Structural parameters of the prepared Co-Er nano ferrite sample.
Synthesis of Cobalt-Erbium nano-ferrites with formulation CoErxFe2-xO4 (x = 0, 0.005, 0.010, 0.015, 0.020, 0.025, and 0.030) using technique of citrate-gel auto-combustion was done. Characterization of prepared powders was done by using XRD, EDAX, FESEM, AFM and FTIR Spectroscopy, DC resistivity properties respectively. XRD Rietveld Analysis, SEM, TEM and EDAX analysis were taken up in studying spectral, structural, magnetic and electrical properties. XRD pattern of CEF nano particles confirm single phase cubic spinal structure. The structural variables given by lattice constant (a), lattice volume (v), average crystallite size (D) and X-ray density(dx), Bulk density (d), porosity (p), percentage of pore space (P%), surface area (s), strain (ε), dislocation density (δ), along with ionic radii, bond length and hoping length were calculated. SEM and TEM results reveal homogeneous nature of particles accompanied by clusters having no impurity pickup. TEM analysis gives information about particle size of nanocrystalline ferrite while EDAX analysis confirm elemental composition. Emergence of two arch shaped frequency bands (ν1 and ν2) that represent vibrations at tetrahedral site (A) and octahedral site(B) was indicated by spectra of FTIR. The samples electrical resistivity (DC) was measured between 30°C -600°C with Two probe method. XRD Rietveld analysis confirm crystallite size lying between 20.84 nm–14.40 nm while SEM analysis indicate formation of agglomerates and TEM analysis indicate particle size ranging between 24 nm–16 nm. DC Electrical measurements indicate continuous decrease in resistivity with increasing temperature while increasing doping decreases curie temperature. 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). The increasing erbium content decreases magnetization thus converting the sample into soft magnetic material. Observations indicated strong dependence of magnetic properties on Erbium substitution and coercivity varies in accordance with anisotropy constant. Due to the presence of magnetic dipole Erbium substituted cobalt ferrites can be used in electromagnetic applications. The present study investigates the effect of different compositions of Er3+ replaced for Fe on structural properties and electrical resistivity of cobalt ferrites.
- Electrical Resistivity Properties
- Co-Er nano-Particles
Vigorous research has been accomplished on the fundamental, technological and potential applications of nano-ferrites. Nanomaterials of spinel ferrite have several applications in technology that include magnetic diagnostics and drug delivery , potential applications that include high density magnetic information storage devices , ferrofluid technology , magneto caloric refrigeration , magnetic recording media, magnetostriction , magnetic sensors, microwave devices and electrical generators etc. Ferrites are also used for catalyst and electronic devices. Ferrites are insulators exhibiting various magnetic and electric properties such as low electrical conductivity, dielectric loss, magnetic loss, relative loss factor, moderate dielectric constant, high initial permeability and saturation magnetization. Low eddy current and high resistivity makes ferrites better choice than metals . Doping and thermal changes during synthesis and processing of cobalt-ferrites alter the distribution of metal ions influencing their structure and magnetic properties . Priya et al.  doped Al ions, with Cobalt ferrite nano particles. They observed that Al doped cobalt ferrite to be suitable for high frequency applications and magnetic memory devices. Nasir Amin et al.  synthesized yttrium substituted cadmium ferrites. They reported yttrium doped Cd nanoferrites can be used in high-frequency microwave absorbing devices. Salma et al.  synthesized ferrite series having formulation SrYbyFe2–yO4 (y = 0.00 to 0.10). They observed the dispersion of frequency of ferrites is responsible for the natural magnetic resonance phenomenon and the domain wall pinning. As per the literature net magnetic moment of lanthanide series elements/ions depend on f-orbital electron number in which Er+3 is of small size (89 pm) with large magnetic moment (7
2. Experimental procedure
Synthesis of Cobalt-Erbium nano-ferrites with citrate-gel auto combustion technique was taken up with starting materials Cobalt Nitrate (Co (NO3)2·6H2O), Ferricnitrate (Fe (NO3)3·9H2O), Erbium Nitrate (Er (NO3).6H2O), Citric Acid (C6H8O7·H2O) and Ammonia solution (NH3) of 99.9% purity after weighing as per stoichiometric ratio. Later liquification of metal nitrates in distilled water was done and the mixture was stirred at 300 rpm for one hour to obtain a clear homogeneous solution. Next citric acid in aqueous form and metal nitrate was maintained in 1:3 ratio for all samples. Now, ammonia solution was added drop by drop to maintain Ph = 7. This solution on stirring was heated at 100°C temperature for ten to twelve hours to form a viscous gel. The water contained in the mixture gets evaporated slowly to form dry gel generating internal combustion to form a black colored desired sample. This sample was manually grinded and subjected to calcinations at 500°C in furnace for 4 hours. The step by step procedure for the synthesis of crystal ferrites is shown in the form of flow chart in Figure 1. Pellet was prepared with KBr hydraulic press (Model: M-15) in 2–3 mm thickness and 10 mm diameter size
Later these samples in pellet or powder form were used to characterize the material. Structural properties were analyzed with XRD (Bruker, CuKα, λ = 0.15406 nm), TEM (Model JEOL 2100F, Japan), Field-emission Scanning Electron Microscope (JEOL JSM-7600 F, Japan), Energy Dispersive X-ray Analyzer (EDAX) and, Atomic Force Microscopy (AFM: VEECO, USA). Two probe method was used to study electric properties.
