99 (1955) 1727. [13] A. Pradeep, P. Priyadharsini, G. Chandrasekaran, J. Magn. Magn. Mater. 320 (2008) 2774. [14] K.W. Wagner, Ann. Phys. 40 (1993) 818. [15] C.G. Koops, Phys. Rev. 83 (1951) 121. [16] E. Veena Gopalan, P.A. Joy, I.A. Al-Omari, D. Sakthi Kumar, Yasuhiko Yoshida, M.R. Anantharaman, J. Alloys Compd. 485 (2009) 711. [17] K. Verma, A. Kumar, D. Varshney, J. Alloys Compd. 526 (2012) 91. [18] M.S. Khandekar, R.C. Kambale, J.Y. Patil, Y.D. Kolekar, S.S. Suryavanshi, J. Alloys Compd. 509 (2011) 1861. [19] V.D. Nithya, R. Kalai Selvan, Physica B 406 (2011) 24. [20] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, second ed., John Wiley & Sons Inc., New Jersey, 2009. [21] R.H. Kodama, A.E. Berkowitz, Phys. Rev. Lett. 77 (1996) 394. [22] E.C. Stoner, E.P. Wohlfarth, Philos. Trans. R. Soc. A 240 (1948) 599.
Please cite this article as: C. Murugesan, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.055i
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural, dielectric and magnetic properties of cobalt ferrite prepared using auto combustion and ceramic route C. Murugesan n, M. Perumal, G. Chandrasekaran Department of Physics, School of Physical, Chemical and Applied Sciences, Pondicherry University, Puducherry 605 014, India
art ic l e i nf o
Keywords: Ferrite Auto combustion Ceramic method Dielectric constant Ac electrical conductivity Saturation magnetization
a b s t r a c t Cobalt ferrite is synthesized by using low temperature auto combustion and high temperature ceramic methods. The prepared samples have values of lattice constant equal to 8.40 Å and 8.38 Å for auto combustion and ceramic methods respectively. The FTIR spectrum of samples of the auto combustion method shows a high frequency vibrational band at 580 cm 1 assigned to tetrahedral site and a low frequency vibrational band at 409 cm 1 assigned to octahedral site which are shifted to 590 cm 1 and 412 cm 1 for the ceramic method sample. SEM micrographs of samples show a substantial difference in surface morphology and size of the grains between the two methods. The frequency dependent dielectric constant and ac conductivity of the samples measured from 1 Hz to 2 MHz at room temperature are reported. The room temperature magnetic hysteresis parameters of the samples are measured using VSM. The measured values of saturation magnetization, coercivity and remanent magnetization are 42 emu/g, 1553 Oe, 18.5 emu/g for the auto combustion method, 66.7 emu/g, 379.6 Oe, and 17.3 emu/g for the ceramic method, respectively. The difference in preparation methods and size of the grains causes interesting changes in electrical and magnetic properties. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Spinel ferrites with general formula MFe2O4 (M¼ Co, Ni, Zn, or other metals) are well known as they continue to be one of the most important scientific and industrial materials by virtue of their electrical and magnetic properties [1]. Among them, cobalt ferrite is a well-known inverse spinel structured hard magnetic material with high coercivity, moderate magnetization and highest magnetocrystalline anisotropy [2,3]. It has Co2 þ ions in B site, Fe3 þ ions distributed in A and B sites equally [4]. It has been reported the properties of ferrites are sensitive to method of preparation and post sintering process [5,6]. Ferrites are prepared using several methods such as co-precipitation, flash combustion, citrate precursor, sol–gel and ceramic techniques. Conventionally ceramic method has been a well approved method for the synthesis of ferrites. Recently researchers focus on the auto combustion method. This method is preferred as it yields ferrite samples with good chemical homogeneity, high purity and crystallinity [7]. An improvement is made over the conventional auto combustion method without using external water in the precursor stage which is reported as novel combustion route [8,9]. It is claimed in the novel combustion method that the ferrite samples are free from water born impurities and have fine particle size of narrow distribution. In the present work, the
n
Corresponding author. Tel.: þ 91 9600564874. E-mail address: [email protected] (C. Murugesan).
improved novel combustion route is employed for the synthesis of cobalt ferrite. To have a direct comparison of impact of nano size of particles, cobalt ferrite is also synthesized using the ceramic method and studied parallely for the structural, dielectric and magnetic properties of the samples with respect to the preparation methods.
