Structural and electrical properties of nanocrystalline cobalt substituted nickel zinc ferrite

Structural and electrical properties of nanocrystalline cobalt substituted nickel zinc ferrite

Journal of Alloys and Compounds 479 (2009) 797–802 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 479 (2009) 797–802

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Structural and electrical properties of nanocrystalline cobalt substituted nickel zinc ferrite M.M. Mallapur, P.A. Shaikh, R.C. Kambale, H.V. Jamadar, P.U. Mahamuni, B.K. Chougule ∗ Department of Physics, Shivaji University, Kolhapur-416004, Maharashtra, India

a r t i c l e

i n f o

Article history: Received 13 November 2008 Received in revised form 13 January 2009 Accepted 18 January 2009 Available online 21 February 2009 Keywords: Nanostructures Chemical synthesis X-ray diffraction Electronic transport

a b s t r a c t The structural and electrical properties of the spinel ferrite system Ni0.7−x Cox Zn0.3 Fe2 O4 (x = 0.0–0.7) have been studied. Samples in the series were prepared through chemical route using sucrose as fuel and PVA as chelating agent. Structural characterization was done by X-ray diffraction technique and infrared absorption spectroscopy. XRD patterns were analyzed to calculate lattice constant (a), X-ray density (dx ) and pore fraction (f). Far infrared absorption spectra show two significant absorption bands, around 600 cm−1 and 425 cm−1 , which are respectively attributed to tetrahedral (A) and octahedral (B) vibrations of the spinel. Scanning electron microscopy was used to study surface morphology. SEM images reveal particles in the nanosize range. DC resistivity (DC ) measurements were carried out as function of temperature using two-probe method and activation energy (E) was determined. It is found that conduction is due to formation of small polarons. The dielectric parameters such as dielectric constant, loss tangent were determined as a function of frequency in the range 100 Hz–1 MHz at room temperature. The compositions exhibit normal dielectric behavior, which is attributed to Maxwell–Wagner type interfacial polarization. AC conductivity was studied as a function of frequency at room temperature. The variation is found to be linear suggesting that conduction is due to small polaron hopping. © 2009 Published by Elsevier B.V.

1. Introduction Nanocrystalline materials are in focus of recent research due to their potential applications and interesting physics involved in them. The ferrite materials substituted with different cations prepared by different techniques have become important from both the fundamental and application point of view [1,2]. The properties of ferrite are very much sensitive to methodology adopted for their synthesis, preparative parameters, initial ingredients, etc. Any change in cation distribution also results into an unexpected electrical and magnetic behavior. Ni–Zn ferrites have been used for many years in the electrical and electronic industries. It is a soft magnetic ceramic that has spinel configurations has a unit cell consisting of eight formula unit of the type [Znx Fe1−x ]A [Ni1−x Fe1+x ]B O4 where ‘A’ represents tetrahedral site and ‘B’ the octahedral site [3]. Among the spinel ferrites cobalt ferrite is especially interesting because of its high cubic magnetocrystalline anisotropy, high coercivity and moderate saturation. The high coercivity is due to the large anisotropy of the Co2+ ion due to its important spin-orbit coupling. The present work is focused on structural and electrical properties of Ni–Zn ferrites doped with Co2+ ions [4].

∗ Corresponding author. Tel.: +91 9850027220 (Mobile). E-mail address: [email protected] (B.K. Chougule). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.01.142

Ceramic processes involving high-temperature solid-state reactions between the constituent oxides or carbonates are commonly used to produce ferrites. The particles obtained by this process are rather large and non-uniform in size. These non-uniform particles, on compacting, result in the formation of voids or low density. In order to overcome these difficulties and obtain advanced ceramic materials with tailor made properties to suit specific applications chemical methods are adopted. Several novel methods of synthesis of nanocrystalline ferrites such as sol–gel [5,6] co-precipitation [7,8] oxidation [9] and reverse micelle technique have been proposed in recent years [10,11]. Among the current methods of synthesis, chemical method using sucrose as fuel stands out as an alternative and highly promising method as it is simple, fast and inexpensive. It is easy to control the stoichiometry and crystallite size, which have an important influence on magnetic and electric properties of ferrites. A salient feature of this method is that the heat required to sustain the chemical reaction is provided by sucrose, which forms one of the main constituents of the initial ingredients [12,13]. 2. Experimental 2.1. Preparation Nanosized mixed ferrite samples were prepared by the self-sustaining method [14,15] using analytical grade iron nitrate Fe(NO3 )3 ·9H2 O, cobalt nitrate Co(NO3 )2 ·6H2 O, nickel nitrate Ni(NO3 )2 ·6H2 O, zinc nitrate Zn(NO3 )2 ·6H2 O and sucrose. Stoichiometric amounts of aqueous metal nitrates were mixed with 2 mol (per metal ion) of aqueous sucrose and 0.8 mol of 10%(w/v) aqueous PVA to produce

