Dielectric, Complex Impedance, and Electrical Transport Properties of

Article pubs.acs.org/JPCC

Dielectric, Complex Impedance, and Electrical Transport Properties of Erbium (Er3+) Ion-Substituted Nanocrystalline, Cobalt-Rich Ferrite (Co1.1Fe1.9−xErxO4) Sandip G. Kakade,† Yuan-Ron Ma,‡ Rupesh S. Devan,‡ Yesh D. Kolekar,† and C. V. Ramana*,§ †

Department of Physics, Savitribai Phule Pune University, Pune 411 007, Maharashtra, India Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan § Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, United States Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on June 26, 2018 at 05:45:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Erbium (Er3+) substituted nanocrystalline, cobalt-rich ferrites, which can be represented chemically as Co1.1Fe1.9−xErxO4 (CFEO; x = 0.0−0.2), were synthesized by the sol−gel autocombustion method. The structural, dielectric, and electrical transport properties of CFEO were investigated in detail. CFEO materials crystallize in a spinel cubic structure for x ≤ 0.10; formation of orthoferrite (ErFeO3) secondary phase was noted for x ≥ 0.15. Microstructural and compositional studies revealed the formation of spherical, elongated grains with stoichiometric presence of Co, Fe, Er, and O. The dielectric constant (ε′) dispersion fits to the Debye’s function for all CFEO ceramics. The relaxation time and spread factor obtained from ε′ dispersion are ∼10−3 s and ∼0.50 (±0.10), respectively. The complex impedance analyses confirm a graininterior mechanism contributing to the dielectric properties. Semiconducting behavior and small polaron conduction mechanism were evident in electrical transport properties of CFEO materials.

■

INTRODUCTION Spinel ferrites, which contain iron oxide, are magnetic ceramics with a vast potential for numerous scientific and technological applications.1−15 Among the spinel ferrites, cobalt ferrite CoFe2O4 (CFO) is the most useful hard ferrimagnetic material, which exhibits unique properties such as strong spin−orbit (L− S) coupling, high Curie temperature, high coercivity, high magneto-crystalline anisotropy, moderate saturation magnetization, large Kerr effect and Faraday rotation, good mechanical hardness, and chemical stability.1−7 Spinel ferrite is chemically represented as M2+Fe23+O4, where M and Fe are divalent and trivalent cations, respectively. The chemistry and physics of M and Fe cations dictate the electrical and magnetic properties of ferrites. Cobalt ferrite (CFO) based materials are useful for the development of microwave and spintronic devices, solar cells, magnetostrictive sensors, drug delivery, transducers, actuators, Li batteries, super capacitors, and memory devices for computers.1−12 Recently, much attention has been paid toward the synthesis of CFO nanoparticles and tailoring their electrical and magnetic properties which could be exploited in numerous applications.2,3,8,10 Furthermore, higher values of magnetostriction make CFO a potential candidate material for “strain sensor and actuator” applications.11 When combined with a piezoelectric/ferroelectric material, CFO can be used for designing the multiferroic/magnetoelectric heterostructures for application in multifunctional and tunable magnetoelectronic devices. Recently, the trivalent ion substitution for Fe3+ in spinel ferrites has been gaining importance for © 2016 American Chemical Society

utilization in high-density magnetic recording, enhanced memory storage, magnetic fluids, and catalysts applications.12−18 The specific nature of the ion/dopant into CFO matrix can modulate the final magnetic and electrical characteristics.13−18 The degree of modification depends on the ionic radius, the electronic configuration of the substituting ion, and its site preference.15−17 The impetus for the present work is to derive a comprehensive understanding the Erbium (Er3+) ion substitution-induced effects on the structural, dielectric, and electrical transport properties of nanocrystalline, Co-rich CFO (Co1.1Fe1.9O4). Erbium ion substitutions in CFO derive the researchers’ attention from a fundamental science perspective since Er3+ ions have a decisive influence on the structure and properties.6 Er3+ is nonmagnetic at room temperature; however, Er3+ for Fe3+ in CFO induces significant saturation magnetization, which is extremely useful to tailor the magnetic properties desired for magnetic recording applications.6,13 Furthermore, while there exists some efforts toward magnetism, the overall attention paid toward the dielectric and electrical transport properties of CFEO in the literature is meager. A better understanding of the dielectric and transport properties of CFEO is, therefore, highly beneficial to tune the properties for desired electronic and electromagnetic applications.18−21 In this context, efforts in this work were directed to synthesize Received: November 15, 2015 Revised: January 13, 2016 Published: January 26, 2016 5682

DOI: 10.1021/acs.jpcc.5b11188 J. Phys. Chem. C 2016, 120, 5682−5693

Article

The Journal of Physical Chemistry C

Figure 1. (a) XRD patterns of CFEO ceramics. Data shown are for variable Er content (x). (b) Magnified view of the (311) peak for Co1.1Fe1.9−xErxO4.

tional/elemental analyses were made using energy-dispersive Xray spectrometry (EDS). The dielectric and electrical measurements were made on the CFEO pellets. The pellet fabrication process for dielectric and electrical measurements involved the following sequential steps. The starting materials were mixed thoroughly in required molar proportions and presintered at 500 °C for 12 h in air. The presintered mixture was then ground and pressed with a pressure of 392 MPa to fabricate the pellets with 10 mm diameter and 2 mm thickness. The samples were finally sintered at 700 °C for 5 h in air, and the furnace was then allowed to cool naturally to room temperature. To ensure the better ohmic contact, prior to electrical measurements, the silver paste was painted onto the polished surfaces of the pellet and baked at 100 °C for 5 h. The frequencydependent ( f = 20 Hz to 1 MHz) dielectric constant (ε′) and dissipation factor (tan δ) were studied using a LCR precision meter bridge (HP-4284A). Alternating current conductivity was calculated using the room-temperature dielectric data. Complex impedance measurements were also carried out for all samples ( f = 20 Hz to 1 MHz) using a LCR precision meter bridge (HP-4284A). The dc resistivity measurements were performed using a two-probe method in the temperature range from room temperature to 500 °C.

nanocrystalline Er-substituted Co-rich ferrites with the chemical formula Co1.1Fe1.9−xErxO4 (x = 0.0−0.2) by sol−gel autocombustion method and to study their structure, chemistry, dielectric, and transport properties. Interestingly, as presented and discussed in this paper, the results demonstrate a significant improvement in the electrical and dielectric properties of Corich CFO with Er substitution.

