Structural, electrical, optical and magnetic properties of chromium substituted Co–Zn nanoferrites Co0.6Zn0.4CrxFe2−xO4 (0 ⩽ x ⩽ 1.0) prepared via sol–gel auto-combustion method

Journal of Molecular Structure 1012 (2012) 162–167

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Structural, electrical, optical and magnetic properties of chromium substituted Co–Zn nanoferrites Co0.6Zn0.4CrxFe2xO4 (0 6 x 6 1.0) prepared via sol–gel auto-combustion method Santosh Bhukal a, Tsering Namgyal a, S. Mor b, S. Bansal c, Sonal Singhal a,⇑ a b c

Dept. of Chemistry, Panjab University, Chandigarh 160 014, India Dept. of Environ. Sci., Panjab University, Chandigarh 160 014, India Dept. of Sci. Tech., New Delhi, India

a r t i c l e

i n f o

Article history: Received 15 November 2011 Accepted 9 December 2011 Available online 17 December 2011 Keywords: Ferrites Magnetic properties Powder X-ray diffraction Electrical properties

a b s t r a c t Chromium substituted Co–Zn nanoferrites having the formula Co0.6Zn0.4CrxFe2xO4 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) have been synthesized using the sol–gel auto-combustion method. Characterized using FT-IR spectroscopy, powder X-ray diffraction, electrical and magnetic properties. XRD patterns indicated the formation of single-phased cubic spinel structure having space group Fd-3m for all the annealed samples. The lattice parameter has been observed to decrease with increasing substitution of Cr3+ ions because of 0 0 3+ 3+ Å) as compared to Fe (0.73 A Å). The electrical resistivity increases the smaller ionic radii of Cr (0.68 A with increasing Cr3+ concentration attributing to the reason that electrical conductivity decreases with decrease in Fe concentration because of the decrease in polaron hopping of Fe2+–Fe3+ ions. The resistivity of all the annealed samples decreases with increasing temperature showing the semiconductor nature of the samples. The drift mobility also increases with increasing temperature because of the enhanced mobility of charge carriers due to thermal activation. The saturation magnetization (Ms) decreases with increasing chromium content, due to the lower magnetic moment of Cr3+ ion (3lB) than that of Fe3+ ion (5lB). Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Magnetic ferrite nanoparticles are the topic of current research interest due to their broad applications in technological fields like magnetic fluids, permanent magnets, microwave devices, disk recording, magnetic refrigeration systems and high density information storage. CoFe2O4 is such a class of magnetic material that has a high coercivity (Hc), high saturation magnetization (Ms), high electromagnetic performance due to which it has emerged as a promising candidate in high density magnetic recording media [1,2]. Cobalt ferrite has inverse spinel structure with Co2+ ions in the octahedral (B) sites and Fe3+ ions equally distributed between tetrahedral (A) and octahedral (B) sites. The interactions between the ions Co2+ and Fe3+ in A and B sites, when they are substituted with various metal cations allows some tunable changes in the electrical and magnetic properties of nanoferrites [3]. The cation distribution mechanism of CoAlxCrxFe22xO4 (x = 0.1–0.5) has been discussed by More et al. [4] and they concluded that both Al3+ and Cr3+ ions occupy the octahedral (B) sites thereby decreasing the

⇑ Corresponding author. Tel.: +91 9872118810. E-mail address: [email protected] (S. Singhal). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.12.019

