Dielectric, magnetic, ferroelectric, and Mossbauer

Dielectric, magnetic, ferroelectric, and Mossbauer properties of bismuth substituted nanosized cobalt ferrites through glycine nitrate synthesis method Krutika L. Routray, Dirtha Sanyal, and Dhrubananda Behera

Citation: Journal of Applied Physics 122, 224104 (2017); View online: https://doi.org/10.1063/1.5005169 View Table of Contents: http://aip.scitation.org/toc/jap/122/22 Published by the American Institute of Physics

JOURNAL OF APPLIED PHYSICS 122, 224104 (2017)

Dielectric, magnetic, ferroelectric, and Mossbauer properties of bismuth substituted nanosized cobalt ferrites through glycine nitrate synthesis method Krutika L. Routray,1 Dirtha Sanyal,2 and Dhrubananda Behera1,a) 1

Department of Physics and Astronomy, National Institute of Technology, Rourkela, India Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Kolkata 700064, India

2

(Received 18 September 2017; accepted 24 November 2017; published online 13 December 2017) CoFe2-xBixO4 nanoferrites with x ¼ 0, 0.05, 0.1, 0.5, and 1.0 have been synthesized by the glycine nitrate process. The present study investigates the effect of Bi3þ substitution on the microstructural, dielectric, ferroelectric, magnetic, and Mossbauer properties of CoFe2O4 nanoparticles. The X-ray diffraction technique was used to confirm the phase purity and estimate the crystallite size which revealed the formation of a secondary phase when Bi3þ concentration exceeds x ¼ 0.5. Transmission electron microscopy indicated the formation of grains by aggregation of small crystallites with a reduction in grain size to 20 nm with an increase in Bi3þ content and also divulged the lattice param˚ , confirming the crystalline nature of the synthesised sample. Dielectric propeter value to be 8.378 A erties performed in the frequency range of 100 Hz to 1 MHz determined that the dielectric behavior is attributed to the Maxwell-Wagner polarization and the activation energy of the specimens is calculated from the dielectric measurements. The hysteresis curve indicated the ferrimagnetic nature of the samples. The samples also exhibited a well saturated P-E loop with gradual lowering in remenant polarization, coercive field, and saturation polarization with an increase in bismuth concentration. M€ ossbauer spectroscopy analysis confirmed the changes in magnetic moment of ions, their coupling with neighbouring ions, and cation exchange interactions. Owing to the high physical, thermal, and chemical stabilities, these magnetic ceramics, CoFe2-xBixO4, possesses tremendous potential in major understanding of magnetism and in magnetic recording applications for high density information storage. Published by AIP Publishing. https://doi.org/10.1063/1.5005169 I. INTRODUCTION

Recently, there has been a resurgence of research interest in nanophase ferrites owing to their remarkable optical, structural, electrical, and magnetic properties which often differ from its bulk counterpart.1–3 These unusual properties of the nanoferrites make them potential candidates for a variety of applications in the high frequency and power devices, especially for microwave control components such as circulators, isolators, and phase shifters.4,5 Spinel ferrites having a general chemical composition of MFe2O4 (where M ¼ Mn, Mg, Zn, Ni, Co, etc.) are among the most extensively used magnetic materials which are associated due to their magnetic behaviour and correlated nature in combination with their structural properties to upsurge their performance in high-frequency devices.6,7 The properties of the spinel ferrite CoFe2O4 (CFO) nanoparticles have been studied extensively.8–11 Cobalt ferrite (CoFe2O4) is a well-known partially inverse spinel, hard ferromagnetic ceramic material with a high specific resistance, low losses in high frequency applications, high coercivity, and a moderate saturation magnetization with positive magneto-crystalline anisotropy.12–16 These properties of the nanoferrite, along with their unique physical and chemical stabilities, make CoFe2O4 (CFO) nanoparticles suitable for applications in the fields of high-density magnetic media, recording colour imaging, magnetic recording applications a)

Author to whom correspondence should be addressed: dbehera@nitrkl. ac.in. Tel.: þ919439785948.

