Journal of Science: Advanced Materials and Devices (2018) 236e242 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Microstructural and magnetic properties of Ni1-xCuxFe2O4 (x ¼ 0.05, 0.1 and 0.15) nano-crystalline ferrites K.Vijaya Babu*, G.V Santosh Kumar, G Satyanarayana, B Sailaja, Ch C Sailaja Lakshmi Advanced Analytical Laboratory, Andhra University, Visakhapatnam 530 003, India a r t i c l e i n f o a b s t r a c t : Article history: Received 12 January 2018 Received in revised form 31 March 2018 Accepted April 2018 Available online 10 April 2018 An attempt was made to synthesize Ni1-xCuxFe2O4 (x ¼ 0.05, 0.1 and 0.15) nano-crystalline ferrites by sole gel auto-combustion method and study their micro-structural, magnetic and dielectric properties From the X-ray diffraction patterns, the lattice constants, average crystallite size, grain size of these compounds were calculated and compared to those of the nickel ferrite The cation distribution obtained from X-ray diffraction shows that the copper occupies only tetrahedral sites in spinel lattice The lattice constant of the ferrites under consideration increases with increasing the copper content The structural parameters such as bond lengths, tetrahedral and octahedral and octahedral edges were varied by copper substitutions The microstructural study was carried out by using SEM technique It implies that the average grain size increases with increasing the copper concentration Similar tendency was observed for the initial permeability (mi) It turns out from the electron spin resonance investigation that the g-factor decreased linearly with increasing magnetic fields as well as dopant concentrations © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Microstructure Initial permeability Spinel structure ESR Introduction Nano-crystalline spinel ferrites attracted special attentions in the field of electronic technology due to their wide applications ranging from microwave to radio wave frequencies Spinel ferrites are made up of a regular combination of oxygen with the general formula AB2O4 In ferrites, cations are distributed between tetrahedral and octahedral sites The most common type is cubic spinel ferrites, which contains tetrahedral (A site) and octahedral (B site) crystalline sites The magnetic and electrical properties of ferrites could be easily tuned by incorporation and suitable distribution of additional cations in the spinel structure The Nickel ferrite (NiFe2O4) has wide applications in various fields thanks to its high saturation magnetization, stability, resistivity and low loss energy over a wide range of frequency Copper is the nonmagnetic divalent metal ion and the Cu-doped nickel ferrites is of interest in fundamental as well as applied researches The substitution of copper in nickel ferrites enhances the magnetic ans dielectric properties,which are useful in many device applications [1e6] In addition, Ni-Cu ferrites are preferred because of their improved high * Corresponding author E-mail address: vijayababu.k@gmail.com (K.Vijaya Babu) Peer review under responsibility of Vietnam National University, Hanoi frequency response and high resistance [7e11] It has been reported by several research groups that, the magnetization in Ni-Cu ferrites decreases with increasing the Cu content and vice-versa Generally, the magnetization of spinel ferrites depends on the distribution of Fe3ỵ in the tetrahedral, octahedral sites and interactions between the cations It was also observed that the preparation technique plays an important role in changing the magnetic properties of spinel ferrites also In this case, a number of investigations on the effect of composition on the electromagnetic wave absorption properties was carried out [12e16] In the present article, we have employed the solegel auto combustion method to investigate the effect of the Cu2ỵ ion substitution on the structural, magnetic and physical properties of NiFe2O4 nano-crystalline ferrites Experimental The Ni1-xCuxFe2O4 (x ¼ 0.05, 0.1 and 0.15) nano-crystalline ferrites were synthesized by using solegel method AR chemicals such as nickel nitrate (Ni(NO3)2$6H2O), ferric nitrate (Fe(NO3)3$9H2O), copper nitrate Cu(NO3) and citric acid (C6H8O7) were used for the synthesis The molar metal nitrates to citric acid ratio were taken as 1:3 Ammonia solution was added to maintain the pH-7 A low pH value was needed for the precipitation