Ferrimagnetic Ni2+ doped Mg-Zn spinel ferrite ...

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Jan 30, 2017 - Email address: [email protected] ... volume ratio show noticeable novel properties in comparison to their bulk counterpart [2].
Accepted Manuscript 2+ Ferrimagnetic Ni doped Mg-Zn spinel ferrite nanoparticles for high density information storage Rohit Sharma, Prashant Thakur, Pankaj Sharma, Vineet Sharma PII:

S0925-8388(17)30429-2

DOI:

10.1016/j.jallcom.2017.02.021

Reference:

JALCOM 40749

To appear in:

Journal of Alloys and Compounds

Received Date: 7 November 2016 Revised Date:

30 January 2017

Accepted Date: 3 February 2017

2+ Please cite this article as: R. Sharma, P. Thakur, P. Sharma, V. Sharma, Ferrimagnetic Ni doped MgZn spinel ferrite nanoparticles for high density information storage, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.02.021. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Ferrimagnetic Ni2+ doped Mg-Zn spinel ferrite nanoparticles for high density information storage Rohit Sharma, Prashant Thakur, Pankaj Sharma*, Vineet Sharma Nanotechnology Laboratory, Department of Physics & Materials Science, Jaypee University

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of Information Technology, Waknaghat, India-173234

Abstract

In the present paper, Ni2+ doped Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) ferrite samples prepared by co-precipitation route have been characterized by XRD, FT-IR,

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FE-SEM, VSM, UV Visible and PL spectroscopy for various physical properties. A transition from superparamagnetic behaviour (x = 0) to soft ferrimagnetic nature (x > 0) with Ni2+

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addition has been reported. With reduction in crystallite size, ferrite nano-particles have shown an increase in coercivity with lower saturation magnetization. An increase in remanence ratio with Ni2+content has been observed. The optical band gap of the prepared samples has been calculated using Tauc plots and have been found to increase with increase in Ni2+content from 4.50 eV (x = 0) to 5.60 eV (x = 0.500). The PL spectra show band edge emission at lower energy than optical band gap energy for all samples. A cation distribution

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has been proposed and based on this some theoretical parameters have been calculated which supports our experimental results. Modest values of coercivity and saturation magnetization indicates that these materials may be used in high density information storage devices. Keywords: Spinel ferrites; Ferrimagnetism; Structural properties; Optical properties.

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Email address: [email protected]

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ACCEPTED MANUSCRIPT 1. Introduction Spinel ferrite nano-particles are of significant interest due to relationship between their magnetic properties and crystal chemistry [1]. Nano-particles with large surface to volume ratio show noticeable novel properties in comparison to their bulk counterpart [2]. With reduction in grain size, ferrite nano-particles show different properties such as higher

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coercivity and lower saturation magnetization [3,4]. The position or occupancy of metal ions (cations) in the lattice sites decides the crystallization of spinel ferrites as normal, inverse or partially inverse spinel structure [5]. At nano level redistribution from inverse spinel to mix or partially inverse spinel structure results in enhancement of magnetization as compared to

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its bulk counterpart [6]. The distinctive properties of ferrites depend upon distribution of metal cations in lattice sites [2]. So the control over distribution of cations in ferrites is an important way to modify their properties. In addition, at nano-scale, particle size is an

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instrumental parameter in tailoring properties of ferrites. The variation in magnetic behavior of cobalt ferrite nano-particles due to change in particle size of nano-particles have been reported [7].

Nano-particles of spinel ferrites are technologically important due to their applications in various fields such as high density magnetic storage, microwave absorption, high frequency

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transformer cores, magnetic resonance imaging, electronic devices, catalysis, ferro fluids, electronic inductors, electromagnets, power transformers, magnetically tunable filters and oscillators [2,7-10].