3. Results and discussion
3.1 XRD analysis
Figure 2 displays the XRD Rietveld Refinement corresponding to samples of CoErxFe2-xO4 with values of x between 0.00 to 0.030 (x = incremented by 0.005). It is observed that the peaks analogous to diffraction planes , , , ,  and  match with usual data (JCPDS card no. 022–1086) confirming FCC cubic spinel structure for samples investigated [12, 13]. Figure 3 shows shift in XRD peaks towards left hand side with increasing concentration of Er+3 ions in CoFe2O4 particles in concurrence with ‘a’ value. Table 1 lists different parameters of XRD calculated for CoErxFe2-xO4 nanoparticles. The values of ‘a’ were calculated from the equation given .
|Compositions||Cell constant(Å)||Cell Volume V(Å3)||Crystallite Size (nm)||X-ray density(||Bulk density(d) (gcm−3)||Porosity P (%)||Surface area(s) (m2/gm)||Packing factor(P)||Strain (||Dis location density (|
|8.361||V = 584.48||20.84||5.3344||3.2113||39.8001||89.61||8.26||2.3||1.9|
|8.367||V = 585.74||20.43||5.3356||3.2120||39.8005||91.39||8.09||2.3||2.0|
|8.373||V = 587.00||19.19||5.3367||3.2127||39.7998||97.34||7.60||2.7||2.5|
|8.379||V = 588.26||19.02||5.3379||3.2134||39.8003||98.18||7.52||2.7||2.5|
|8.386||V = 589.74||17.73||5.3370||3.2129||39.7995||105.38||7.01||3.1||3.2|
|8.392||V = 591.01||15.56||5.3381||3.2135||39.8006||119.99||6.14||4.1||4.7|
|8.398||V = 592.28||14.40||5.3392||3.2141||39.8018||132.58||5.68||4.8||5.9|
where cell constant is given by ‘a’, inter planer spacing calculated from Bragg’s equation (2 dsin
It was reported that, low concentration RE (rare earth) doping in spinel ferrite experience phase separation and grain boundary diffusion giving rise to precipitation of additional crystalline phases like hematite (a-Fe2O3), metal monoxides and orthoferrites (REFeO3) [15, 16, 17]. Hence in case of rare earth doped ferrites, Er+3doped CFO having no impurity phase (
where ‘λ’ = wavelength of x ray,‘β’ = peak width at half maximum height and constant ‘K’ = 0.9. The data related to intense peak (311) was used in estimating size (L). The results indicated reduction in size of crystallite from 20.84nmto14.40 nm (for x = 0.0 to 0.030). Further, the high intense peak (311) shifts towards the lower angle with increasing values of
X-ray density value is found to increase from 5.3344gm/cm3 to 5.3392gm/cm3 (
where pellets mass, thickness and radius are given by ‘
Here d and dx are apparent and experimental densities. Surface area was calculated by using the Eq. (16).
Here, S = area of surface, D = crystallite size, d = Bulk density.
Strain was calculated by using the following Equation .
Dislocation Density calculated by using following equation
Here L is crystallite size, d is inter planner spacing.
Cationic distributions that depend on factors like synthesis, total energy and thermal history are useful in understanding spinel ferrites behavior (electric and magnetic). Cationic calculations play important role in this regard. Average ionic radii of A, B sites were calculated from Stanley’s equations:
Here Ro is the radius of the oxygen ion (1.35 Å), ‘u’ is the oxygen parameter whose ideal value is 0.375Å and experimental value is 0.383Å.
Bonding lengths and hopping lengths are calculated by using following formulas .