2. Experimental 2.1. Synthesis of cobalt ferrite using auto combustion route Required amount of cobalt nitrate, ferric nitrate and citric acid are taken in a beaker as starting materials and mixed magnetically to obtain a homogeneous mixture. The precursor mixer in the form of gel is heated on a hot plate under constant stirring at temperature of about (80–90) 1C to remove inherent water present. The dried gel is heated on a hot plate until self ignited. The entire preparation processes completed within 30 min. The final product is annealed at 300 1C for 1 h to remove the volatile compound present in the final product. The procedure of synthesis reported here involves low temperature, short duration and less cost over the similar methods reported earlier [10]. 2.2. Synthesis of cobalt ferrite using ceramic route Oxides of cobalt and iron are taken in stoichiometric ratio and ground in a mortar and pestle for 1 h. The homogeneous mixture
http://dx.doi.org/10.1016/j.physb.2014.04.055 0921-4526/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: C. Murugesan, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.055i
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is subjected to pre sintering in air at 1100 1C for 15 h and furnace cooled at a rate of 100 1C per hour. The product is ground, pelletized and post sintered at 1150 1C for 8 h and furnace cooled. Hereafter, the synthesized CoFe2O4 particles using auto combustion and ceramic routes are termed as CoFe2O4-A and CoFe2O4-B respectively. 2.3. Characterization Structure of CoFe2O4 powder is identified using X-Ray Diffraction (XRD) study in PANalytical X'pert PRO diffractometer. The diffraction pattern recorded using Cu-Kα1 radiation with the wavelength of 1.5406 Å. Fourier Transform Infrared (FTIR) spectra is observed using a Shimadzu-8700 FTIR spectrometer over the range of 1200–400 cm 1 at room temperature. Surface morphology of the samples is analyzed using Scanning Electron Microscope (SEM) – Hitachi S-3400 N. Dielectric measurements are carried out over the frequency range of 1 Hz to 2 MHz using Broadband Dielectric Spectrometer (BDS) – Novocontrol TechnologiesConcept-80. Magnetic properties are investigated using Vibrating Sample Magnetometer (VSM) Lakeshore VSM-7410. All the measurements were performed at room temperature.
Fig. 2. FTIR spectrum of CoFe2O4 prepared using auto combustion and ceramic route.
3. Results and discussion 3.1. Structural and morphological analysis 3.1.1. X-ray diffraction study The room temperature XRD patterns of CoFe2O4 samples obtained from both the synthesis routes are shown in Fig. 1. The XRD patterns of the samples conform with JCPDS no. 22-1086 for cobalt ferrite. The crystallite size is calculated using Scherrer's formula [11] for CoFe2O4-A. D ¼ 0:9λ=β cos θ
ð1Þ
here, D is crystallite size, λ is X-ray wavelength, β is full width at half maximum and θ is Bragg angle. The size of the crystallite is 28 nm for CoFe2O4-A and that of CoFe2O4-B is 301 nm samples. Lattice constant calculated using the following relation: "
a¼
λ h2 þ k2 þl2 2 sin 2 θ
#1=2 ð2Þ
here, a is lattice constant, λ is wavelength, and hkl are Miller indices. The calculated value of lattice constant is 8.40 Å for CoFe2O4-A and 8.38 Å for CoFe2O4-B. The variation in the lattice constant is attributed to the change in the kinetics of the reaction methods which may alter substantially site occupancy of cations in A and B sites of cobalt ferrite and nature of cations [5].
Fig. 1. XRD pattern of CoFe2O4 prepared using auto combustion and ceramic route.
3.1.2. FTIR analysis The infrared spectra of samples observed at room temperature are shown in Fig. 2. Two persistent and characteristic absorption bands are found in the range of frequency from 590 to 409 cm 1. The frequencies of IR bands are in agreement with the report of Waldron [12] for similar ferrites. In ferrites the metal ions are situated in two different sub-lattices designated as tetrahedral (A) and octahedral (B) sites. The stretching of these geometrical configurations is attributed to the band at around 600 cm 1 for tetrahedral group and 450 cm 1 for octahedral group. The information about the presence of tetrahedral and octahedral coordination confirms the formation of spinel structure. The FTIR spectrum of samples of the auto combustion method shows a high frequency vibrational band at 580 cm 1 assigned to tetrahedral site and a low frequency vibrational band at 409 cm 1 assigned to octahedral site which are shifted to 590 cm 1 and 412 cm 1 for the ceramic method sample. It is noticed that the vibrational frequencies are shifted to the low frequency region for CoFe2O4-A when compared with CoFe2O4-B. The shift in frequency occurs due to change in bond length between the ions in their coordinated structures [13].