■

EXPERIMENTAL DETAILS A. Synthesis. Nanoparticles of Co1.1Fe1.9−xErxO4 (CFEO) with x = 0.00−0.20 were synthesized by th sol−gel autocombustion method. Analytical reagent (AR) grade Co(NO3)2·6H2O, Fe(NO3)3·9H2O, Er(NO3)3·5H2O, and citric acid (C6H8O7·H2O) were used as raw precursors. Stoichiometric amounts of metal nitrates and citric acid with 1:1 ratio were dissolved in deionized water to form the citrate−nitrate solution. The liquor ammonia was then added drop by drop into the citrate−nitrate solution to adjust the pH (∼7). The final solution was continuously stirred on a magnetic stirrer at 60 °C to homogenize the solution and to form the redox mixture of citric acid and metal nitrates for combustion reaction. Then the resultant solution was dehydrated slowly on a hot plate at 80 °C with continuous stirring until the viscous xerogel was formed. On further heating, the temperature of the gel increased and at a certain temperature autoignition of gel took place with the evolution of gases and a black powder was obtained. All samples were finally sintered at 700 °C for 5 h. The balanced chemical reaction for Co1.1Fe1.9O4 can be written as

■

RESULTS AND DISCUSSION A. Structure and Morphology. The XRD patterns of Co1.1Fe1.9−xErxO4 (x = 0.0−0.2) are shown in Figure 1a. Indexing of the patterns was made using the JCPDS data: 221086 (CoFe2O4) and 47-0072 (ErFeO3). It is evident (Figure 1a) that the XRD data confirms the formation of a facecentered cubic spinel structure with the space group Fd3m-Oh.7 An interesting observation is the fact that the formation of a single-phase cubic spinel structure occurs for the CFEO ceramics with lower Er content, x ≤ 0.10, where no other diffraction peaks corresponding to the impurity or any other phase present. The remarkable effect of Er3+ substitution is reflected in two important observations in XRD analyses. An increase in the lattice constant is first, and reduction in the

1.1Co(NO3)2 ·6H 2O + 1.9Fe(NO3)3 ·9H 2O + C6H8O7 ·H 2O + 2NH3 Δ

·H 2O → Co1.1Fe1.9O4 + 6CO2 + 3N2 + 4NO2 + 34H 2O

B. Characterization. The crystal structure of the CFEO samples was studied using X-ray diffraction (XRD) employing Cu Kα radiation (λ = 1.54178 Å; D8 Advance, Bruker Inc., Karlsruhe, Germany). The microstructure of the CFEO samples was analyzed employing a field emission scanning electron microscope (FESEM, JEOL-JSM-6360). The composi5683


Article


sol−gel chemical technique is more advantageous than the others for the preparation of RE-substituted spinel ferrite systems. B. Dielectric Properties. The dielectric behavior of Febased oxides is due to the electric dipole moments developed due to the charge transfer between the di- and trivalent metal cations (ion exchange) within the spinel structure. The dielectric properties of ferrites also depend on the microstructure, method of preparation, sintering time and temperature, chemical composition, and site occupancy of metal cations among the A or B sites.4,14,19 The dielectric behavior of ferrites as a function of frequency provide valuable information about the behavior of localized electric charge carriers and better understanding of the mechanism of dielectric polarization in ferrites.4,24,25 The variation of the dielectric constant of CFEO ceramics with Er content (x) is presented in Figure 4. The data shown are at three different constant frequency values. It is evident from Figure 4 that the Er content strongly influences the dielectric constant of the samples. The dielectric constant increases with increasing x values in Co1.1Fe1.9−x ErxO4. However, it can be noted that the dielectric constant increases, becomes maximum at x = 0.15, and decreases thereafter. The increase in the dielectric constant with the inclusion of Er3+ ions could be due to the fact that, with the inclusion of rare-earth ions (RE3+), the Co−ferrite lattice is distorted and increases in Fe−O and RE−O bond lengths at B site giving rise to increases in the atomic polarizability and subsequently the dielectric constant.9,16,17 In addition, formation of a small amount of the ErFeO3 phase at the grain boundaries leads to the accumulation of charges at the grain boundaries, resulting the interfacial polarization, which contributes to the additional increase in dielectric constant with increasing Er content. Earlier, we reported such behavior for a wide variety of RE3+ ions into either Co−ferrite or Ni− ferrite.5,9,17,25 Also, it is widely accepted in the literature that the incorporation of RE3+ ions with larger ionic size (compared to Ni/Co/Fe ions) into ferrites increases the resistivity and dielectric constant.5,9,17,26,27 However, in the present case of Er incorporation into CFO to increase the dielectric constant is not very significant, although comparable in some cases, compared to other ions such as La, Gd, Ho, or Sm.9,12−17 This can be attributed to the ionic radius of Er3+ ion, which is relatively small compared to that of La, Gd, Ho, or Sm ions. Further analysis of the dielectric properties and the effect of Er content on them is evaluated by considering the frequencydependent dielectric constant and electrical conductivity of the CFEO ceramics. The dielectric constant (ε′) data of CFEO ceramics measured over a wide frequency range (20 Hz−1 MHz) are shown in Figure 5a. It can be noted that ε′ undergoes a sharp decrease with frequency up to 1 kHz, at which point ε′ decreases slowly with further increase in frequency and remains constant at higher frequencies (≥10 kHz). The larger value of ε′ at lower frequencies is mainly due to the contributions from various polarizations such as ionic, space charge, and interface. At higher frequencies, ε′ is independent of frequency due to the inability of electric dipoles to follow the fast variation of the alternating applied electric field. Thus, the dielectric dispersion occurring in the lower frequency regime is attributed to the interfacial polarization, because the electronic and atomic polarizations remain unchanged at this frequencies.28 The frequency dependence of the dissipation factor or loss tangent (tan δ) for CFEO samples is shown in Figure 5b. Here