Among spinel ferrites, Mg-Zn ferrites are one of the vital magnetic oxides due their high

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Curie temperature, low cost, high electrical resistivity and better environmental stability [11]. Commercially, Mg-Zn ferrites are known as Ferrocube-Z [12]. These are used as core

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material in different applications, where there is a requirement of effective coupling between electric current and magnetic flux [12]. Mg-Zn ferrites have been used in microwave devices, transformer cores, high definition TV deflection yokes, telecommunication and in many more devices of technological importance [13]. Soft magnetic behavior and low losses in Mg-Zn ferrites make them useful as core material in transformers and television deflection yokes [11,13]. The diamagnetic Zn2+ ions in tetrahedral sites reduce the A-B exchange interaction [14]. The substitution of any magnetic ion by replacing Zn2+ in Mg-Zn ferrites plays an important role in modifying its properties. Ni2+ is soft magnetic ion with magnetic moment of 2 µB and strong preference for octahedral sites [15]. Padmapriya et al. have reported ferromagnetic behavior and increase in coercivity with Ni2+ doping on replacing Zn2+ in Zn2

ACCEPTED MANUSCRIPT ferrites [16]. Jalaiah et al. [17] and Topkaya et al. [18] have reported increase in coercivity with Ni2+ substitution in Mn-Zn and Mn ferrites respectively. There are reports for doping of Ni2+ in various ferrite systems, where it leads to variation in structural, magnetic and dielectric properties [16,17]. The nickel doped ferrites have shown promise in memory storage devices [19,20].

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Ni2+ doped zinc ferrites are also reported to show increased photo-catalytic activity [16]. In addition to showing more photo-catalytic activity, Ni2+ doped ferrites have also shown application in gas sensing [21]. Ni2+ based ferrites nano-particles have also shown their usefulness in hyperthermia applications [22,23]. To the best of our knowledge, Ni2+

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doping in Mg-Zn ferrites have been scarcely reported. The substitution of Ni2+ in Mg-Zn ferrites may play an important role in tuning various properties of Mg-Zn ferrites. The change in temperature and route of synthesis plays an important role in deciding cation distribution

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and several physical properties of ferrites [24-26]. In order to synthesize spinel nano ferrites, various methods have been used such as combustion, co-precipitation, citrate, sol gel, hydrothermal etc. [5,8,26-28]. Among these, co-precipitation has been regarded as appropriate and flexible technique, due to its high homogeneity and purity [29]. The availability of different parameters in co-precipitation route to control size and size

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distribution make it a preferable route of synthesis [30]. In present work co-precipitation route has been used for the synthesis of Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) ferrite samples. The synthesized samples have been characterized for structural, morphological, magnetic, and optical properties.

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2. Experimental details 2.1 Synthesis

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Magnesium (II) chloride hexahydrate (MgCl2.6H2O, ≥ 98 %), Zinc (II) chloride (ZnCl2, ≥ 98%) and Iron (III) chloride hexahydrate (FeCl3.6H2O, ≥ 99%) metal salts procured from Merck India are of AR grade and Nickel (II) chloride hexahydrate (NiCl2.6H2O, 98 %) purchased from Alfa Aesar have been used for synthesis as received. The aqueous solution has been prepared by taking (MgCl2.6H2O, ZnCl2, NiCl2.6H2O) and (FeCl3.6H2O) in 1:2 molar ratio in 100 ml double distilled water. The aqueous solution of metal salts has been stirred constantly, until clear solution is obtained. The clear aqueous solution of metal salts have been mixed with precipitating agent NaOH and then subjected to constant stirring at 353 K for 45 minutes. During the reaction process, the pH level has been kept between 11 to 12. After stirring the solution, the precipitates have been obtained. The precipitates have been collected and the excess water on the top has been discarded. The precipitates have been 3

ACCEPTED MANUSCRIPT washed many times with double distilled water. The washed precipitates have been dried in an oven at 373 K for 12 hrs and further crushed with granite mortar pestle into powdered form. Finally, the crushed powder has been sintered at 1173 K in an automatic muffle furnace for 3 hrs. Fig. 1 shows schematic diagram for the preparation of Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) samples.

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2.2 Characterization The crystal structure and phase identification of samples have been carried out by the means of X-ray diffraction at room temperature using Shimadzu 6000 diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). Data has been acquired at scan speed of 2

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degree/minute, over the range 10o-80o. The diffractometer has been operated at 40 kV and 30 mA. For the confirmation of vibrational bands of spinel structure in synthesized samples, FTIR spectra have been recorded in the range 400-1000 cm-1 using Perkin Elmer

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spectrophotometer model-65, by mixing powder sample with solid KBr. Microstructural, surface morphological and elemental observations have been carried using Field Emission Scanning Electron Microscope, (FE-SEM, Quanta 200 FEG) and energy dispersive spectroscopy (EDS, Oxford Instruments) respectively. PAR-155 vibrating sample magnetometer (VSM) has been utilized for magnetic measurements. The magnetic

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measurements have been carried from -10 kOe to + 10 kOe at room temperature. Perkin Lambda 750 spectrophotometer has been utilized for UV-Visible absorption studies in the range 200 nm to 400 nm. For photoluminescence studies, Perkin LS-55 spectrophotometer has been employed to record PL spectrum in the range 250 nm to 425 nm at an excitation

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wavelength of 220 nm at room temperature.