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Fig. 4. Frequency dependent dielectric constant of CoFe2O4 prepared using auto combustion and ceramic route.
Fig. 3. Typical SEM image of CoFe2O4 prepared using, (a) auto combustion route and (b) ceramic route.
3.1.3. SEM analysis It is clear from room temperature SEM micrographs shown in Fig. 3(a) and (b) that the CoFe2O4-A shows agglomerated and spherical shaped grains but CoFe2O4-B shows agglomerated and irregular shaped grains. The grains of CoFe2O4-B are larger as they are prepared through a prolonged and high temperature heat treatment compared with CoFe2O4-A. The explanation is in agreement with XRD analysis, that is, the crystallite size are smaller for CoFe2O4-A than CoFe2O4-B. 3.2. Electrical analysis 3.2.1. Dielectric constant Dielectric constant as a function of frequency is shown in Fig. 4. It is observed that the dielectric constant decreases rapidly at low frequency and attains a frequency independent nature at higher frequencies, which is a normal behavior in the ferrite system. This is due to space charge polarization or Maxwell–Wagner type interfacial polarization resulting from the inhomogeneous nature of dielectrics [14,15]. This model suggests that the dielectric medium is made up of well conducting grains separated by poorly conducting grain boundaries. In cobalt ferrite, hopping of electron between Fe2 þ and Fe3 þ ; hole hopping between Co2 þ and Co3 þ contribute to the polarization [16]. Dielectric constant of CoFe2O4-A is one order of magnitude higher than that of CoFe2O4-B.
Fig. 5. Frequency dependent ac conductivity of CoFe2O4 prepared using auto combustion and ceramic route.
3.2.2. Ac electrical conductivity The measurement of ac electrical conductivity of the samples with frequency at room temperature made and the plot of them is shown in Fig. 5. It is observed in Fig. 5 that the increase of conductivity of the sample is less at low frequency, but it is rapid at high frequency. This frequency dependent ac conductivity has been explained on the basis of Maxwell–Wagner double layer model for dielectrics. According to that, the resistive grain boundaries are more active at low frequencies. At higher frequencies the conductive grains become more active thereby increasing hopping of charge carriers [17]. It is explained that, in small polaron hopping ac conductivity increases with frequency whereas in large polaron hopping ac conductivity decreases with frequency [18]. In our investigation both samples show an increase in conductivity with frequency due to the small polaron conduction. It is further noticed that the ac electrical conductivity of CoFe2O4-A is higher than that of CoFe2O4-B. The obtained conductivity spectra obey the following Jonscher's power law:
sðωÞ ¼ sdc þ Aωs
ð3Þ
Here s(ω) is ac conductivity, sdc is the dc limit of s(ω), A is temperature dependent parameter and ω is frequency. A nonlinear
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The FTIR spectrum shows high and low frequency absorption bands corresponding to stretching vibrations in tetrahedral and octahedral sites. Cobalt ferrite synthesized using the combustion method shows an increase in the frequency dependent dielectric constant and ac conductivity one order of magnitude when compared with the ceramic method. Cobalt ferrite prepared using the auto combustion method is having higher coercivity and the ceramic method having higher saturation magnetization. Comparison of the structural, morphological, electrical and magnetic properties is in support of cobalt ferrite synthesized by two routes satisfy that the novel combustion route can yield materials with less cost and short time for making memory devices. The present work concludes that the cobalt ferrite samples of the ceramic method yields bulk particles and the auto combustion method successfully provides nanoparticles as evidenced by their structural, electric and magnetic properties and may be exploited for appropriate applications. Fig. 6. Hysteresis curves of CoFe2O4 prepared using the auto combustion and ceramic method.