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a homogenous metal ion–sucrose–PVA complex solution. The mixture was heated till dry and a very fragile foam was obtained. The dry foam was crushed to fine powder and sintered at 600 ◦ C for 4 h. The sintered powder was pelletized into the shape of a disc of 10 mm diameter and 2–3 mm thickness using hydraulic press. The pellets were used for further characterization. 2.2. Characterization X-ray diffractograms of the samples were recorded using X-ray diffractometer (Philips Model PW 3710) using CuK␣ radiation ( = 1.5418 Å). The scanning electron microscope (Model JEOL JSM 6500F) was used to record the microstructure of the samples. The IR absorption spectra of the powders were recorded in the wave number range of 700–350 cm−1 , by using PerkinElmer IR spectrometer [Model 783] in the KBr medium. The electrical resistivity was measured by two-probe method as a function of temperature in the range room temperature to 650 ◦ C. The dielectric parameters such as dielectric constant, loss tangent and AC conductivity was measured in the frequency range 100 Hz to 1 MHz at room temperature using LCR meter Bridge (Model HP 4284 A). The samples were painted on either side with silver paste to ensure good electric contacts.

3. Results and discussion 3.1. XRD analysis The X-ray diffraction patterns of the samples are shown in Fig. 1. They correspond to well-defined crystalline FCC phase and confirm the spinel structure. No additional peaks are found ensuring phase purity. The peaks were indexed by comparing the interplaner distance with JCPDS data (JCPDS card no 10-325, 22-1086). The lattice constant was calculated for each composition by analyzing the XRD patterns and are listed in Table 1. The lattice constant is found to be insensitive to the compositional variation. The crystallite size, bulk density, X-ray density and pore fraction were calculated using the standard relations and are listed in Table 1. 3.2. Scanning electron microscopy Fig. 2 shows scanning electron micrographs of the samples. It is seen that ferrite particles are of uniform size (of the order of 80–100 nm). Some of them are found to be agglomerated. 3.3. IR spectral analysis The infrared absorption spectra of the compositions are depicted in Fig. 3. The spectra indicate the presence of two absorption bands in the range of 350–700 cm−1 which is a common feature of the spinels. According to Waldron [16] the band 1 is attributed to the stretching vibration of Fe3+ –O2− in the tetrahedral complexes and 2 to that of bending vibrations in octahedral complexes. The band frequencies are given in Table 1.The position and intensities of 1 and 2 vary slightly due to the difference in the Fe3+ –O2− distances for the tetrahedral and octahedral sites.

Fig. 1. X-ray diffraction patterns of Ni0.7−x Cox Zn0.3 Fe2 O4 , where, (a) x = 0.0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3 (e) x = 0.4, (f) x = 0.5 (g) x = 0.6 and (h) x = 0.7.

3.4. DC resistivity The variation of DC resistivity with temperature for all the compositions is represented in Fig. 4. The plots show two slopes with a single transition at a temperature, which is close to the Curie temperature of the ferrites [17]. The values of Curie temperatures noted from plots are listed in Table 1 along with resistivity values of the samples at room temperature. The temperature dependence of resistivity is given by the Arrhenius equation viz.  = 0 exp

E  a

(1)

kT

where 0 is the pre-exponential factor with the dimensions of -cm, k is the Boltzmann constant, Ea is the activation energy and T is the absolute temperature. The activation energies for conduction were computed from log  vs. 1000/T plots and are presented in Table 1. The activation energy increases on changing from the ferrimagnetic to

Table 1 Data on crystallite size (D), lattice parameter (a), X-ray density (dx ), actual density (d), pore fraction (f), absorption band (1 ), absorption band (2 ), resistivity (), Curie temperature (Tc ), activation energy (E1 ) in ferri (E2 ) in para regions and E = E2 − E1 . Parameters

Crystallite size (D) (nm) a (Å) dx (g/cm3 ) d (g/cm3 ) f (pore fraction) 1 (cm−1 ) 2 (cm−1 ) DC × 1010 ( cm) (at 300 K) Curie temp. (Tc ) (K) Ferri E1 (eV) Para E2 (eV) E = E2 − E1 (eV)

Cobalt concentrations x = 0.0

x = 0.1

x = 0.2

x = 0.3

x = 0.4

x = 0.5

x = 0.6

x = 0.7

24.89 8.352 5.38 4.15 0.22 583.16 407.95 2.8 588 0.03 0.81 0.78

24.73 8.356 5.38 4.40 0.18 596.01 374.77 2.5 590 0.04 0.85 0.80

25.27 8.366 5.36 4.82 0.10 599.09 405.03 2.0 595 0.08 0.51 0.42

25.20 8.371 5.35 4.83 0.09 599.08 375.53 1.9 594 0.21 0.59 0.38

25.31 8.373 5.35 4.71 0.11 599.52 374.04 1.8 594 0.17 0.42 0.15

25.19 8.374 5.34 4.84 0.09 600.05 374.88 1.5 593 0.17 0.68 0.51

25.20 8.380 5.33 4.78 0.10 600.12 374.05 1.1 598 0.23 0.74 0.51

25.14 8.398 5.31 4.48 0.15 603.7 374.04 0.9 605 0.13 0.69 0.56

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Fig. 2. SEM micrographs of Ni0.7−x Cox Zn0.3 Fe2 O4 , where, (a) x = 0.0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3 (e) x = 0.4, (f) x = 0.5 (g) x = 0.6 and (h) x = 0.7.