of ingredients in the system and that can completely precipitate at a pH value of 7.0 The collected https://doi.org/10.1016/j.jsamd.2018.04.003 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) K.Vijaya Babu et al / Journal of Science: Advanced Materials and Devices (2018) 236e242 compounds were then dried in oven for h When ignited at any point of the gel, the dried gel burnt in a self-propagating combustion manner until all gels are completely burnt out to form a fluffy loose powder The burnt powder is then ground in an agate mortar to get fine ferrite powder The powder samples added with 10% polyvinyl alcohol (PVA) as a binder are ground and then pressed at tons/ pressure into a circular disk shaped pellet The synthesized powder and pellet is sintered at 1200 C for h and then used for further investigations of structural, morphological, magnetic and electrical properties The surface layers of the sintered pellet are carefully polished and washed in acetone and then the pellet is coated with silver paste on the opposite faces which act as electrodes The synthesized nanocrystalline Ni1-xCuxFe2O4 ferrites are characterized by standard techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), electron spin resonance (ESR) and LCR meter The XRD patterns are recorded at room temperature in the 2q range of 10 e70 using Cu-Ka radiation (l ¼ 1.5405 Å) The particle morphology of the powders is observed using scanning electron microscopy images taken from JEOL JSM6610L The magnetic properties are measured using JEOL-JESFA100 electron spin resonance (ESR) spectrometer with X-band at room temperature Initial permeability is determined by 10 turns of SWG enamelled copper wire on toroid's and inductance measurement is carried out at various frequencies using Wayne-Kerr Network Analyzer model 65120 in the frequency range 20 Hze8 MHz at room temperature Results and discussion planar spacing (d) values are calculated for the recorded peaks using Bragg's law and the lattice constant ‘a’ is calculated for each compound The lattice parameter of the synthesised samples is estimated using the relation: a ¼ dhkl q h2 ỵ k2 ỵ k2 The lattice constant ‘a’ increases from 8.3156 Å to 8.3259 Å by copper substitution The cell volume has also been observed to increase from 575 to 577 Å3 (see Table and Figure 2) The increase in lattice constant is attributed to the larger ionic radius of Cu2ỵ (0.73 ) than Ni2ỵ (0.69 ) Since larger cations (Cu2ỵ) replaced the smaller cations (Ni2ỵ), the increase in lattice constant is justied The bulk densities of all samples are found to be compared with Xray densities This indicates the existence of pores in nanocrystallites which are formed and developed during preparation process [17,18] The extrapolation function F(q) (called as NelsoneRiley function) for each reflection of the invesitigated samples is calculated based on FðqÞ ¼ The X-ray diffraction patterns of all the synthesised nanocrystalline ferrites are given in the Fig The X-ray diffraction patterns show the formation of single phase cubic spinel structure without any impurity peak The peaks could be indexed by comparing with the standard JCPDS card #74-2081 of nickel ferrite as (111), (220), (311), (222), (400), (422), (511) and (440) The inter- ! cos2 q cos2 q ỵ sin q q The lattice strain or micro strain induced inside the samples has been estimated using the WilliamsoneHall plot relation b cos q ¼ 3.1 X-ray diffraction 237 kl ỵ sin q D where D is the average crystallite size, k is a constant, l is the wave length, b is the full width at half maximum of diffraction peaks, ε is the strain induced inside the samples and q is the Bragg's angle The calculated values of crystallite size of (x ¼ 0.05, 0.1 and 0.15) ferrites are listed in Table It is observed that the crystallite size is increasing with copper concentration This increase is attributed to the difference in the ionic radii of copper and nickel ions [19,20] It Fig XRD patterns of Ni1-xCuxFe2O4 (x ¼ 0.05, 0.10 and 0.15) nanocrystalline ferrites 238 K.Vijaya Babu et al / Journal of Science: Advanced Materials and Devices (2018) 236e242 Table Lattice constant, cell volume, X-ray density, strain and crystallite size of Ni1-xCuxFe2O4 nanocrystalline ferrites x Lattice constant (Å) Volume (Å)3 X-ray density (g cmÀ3) Crystallite size (nm) F(q) Strain  10À6 0.05 0.10 8.3156 8.3234 8.3259 575.0241 576.6519 577.1691 5.4077 