0021-8979/2017/122(22)/224104/12/$30.00

such as audio and videotape, high-density digital recording disks, ferrofluids, high-frequency devices, and magnetic refrigeration.3,17–19 These useful properties of spinel nanoferrites rely on the selection of the cations along with Fe2þ and Fe3þ ions and their occupancy in tetrahedral (A) and octahedral (B) sites of the spinel lattice.20 In case of CFO, the cation distribution is given by [Co1-xFex] A [CoxFe2x] B, where for x ¼ 1, octahedral (B) site is occupied by cobalt ion (inverse spinel state) and for x ¼ 0, octahedral (B) site is occupied by iron ion (inverse spinel state).21–23 When shrunk to nanoscale, CFO accedes to partial inverse spinel structure because of the large change in cation distribution that takes place as soon as the particle size approaches nano dimension which affects the magnetic and electric behaviour of CFO system.24,25 At present, substitution of non-magnetic ions like Al3þ, 2þ Zn ,Cr3þ, Cd2þ, and many others in spinel ferrites has been widely studied to investigate the structural, electric, and magnetic properties.8–11 A very few literatures have reported the effect of substituting Bi3þ in CFO. It is anticipated that substituting Bi3þ in a very small amount does not alter the spinel structure of CFO; however, when the amount of Bi3þ concentration is increased to 0.1 and above, Bi3þ replaces Fe3þ in octahedral site resulting in the enhancement of electrical properties, reduction in magnetic anisotropy, and decrease in the saturation magnetization.26,27 Possessed with high electrical resistivity, low magnetic, and dielectric losses, Bismuth Ferrite is one of the most important single phase multi-ferroic materials.28 With these

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great physicochemical properties and thermal stability of Bismuth and its substituted compounds find suitability in several fields such as development of cosmetic products, lubricating oils, medicines, pigments, electronics, semiconductors, alloys industry, and recycling of uranium nuclear fuels.29 In particular, Bismuth doped Cobalt nanoferrites are preferred in magnetic recording applications such as audio, videotapes, and high-density recording devices.27 It is anticipated that Bi3þ can occupy both tetrahedral and octahedral sites. Replacing Fe3þ ions with Bi3þ can modify the saturation magnetization, decreases the magnetic anisotropy constant, and enhances the electrical properties.30 Hence, Bi3þ is a potential substitution for improving the structural, electrical, and magnetic properties of ferrites. Synthesizing CFO nanoparticles of preferred size, pure phase, electric, and magnetic properties has been attempted by various synthesis routes such as solid-state route, chemical reduction, glycine nitrate process, sol-gel, auto combustion reaction, micro-emulsion, hydrothermal, mechanical milling, and co-precipitation.31–35 Among all these techniques, glycine nitrate process is one of the simple, inexpensive, and versatile techniques, which quickens the synthesis of compounds and brings crystal homogeneity.36 In this contribution, we report the comprehensive study of the effects of Bi3þ on the structural, dielectric, Mossbauer, and magnetic properties for a series of sintered Bi-substituted CFO nanoparticles of composition x ¼ 0, 0.05, 0.1, 0.5, and 1.0, employing glycine nitrate process is described. These synthesised samples were subsequently characterized by a number of microscopic techniques including X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Selected Area Electron Diffraction (SAED). In addition, we report the dielectric, magnetic, and ferroelectric behaviour of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) as a function of frequency for a range of selected temperatures and Mossbauer spectra at room temperatures. Thus, the present work is motivated to understand the effect of Bi substitution in CFO using various microscopic, dielectric, and magnetic analyses so as to make it preferable to be used in magnetic recording applications for high-density information storage.

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and self-sustaining combustion were completed, yielding the black-coloured ashes. The as-prepared powders of all the samples were calcined separately at 600 C for 3 h to get the final product. Figure 1 shows the flowchart of the synthesising procedure. B. Characterization techniques

The crystal structure, phase identification, and particle size of the synthesised CFO and its modified nanoparticles were determined by qualitative X-ray powder diffraction measurements with the help of Rigaku Ultima-IV X-Ray ˚ ) at room Diffractometer (using Co-Ka radiation, k ¼ 1.79 A temperature. The XRD patterns of samples were recorded in the range 2h ¼ 20 80 using a step size of 0.02 and a counting time of 2 s per step. The lattice parameters, the oxygen position, and the cation distribution were determined by means of Rietveld refinement. The crystallite size was further refined by Rietveld refinement. The size and morphology of the particles were estimated by FEI MODEL-TECNAI TF 30 G2 Transmission electron microscopy (TEM). Dielectric measurement was measured using H20KI LCR meter (IM3570) at the room temperature. Magnetic measurements were carried out using Vibrating Sample Magnetometer (VSM) at S. N. Bose National Centre for Basic Sciences. Polarizations vs. electric field (P-E) hysteresis loops at room temperature were measured using Radiant precision premier II. Further, the Mossbauer study of the samples was conducted by Mossbauer spectrometer 57Co Source at VECC, Kolkata, to observe the distribution of the Cobalt and Bismuth in the spinel ferrite. III. RESULTS AND DISCUSSION A. Structural and morphological evolution 1. XRD analysis