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Fig. 3. IR absorption spectra of Ni0.7−x Cox Zn0.3 Fe2 O4 ferrites.

paramagnetic region. According to the theory of magnetic semiconductors, one expects such an increase in the activation energy as the system undergoes the transition from the ferrimagnetic state to the paramagnetic state. This is due to the fact that the ferrimagnetic state is an ordered state while the paramagnetic state is disordered; thus charge carriers require more energy for the conduction. The high value of the activation energy in the paramagnetic state as compared to the ferrimagnetic state is due to the volume expansion of the samples during the magnetic transition [18–20]. The activation energies in the ferrimagnetic region are much higher than the ionization energies (Ei = 0.1 eV) of donors or acceptors and also higher than the transition energy of Fe2+ and Fe3+ (Ee = 0.2 eV), which indicates that the polaron type conduction mechanism is favored. The AC conductivity of the samples was calculated from the dielectric parameters using the relation ac = ε ε0 ω tan ı

(2)

where ε is dielectric constant, ε0 permittivity of free space, ω the angular frequency, tan ı is the loss tangent. Fig. 5 shows frequency dependent variation of AC conductivity. The conductivity is observed to increase with increase in frequency for all the compositions under study. Linear variation of AC conductivity indicates that the conduction occurs by hopping of charge carriers among localized states. Adler and Feinleib [21] have shown that for conduction

by small polarons, AC conductivity increases with frequency linearly. Hence it is concluded that the conduction is due to small polarons. 3.5. Dielectric properties The variation of dielectric constant for the compositions as a function of frequency is shown in Fig. 6. The dielectric constant was calculated using the relation, ε =

Cd ε0 A

(3)

where C is capacitance, d the thickness of pellet, ε0 the permittivity of free space and A is the area of pellet. The dielectric constant decreases as the frequency increases. The decrease is rapid at lower frequencies showing dispersion in the lower frequency region and remains constant at higher frequencies. The decrease takes place when the jumping frequency of electric charge carriers cannot follow the alternation of applied AC electric field beyond a certain critical frequency. Dielectric dispersion in ferrite can be explained on the basis of space charge polarization, which is a result of the presence of higher conductivity phases (grains) in the insulation of matter (grain boundaries) of a dielectric causing of localized accumulation of charges under the influence of an electric field [22]. The samples show dispersion due to Maxwell Wagner type interfacial polarization in agreement with Koops phenomenological theory

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801

Fig. 4. Variation of DC resistivity with temperature for Ni0.7−x Cox Zn0.3 Fe2 O4 ferrites.

[23]. The large value of dielectric constant at lower frequencies is attributed to different type of polarizations (electronic, atomic, interfacial, ionic, etc.) and as frequency increases ionic and orientation sources of polarizability decrease and finally disappear due to inertia of the molecules and ions. In practice, there is relaxation time for charge transport. The mechanism of this dielectric polarization may also be attributed to the dipoles resulting from

Fig. 5. Variation of AC conductivity of Ni0.7−x Cox Zn0.3 Fe2 O4 with frequency.

the change in valence of cations, such as Fe3+/ Fe2+ . The polarization at lower frequencies may result from the electron hopping between Fe3+/ Fe2+ ions in ferrite lattice. Fig. 7 shows variation of Tan ı with frequency for the ferrites, which shows a similar dispersion as that of dielectric constant with frequency. The low value of loss tangent indicates qualities the substance for high frequency applications.

Fig. 6. Variation of dielectric constant Ni0.7−x Cox Zn0.3 Fe2 O4 of with frequency.

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Acknowledgements The authors are thankful to the management of Tilak High School and Junior College, Karad for their encouragement. The author (MMM) expresses her gratitude to A.D. Sheikh and Rupesh Deven for their help. References

Fig. 7. Variation of dielectric loss (tan ı) Ni0.7−x Cox Zn0.3 Fe2 O4 of with frequency.

4. Conclusion A chemical method using sucrose as fuel and PVA as chelating agent was used to prepare the cobalt substituted Ni–Zn ferrites. XRD patterns reveal the single-phase formation of the ferrites. SEM micrographs confirm the nanosized nature of the particles. Infrared absorption spectra show two significant absorption bands characteristic of tetrahedral and octahedral vibrations. The variation of DC resistivity with temperature shows change in conduction behavior at Curie temperature. The dielectric dispersion at lower frequencies is attributed to interfacial polarization. The variation of AC conductivity with frequency is linear suggesting that the conduction is due to small polarons.

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