Fe3+ ion content in the B sites. Sharma et al. [5] reported the synthesis of Cr3+ substituted nanoparticles of cobalt zinc ferrite by the co-precipitation method and observed an exponential decrease in the super paramagnetic blocking temperature with increase in Cr3+ concentration for all the samples. Javed Iqbal et al. [6] studied the electrical and magnetic properties of chromium substituted cobalt ferrite nanomaterial and found that the magnetic saturation has maximum value for the composition CoCr0.2Fe1.8O4 while the coercivity decreases with increasing Cr3+ concentration in the sample, indicating the loss of magneto crystalline anisotropy. Hankare et al. [7] synthesized CoCrxFe2xO4 (0.0 6 x 6 2.0) and observed a decreasing trend of saturation magnetization, coercivity and remanent magnetization with increase in Cr3+ content attributing to the weakening of sublattice interactions which lowers the magnetic moment of the unit cell. Kumar et al. [8] studied the environment of Fe in Co0.5CdxFe2.5xO4 (0.0 6 x 6 0.5) using Mossbauer spectroscopy and observed a decrease in hyperfine magnetic field as the cadmium doping increases in the A and B site. Farea et al. [9] reported that the dielectric constant, dielectric losses and AC conductivity increase with increase in the cadmium concentration upto 20% and decrease thereafter, which was explained on the basis of Maxwell and Wagner two-layer model. Effect of chromium substitution on the properties of Ni–Zn ferrites was studied by

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200

(b)

100

(a) 25

3.2. X-ray diffraction studies Fig. 1 depicts the typical XRD pattern of Co0.6Zn0.4Cr0.2Fe1.8O4 after annealing at 400 °C, 600 °C, 800 °C and 1000 °C to analyze the formation of various phases. The evolution of the spinel phase having fcc structure with space group Fd-3m was observed early for the annealed samples at 400 °C. So, all the samples annealed at 400, 600, 800 and 1000 °C show characteristic peaks corresponding to cubic spinel lattice of cobalt ferrite, with the peaks getting sharper with annealing temperature. The increase in particle size with annealing temperature is hence confirmed using the famous Debye Scherrer equation [13]. The spinel cubic structure was also observed for all the annealed samples at 1000 °C and

40

50

60

70

Angle (2θ) Fig. 1. XRD patterns of Co0.6Zn0.4Cr0.2Fe1.8O4 nanoferrite annealed at (a) 400, (b) 600, (c) 800 and (d) 1000 °C.

the peaks can be indexed to (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes of the cubic unit cell which correspond to the cubic structure (Fig. 2). The average crystallite size for all the samples has been calculated from the line broadening of the most intense peak corresponding to (3 1 1) plane of the spinel structure using the classical Scherrer equation.

Dhkl ¼ 0:9k=B cos h where Dhkl is the grain diameter, B is the half maximum line width, k is the wavelength of the radiation used, h is the angle of diffraction. Its values are listed in Table 1. The diffraction peaks become narrower and sharper with increasing temperature, suggesting increase in the grain size with the increasing annealing temperature, The Le-Bail refinement method was used to calculate the lattice parameter ‘a’ whose values are tabulated in Table 1. It was observed that the lattice parameter ‘a’ decreases with increasing chromium concentration due to the smaller ionic radii of chromium as compared to iron ion. 3.3. Electrical properties Spinel ferrite structure consists of cubic close packed oxygen lattice with cations at the octahedral (B) and tetrahedral sites (A). The distance between the two metal ions at B site is smaller than the distance between a metal ion at a B site and another metal ion at an A site. Conduction of ferrites is mainly due to the polaron hopping of Fe3+ to Fe2+ ions that result in the local displacement of

1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

440

The FT-IR transmission spectra of Cr3+ substituted cobalt ferrites show two dominant absorption bands in the range of 400– 600 cm1. The high frequency band at 490 cm1 is assigned to stretching vibrations of the tetrahedral groups and lower frequency band at 450 cm1 is attributed to the stretching mode of the octahedral M–O groups in the ferrites [11,12]. The difference in positions of frequency bands is due to the difference in the Fe3+– O distance for octahedral and tetrahedral complexes.

30

511

3.1. FT-IR characterization

(c)

300

400

3. Results and discussion

(d)

400

311

Fourier Transform infrared (FT-IR) spectra have been recorded using Perkin Elmer RX-1 FT-IR spectrophotometer with KBr pellets in the range 4000–400 cm1. Powder X-ray Diffraction (XRD) studies have been carried out using a Bruker AXS. The electrical properties have been carried out using a two-probe instrument. The magnetic properties have been measured at room temperature by a Vibrating Sample Magnetometer (VSM) (155,PAR) up to a magnetic field of ±10 kOe. UV–Visible spectrum was recorded using a Hitachi 330 UV–VIS–NIR spectrophotometer.