fitting was carried out on the basis of Jonscher's power law for all the conductivity curves. The calculated frequency exponents have values of 0.54 for CoFe2O4-A and 0.93 for CoFe2O4-B. If s o1, the hopping of the charges involves a translation motion with a sudden hopping [19]. 4. VSM study The hysteresis curves of cobalt ferrite are shown in Fig. 6. The measured values of saturation magnetization, coercivity and remanent magnetization are 42 emu/g, 1553 Oe, and 18.5 emu/g for CoFe2O4-A and 66.7 emu/g, 379.6 Oe, and 17.3 emu/g for CoFe2O4-B, respectively. The coercivity of the CoFe2O4-A is higher than CoFe2O4-B. It is reported that coercivity is closely related to the nature of packing of the grain. The higher coercivity of CoFe2O4-A is due to the increase of magneto crystalline anisotropy and surface anisotropy when compared with CoFe2O4-B [2,20]. The saturation magnetization of CoFe2O4-B is higher than CoFe2O4-A. It is due to the increase of spin disorder at the surface of the CoFe2O4-A [21]. The squareness ratio of CoFe2O4-A (0.44) is higher due to the pseudo-single domain nature. The squareness ratio of CoFe2O4-B (0.26) is low due to the multi domain nature of magnetic domains [22]. CoFe2O4 prepared using the auto combustion method has a reduction in size. Hence it has higher coercivity that will be more suitable for magnetic field memory applications as they can store more magnetic energy. 5. Conclusion The structural, dielectric and magnetic properties of cobalt ferrite, synthesized by two different methods such as auto combustion and ceramic methods are investigated. The value of lattice constant is higher for the sample prepared by the ceramic method.
Acknowledgment The authors thank the Central Instrumentation Facility, Pondicherry Universityand DST-FIST, Government of India for funding the facilities utilized in the present work. C. Murugesan thanks UGC for the financial assistance in the form of Rajiv Gandhi National Fellowship (RGNF) (Grant No. F. 14-2(SC)/2010 (SA-III)). References [1] A. Goldman, Modern Ferrite Technology, second ed., Springer, Pittsburgh, 2006. [2] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, J. Magn. Magn. Mater. 308 (2007) 289. [3] T. Niizeki, Y. Utsumi, R. Aoyama, H. Yanagihara, J. Inoue, Y. Yamasaki, H.N. Koike, E. Kita, Appl. Phys. Lett. 103 (2013) 162407. [4] K.K. Bharathi, R.J. Tackett, C.E. Botez, C.V. Ramana, J. Appl. Phys. 109 (2011) 07A510. [5] M.A. Ahmed, N. Okasha, S.I. El-Dek, Nanotechnology 19 (2008) 065603. [6] L. Ai, J. Jiang, Curr. Appl. Phys. 10 (2010) 284. [7] A. Sutka, G. Mezinskis, Front. Mater. Sci. 6 (2) (2012) 128. [8] P. Priyadharshni, A. Pradeep, P.S. Rao, G. Chandrasekaran, Mater. Chem. Phys. 116 (2009) 207. [9] Chyi-Ching Hwang, Tsung-Yung Wu, J.u.n. Wan, Jih-Sheng Tsai, Mater. Sci. Eng. B 111 (2004) 49. [10] A. Franco, E. Lima Jr., M.A. Novak, P.,R. Wells Jr, J. Magn. Magn. Mater. 308 (2007) 198. [11] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, Prentice-Hall Inc., Upper Saddle River, New Jersey, 2001. [12] R.D. Waldron, Phys. Rev. 99 (1955) 1727. [13] A. Pradeep, P. Priyadharsini, G. Chandrasekaran, J. Magn. Magn. Mater. 320 (2008) 2774. [14] K.W. Wagner, Ann. Phys. 40 (1993) 818. [15] C.G. Koops, Phys. Rev. 83 (1951) 121. [16] E. Veena Gopalan, P.A. Joy, I.A. Al-Omari, D. Sakthi Kumar, Yasuhiko Yoshida, M.R. Anantharaman, J. Alloys Compd. 485 (2009) 711. [17] K. Verma, A. Kumar, D. Varshney, J. Alloys Compd. 526 (2012) 91. [18] M.S. Khandekar, R.C. Kambale, J.Y. Patil, Y.D. Kolekar, S.S. Suryavanshi, J. Alloys Compd. 509 (2011) 1861. [19] V.D. Nithya, R. Kalai Selvan, Physica B 406 (2011) 24. [20] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, second ed., John Wiley & Sons Inc., New Jersey, 2009. [21] R.H. Kodama, A.E. Berkowitz, Phys. Rev. Lett. 77 (1996) 394. [22] E.C. Stoner, E.P. Wohlfarth, Philos. Trans. R. Soc. A 240 (1948) 599.
Please cite this article as: C. Murugesan, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.055i