Figure 2 shows the XRD patterns of CoFe2-xBixO4 nano ferrite obtained for x ¼ 0.0, 0.05, 0.1, 0.5, and 1.0. The XRD micrographs confirm the diffraction peaks of cubic spinel cobalt ferrite type when 0 x 0.1, whilst when x 0.5,

II. EXPERIMENTAL PROCEDURE A. Synthesis

A series of Bismuth substituted CFO nano-particles having the chemical formula CoFe2-xBixO4 (where x ¼ 0, 0.05, 0.1, 0.5, 1.0) were synthesized by the glycine nitrate process36 using glycine as a fuel and nitrates as oxidants. The metal nitrates that were used for the synthesis were Cobalt Nitrate [Co(NO3)26H2O], Bismuth Nitrate [Bi(NO3)35H2O], and Ferric Nitrate [Fe(NO3)39H2O]. The stoichiometric amount of all metal nitrates was dissolved in a glass beaker with double distilled water to obtain the precursor solution. A specified amount of glycine was then added into the nitrate solution at a molar ratio of 1:4 for fuel oxidant (stoichiometric combustion) and mixed together with the help of magnetic stirrer placed on a hot plate to evaporate excess water at about 150 C with uniform stirring to obtain a highly homogeneous viscous brown gel. The auto-ignition

FIG. 1. Flow chart for the synthesis of CoFe2-xBixO4 nanoferrite particles by glycine nitrate process.

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substitution of Bi3þ goes beyond (for x ¼ 0.5) may be due to the electronic configuration and higher ionic radius of the Bi3þ as compared to Fe3þ for which Bi3þ occupies tetrahedral or octahedral sites more conveniently. Parameters including interplanar spacing (d), lattice parameter, crystallite size, volume of unit cell, and dislocation line density prepared samples are obtained from XRD data are shown in Table II and Table III. The inter-planar spacing value is calculated from Bragg’s law, which is as follows:37 nk ¼ 2d sin h:

(1)

The lattice parameter value has been calculated from the following equation:38 FIG. 2. The XRD pattern of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites along with the figure showing the shifting of the most intense (311) peak. TABLE I. Diffraction angle (2h), percentage (%) of most intensive peak, and Miller indices (hkl) values of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites. Diffraction angle (2h)

% of most intense peak

Miller indices (h k l)

30 100 8 20 10 30 40

(220) (311) (222) (400) (422) (511) (440)

35.081 41.393 43.310 50.448 62.958 67.266 74.195

additional peaks corresponding to Bismuth are observed to be evolved. All the diffraction peaks with the planes (220), (311), (222), (400), (422), (511), and (440) are precisely in match with the standard cobalt ferrite JCPDS card No. 22-1086. The corresponding intensities of the peaks obtained from the XRD pattern with the percentage (%) of the most intensive peak positioned with their respective miller indices are shown in Table I. The most intense peaks (311) of CFO are shifted to higher angles as marked in Fig. 2. This shifting is because of the recurrent structural distortion taking place due to doping. In addition to this, it is also observed that the synthesised bismuth-doped cobalt ferrites have the inverse cubic spinel structure, belonging to Fd-3m space group. Small and more intense peaks are detected in the obtained XRD patterns which indicate higher crystallinity, signifying both the doped and undoped ferrites were polycrystalline. The secondary phase that is observed when

1

a ¼ dðh2 þ k 2 þ l2 Þ2 :

(2)

The crystallite sizes of all compositions have been determined from broadening of the most intense peak (311) of XRD pattern using the Scherrer formula D¼

Kk ; B cos h

(3)

where D is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size; K is a dimensionless shape factor having value 0.9; k is the X-ray wavelength; b is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians; and h is the Bragg angle (in degrees). The dislocation line density of the prepared samples has been calculated using the following formula:39 l¼