crystallite size is later. The lattice constant increases from 8.3784 (x = 0.0) to 8.4012 Å (x = 0.2) with increasing Er content. The corresponding crystallite size decreases from 42.0 (x = 0.0) to 22.5 nm (x = 0.2). Furthermore, the intense (311) peak shows a negative shift (i.e., lower angle) with increasing Er content (Figure 1b). The (311) peak negative shift and intensity reduction with Er content are shown in Figure 1b, where the lines and arrows illustrate the induced Er effect. The (311) peak shift coupled with a linear increase in the lattice constant is mainly due to the replacement of smaller Co2+ (0.78 Å) and Fe3+ (0.64 Å) by the larger Er3+(0.90 Å) in CFEO spinel lattice. Differences in the ionic radii of Co2+, Fe3+, and Er3+ ions can result in lattice strain,6,14,22 which accounts for the lattice parameter and, hence, unit cell expansion. Such effect is mainly due to the larger Er3+ ion substitution for Fe3+in CFO. The lattice parameter, density, and average size values determined from XRD are listed in Table 1. Table 1. Values of Lattice Constant, Density, and Particle Size As Determined from XRD for Co1.1Fe1.9−xErxO4 Er content (x)

lattice constant (Å)

density (gm/cm3)

particle size nm

0.00 0.05 0.10 0.15 0.20

8.3784 8.3881 8.3916 8.3970 8.4012

5.3056 5.4126 5.5310 5.6453 5.7614

41.99 29.07 27.92 24.84 22.50

The FE-SEM micrograph shown in Figure 2 displays the surface morphology for all CFEO samples. It is seen that most of the CFEO particles exhibit a nearly spherical, elongated granular structure. The particle size reduces with increasing Er content in CFEO (Figure 2). Er3+ ions may segregate closer to the textural boundaries of Co1.1Fe1.9O4 and minimize the possibility of decreasing the size by the process of agglomeration or grain growth inhibition. Energetics may also play a role; the energy required to incorporate the RE ions with larger ionic radii in place of Fe3+ and to form the RE−O bond is high.22,23 Therefore, in the presence of Er3+ ions, grain growth is inhibited leading to the reduced CFEO particle size. Similar results are reported for RE-ion-substituted cobalt ferrite.6,22,23 The EDS spectra of CFEO samples as a function of Er content are shown in Figure 3. The EDS spectra confirm the stoichiometric presence of Er, Co, Fe, and O elements. Furthermore, the relative increase in intensity of the peak assigned for Er confirms the expected Er concentration increase in CFEO. The significance of the present results and underlying chemistry can be understood as follows. As reported in the literature, doping of rare-earth (RE) ion into the spinel lattice suffers the phase segregation and diffusion of the ion species to the grain boundaries even for very low content, i.e., lower x values. In most cases such phenomena cause the precipitation or formation of extra amorphous or crystalline phases like orthoferrite (RE−FeO3), hematite (α-Fe2O3), and metal monoxides.5,15 However, in the present case, the autocombustion-based sol−gel chemistry procedure and the synthetic conditions adopted are quite successful to produce the spinel phase of Er-substituted cobalt ferrite without formation of any of the impurity phase. Formation of such secondary phase is noted only at x ≥ 0.15, which is significantly higher compared to the traditional conventional solid state ceramic or hightemperature processing methods. Thus, the combustion-based 5684


Article


Figure 2. FE-SEM micrographs of Co1.1Fe1.9−xErxO4.

angle “δ” is the phase difference between the applied electric field and the induced current.19,25 Similar to ε′, the tan δ values decrease with increasing frequency and become constant at higher frequencies (≥10 kHz) due to the decreased polarization at higher ac fields. The observed tan δ variation with frequency can be attributed to the conduction mechanism in ferrites,12 which is analogous to Koop’s phenomenological model.29 The loss factor curve is considered to be caused by domain wall resonance. At higher frequencies, losses are found to be low if the domain wall motion is inhibited as per Rezlescu model.28,30 Furthermore, it is known that the dielectric loss arises when the polarization lags behind the applied alternating field and may be caused by the grain boundaries, impurities, and imperfections in the crystal lattice.15,24 It is evident (Figure 5b) that the dielectric loss is maximum for x = 0.05 and minimum for Co1.1Fe1.9O4. Interestingly, the sample with x = 0.15 showed the lower dielectric loss with maximum dielectric constant. This variation may be due to the change in resistance of samples with the Er concentration. In order to gain further insight into the dielectric properties of CFEO, attempts were made to further analyze the ε′-dispersion behavior. The experimental data can be theoretically calculated using the

modified Debye’s function that considers the possibility of more than one ion (O2−, Fe3+, Co2+, and Er3+) contributing to the relaxation.25 Thus, the observed ε′ndispersion can be modeled using ε′ = ε′∞ +

(ε′0 − ε′∞) [1 + (ωτ )2(1 − α)]

(1)

where ε′ is the real part of the dielectric constant, ε′∞ is the dielectric constant at higher frequency (1 MHz), ε′0 is the dielectric constant at the lowest possible frequency (20 Hz), ω = 2πf is the angular frequency, f is the applied frequency to the signal of 1 V, τ is the mean relaxation time, and α is the spreading factor of the actual relaxation times about the mean value. The pioneering work of Cole−Cole with the standard procedure31 were adopted to fit the experimental data and to obtain the spreading factor “α” value for CFEO materials. However, we considered the real part of the dielectric constant instead of the complex part of the dielectric constant and plotted the graph of ln(ε′0 − ε′/ε′ − ε′∞) versus ln(ω) for all compositions (Figure 6). Such analysis was found to be quite useful to understand the dielectric constant dispersion behavior 5685


Article


Figure 3. EDS spectra of Co1.1Fe1.9−xErxO4..