5.3192 5.4350 8.9 8.9 10.2 2.0314 2.0381 2.0218 0.6529 0.6503 0.6610 where rA is tetrahedral site radius rB is the octahedral site radius and ro is the radius of oxygen ion (1.32 Å) The ionic radius of the tetrahedral (rA) and octahedral (rB) sites are calculated by using the following relation rA calị ẳ jCNi rNi2ỵ ỵ CCu rCu2ỵ ỵ CFe rFe3ỵ j rB calị ẳ Fig Variation of lattice constant and unit cell volume with dopant concentration is also clear seen from the Table that the lattice parameter and the strain follow the same trend with crystallite size The hopping length (LA and LB) between magnetic ions (the distance between the ions) in the tetrahedral A-site and octahedral B-site can be calculated using following relation: LA ¼ pffiffiffi a LB ¼ pffiffiffi a The obtained values of hopping lengths are listed in Table The variation of the hopping length is increased as a function of cobalt substitution similar behaviour as that of lattice constant The result is explained on the basis of the variation of the lattice constant with dopant concentration rA ¼ rB ¼ In order to verify the cation distribution, the theoretical lattice parameter was calculated from following relation and compared with experimental data ath ẳ 2ỵ 3ỵ where r2ỵ Ni , rCu and rFe are the cationic radius of Ni, Cu and Fe ions taken from the work of Shannon [21] From the Table 3, it is clearly observed that the proposed cation distribution from X-ray intensity calculations is in close agreement with the real distribution It is shown that rA increases and rB decreases with increasing copper concentration The theoretical and experimental lattice parameter is maintaining the same increasing trend The difference found that the theoretical lattice constant values are found to be a little smaller than those of experimentally determined values This variation is due to the occupation of the copper ions to the A-site by replacing cobalt ions in B-site From the other hand, the mean radius of the ions at tetrahedral site and octahedral site is given by uÀ 3.2 Theoretical lattice constant p p rA ro ị ỵ 3rB þ ro Þ 3 Table Hopping length LA, LB, Tetrahedral bond (dAL), octahedral bond (dBL), tetra edge (dAE) and octahedral edge (dBE) (shared and unshared) of Ni1-xCuxFe2O4 (x ¼ 0.05, 0.10 and 0.15) nanocrystalline ferrites x-value LA (Å) LB (Å) dAL dBL dAE dBE dBEU 0.05 0.10 0.15 3.6007 3.6041 3.6052 2.9400 2.9427 2.9436 1.9878 1.9091 1.8309 1.9766 2.0209 2.0652 3.2460 3.1176 2.9899 2.6339 2.7678 2.8973 2.9479 2.9453 2.9438 jC r 2ỵ ỵ CCu rCu2ỵ ỵ CFe rFe3ỵ j Ni Ni p a À ro À u a À ro where rA represents radius of tetrahedral (A) site cation, rB represents radius of tetrahedral (B) site cation, u-oxygen positional parameter and ro represents radius of oxygen anions The oxygen parameter (u-value) is determine by using this formula ! 1 u ẳ rA ỵ ro ị p ỵ 3a In spinel structure, the oxygen positional parameter has a value in the neighbourhood of 0.375 Å for which the arrangement of O2À ions are equals exactly a cubic closed packing In our case, however, the u-value is varies from 0.3610 to 0.3616 Å, which is smaller than the ideal value (u ¼ 0.375 Å) It may be attributed to small displacement of anions from the ideal situation to form extended tetrahedral interstices [22,23] Table Ionic radii of tetrahedral A-site (rA), octahedral B-site (rB), theoretically lattice constant (ath) and oxygen positional parameter (u) of Ni1-xCuxFe2O4 x rA (Å) rB (Å) ath (Å) u-value (Å) Atomic weight (g/mole) 0.05 0.10 0.15 0.618 0.591 0.564 0.6875 0.7025 0.7175 8.3450 8.3494 8.3525 0.3880 0.3824 0.3769 234.8643 235.3496 235.8349 K.Vijaya Babu et al / Journal of Science: Advanced Materials and Devices (2018) 236e242 239 Table Cation distribution and intensity ratios of Ni1-xCuxFe2O4 (x ¼ 0.05, 0.10 and 0.15) x-value Cation distribution [Cu0.05Fe0.9][Ni0.95Fe1.1]O4 [Cu0.1Fe0.8][Ni0.9Fe1.2]O4 [Cu0.15Fe0.7][Ni0.85Fe1.3]O4 0.05 0.10 0.15 I(220)/I(400) I(422)/I(440) Cal Obs Cal Obs Cal 0.7053 0.9591 0.8232 0.7244 0.9638 0.8845 0.6214 0.5276 0.9071 0.6028 0.5056 0.9646 0.5964 0.5870 0.7993 0.5992 0.5476 0.8442 3.3 Interionic distances Using the experimental values of lattice parameter ‘a’ and oxygen positional parameter ‘u’, the interionic distances i.e., tetrahedral and octahedral bond length dAL and dBL, tetrahedral edge, shared and unshared octahedral edge (dAE, dBE and dBEU) are calculated by using the following equations