1 : D2

(4)

The lattice parameters and crystallite sizes as a function of Bi3þ concentration are displayed in Fig. 3. As observed with increasing Bi3þ concentration, the lattice parameter increases which is due to the relative sizes of their ionic radii. In particu˚ ) ions is larger than the lar, the ionic radius of Bi3þ (1.03 A ˚ ) ion; therefore, replacement of ionic radius of Fe3þ (0.76 A smaller Fe3þ cations by larger Bi3þ in the cobalt ferrite is more convenient. It is in well agreement with the reported literature.27 It is also seen that the crystallite size decreases approximately from 56.0 to 17.0 nm. As the full width at half maximum (FWHM) increases, the crystallite size decreases. The presence of additional bismuth peaks in the spinel structure is manifested from the XRD patterns, and this results in an

TABLE II. Parameters obtained from XRD data for CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nano ferrite.

X 0.0 0.05 0.1 0.5 1.0

Composition

Inter planar spacing ˚) (d) (A

Lattice parameter ˚) (a) (A

Crystallite size (D) (nm)

Volume of unit cell (a3) (cm3)

Dislocation line density (l)

CoFe2O4 CoFe1.95Bi0.05O4 CoFe1.90Bi0.1O4 CoFe1.5Bi0.5O4 CoFe1.0Bi1.0O4

2.359 2.450 2.452 2.487 2.456

8.316 8.322 8.329 8.347 8.413

56.27 49.67 48.12 39.79 17.69

575.10 576.35 577.80 581.56 595.46

3.2 4.1 4.3 5.6 6.5

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FIG. 3. The lattice parameters and crystallite sizes as a function of Bi3þ concentration.

enhanced dislocation line density and induces micro-strain in the material. 2. TEM analysis

The particle morphology, microstructure, and crystal structure have been investigated using a high resolution transmission electron microscopy (HRTEM). The images elucidate the nanoscale nature of the synthesised ferrite particles. Figure 4 shows the TEM image, lattice fringe patterns of the nanoparticle, and Selected Area Electron Diffraction (SAED) patterns of the synthesised CoFe2-xBixO4 nano ferrite.

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TEM images show the presence of particles which are spherically cubic shaped crystallites of size approximately 50 nm and varies to less than 20 nm with an increase in Bi3þ ion substitution (Fig. 5). This corroborates well with the results obtained from XRD according to the Scherrer’s formula. Agglomerated particles as well as separated ones are observed in the images of the sample. This tendency of forming agglomerates is because of the permanent magnetic moment that the samples experience. The SAED pattern’s inset in Fig. 4 confirms the crystalline nature of the prepared sample. Diffraction rings can be clearly seen from the SAED pattern. For instance, from the fringe patterns of the nanoparticle for x ¼ 0.05, we obtain the particle size of 50 nm to 17 nm and it is confirmed that the crystal planes of a particular orientation (311) are confined within a particle. The plane spacing d for the plane (311) calculated from lattice fringes of all the specimen is nearly equal to 0.250 nm which is agreeing with the calculated d spacing of spinel phase obtained from the XRD analysis. The lattice parameter value ˚ , which is also in good agreement with obtained is 8.378 A the standard value and XRD pattern. 3. Elemental analysis

The energy dispersive x ray (EDX) analysis of CoFe2-x BixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites was carried out to confirm the replacement of divalent Co2þ by trivalent Bi3þ cations as well as quantify the Bi3þ substitution in CoFe2O4. Figure 6 shows the EDX spectrums of CoFe2-xBixO4

FIG. 4. TEM image and its corresponding SAED pattern (inset) of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

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FIG. 5. HRTEM images of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

(x ¼ 0, 0.05, 0.1, 0.5, 1.0). Co, Fe, and O are major elements detected in pure CoFe2O4, while Bi was detected in the substituted cobalt ferrite. The EDX analysis confirmed that the synthesized ferrites are pure in phase and structure, and Bi3þ-doping was successfully achieved. The obtained atomic percentages of O, Fe, Co, and Bi in the pristine and doped

CoFe2O4 ferrites are given in Table IV. It is observed that Co2þ concentration is almost constant in all the specimens, while with the increase in the Bismuth doping, Bi3þ concentration increases signifying Bi3þ-doping with the displacement of Fe3þ in CoFe2O4 without disturbing Co2þ as Co2þ concentration does not vary.