Bonding lengths: Hoping lengths:
The difference in ‘u’ value in comparison with its ideal value on substituting Er+3 ions has been explained with rA values. Increasing rA values increase ‘u’ showing distortion in CoFe2O4 spinel lattice. Calculated values of ionic radii for B-sites are slightly higher than A-site because more Er+3 ions reside at B-site than A-site. Hopping length is the gap between magnetic ions at A, B sites. The hopping lengths between magnetic ions at A, B sites are denoted by LA and LB whose values reduce with addition of Er+3 content and is consistent with variation in lattice constant on adding Er+3 ions. The determined values from the formulas (10), (11), (17) and (18) are listed in Table 1.
By using the relations below structural parameters associated with A, B sites are calculated. Magnetic interactions and their strengths among AA, BB and AB sites mainly depend on bond length and bond angle existing between positive and negative ions. Increase in bond angle increases magnetic interaction strength while it reduces with increasing bond length as the strength has direct relation with bond angle and inverse relation with bond length. Table 3 summarizes different bond lengths of A, B sites (dA-A, dB-B, dA-B, dA-OA, dB-OB) which depict an increase in bond lengths of tetrahedral and octahedral sites which is due to Er+3 ion doping in spinel lattice which might be due to larger Er+3 ions replacing smaller Fe+3 ions.
|X = 0.000||12.40||34.43||0.000||26.86|
|X = 0.005||12.37||34.27||0.094||26.80|
|X = 0.010||12.34||34.10||0.18||26.74|
|X = 0.015||12.31||33.93||0.28||26.67|
|X = 0.020||12.28||33.77||0.37||26.61|
|X = 0.025||12.25||33.60||0.46||26.55|
|X = 0.030||12.23||33.44||0.55||26.49|
3.2 EDAX analysis
Figure 4 displays the EDAX spectrums that analyzed elemental and atomic percentages of CoFe2-xErxO4nanoparticles for x = 0.0, 0.005, 0.010, 0.015, 0.020, 0.025 and 0.030. It confirmed the presence of Co, O, Fe and Er. Er peak confirms Erbium substitution in the Fe2-x lattice.
Table 2 summarizes atomic percentages of individual in CoFe2-xErxO4 nanoparticles. EDAX confirmed the effect of incorporating Er3+ into CFO and stoichiometric amount of O, Fe, Co, and Er atoms.
3.3 Field emission scanning Electron microscopy (FE-SEM)
Figure 5 shows studies on surface morphology of ferrite powders with the help of FE-SEM. The nature of ferrite particle in the samples is uniform indicating fine form of agglomeration and grain growth. Agglomerate formation specifies strong magnetic nature of erbium doped ferrites. These studies also confirm microstructure changes on doping Er+3. A close look at these microstructures indicate improvement in microstructure and spherical shaped grains in all samples. Apart from this Erbium doping increases percentage of porosity in small range between 39.8001to 39.8018 illustrating individual grains and grain boundaries are separated.
3.4 Atomic force microscopy (AFM)
AFM was used to characterize the surface roughness of CoErxFe2-xO4 nano ferrite samples of the synthesized nanoparticles. The three-dimensional arrangement of the spherical nanoparticles and diameter are shown in Figure 6. The surface roughness increased when the coercivity increases, but in this work all the parameters crystallite size, saturation magnetization, remanent magnetization, coercivity decreased with the increasing of Er dopant from x = 0.00 to 0.030 in the cobalt ferrite. In view of the above, the largest surface roughness is observed for x = 0.0 sample and the lowest surface roughness is obtained for Er (x = 0.030) doped samples. This indicates that the surface activity of x = 0.0 ferrite has higher values compared to the range x = 0.005–0.030 ferrite samples. The largest surface roughness is observed for x = 0.0 sample that is, it behaves like hard ferrite and the lowest surface roughness is obtained for Er (x = 0.030) doped samples. That is, it behaves like soft ferrite, hereby the ferrite is transformed from hard ferrite to soft ferrite due to the doping of Er content.
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 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.
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−1 and 1000 cm−1 was displayed by Figure 8 in which two important broad bands (1 in the range 500 cm−1 − 600 cm−1 and 2 in the range 400cm−1 − 500 cm−1) 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−2 at A, B sites . 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  whose values are listed in Table 4.
|Compositions||Wave number v1(cm-1)||Wave number v2(cm-1)||FT×105(dynes/cm)||FO×105(dynes/cm)|
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
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 (
|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)|
where Mr is Remanent magnetization and Ms is saturation magnetization.
where Mω are samples molecular weight and saturation magnetization.
K (magnetic anisotropic constant) is related to the Ms (saturation magnetization) and Hc (magnetic coercivity)  by following relation
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,
Thanks to CSIR, New Delhi, India for Research Fellowship (CSIR-JRF). The authors are grateful to the Prof. Syed Rahman, Head Department of Physics, University College of Science, Osmania University Hyderabad for his constant encouragement.