of Mn- or Hf-substituted CFO materials as reported elsewhere.4,21 The data obtained for CFEO samples yield a straight line (Figure 6). The parameter (1 − α) is determined from the slope of the linear part; α values, thus, estimated were used to determine the values of the relaxation time (τ). The calculated values of α and τ are listed in Table 2 for all CFEO compositions. Thus, using α and τ, the experimentally measured room-temperature dielectric constant (ε′) is fitted with the model, eq 1, as shown in Figure 7. The experimental data is in good agreement with the calculated data (Figure 7), indicating the validity of modified Debye’s function with the possibility of more than one ion contributing to the relaxation process. The dielectric relaxation process observed in Ersubstituted CFO ceramics and validity of the Debye’s-modified function with a contribution from multiple ions seems to be unique to the RE-ion-incorporated ferrites due to the fact that RE ion with size larger than either Co or Fe ions in CFO introduces the structural distortion.4,9,16−19,22

A more detailed account of the Er-induced effect on the dielectric properties can be derived if the ε′−x relation is considered in CFEO. ε′ increases (Figure 5a) from 200 to 700 for an increase in x from 0.00 to 0.15. This is due to the resistivity decrease in CFEO with increasing Er content as discussed later under dc electrical studies. Note that the dielectric permittivity is directly proportional to the square root of conductivity.30 The other factor that contributes to the ε′ enhancement is the grain size reduction with Er content as evidenced in XRD and SEM results. For cobalt ferrite, it has been reported that ε′ increases with the decrease in particle size.12,16,19,20 Thus, ε′ increase until x reaches 0.15 is due to the reduction effect of size and electrical resistivity. However, for x = 0.2, the decrease in ε′ may be due to the change in cation distribution and the presence of impurity phase, i.e., ErFeO3. The presence of orthoferrite secondary phase increases the resistivity. Furthermore, there may be migration of some Co2+ and Fe3+ ions among the A and B sites due to the induced strain at the B site with Er substitution and at the A site, with the migration of some CO2+ from B site to A site. This may 5686


Article


Figure 5. (a) Variation of dielectric constant (ε′) with frequency (log f). (b) Variation of dielectric loss tangent (tan δ) with frequency (log f) for Co1.1Fe1.9−xErxO4..

Figure 4. Dielectric constant variation with Er content (x) in CFEO ceramics. (a) Dielectric constant data for frequencies < 1 kHz. (b) Dielectric constant data for frequencies ≥ 1 kHz.

result in the resistivity increase. In general, the resistivity of a material increases as a result of imperfections, such as defects, impurities, grain boundaries, and dislocations.24,25 Having understood the Er-induced improvement in dielectric constant of CFO, it is imperative to compare the data with the literature to gain some additional insight into the underlying general mechanism of RE-ion influence on the dielectric properties of CFO. A comparison of the present data of Er3+ ions incorporated into CFO with that of other RE ions is presented in Table 3. While comparison of the data for other ions obtained specifically by the sol−gel autocombustion method is not readily possible due to limited data available in the literature, an effort is made in Table 3 to present a reasonable comparison of the data with that in the literature, although samples prepared by other methods are also considered. It is evident from Table 3 that the effect of Er3+ and Gd3+ ions is very similar, especially for materials synthesized by the sol−gel autocombustion method. ε′ increases as the Er/Gd content increases. Therefore, it appears that the ε′ increase due Er content in CFO is also due to the small polaron hopping similar to the Gd-doped CFO.16,32 It is well known that the charge localization with strong lattice distortion can occur due to the strong electron−lattice interaction, i.e., due to the small polaron formation.4,9,16,17,26,32 This small polaron formation allows the charge carriers to locally distort the lattice around it, subsequently trapping itself in the resulting potential well.

Figure 6. Variation of dielectric constant ln(ε′0 − ε′/ε′ − ε′∞) with frequency (ln(ω)) for Co1.1Fe1.9−xErxO4 at room temperature.

Table 2. Fitted Values of Spreading Factor (α) and Relaxation Time (τ), Grain Resistance (Rg), Grain Capacitance (Cg) Obtained from Fitting the Impedance Data, and Activation Energy (Eg) Values Obtained from dc Resistivity Measurement for Co1.1Fe1.9−xErxO4 Er content (x)

α

τ × 10−3 s

Rg (MΩ)

Cg (pF)

Eg (eV)

0.00 0.05 0.10 0.15 0.20

0.5943 0.5409 0.5567 0.4085 0.5492

0.7520 0.6977 0.7325 0.9731 0.9520

1.690 0.405 0.615 0.345 0.980

63.0 71.0 73.9 89.0 75.0

0.2391 0.2426 0.2453 0.2481 0.2517

C. Complex Impedance Analysis. It is well known that the conduction in ceramics is mainly due to the response of 5687


Article


Figure 7. Room-temperature frequency-dependent real part of the dielectric constant (ε′) for Co1.1Fe1.9−xErxO4 ferrites. Data are fitted with the modified Debye model taking the spreading factor (α). Observed and calculated data are in good agreement.

grain and grain boundaries to externally applied factors, viz. frequency and temperature.28,30,33 In ferrites, the grain boundaries (poorly conducting) are more active and hence the hopping of Fe2+ and Fe3+ ions is not effective at lower frequencies.9,17,18,28,30,33 As the frequency of applied field increases, the conductive grains become more active, thereby promoting the hopping between Fe2+ and Fe3+ ions. Thus, it is important to understand the conduction mechanism whether it is due to the grain or grain boundary or stray charges developed across the electrode. Impedance spectroscopy, which is a welldeveloped tool to separate the bulk (grain) and grain boundary contribution to the total conductivity, has been employed to evaluate CFEO. The complex impedance data obtained at room temperature for CFEO are shown in Figure 8. The Nyquist plots show the presence of a single semicircular arc for all

samples in the frequency range from 20 Hz to 1 M Hz. The diameter of the semicircle corresponds to the resistance of the grain.25,28,33 The semicircle diameter decreases with Er content, indicating grain resistance decrease in CFEO.28 This result is in correlation with structural data, where the grain size reduction with Er content is evident. Thus, the grain with fine size in Ersubstituted Co-rich CFO becomes more conductive.34 The experimental impedance spectra were further analyzed considering an ideal equivalent circuit consisting of a resistor R and a capacitor C.15,25,33 In this work, the observed semicircle can be modeled by an equivalent circuit consisting of a parallel combination of grain resistance (Rg) and grain capacitance (Cg) (inset, Figure 8) which accurately fits with the Cole−Cole semicircle. Impedance for the circuit is 5688