FIG. 6. EDX spectra of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

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TABLE III. Structural parameters of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nano ferrites.

X 0.0 0.05 0.1 0.5 1.0

Composition

Molecular weight (gm/mol)

Density (gm/cc) (X-ray)

Bulk density (gm/cc)

Porosity (%)

Tetrahedral site ˚) (dA) (A

Octahedral site ˚) (dB) (A


234.62 242.29 249.94 311.19 387.77

5.41 5.58 5.74 7.10 8.65

4.79 4.86 4.97 6.96 7.89

0.11 0.13 0.14 0.06 0.02

3.601 3.603 3.606 3.614 3.642

2.940 2.941 2.944 2.950 2.974

TABLE IV. Atomic percentage of O, Fe, Co, and Bi in CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites. X 0.0 0.05 0.1 0.5 1.0

Composition

Co

Fe

O

Bi

Total


26.63 22.48 23.05 23.74 27.30

45.25 44.20 39.55 35.58 33.12

28.12 30.05 32.74 25.85 22.61

0 3.27 4.66 14.83 16.97

100 100 100 100 100

B. Dielectric properties 1. Temperature dependent variation

The temperature dependence plot of the dielectric constant and dielectric loss for CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites measured by dielectric spectroscopic at different frequencies, viz., 100 Hz, 0.5 kHz, 1 kHz, 0.5 MHz, and 1 MHz is represented in Fig. 7. All the samples exhibit an increase in dielectric constant with increasing temperature, which may be attributed to the increase of space charge polarization and conductivity. This is because of the increase of mobility of space charge carriers. A sudden increase in the dielectric constant at 150 C is observed, which marks the ferroelectric phase. When compared with the frequency, the

dielectric constant value varies inversely with the frequency. This inverse variation is due to the electron hopping of Fe2þ and Fe3þ ions present in the octahedral sites. With an increase in temperature, this hopping gets thermally activated and results in high dielectric loss at higher temperatures. 2. Frequency dependent variation

Figure 8 illustrates the variation of dielectric constant (e0 ) and dielectric loss (e00 ) as a function of applied frequency of the CFO samples possessing at different Bi3þ ions concentrations at room temperature. As shown in the figure, dielectric constant (e0 ) and dielectric loss (e00 ) decrease abruptly at lower frequencies and remain almost constant at higher frequencies, indicating the usual dielectric dispersion phenomenon as observed in case of ferrites.40–42 It is observed that all the dielectric constants and dielectric losses are in decreasing trend with an increase in the frequency up to 104 Hz, and then it remains almost constant in the frequency range of 104–106 Hz. With an increase in Bi3þ ions concentration, the dielectric constant increases which can be attributed to the lattice distortion that leads to the enhancement in the atomic polarizability because of the changes in valence states of cations and space charge polarization. This lattice distortion occurs due to the incorporation of Bi3þ ions

FIG. 7. Temperature dependence of dielectric constant and dielectric loss CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

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FIG. 8. (a) Dielectric constant and (b) dielectric loss of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

in CFO nanoferrite particles and hence enhances the bond length in the octahedral (B) site. It is noticed that with an increase in the frequency, the dielectric constant remains independent of frequency because of the electric dipoles that lack the ability to respond to the fast variation of the applied alternating electric field. This interfacial polarization leads to the dispersion phenomenon.43 The parameter e00 decreases exponentially with the increase of frequency and attains a constant value at a high frequency similar to the type of behaviour that is observed in case of dielectric constant (e0 ). The variation in dielectric constant for different concentrations of Bi3þ ion in CFO nanoparticles as a function of frequency at different temperatures is shown in Fig. 9. This shows the value of dielectric constant to be higher at the lower frequency region and gradually decreases at the higher frequency up to 104 Hz and then the changes become insignificant beyond the frequency limit of 104 Hz. This type of behaviour is usually noticed in mixed ferrites. This observed variation of dielectric constant with frequency is akin to dispersion phenomenon due to Maxwell–Wagner type interfacial polarization and is in well agreement with Koop’s phenomenological

