**3.5 TEM analysis**

Phase structure and morphology studies for the investigating synthesized samples were taken up through TEM analysis. **Figure 7** shows the TEM images and their

**Figure 6.** *AFM Micrographs of CoFe2O4 (x = 0.000) and CoFe2-xErxO4 (x = 0.005 to 0.030).*

respective SAED images with particle size distribution chart of the samples got x = 0.0, 0.005, 0.01, 0.015, 0.02, 0.025 and 0.03 respectively. TEM and SAED images demonstrated spherical shape and less thickness for majority of the nanoparticles along with few elongated particles. Observation of TEM images confirm well distanced particles for lower concentration of Er+3 ions and increase in Er+3 ion substitution leads to agglomeration of particles because of magnetic nano particle interaction which makes the particles to be stacked on top of each other. The particle size measured from TEM images are in the range 16nm–24 nm.

*Crystal Chemistry, Rietveld Analysis, Structural and Electrical Properties of Cobalt… DOI: http://dx.doi.org/10.5772/intechopen.98864*

**Figure 7.** *TEM/SAED images of CoFe2O4 (x = 0.000) and CoFe2-xErxO4 (x = 0.005 to 0.030).*

### **3.6 FTIR analysis**

FTIR (Fourier Transform Infrared) spectroscopy is a very useful technique that estimates cationic redistribution at A and B sites of spinel ferrites. FTIR spectra for samples between 400 cm�<sup>1</sup> and 1000 cm�<sup>1</sup> was displayed by **Figure 8** in which two important broad bands (1 in the range 500 cm�<sup>1</sup> � 600 cm�<sup>1</sup> and 2 in the range 400cm�<sup>1</sup> � 500 cm�<sup>1</sup> ) were observed. As per Waldron suggestion intrinsic vibrations of M–O complexes was shown by band 1 at site A site and band 2 at site B. This difference between 1 and 2 was because of variation in bond length of Fe+3-O�<sup>2</sup> at A, B sites [29]. Observations indicate shift in octahedral (2) and tetrahedral (1) bands towards higher frequency with the addition of Er+3 ions due to bond length variation, expansion in A, B sites and cation migration between two sites. The residency of Er+3 ions at B-site was also confirmed. FT-IR spectra of CoFe2O4 (x = 0.00) and CoErxFe2-xO4 (x = 0.005 to 0.030) nanoparticles are shown in **Figure 8**. The values of force constant at tetrahedral and octahedral (Ft&Fo) sites were determined using the formulas below [30] whose values are listed in **Table 4**.

$$\mathbf{F}\mathbf{t} = 4\pi^2 \mathbf{c}^2 \nu\_1^2 \mathbf{u} \tag{19}$$

$$\mathbf{Fo} = 4\pi^2 \mathbf{c}^2 \nu\_2^2 \mathbf{u} \tag{20}$$

**Figure 8.** *FTIR spectra of CoFe2O4 (x = 0.000) and CoFe2-xErxO4 (x = 0.005 to 0.030).*


#### **Table 4.**

*Summarizes FTIR modes(v1,v2) and force constants (FT, FO) of CoFe2-xErxO4 nano particles.*

*Crystal Chemistry, Rietveld Analysis, Structural and Electrical Properties of Cobalt… DOI: http://dx.doi.org/10.5772/intechopen.98864*

where vibrational frequencies of A, B sites are denoted by v1, v2, reduced mass of Fe3+ and O2� ions is u, speed of light = c. Because of changes in bond lengths of Fe3+ and O2� ions at A, B sites variation in values of force constant was determined.

#### **3.7 Resistivity analysis**

Resistivity figures signify distinct log ρ vs. 1000/T for various compositions of CoFe2-xErxO4. Resistivity reducing while increasing temperature, this behavior indicates that the semiconducting behavior of the prepared samples. Mobility of charge carriers (drift) reduces resistivity with temperature. Enhancement in temperature boosts enough energy to improve charge carriers hopping from one cationic site to other. The *μD* is growing up with the raise of Er+3 content the low drift mobility means temperature has not supplied sufficient potential to develop charge carriers to click from one site to another. Enrichment in *μD* with the boost of Er+3contents advocate the enhancement of hopping from one cationic site to other for all nano ferrites synthesized particles. DC resistivity and drift mobility have inverse relation with each other. Observed resistivity figures indicated increase in resistivity initially for x = 0.000 and later decreases with increasing Er for x = 0.005 to 0.030. It is evident that all specimens contain a fixed quantity of Co. Resistivity vs. temperature curves of Er-substituted CoFe2O4 nanoparticles are shown in **Figure 9**. The resistivity is calculated from the following formula.

$$
\rho = \frac{RA}{l} \tag{21}
$$

Here R is the resistance, A is area of the pellet, l is length of the pellet.