Article


constant, tan δ is the loss factor, and σac is the ac conductivity. 15,24 Figure 9 shows the variation of ac conductivity of CFEO ceramics. It is well known that the frequency-dependent ac conductivity decreases for large polaron hopping, whereas it increases for small polaron hopping.19 In the present case, the ac conductivity plots are linear, indicating the small polaron conduction. This linear increase in ac conductivity relates to the conduction by electron exchange between the ions of same element but different valence states. As ferrite structure is cubic close-packed lattice with cations at the octahedral (B) site and tetrahedral (A) site, the exchange of electric charge between A−B sites is more important as compared to B−B site exchange. On the application of an ac electric field, this electron exchange increases, thereby increasing the ac conductivity.19 In addition, the frequency dependence of ac conductivity can be explained with the help of the Maxwell−Wagner two-layer model. According to this model, grain boundaries are more active at lower frequencies; hence, the hopping frequency of the electron between Fe3+ and Fe2+ ion is less at lower frequencies. As the frequency of applied field increases, the conductive grains become more active, thereby promoting electron hopping between two adjacent octahedral sites (B sites) and a transition between Fe2+and Fe3+ ions. As a result, conductivity gradually increases with frequency.15,19 Furthermore, variation in ac conductivity is in accordance with the decrease in resistance of CFEO with Er content as confirmed from both dc resistivity (section 3.4) and impedance measurements. The results obtained in the present work for Er3+ ion-substituted cobalt ferrite are very similar to the trend noted for Gd3+ ions with comparable ionic radius.19,26,37 Specifically for Gd3+ ionsubstituted CFO prepared by the same chemical method, i.e., sol−gel autocombustion, the hopping of charge carriers due to active grains with increasing frequency is evident,26 similar to the effect of Er3+ ions noted in this work. Rahman at al. also reported similar results and a small-polaron conduction mechanism and explained their results considering a two-layer heterogeneous model for Gd-substituted CFO prepared by high-temperature solid state reaction method.35 In addition, Hashim et al. demonstrated the validity of such model and polycrystalline heterogeneous two-layer structure to explain the ac electrical properties of ferrite composites and mixed Zn/Mnbased ferrites.38 E. Direct Current Electrical Resistivity (ρ). The variation of dc electrical resistivity (ρ) with temperature for CFEO samples is shown in Figure 10. The resistivity decreases exponentially with temperature which indicates the semiconducting nature of CFEO samples. Such behavior along with the characteristic of electrical conductivity decrease with RE-ion substitution was also noted for several ions such as Gd3+, Sm3+, and Ho3+ ion substituted Co−/Ni−ferrites.9,17,26,35 The temperature dependent electrical conductivity in ferrites is due to both hopping of electrons and charge transport via excited states which can be expressed as38−40

Table 3. Comparison of the Dielectric Constant of Er3+ Ion with Other Rare-Earth Ions in Cobalt Ferrite dielectric constant at different frequencies rare-earth ion incorporated into CFO

content (x)

Er3+

Gd3+

Gd3+ Dy3+ Gd3+ La3+

20 Hz 100 Hz

100 kHz

1 MHz

0

207

129

16

11

0.05 0.10 0.15 0.2 0 0.05 0.10 0.15 0.2 0.05 0.1 0.2 0.2 0.2

333 379 641 326

235 237 389 169 230 313 465 616 501

17 19 20 18

10 12 14 12 10−15 10−15 10−15 10−15 10−15 16.1 13.6 15 18 30

Z = Z′ − jZ″

27 25 48

ref(s) present work

26

32 16

(2)

where Z′ =

Z′ =

Rg 1 + R g 2ω 2Cg 2

(3)

R g 2ωCg 1 + R g 2ω 2Cg 2

(4)

The observed and calculated complex impedance data matches fairly well for CFEO ceramics. The parameters (Rg and Cg) obtained from the best fitting are listed in Table 2. It is observed that the grain capacitance (Cg) strongly depends on the Er content, becomes maximum (89 pF) for x = 0.15, and then decreases (75 pF) for x = 0.20. This observation confirms the increase in dielectric constant of CFEO and is associated with the grain capacitance (Cg). The low-frequency semicircle, which contributes the grain boundary effect, might be absent due to the reduced grain size (nanoscale dimensions). While one complete semicircle indicates the contribution to the conduction process by a grain interior conduction mechanism, comparison of the present complex impedance spectroscopic data with the literature is not readily possible due to the fact that such studies on Er3+-substituted CFEO are not available at this time. Note that as Bulai et al. argued recently while the presence of the second ortho-ferrite phase at grain boundaries could lead to an increase of permittivity through interfacial polarization mechanisms, the air-filled regions have a negative contribution to the dielectric response.35 Therefore, controlled micro/nanostructure of the RE-ion-substituted cobalt ferrite is also an important factor that contributes to the overall dielectric and magnetic properties of the resulting materials.35,36 D. Alternating Current Conductivity (σac). The ac conductivity can be determined using the relation15

σac = 2πε0ε′f tan δ

⎡ −ΔEa ⎤ σdc = σ0 exp⎢ ⎥ ⎣ kBT ⎦

(6)

where σ0 is a pre-exponential factor, ΔEa is the activation energy, kB is Boltzmann’s constant, and T is the absolute temperature. While the increase in conductivity with temperature indicates the semiconducting behavior of the ceramics,

(5)

where f is the frequency of the applied field, ε0 is the permittivity of free space, ε′ is the real part of dielectric 5689


Article


Figure 8. Nyquist plots for Co1.1Fe1.9−xErxO4 (experimental and fitted data at 300 K).

Figure 10. Variation of dc resistivity (ln ρdc) with temperature (1000/ T) for Co1.1Fe1.9−xErxO4.