theory.44 According to this theory, dielectric materials comprises well conducting grains, which are separated by insulating grain boundaries. Hopping phenomenon between Feþ2 and Feþ3 takes place resulting in the electrons to reach the grain boundary and if the resistance of the grain boundary is high enough, electrons mass up at the grain boundaries enhancing the polarization. Hence, the dielectric constant increases at the lower frequency as seen in Fig. 9. Conversely, when the frequency of the applied field is amplified to a certain value, the electron hopping lacks the ability to respond to the applied alternating electric field resulting in the decrease of polarization and, hence, dielectric constant at higher frequency. The enhancement in dielectric constant with increasing temperature from the room temperature is because of the production of extra thermal energy, which increases the charge carrier mobility resulting in enhanced polarization and hence higher dielectric constant. This may be due to the inadequate thermal energy, which lags the mobility of charge carriers. Figure 10 shows the variation in dielectric loss as a function of frequency for temperature ranging from room temperature to 300 C, which also exhibits the same trend as seen

FIG. 9. Variation in dielectric constant as a function of frequency of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

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FIG. 10. Variation in dielectric loss as a function of frequency of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

in dielectric constant. It is found that for all the nanoferrite samples, the dielectric loss decreases gradually with an increase in frequency for different temperature ranges. No resonance peaks were observed for a small amount of Bi3þ concentration; however, when Bi3þ concentration increases, a peaking behaviour is observed which is ascribed to the hopping frequency of electrons between Fe2þ and Fe3þ which matches with the frequency of the applied electric field.45 Figure 11 illustrates the variation of loss tangent (tan d) with applied frequency. A similar behaviour to that of dielectric constant (e0 ) is observed in case of loss tangent; tan d decreases when the frequency increases from 100 Hz to 1 MHz. An abnormal dielectric behaviour of tan d curves where dielectric relaxation processes (peaks) take place was observed. This abnormal dielectric behaviour observed can be associated with the collective contribution of both types of electric charge carriers, viz. electrons and holes to the

FIG. 11. Plot of dielectric loss tangent as a function of frequency for CoFe2-x BixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites.

dielectric polarisation which can be explained on the basis of the Rezlescu model.44 The predominant electric conduction mechanism in ferrites is due to the electron exchange occurring between Fe2þ and Fe3þ ions and hopping of hole between Co2þ and Co3þ ions in the octahedral (B) sites. This can be explained by the presence of cobalt on the octahedral site of the spinel ferrite that favours a conduction mechanism: Co2þ þ Fe3þ $ Co3þ þ Fe2þ. The reciprocal temperature dependent electrical conductivity (rdc) of the CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites is represented in Fig. 12. From the plot, it can be revealed that the conductivity divulges an Arrhenius-type temperature dependence curve, given by the relation, rdc ðT Þ Edc ¼ r0 exp KB T , where r0 is the pre-exponential factor, Edc is the activation energy, KB is the Boltzmann constant, and T is the temperature in Kelvin.46 The activation energy is found from the linear fit of the Arrhenius equation s ¼ s0 exp KEBaT ,

FIG. 12. Ln s vs. 1000/T for CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0).

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FIG. 13. (a) Hysteresis loops for CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoferrites. (b) Variation of saturation magnetization (Ms) with different bismuth concentrations.

where s is the relaxation time, Ea is the activation energy, s0 is the pre-exponential factor, and KB is the Boltzmann constant. The activation energies were found to be 1.01 eV (for x ¼ 0.05), 1.28 eV (for x ¼ 0.1), 1.35 eV (for x ¼ 0.5), and 1.43 eV (for x ¼ 1). The electrical conduction and dielectric polarization in cobalt ferrite occur due to the electron hopping between Fe2þ $ Fe3þ ions and hole hopping between Co3þ $ Co2þ ions in octahedral sites. Doping Bismuth in Fe site may lead to charge mismatch as Fe possesses two oxidation states Fe2þ and Fe3þ and so there arises oxygen vacancy in order to maintain the charge balance within the system. In this work, we found that the electrical conductivity as well as the activation energy increases. As higher Bi3þ is incorporated into CFO, the transfer of Fe3þ ions from the tetrahedral sites to the octahedral sites takes place. In addition, with the formation of a large amount of the FeO3 secondary

phase in CoFe1.0Bi1.0O4, the number of Fe ions, and ultimately Fe2þ $ Fe3þ ion pairs, increases. Since the hopping between Fe2þ $ Fe3þ ions is responsible for electrical conduction, an increase in Fe2þ $ Fe3þ ion pairs increases the hopping of electrons and also the electrical conductivity of CoFe1.0Bi1.0O4. Therefore, it may be concluded that, as the crystallite size decreases, the migration of Fe3þ from tetrahedral sites to octahedral sites plays an important role in the conduction process of Bismuth doped cobalt ferrite nanoparticles. 3. Magnetic properties