and the values are listed in Table pffiffiffi dAL ¼ a 3ðu 0:25ị dBL r 11 43 uỵ ẳ a 3u2 64 p dAE ẳ a 22u 0:5ị dBE p ẳ a 21 2uị r 11 4u2 3u ỵ 16 dBEU ẳ a It is clear that the values of dAL, dBL, dAE, dBE, and dBEU increase with increasing cooper concentration This variation may be attributed to the substitution of copper and the cation distribution 3.4 Suggested cation distribution The investigations of cation distribution provide very useful information for development of materials with desired properties The cation distribution in spinel ferrites can be obtained from the analysis of X-ray diffraction pattern by the Bertaut method This method selects a few pairs of reflections according to the expression R¼ I(220)/I(440) Obs X hkl hkl À Ical Iobs Ihkl ¼ jFj2hkl pLp Lp ¼ þ cos2 2q sin2 q:cos q here F is structure factor, p is multiplicity factor, and Lp is Lorentzpolarization factor From the table, the results reveal that Ni2ỵ ions occupy B-sites whereas Co2ỵ ions occupy tetrahedral A-site Cu2ỵ ions preferably replace Co2ỵ from tetrahedral site The suggested cation distribution with intensities is shown in Table 3.5 FTIR Fig shows the FTIR spectra of the synthesized nanocrystalline Ni1-xCuxFe2O4 (x ¼ 0.05, 0.1 and 0.15) ferrites recorded in the range of 400e1300 cmÀ1 The two characteristic bands of spinel ferrites v1 and v2 are observed in the spectra The v1 band locates in the range 570e596 cmÀ1 whereas the v2 band are observed in the range of 400e450 cmÀ1 The band v1 is assigned to intrinsic stretching vibrations of the tetrahedral A-sites metal ion oxygen bonding corresponding to the highest restoring force and v2 to intrinsic vibrations of the octahedral B-sites metal ioneoxygen complexes which are bond bending vibrations The existence of the absorption bands v1 and v2 in FTIR spectra confirms that the synthesised samples have a single phase cubic spinel structure In the present case, it is observed that as Cu2ỵ content increases both v1 shifts to the lower frequency range This may be due to changes that occurred in cation distribution of the spinel lattice This can be explained as the entrance of Cu2ỵ ions (with larger ionic radius) into the tetrahedral lattice sites (A) of the lattice induces a partial migration of Fe3ỵ ions from A-site to B-site The values of v1 are higher than v2 This might be realated to the fact that the normal mode of vibration of the tetrahedral cluster is higher than that of the octahedral cluster Indeed, it can also be attributed to the shorter bond length of the A-site clusters than that of the B-site clusters [24] (see Table 2) hkl The best information on the cation distribution is achieved when comparing experimental and calculated intensity ratios for reflections whose intensities (i) Are nearly independent of the oxygen parameter (ii) Vary with the cation distribution in opposite ways and (iii) Do not differ significantly In the present work, the (220), (400) and (440) planes was taken to calculate the theoretical and experiential intensity ratios These reflections was assumed to be sensitive to the cation distribution To make the comparison, the intensity of the crystal plates was extracted from the diffraction patterns as the observed intensity and the theoretical intensity was calculated by Fig FTIR spectra of Ni1-xCuxFe2O4 (x ¼ 0.05, 0.10 and 0.15) nanocrystalline ferrites 240 K.Vijaya Babu et al / Journal of Science: Advanced Materials and Devices (2018) 236e242 3.6 SEM SEM microstructure of nanocrystalline Ni1-xCuxFe2O4 (x ¼ 0.05, 0.1 and 0.15) ferrites is shown in Fig 4(aed) It can be seen that the grain size and shape are significantly affected by the copper substitution The particle's shape is similarly spherical for the sample with x ¼ 0.05 The deformation, however, is developed with increasing the copper concentration in the synthesized ferrites The average particle sizes of the ferrites are in the range of 4e6 mm, which is quite larger than that of the crystallite size determined by XRD This implies that all particles are formed by a number of crystallites or grains [25e27] The chemical compositions are determined by energy dispersive spectra (EDS) for the samples after final sintering The obtained results show the presence of the elements (Ni, Cu, Fe and O) without impurities In addition, the observed quantitative results indicate that the copper concentration increases in the samples as expected based