440

500

220

2.2. Physical measurements

400

600

0

Relative Intensity

Synthesis of Co0.6Zn0.4CrxFe2xO4 ferrites was carried out by the sol–gel auto-combustion method. The AR Grade Fe(NO3)39H2O, Co(NO3)26H2O, Cr(NO3)39H2O, Zn(NO3)26H2O and citric acid were weighed in desired stoichiometric proportions and were dissolved in the minimum amount of distilled water. The individual solutions were then mixed together and the pH value was adjusted to 5–7 by adding NH4OH solution. The solution was then burnt to self ignition to obtain a loose powder. The powders were annealed at 400 °C, 600 °C, 800 °C and 1000 °C in a muffle furnace for 2 h.

220

700

511

800

2. Experimental 2.1. Preparation of Co0.6Zn0.4CrxFe2xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoferrites

311

900

Relative Intensity

El-Sayed [10] and a decreasing behavior of the grain size, porosity and shrinkage was reported, while the lattice parameter and density values increase with increase in Cr concentration. The simultaneous investigation on the electrical, optical and magnetic properties of Cr and Zn co-doped cobalt ferrite using the sol–gel auto-combustion method have not been reported so far. Therefore, in this work Co0.6Zn0.4CrxFe2xO4 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoferrites were synthesized using the sol–gel auto-combustion method and investigated the effect of Cr3+ ion substitution on the electrical, magnetic and optical properties of Co0.6Zn0.4Fe2O4 nanoferrite.

(f) (e) (d) (c) (b) (a)

25

30

40

50

60

70

Angle (2θ) Fig. 2. XRD patterns of Co0.6Zn0.4CrxFe2xO4 where (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6, (e) x = 0.8 and (f) x = 1.0 after annealing at 1000 °C.

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Table 1 Lattice parameter (a), activation energy, crystallite size (D), saturation magnetization (Ms), coercivity (Hc), magnetic moment (lB) and energy band gap (Eg) values of the nanoferrites annealed at 1000 °C. Ferrites composition

a (Å)

Activation energy (eV)

D (nm)

Ms (emu/g)

Hc (Oe)

lB (BM)

Eg (eV)

Co0.6Zn0.4Fe2O4 Co0.6Zn0.4Cr0.2Fe1.8O4 Co0.6Zn0.4Cr0.4Fe1.6O4 Co0.6Zn0.4Cr0.6Fe1.4O4 Co0.6Zn0.4Cr0.8Fe1.2O4 Co0.6Zn0.4CrFeO4

8.399 8.355 8.349 8.299 8.295 8.292

2.45 2.66 3.00 3.33 5.00 6.00

44.5 55.9 33.76 50.5 46.8 40.10

100.79 57.26 45.70 30.95 7.92 2.53

474.0 481.5 475.5 478.0 264.0 479.5

4.27 2.42 1.92 1.30 0.33 0.10

3.0 5.0 4.9 4.8 4.8 3.1

Table 2 DC resistivity and drift mobility at 323 K of all the ferrites annealed at 1000 °C. Ferrites composition

Resistivity (109) (ohm cm)

Drift mobility (1013) (cm2 V1 s1)

Co0.6Zn0.4Fe2O4 Co0.6Zn0.4Cr0.2Fe1.8O4 Co0.6Zn0.4Cr0.4Fe1.6O4 Co0.6Zn0.4Cr0.6Fe1.4O4 Co0.6Zn0.4Cr0.8Fe1.2O4 Co0.6Zn0.4CrFeO4

7.67 13.64 32.4 87.36 182.43 245

6.96 3.52 1.64 0.78 0.43 0.38

(f)