The magnetic properties of CoFe2-xBixO4 with x ¼ 0, 0.05, 0.1, 0.5, and 1.0 were analyzed using a VSM at room temperature with an applied field of 15 kOe H 15 kOe. These plots (Fig. 13) show that an increase in Bi3þ doping yields a

FIG. 14. P-E Hysteresis loops of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoparticles.

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4. Ferroelectric behavior

FIG. 15. M€ossbauer spectra of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoparticles.

reduction in the saturation magnetization of CoFe2-xBixO4, which may be attributed to the occupancy of Bi3þ ions in the octahedral sites. As the magnetic moment gB per ion for Bi3þ (5 gB) is more than Co2þ ions (3 gB), the increasing Bi3þ concentration results in a reduced magnetic moment. Thus, saturation magnetization also decreases. The magnetic moment per formula unit in Bohr magneton (gB) was calculated by using the following relation: gB ¼

M MS ; 5585

where M is the molecular weight of particular composition and MS is the saturation magnetization (emu/gm).47 Figure 13(b) reveals that gB decreases linearly with an increase in Bi3þ ion concentration. The coercivity is found to increase with Bi3þ doping as can be observed from Fig. 13(a). This increase may be attributed to the increase in anisotropy and magnetostriction constant. However, the obtained value of coercivity obtained is very low which specifies that the domain wall motion follows the dominant magnetization mechanism and the sample specimens tend to become magnetically softer, which may find suitability for magnetic recording devices.48,49

The hysteresis loops (P-E) for CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoparticles at room temperature are illustrated in Fig. 14. Generally, the ferroelectric properties of a material are dependent on its composition, homogeneity, defects, external field, and orientation of domains. As the grain size plays a vital role on the observed polarization and ferroelectric behavior, observed TEM image depicts the reduction in grain size. This reduces the polarization value as it induces less strain. From the hysteresis loops, we can conclude that when the concentration of bismuth 0.1, lowquality hysteretic behavior is observed (Fig. 14). This kind of behavior is attributed to the existence of conduction mechanisms that are intrinsic of magnetic compounds, which correspond to the ferroelectric response of the material. These phases usually have high conductivities values because of the oxygen vacancies created due to the polaron mechanism where Fe3þ and Fe2þ simultaneously exist.50 So when Bi3þ concentration increases, with the applied electric field, an increase in current occurs which leads to high-loss and unsaturated hysteresis loop. 5. Mossbauer spectroscopy

Figure 15 depicts the room temperature M€ossbauer spectra of CoFe2-xBixO4 nanoparticles for x ¼ 0.0, 0.05, 0.1, 0.5, 1.0. It is clear from these figures that all the samples display ferromagnetic nature (six line pattern of M€ossbauer spectra). From the M€ossbauer spectra, the isomer shift (d), line width, and quadrupole splitting (QS) corresponding to the tetrahedral (A) and octahedral [B] sites have been calculated by the standard least square fitting program NORMOS.51 In order to understand the ionic state of Iron, i.e., Fe2þ or Fe3þ, the value of Mossbauer parameter (isomer shift) is a good measure. The Fe2þ and Fe3þ contain six numbers of 3d electrons and five numbers of 3d electrons, respectively. Thus, the effective s-electron density at the Fe nucleus is less in case of Fe2þ state than Fe3þ state, because of 3d electron screening. As a result of which, the isomer shift in Fe2þ state (ferrous) is more than Fe3þ state (ferric). In our present Mossbauer data, the value of isomer shift is 0.33 mm/s for x¼ 0 and 0.05

FIG. 16. (a) Isomer shift, line width, and quadrupole splitting of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoparticles; (b) hyperfine field of CoFe2-xBixO4 (x ¼ 0, 0.05, 0.1, 0.5, 1.0) nanoparticles.