on synthesis method 3.7 Initial permeability The initial permeability (mi) of the toroid ferrites is determined by using the formula: mi ¼ h L 0:0046N h log dd2 i Fig SEM and EDS images of Ni1-xCuxFe2O4 a) x ¼ 0.05, b) x ¼ 0.10 and c) x ¼ 0.15 nanocrystalline ferrites K.Vijaya Babu et al / Journal of Science: Advanced Materials and Devices (2018) 236e242 Fig Variation of initial permeability with frequency of Ni1-xCuxFe2O4 (x ¼ 0.05, 0.10 and 0.15) nanocrystalline ferrites where L - inductance in mH, N - number of turns, d1 - inner diameter in cm, d2 - outer diameter in cm and h - height of the core in cm The permeability is intrinsic property of material, which charaterizes the ability to induce the magnetic field in materials Fig plots the initial permeability as a function of frequency for Ni1-xCuxFe2O4 (x ¼ 0.05, 0.1 and 0.15) nanocrystalline ferrites The initial permeability (mi) increases with increasing copper concentration This increase can be attributed to the enhancement of the magnetization It is well known that permeability in ferrites affected by the preparation method and impurities presented in chemical compositions They in turn affect the real stoichiometry, grain size, saturation magnetization and magnetostriction The high frequency dispersion and absorption in initial permeability is attributed to the rotational resonance in the combined anisotropy and demagnetizing fields while low frequency dispersion is related to the domain wall displacement [28,29] 3.8 ESR ESR is a technique of observing resonance absorption of microwave power by unpaired electron spins aligned with magnetic fields The spin angular momentum S gives rise to a magnetic moment m ¼ À g mB S, where g is the g-factor, its value for free electrons is g ¼ 2.0023 and mB is the Bohr magneton When the electrons are subjected to an external field H (it is customary to place the field along the z axis), the energy levels of the generate spin states split depending on their quantum magnetic moment ms ¼ ±1=2 and the strength of the magnetic field The g-value is one of the most important concepts in ESR studies Usually peaks in ESR (which have an alternating current shape) are described using gvalues, which are a measure of the magnetic fields at which they occur The room temperature ESR spectrum of investigated Ni1-x CuxFe2O4 nanocrystalline ferrites are shown in Fig The spectra exhibit a single broad signal indicating the presence of Fe3ỵ, Ni2ỵ and Cu2ỵ ions with g ẳ 2.00 The broadness of the ESR resonance field is related to the random orientation of ferromagnetic particles in low nickel content samples, which scatter in directions of the anisotropic field of the nanoparticles The variation is possible only by the strengthening dipolar interactions among cations through oxygen [30,31] The obtained g-value and resonance field of the Ni1-x CuxFe2O4 (x ¼ 0.05, 0.1 and 0.15) nanocrystalline ferrites are listed in Table Note that these data decrease with increasing the Cu concentration 241 Fig ESR spectra for Ni1-xCuxFe2O4 (x ¼ 0.05, 0.10 and 0.15) nanocrystalline ferrites Table The g-value and resonance field for Ni1-xCuxFe2O4 (x ¼ 0.05, 0.10 and 0.15) nanocrystalline ferrites x-value g-value Resonance field (mT) 0.05 0.10 0.15 2.0717 2.0025 1.9856 398.705 366.161 364.144 Conclusion The copper substituted NiFe2O4 nanocrystalline ferrite, synthesized by the solegel auto combustion method, is a promising material for high frequency applications Their structural parameters such as bond lengths, tetrahedral and octahedral edges were found to be varied by the copper substitution X-ray diffraction data, cation distribution and IR bands reveal that the Cu2ỵ ions have the stronger 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[Cu0 .15 Fe0.7][Ni0.85Fe1.3]O4 0. 05 0. 10 0 .15 I(2 20) /I( 400 ) I(422)/I(4 40) Cal Obs Cal Obs Cal 0. 705 3 0. 95 91 0. 8232 0. 7244 0. 9638 0. 8845 0. 6 214 0. 5276 0. 907 1 0. 602 8 0. 505 6 0. 9646 0. 5964 0. 58 70 0.7993... (dAE) and octahedral edge (dBE) (shared and unshared) of Ni1- xCuxFe2O4 (x ¼ 0. 05, 0. 10 and 0. 15 ) nanocrystalline ferrites x- value LA (Å) LB (Å) dAL dBL dAE dBE dBEU 0. 05 0. 10 0 .15 3. 600 7 3. 604 1 3. 605 2... for Ni1- xCuxFe2O4 (x ¼ 0. 05, 0. 10 and 0. 15 ) nanocrystalline ferrites Table The g-value and resonance field for Ni1- xCuxFe2O4 (x ¼ 0. 05, 0. 10 and 0. 15 ) nanocrystalline ferrites x- value g-value Resonance