Figure 9. Variation of ac conductivity with frequency for Co1.1Fe1.9−xErxO4. 5690


Article


and chemistry of the ion under question. While x = 0.15 is the limit for Er+ ions into CFO as determined in this work for materials synthesized by the sol−gel autocombustion method, Y3+ ions can readily adopt to the structure and solubility up to x = 0.20.41 Formation of secondary phases was reported only for x = 0.25.41 On the other hand, Gd3+ ions exhibit the similar trend and behavior compared to Er when synthesized by the sol−gel autocombustion method.26 From the viewpoint of properties, it is obvious that the sol−gel autocombustion-based chemical method is able to produce materials with controlled chemistry and size, which results in property enhancement.26,27,39−41 For CFYO, enhancement in the infrared radiation properties are strongly evident, whereas the dielectric property enhancement is observed in Gd- or Er-substituted CFO. Thus, while enhanced property is clearly demonstrated for RE-ion incorporation into CFO using sol−gel autocombustion method, comparison of the present data with those reported for Y3+ and Gd3+ ions clearly indicates that the RE-ion chemistry and processing temperature are the two key factors that decide the structure and ultimate properties and phenomena. In fact, not only for RE ions but also sol−gel autocombustion method was also found to be quite attractive to control the chemistry of molybdenum ions (Mo3+ vs Mo6+ ions) so that the structural stability of CFO can be retained up to a larger amount of Mo, where x = 0.3, which is certainly higher compared to any other ions considered.42 Furthermore, Murugesan et al. recently examined and compared the properties of Gd-substituted CFO samples prepared by sol− gel autocombustion method and the standard solid-state ceramic method, respectively.43 It was reported that the frequency-dependent dielectric constant was much higher for sample prepared by sol−gel autocombustion compared to that of the ceramic method.43 Also, the measured values of saturation magnetization, coercivity, and remanent magnetization are 42 emu/g, 1553 Oe, and 18.5 emu/g for the autocombustion method and 66.7 emu/g, 379.6 Oe, and 17.3 emu/g for the ceramic method, respectively. The authors attribute the difference in preparation methods and size of the grains causes interesting improvement in electrical and magnetic properties.43 While a more detailed account of the RE-ion-substituted ferrites family at the nanoscale dimensions and implications for their property and performance tuning calls upon further investigations, the results presented in this work coupled with those reported in the literature undoubtedly suggest the promising aspects of the sol−gel autocombustion method to control over the “phase, chemistry, and size”, which are fundamental to the practical applications of either intrinsic or doped ferrites.

the activation energy can be calculated using the temperaturedependent electrical data at a given or constant frequency. The observed electrical behavior can be attributed to the increase of thermally activated drift mobility of charge carriers according to the hopping conduction mechanism. In ferrites, the electron hopping between Fe2+ and Fe3+ and hole hopping between Co3+ and Co2+ ions at the B site decides the electrical resistivity. It is found that ρ is minimum for x = 0.15 and 0.05, while it increases for x = 0.10 and 0.20. This may be due to the cation occupancy of Co2+, Fe3+, and Er3+ and their migration among the A and B sites in CFEO due to Er substitution. Upon Er substitution for Fe, a part of Er3+ ions will occupy the octahedral sites, displacing some Co2+ from the octahedral site to the tetrahedral site (B to A), which is accompanied by the opposite transfer of an equivalent number of Fe3+ ions from A to B sites in order to relax the strain.20 This increase in Fe3+ ions at the octahedral sites will increase the number of Fe2+ and Fe3+ pairs and hence increases the conductivity of CFEO. Therefore, ρ decreases for Er content, x ≤ 0.15. For x = 0.20, ρ increases slightly. Perhaps, the presence of Er ions at the B site increases the separation between Fe3+ to Fe2+ ions in proportion with their ionic radius. It hinders the electron transfer between Fe3+ and Fe2+ ions. Accordingly, ρ increases slightly for x = 0.20.20 A direct correlation between the dc resistivity, ac conductivity, and impedance spectroscopy data is well noted for CFEO ceramics. The activation energy values determined from the slope of log ρ versus 1000/T plots (Figure 10) are presented in Table 2. The increased values of activation energy with Er content suggest that more energy is required to promote charge transport in CFEO, which is a direct consequence of larger Er3+ion incorporation into the Co-rich CFO lattice.6,23 The activation energy values calculated from the slopes of Arrehenius-type resistivity plots for CFEO ceramics are in the range of 0.24−0.26 eV. A wide range of activation energy values are reported in the literature for intrinsic and ion-doped/ substituted ferrites. The typical range is 0.2−0.6 eV. However, it should be noted that the activation energy in these ferrite materials is very sensitive to the impurities and their crystal qualities.9,26,35−38 The low end of activation energies obtained in this work for Er3+ ion-substituted cobalt ferrite may be due to the fact that the charge transport is in the nanosized grains of CFEO. While comparison of the activation energy values for Er-substituted Co1.1Fe1.9O4 is not possible at this time with those obtained by other methods, similar results were reported for Gd3+-substituted CFO obtained by the sol−gel autocombustion method.26 Lower activation energy values and enhanced electrical conductivity of Gd3+ ion-substituted CFO is attributed to the cation distribution in the smaller size (nano) grains when synthesized by the sol−gel autocombustion method.26 We, therefore, believe that the size reduction as observed in XRD results coupled with Er concentration in CFO is responsible for the observed dc electrical parameters for CFEO. Finally, some general remarks can be made about the sol−gel autocombustion chemistry and its applicability to the rare-earth (RE) ion substitution in cobalt ferrites. We consider the available literature data and make use of such data for specific ions considered in the literature to shed some light on the underlying chemistry and physics of RE-ion-substituted CFO. The noteworthy point is the fact that the single-phase formation and the extent of RE-ion content for complete solubility into the spinel ferrite lattice depends on the ionic size