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Bismuth concentration, 0.61 for x ¼ 0.1 and 0.5 Bismuth concentration, and 0.32 for Bismuth concentration (x ¼ 1). The isomer shift values for x ¼ 0, 0.1, and 1.0 suggest the ferric state (Fe3þ). In case of ferrous the value is much higher, i.e., for x ¼ 0.1 and 0.5. Figure 16(a) represents the variation of the isomer shift, the line width, and the quadrupole splitting with various concentrations of Bi in CoFe2-xBixO4 ferrites for x ¼ 0.0, 0.05, 0.1, 0.5, and 1.0 samples. The Fe content of the samples with the values of x is almost in a same trend and significant M}ossbauer absorption is observed in all the proportions. Each spectrum consists of a normal magnetic sextet due to Fe3þ at the tetrahedral (A) sites and another due to Fe3þ at octahedral [B] sites. This indicates that all ferrites are magnetically ordered ferrites at room temperature. In case of CoFe1.90Bi0.1O4, the spectrum exhibits a central doublet on broad magnetic sextets, signifying the partial transformation to an ordered magnetic structure. The existence of ferrimagnetic particles may be ascribed to the wide size distribution of the dopants and the pristine material or effect of synthesizing technique. Substitution of Bi3þ in cobalt ferrite alters the isomer shift (d) values at both the [A] and [B] sites. However, the same values of isomer shift (d) have been calculated for x ¼ 0, 0.05, and 0.1, which indicates that the s-electron distribution of the Fe3þ ions could be unresponsive to the Bi3þ content, but for x > 0.1, we observe a change in isomer shift values. This indicates that the s-electron distribution of the Fe3þ ions is exaggerated by the substitution of Bi3þ. Studies revealed that with increasing Bi3þ concentration, Bi3þ can occupy both the tetrahedral (A) and octahedral [B] sites. For when x > 0.1, Bi3þ ions preferably occupy octahedral [B] sites.27 The decrease in d value at the [B] site for x ¼ 1 may be due to the bonding nature of Fe3þ with Co2þ and Bi3þ at both sites. As calculated from the Mossbauer data, small values of QS ranging from 0.02 to 0.04 for Bi3þ substituted cobalt ferrite at A and B-sites are obtained, which illustrates the presence of cubic symmetry.52 Figure 16(b) represents the hyperfine field (hf) of various concentrations of Bi in CoFe2-xBixO4. From the microstructural analysis, we have observed a decrease in particle size with an increase in Bi3þ concentration. Considering hf, for pure cobalt ferrite hf at A-site is larger than hf at B-site for the Bi3þ substituted material. As there is reduction in particle size with an increase in Bi3þ concentration, the occurrence of collective magnetic excitations shrinks the hyperfine field of the sample besides the superparamagnetic transformation.53 Hence, the increase in Bi3þ concentration leads to reduction in particle size and enhancement in lattice distortion, leading to decrease in hyperfine field. The size of the crystal grain decreases, whereas lattice distortion and concentration of deficiency both increase, resulting in decrease of the hyperfine field. IV. CONCLUSION

Bismuth substituted cobalt nanoferrites with a composition of CoFe2-xBixO4 (x ¼ 0.0, 0.05, 0.1, 0.5, and 1.0) were successfully synthesised by the glycine nitrate process. The microscopic results of XRD and HRTEM divulge that all the

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samples are well crystalline nanosized spinel ferrites though morphology depends on Bi3þ concentration. The average particle size obtained was around 20 nm to 110 nm, which is in good agreement with the XRD results. The morphological study reveals that the introduction of Bi3þ ions in CFO nanoparticles modifies the particle size. The dielectric study reveals that the dielectric constant and the dielectric loss abruptly decrease and then remain independent at a higher value of frequencies with increasing Bi3þ concentration as well as with the temperature, indicating the usual dielectric dispersion phenomenon. Analysis of the magnetic hysteresis loop leads to the conclusion that the influence of Bi3þreduces saturation magnetisation, coercivity, remenant magnetisation, and magnetic moment. Mossbauer spectroscopy analysis exhibits ferrimagnetic behavior with Bi3þ concentration. It was also observed that the isomer shift is increased at the [A] site, while it is decreased at the [B] site because s-electron distribution of the Fe3þ ions is exaggerated by substitution of Bi3þ. 1

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