#### **3.8 Magnetic properties**

M-H curves (Hysteresis Loops) are plots drawn between magnetization (M) and applied field (H) which helps us in analyzing magnetic response and magnetic parameters of ferrites under investigation. The M-H loops of all nanoparticles, that is CoErxFe2-xO4 (*x* = 0.00–0.030) heated at 500°C are displayed in **Figure 10**. The

**Figure 9.** *Resistivity vs. temperature curves of Er-substituted CoFe2O4.*

**Figure 10.** *The magnetic hysteresis curves of Er-substituted CoFe2O4 nano particles at room temperature.*

measured magnetic parameters are displayed in **Table 5**. The Magnetic parameters such as Saturation magnetization (Ms), Remanent magnetization (Mr), Coercivity (Hc) and Squareness ratio (R = Mr /Ms), Magnetic moment (nB) were altered by doping of Er+3 content in the increasing order (x = 0.00 to 0.030). Generally, dopant type, concentration and morphology will affect magnetic properties of soft ferrite sample. At the same time variation in magnetic parameters was seen due to microstructure with noting of higher saturation magnetization with higher grain size [31, 32]. **Table 5** indicate high saturation magnetization and coercivity due to large grain size in CoFe2O4 ferrites as depicted by the hysteresis loop in **Figure 10**. Ms. value decreased from 60 emu/g to 44 emu/g with decrease in grain size due to increased Er content in cobalt ferrite which may be due to increase of erbium

**Composition Lattice parameter (a) Crystallite Size (nm) Hc (c) Ms (emu/g) Mr (emu/g) R=Mr/ Ms K (erg/Oe) Magnetic moment (μB/f.u)** CoFe2O4 8.361 20.34 2998 60.6739 31.19 0.5141 189,479.783 2.5488 CoEr0.005Fe1.995O4 8.392 20.43 2997 58.7486 31.07 0.5289 183,405.941 2.4738 CoEr0.010Fe1.990O4 8.407 19.19 2996 56.9560 28.86 0.5067 177,750.433 2.4040 CoEr0.015Fe1.985O4 8.367 19.02 2995 55.4902 27.66 0.5136 173,117.863 2.2784 CoEr0.020Fe1.980O4 8.367 17.73 2993 53.1555 26.83 0.5208 165,723.033 2.1853 CoEr0.025Fe1.975O4 8.386 15.56 2991 49.5845 25.15 0.5240 154.486.957 2.0400

CoEr0.030Fe1.970O4 8.398 14.4 2989 44.8444 22.88 0.5275 139,625.157 1.8480

*Crystal Chemistry, Rietveld Analysis, Structural and Electrical Properties of Cobalt… DOI: http://dx.doi.org/10.5772/intechopen.98864*

**Table 5.**

*The Magnetic Properties of Er-substituted CoFe2O4.*

cations in ferrite lattice site [33]. Particularly, high magnetic moment (5 μB) ferrite cations were replaced by erbium cations of magnetic moment 7 μB at B sites. In addition, increasing erbium cations may decrease ratio of ferric and ferrous ions at A, B sites thereby decreasing the magnetic exchange interaction between two sites [34] reducing the Msvalue. It was also observed that increase of erbium content reduced value of Hc from 18998 Oe to 18990Oeinitiating the fact that magnetic moment can be changed with low coercive field, hence coercivity variation is in agreement with variation in anisotropy constant. Henceforth, value of anisotropy constant 'K' will decrease further which decreases the energy of magnetic domain wall. Remanent magnetization values decreased from 31 emu/g to 22 emu/system supporting soft magnetic nature due to low coercivity in erbium doped cobalt ferrites [35]. **Table 5** indicates decrease in magnetic moment with increased erbium content which may be assigned to more probable chance of erbium cations to occupy B sites. As per the revealed data increasing erbium content decrease magnetization converting the sample into soft magnetic material. It is understood that increase in erbium content decreases value of 'K'. M–H loops indicated that soft magnetic Co-Er nano ferrites can be easily magnetized and demagnetized. Squareness ratio (R = Mr/Ms) was estimated from

$$\mathbf{R} = \frac{\mathbf{M}\_{\mathbf{r}}}{\mathbf{M}\_{\mathbf{S}}} \tag{22}$$

where Mr is Remanent magnetization and Ms is saturation magnetization. Magnetic moment per unit (ηB) was calculated from [31, 32].

$$m\_B = \frac{M\_{\text{av}} \times M\_S}{5585} \tag{23}$$

where Mω are samples molecular weight and saturation magnetization.

K (magnetic anisotropic constant) is related to the Ms (saturation magnetization) and Hc (magnetic coercivity) [28] by following relation

$$k = \frac{M\_{\text{S}} \times H\_{\text{c}}}{0.96} \tag{24}$$

### **4. Conclusions**

Synthesis and characterization of erbium substituted cobalt ferrites along with conglomeration was done using citrate-gel auto combustion method. Significant

induced effect of Erbium was observed on the structure of crystal structure, dielectric constant, morphology and electrical transport properties of cobalt ferrite material. Copy of secondary ErFeO3 along with primary spinel cubic structure occur only for Er-content, *x* = 0.015,0.020 and regains its primary spinal structure for Er content x = 0.025,0.030 while the crystallite size decreased from 20.84 nm– 14.40 nm. According to the SEM analysis the growth in grain along with agglomeration form was found for all samples. With the Erbium substitution which is a combined effect of decrease in resistivity. Small polaron hopping as well as thermally activated mobility of charge carriers was operative in CFEO ceramics and confirmed by DC electrical measurements. Observations indicated strong dependence of magnetic properties on Erbium substitution and coercivity varies in accordance with anisotropy constant. The presence of magnetic dipole could be useful for considering the Erbium substituted cobalt ferrites in electromagnetic applications. The studies of CoErxFe2-xO4 for compositions with cobalt content x = 0.0 to 0.030 with increasing order of x = 0.005 indicated decreasing crystallite size with increasing erbium content and increase in surface area of the particle makes it a good adsorbent. Hence these adsorbents can be used in gas sensors and waste water treatment etc. …