■

CONCLUSIONS Erbium-substituted cobalt-rich ferrites were synthesized with nanoscale dimensions, and we systematically investigated the influence of Er content on the structural, electrical, dielectric, and impedance characteristics. Results indicate that the spinel cubic structure formation can be possible only within the solubility limit of Er, x ≤ 0.10. Ortho-ferrite (ErFeO3) secondary phase nucleates at x ≥ 0.15. All samples exhibit nearly spherical, elongated nanogranular morphology with the stoichiometric chemical composition. The dielectric constant (ε′) exhibits the usual dielectric dispersion; ε′ enhancement is maximum for Er, x = 0.15. The observed dielectric data fits to the modified Debye’s function, indicating the contribution of more than one ion in the dielectric relaxation process. The 5691


Article

The Journal of Physical Chemistry C relaxation time and spread factor obtained from ε′ dispersion are ∼10−3 s and ∼0.50 (±0.10), respectively. Complex impedance analyses confirm the contribution from grain interior conduction. Variation of dc resistivity with temperature also correlates with the dielectric and impedance behavior of the samples. The results presented are helpful to understand the effect of Er3+on the transport properties of CFEO and provide a roadmap for utilizing these materials in magnetoelectric sensors and high-frequency electromagnetic devices.

■

synthesized by co-precipitation method. J. Alloys Compd. 2010, 492, 590−596. (13) Kahn, M. L.; Zhang, Z. J. Synthesis and magnetic properties of CoFe2O4 spinel ferrite nanoparticles doped with lanthanide ions. Appl. Phys. Lett. 2001, 78, 3651−3653. (14) Pandit, R.; Sharma, K. K.; Kaur, P.; Kumar, R. Cation distribution controlled dielectric, electrical and magnetic behavior of Ln3+ substituted cobalt ferrites synthesized via solid-state reaction technique. Mater. Chem. Phys. 2014, 148, 988−999. (15) Mahalakshmi, S.; Srinivasa, M. K.; Nithiyanantham, S. Electrical properties of nanophase ferrites doped with rare earth ions. J. Supercond. Novel Magn. 2014, 27, 2083−2088. (16) Dascalu, G.; Popescu, T.; Feder, M.; Caltun, O. F. Structural, electric and magnetic properties of CoFe1.8RE0.2O4 (RE = Dy,Gd,La) bulk materials. J. Magn. Magn. Mater. 2013, 333, 69−74. (17) Bharathi, K. K.; Ramana, C. V. Improved electrical and dielectric properties of La-doped Co ferrite. J. Mater. Res. 2011, 26, 584−591. (18) Alcantara, G. B.; Paterno, L. G.; Fonseca, F. J.; Pereira-da-Silva, M. A.; Morais, P. C.; Soler, M. A. G. Dielectric properties of cobalt ferrite nanoparticles in ultrathin nanocomposite films. Phys. Chem. Chem. Phys. 2013, 15, 19853−19861. (19) Pervaiz, E.; Gul, I. H. Structural, electrical and magnetic studies of Gd3+ doped cobalt ferrite nanoparticles. Int. J. Curr. Eng. Technol. 2012, 2, 377−387. (20) Kadam, A. A.; Shinde, S. S.; Yadav, S. P.; Patil, P. S.; Rajpure, K. Y. Structural, morphological, electrical and magnetic properties of Dy doped Ni−Co substitutional spinel ferrite. J. Magn. Magn. Mater. 2013, 329, 59−64. (21) Kolekar, Y. D.; Sanchez, L.; Rubio, E. J.; Ramana, C. V. Grain and grain boundary effects on the frequency and temperature dependent dielectric properties of cobalt ferrite−hafnium composites. Solid State Commun. 2014, 184, 34−39. (22) Naik, S. R.; Salker, A. V. Change in the magneto structural properties of rare earth doped cobalt ferrites relative to the magnetic anisotropy. J. Mater. Chem. 2012, 22, 2740−2750. (23) Prathapani, S.; Monaji, V. R.; Jayaraman, T. V.; Das, D. Structural and ambient/sub-ambient temperature magnetic properties of Er-substituted cobalt-ferrites synthesized by sol-gel assisted autocombustion method. J. Appl. Phys. 2014, 116, 023908 (1−9). (24) Jonker, G. H. Analysis of the semiconducting properties of cobalt ferrite. J. Phys. Chem. Solids 1959, 9, 165−175. (25) Kolekar, Y. D.; Sanchez, L. J.; Ramana, C. V. Dielectric relaxations and alternating current conductivity in manganese substituted cobalt ferrite. J. Appl. Phys. 2014, 115, 144106. (26) Murugesan, C.; Chandrasekaran, G. Dielectric relaxations and alternating current conductivity in manganese substituted cobalt ferrite. RSC Adv. 2015, 5, 73714−73725. (27) Shobana, M. K.; Nam, W.; Choe, H. Yttrium doped cobalt nanoferrites prepared by sol-gel combustion and their characterization. J. Nanosci. Nanotechnol. 2013, 13, 3535−3538. (28) Kambale, R. C.; Shaikh, P. A.; Bhosale, C. H.; Rajpure, K. Y.; Kolekar, Y. D. The effect of Mn substitution on the magnetic and dielectric properties of cobalt ferrite synthesized by an autocombustion route. Smart Mater. Struct. 2009, 18, 115028 (1−7). (29) Koops, C. G. On the dispersion of resistivity and dielectric constant of some semiconductors at audio-frequencies. Phys. Rev. 1951, 83, 121−124. (30) Bhavikatti, A. M.; Kulkarni, S.; Lagashetty, A. Evaluation of AC conductivity and dielectric behavior of cobalt ferrite. Int. J. Eng. Sci. Technol. 2014, 3, 5985−5991. (31) Kole, K. S.; Cole, R. H. Dispersion and absorption in dielectrics I: Alternating current characteristics. J. Chem. Phys. 1941, 9, 341−351. (32) Pervaiz, E.; Gul, I. H. Influence of rare earth Gd3+ on structural, gigahertz dielectric and magnetic studies of cobalt ferrite. J. Phys. Conf. Series 2013, 439, 012015. (33) Devan, R. S.; Kolekar, Y. D.; Chougule, B. K. Effect of cobalt substitution on the properties of nickel−copper ferrite. J. Phys.: Condens. Matter 2006, 18, 9809−9821.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1-9157478690. Fax: (915)747-8690. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Y.D.K. acknowledges the financial assistance from Department of Science and Technology (Government of India) (Grant No.: SR/FTP/PS-040/2010). S.G.K. thankfully acknowledges the University Grants Commission (UGC) for providing a Teacher Fellowship under XIIth Plan. C.V.R. acknowledges, with pleasure, the support from the National Science Foundation (NSF) with NSF-PREM grant no. DMR-1205302.