× 10 9 (ohm cm)

charge causing polarization [14]. The electron hopping between B and A sites has very smaller probability compared to that for B–B hopping. The hopping between A–A sites does not exist, due to the fact that there are only Fe3+ ions at A sites and Fe2+ ions formed during the process preferentially occupy B-sites [15]. Chromium ions have a preference for octahedral site, thus they reduce the Fe3+ ions in the B-site and in turn reducing Fe3+–Fe2+ ions polaron hopping [16,17]. Therefore, an increase in resistivity values with increasing Cr content due to the substitution of stable Cr3+ ions with Fe3+ ions have been observed shown in Fig. 3 and data is given in Table 2. Due to the paramagnetic nature of chromium ion, it exists only in one valence state and limits the extent of conduction through the Fe3+ to Fe2+ without participating in the conduction mechanism. The electrical resistivity in the ferrites mainly depends on the density, porosity, grain size, chemical composition and crystallography of the sample [18]. The electrical resistivity values obtained are well consistent with the results reported by Laishram et al. [19]. The variation of d.c resistivity with temperature is shown in Fig. 4 and an increasing trend of resistivity with increase in temperature was observed, indicating semiconducting nature of the ferrites. The activation energy (Ea) has been calculated using the Arrhenius type equation,

(e)

(d)

q ¼ q0 expðEa =kTÞ where qo is the resistivity at infinitely high temperature, k is the Boltzmann constant, Ea is the activation energy. The slope of (log q) vs (1/kT) gives the values of Ea values for all the samples shown in Fig. 5 and the respective values has been tabulated in Table 1.

(c) (b) (a)

Temperature (K) Fig. 4. Variation of DC resistivity with temperature of Co0.6Zn0.4CrxFe2xO4 (0  x  1.0) with (a) x = 0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6, (e) x = 0.8 and (f) x = 1.0 annealed at 1000 °C.

1000/T (k-1) Chromium Content Fig. 3. Variation of DC resistivity with chromium concentration.

Fig. 5. Arrhenius plots of Co0.6Zn0.4CrxFe2xO4 (0  x  1.0) with (a) x = 1.0, (b) x = 0.8, (c) x = 0.6, (d) x = 0.4, (e) x = 0.2 and (f) x = 0 annealed at 1000 °C.

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The drift mobility (l) of the charge carriers in the synthesized samples is calculated by the following equation [20] listed in Table 2.

l ¼ 1=neq where n is the number of charge carriers, e the charge on electron and q is the resistivity at a given temperature. The drift mobility shows increasing behavior with increasing temperature which represents enhanced mobility of the charge carriers due to thermal activation (Fig. 6) [21]. The variation of drift mobility with concentration of chromium from x = 0.0 to 1.0 is shown in Fig. 7 which reveals that the drift mobility decreases with increase in Cr concentration. The activation energy has been calculated from the slope of plot of resistivity vs temperature and its value is found to be 4.5 eV (Fig. 7). 3.4. UV–Visible spectroscopic studies UV–Visible spectroscopic data has been analyzed to obtain the optical band gap values of the nanoferrites. The energy band gap values were obtained by plotting the graphs between (aht)2 vs (ht) as shown in Fig. 8. The calculation of the energy band gap involves the extrapolation of linear part of the curve obtained by plotting (aht)2 vs (ht) to cut the energy axis. The absorption coefficient, a of the nanoparticles has been calculated using the fundamental relationships [22]

Fig. 7. Variation of drift mobility with concentration of chromium in Co0.6Zn0.4CrxFe2xO4 (0  x  1.0) nanoferrites annealed at 1000 °C.

450

I ¼ I0 eat

400 350

and

300

a ¼ 2:303ðA=tÞ where A is the absorbance and t is the thickness of the sample. The values of energy band gap for all the samples annealed at 1000 °C are listed in Table 1. It is observed that as the particle size decreases, the energy band gap increases. This can be explained on the basis of Bras effective mass model [23,24] according to which the measured band gap, Eg can be expressed as a function of particle size as

( h )2 eV cm-1

A ¼ logðI0 =IÞ

250 200 150 100 50 0

  2  p2 1 h 1 1:8e2  Eg ffi Ebulk þ þ g 2 2er me mh 4pee0 r 0

2

2.5

3

3.5

4

4.5

5

5.5

h (eV)

where Ebulk is the bulk energy gap, r is the particle size, me is the g effective mass of electrons, mh is the effective mass of holes, e is the relative permittivity, eo is the permittivity of free space, ⁄ is the Planck’s constant divided by 2p and e is the charge on electron.