■

REFERENCES

(1) Spaldin, N. A.; Fiebig, M. The renaissance of magnetoelectric multiferroics. Science 2005, 309, 391−392. (2) Limaye, M. V.; Singh, S. B.; Date, S. K.; Kothari, D.; Reddy, V. R.; Gupta, A.; Sathe, V.; Choudhary, R.; Kulkarni, S. K. High coercivity of oleic acid capped CoFe2O4 nanoparticles at room temperature. J. Phys. Chem. B 2009, 113, 9070−9076. (3) Mazario, E.; Menéndez, N.; Herrasti, P.; Cañete, M.; Connord, V.; Carrey, J. Magnetic hyperthermia properties of electrosynthesized cobalt ferrite nanoparticles. J. Phys. Chem. C 2013, 117, 11405−11411. (4) Ramana, C. V.; Kolekar, Y. D.; Bharathi, K. K.; Sinha, B.; Ghosh, K. Correlation between structural, magnetic, and dielectric properties of manganese substituted cobalt ferrite. J. Appl. Phys. 2013, 114, 183907. (5) Kambale, R. C.; Song, K. M.; Koo, Y. S.; Hur, N. Low temperature synthesis of nanocrystalline Dy doped cobalt ferrite: Structural and magnetic properties. J. Appl. Phys. 2011, 110, 053910. (6) Prathapani, S.; Monaji, V. R.; Jayaraman, T. V.; Das, D. Structural and ambient/sub-ambient temperature magnetic properties of Ersubstituted cobalt-ferrites synthesized by sol-gel assisted autocombustion method. J. Appl. Phys. 2014, 116, 023908 (1−9). (7) Valenzuela, R. Novel application of soft ferrites. Phys. Res. Int. 2012, 2012, 1−9. (8) Mukherjee, D.; Hordagoda, M.; Hyde, R.; Bingham, N.; Srikanth, H.; Witanachchi, S.; Mukherjee, P. Nanocolumnar interfaces and enhanced magnetic coercivity in preferentially oriented cobalt Ferrite thin films grown using oblique-angle pulsed laser deposition. ACS Appl. Mater. Interfaces 2013, 5, 7450−7457. (9) Kamala Bharathi, K.; Markandeyulu, G.; Ramana, C. V. Structural, magnetic, electrical, and magnetoelectric properties of Sm- and Ho-substituted nickel ferrites. J. Phys. Chem. C 2011, 115, 554−560. (10) Mikalauskaitė, A.; Kondrotas, R.; Niaura, G.; Jagminas, A. Goldcoated cobalt ferrite nanoparticles via methionine-induced reduction. J. Phys. Chem. C 2015, 119, 17398−17407. (11) Monaji, V. R.; Das, D. Influence of Zr doping on the structural, magnetic and magnetoelastic properties of cobalt- ferrites. J. Alloys Compd. 2015, 634, 99−103. (12) Shaikh, P. A.; Kambale, R. C.; Rao, A. V.; Kolekar, Y. D. Structural, magnetic and electrical properties of Co−Ni−Mn ferrites 5692


Article

The Journal of Physical Chemistry C (34) Gangatharan, S.; Chidambaram, V.; Kandasamy, S. Synthesis, structural and dielectric studies of nickel substituted cobalt-zinc ferrite. Mater. Sci. Appl. 2010, 1, 19−24. (35) Bulai, G.; Diamandescu, L.; Dumitru, I.; Gurlui, S.; Feder, M.; Caltun, O. F. Effect of rare earth substitution in cobalt ferrite bulk materials. J. Magn. Magn. Mater. 2015, 390, 123−13. (36) Amiri, S.; Shokrollahi, H. Magnetic and structural properties of RE doped Co-ferrite (RE=Nd, Eu, and Gd) nano-particles synthesized by co-precipitation. J. Magn. Magn. Mater. 2013, 345, 18−23. (37) Rahman, Md. T.; Vargas, M.; Ramana, C. V. Structural characteristics, electrical conduction and dielectric properties of gadolinium substituted cobalt ferrite. J. Alloys Compd. 2014, 617, 547−562. (38) Hashim, M.; Alimuddin; Kumar, S.; Shirsath, S. E.; Mohammed, E. M.; Chung, H.; Kumar, R. Studies on the activation energy from ac conductivity measurements of rubber ferrite composites containing manganese zinc ferrite. Phys. B 2012, 407, 4097−4103. (39) Mott, N. F.; Davis, E. A. Electronic Processes in Non-Crystalline Materials, 2nd ed.; Clarendon: Oxford, U.K., 1979. (40) Vemuri, R. S.; Kamala Bharathi, K.; Gullapalli, S. K.; Ramana, C. V. The effect of size and structure on the electrical properties of nanocrystalline WO3 films. ACS Appl. Mater. Interfaces 2010, 2, 2623− 2628. (41) Wu, X.; Yu, H.; Dong, H. Enhanced infrared radiation properties of CoFe2O4 by doping with Y3+ via sol−gel autocombustion. Ceram. Int. 2014, 40, 12883−12889. (42) Mohamed, M. B.; Wahba, A. M.; Yehia, M. Structural and magnetic properties of CoFe2‑xMoxO4 nanocrystalline ferrites. Mater. Sci. Eng., B 2014, 190, 52−58. (43) Murugesan, C.; Perumal, M.; Chandrasekaran, G. Structural, dielectric and magnetic properties of cobalt ferrite prepared using auto combustion and ceramic route. Phys. B 2014, 448, 53−56.

5693


Dielectric, Complex Impedance, and Electrical Transport Properties of

Recommend Documents