Fig. 8. Plot of (aht)2 vs ht for Co0.6Zn0.4Cr0.4Fe1.6O4 nanoferrite annealed at 1000 °C.

(b)

(b)

(a)

Magnetic Moment (emu/g)

µ × 10-13 cm2 V-1 s-1

(a)

(c) (d) (e)

(f)

Temperature (K) Magnetic Field (G) Fig. 6. Variation of drift mobility with temperature of Co0.6Zn0.4CrxFe2xO4 (0  x  1.0) with (a) x = 0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6, (e) x = 0.8 and (f) x = 1.0 annealed at 1000 °C.

Fig. 9. Hysteresis loops of Co0.6Zn0.4Cr0.6Fe1.4O4 nanoferrites annealed at (a) 400 and (b) 1000 °C.

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Magnetic Moment (emu/g)

(x = 0.0)

(x = 0.2) (x = 0.4)

(x = 0.6) (x = 0.8) (x = 1.0)

Magnetic Field (G) Fig. 10. Hysteresis loops of Co0.6Zn0.4CrxFe2xO4 (0.0  x  1.0) annealed at 1000 °C.

3.5. Magnetic measurements Typical hysteresis loops of Co0.6Zn0.4Cr0.6Fe1.4O4 annealed at 400, 600, 800 and 10000C are shown in Fig 9. With the increase in the temperature, there is a gradual increase in crystallinity and particle size and hence the saturation magnetization increases [25]. In ferrites, the magnetization of the tetrahedral clusters (A-sites) is found to be anti-parallel to those of the octahedral clusters (B-sites). The A–B super-exchange interactions predominate the A–A and B–B interactions, according to Neel’s molecular field model [26]. Therefore, the net magnetic moment in ferrites is given by the equation M = MB  MA. Depending upon the distribution of the various cation substituents in these sites, the magnetic properties of the ferrites vary and can be modified according to the research interest. Fig. 10, shows the saturation magnetization as a function of the chromium concentration and it is observed that the saturation magnetization decreases with increase in the chromium concentration. This decreasing trend is due to the smaller magnetic moment of Cr3+ ion (3lB) as compared to that of Fe3+ ion (5lB). However a sharp decrease in the saturation magnetization for x = 0.8 and 1.0, may be due to the occupation of Cr3+ in the tetrahedral site after x = 0.8. The decrease in the saturation magnetization in the case of nanoparticles may also be explained on the basis of canted spin or spin glass like layer [27,28] due to the larger fraction of surface to volume atoms in the smaller particles at the surface of nanoparticles. No true saturation is achieved by the particles even at the high applied field due to the canted spins at the surface of nanoparticles which leads to the reduction of magnetization in the smaller particles [29]. The magnetic moment of the samples has been calculated using Bohr magnetron (lB) by following relationship [30] and is tabulated in Table 1.

gB ¼ M  Ms =5585

ð1Þ

where M is the molecular weight of the composition and Ms is the saturation magnetization (emu/g). 4. Conclusion Co0.6Zn0.4CrxFe2xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoferrites were successfully synthesized by the sol–gel auto-combustion method. The FT-IR studies confirmed the formation of the metal

oxide bonds in ferrites. The single phase cubic spinel structure formation was shown by X-ray powder analysis. A decreasing trend of the lattice parameter ‘a’ was observed with increasing Cr3+ concentration attributing to the smaller ionic radii of Cr3+ ion as compared to Fe3+ ion. However with increasing Cr3+ ion content, the electrical resistivity increases, as the conductivity decreases due to the substitution of stable Cr3+ ions with Fe3+ ions. The drift mobility of the charge carriers increases with increasing temperature due to thermal activation. The optical band gap values were found to be around 5 eV for all the samples annealed at 1000 °C. The saturation magnetization was found to decrease with increasing Cr3+ ion concentration due to the lesser magnetic behavior of the chromium than that of iron. However, the saturation magnetization decreased sharply when x = 0.8 indicating that Cr3+ occupies tetrahedral site after x = 0.8.

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