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3. Results and discussion 3.1 X-ray diffraction

Fig. 2 shows the X-ray diffraction patterns of Ni2+ doped Mg0.5Zn0.5-xNixFe2O4 (x = 0,

0.125, 0.250, 0.375, 0.500) samples. Peaks obtained in X-ray diffraction patterns have been indexed as (111), (220), (620), (311), (400), (211), (422), (511), (440). The peaks indexed as (111), (220), (620), (311), (400), (422), (511), (440) corresponds to characteristic planes of single phase cubical spinel structure [30,31]. For higher nickel concentration (x > 0.125), the plane indexed as (211) emerge, indicating the presence of magnesium oxide [32]. At higher Ni2+ content (x > 0.125) in Mg-Zn ferrite the presence of magnesium oxide indicates the conversion of magnesium hydroxide into magnesium oxide [33] and lesser solubility of magnesium with nickel metal ions. The crystallite size (D) has been calculated using line 4

ACCEPTED MANUSCRIPT broadening of most intense (311) peak with the help of Scherrer’s equation ;

D = 0.9λ / β cosθ , where D is the crystallite size, β is the full width half maximum, λ is the X-ray wavelength and θ is the Bragg’s angle. The crystallite size decreases from 59.40 nm to 43.58 nm with increase in Ni2+ content (Table 1). The decrease in crystallite size is due the replacement of Zn2+ with Ni2+, as Ni2+ has smaller ionic radius (0.69 Å) than Zn2+ (0.82 Å)

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[34,35]. Manikandan et al. have also reported similar behavior of crystallite size variation with smaller ion doping [36]. The XRD peaks show a shift in prominent peaks towards higher angle with increase in Ni2+ content which may be attributed to the smaller Ni2+ metal ions replacing the larger Zn2+ metal ions [37]. The most prominent peak (311) shifts from 2θ =

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35.37 for x = 0 to 2θ = 35.67 for x = 0.500 (see inset in Fig. 2). The d-spacing (dhkl) and lattice constant (a) have been calculated using most intense reflection (311) from equation

a = dh kl (h2 + k 2 + l 2 )1/ 2 , where h, k ,l are the miller indices corresponding to

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[38];

respective planes. The lattice constant and inter-planar spacing have been found to decrease with an increase in Ni2+ concentration (Table 1). This is attributed to the difference in ionic radii of Ni2+ (0.69 Å) and Zn2+ (0.82 Å) metal ions. Packing factor (p) has been calculated using the equation [39]; p = D / d . Packing factor decreases with increase in Ni2+ concentration (Table1). The variation in packing factor shows dependence on crystallite size.

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The sample x = 0.500, has minimum crystallite size with least packing factor. The uniform strain (εu), for the prepared samples have been calculated using the relation [40];

ε u = c(d f − di ) / di = c∆d / di , where c is material dependent constant and ∆d is change in

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inter-planar spacing between pure i.e. un-doped (di) and Ni2+ doped (df) samples. The strain value increase from -3.233 × 10-3 for x = 0.125 to -8.710 × 10-3 for x = 0.500 (Table1). The

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presence of compressive strain has been observed for all Ni2+ doped samples. Similar compressive strain has also been reported in literature [40]. It has been observed that the strain increases with decreases in crystallite size. The decrease in crystallinity with reduction in crystallite size may be the major reason for increase in strain values. 3.2 Cation distribution The cation distribution has been proposed using the X-ray diffraction results when agreement factor (R) has minimum value. Agreement factor is the difference between ratio of intensity obtained for observed diffraction pairs and calculated diffraction pairs. The reflections indexed as (220), (440) and (400) have been used for calculation of agreement factor as these reflections have been considered most sensitive towards cation distribution 5

ACCEPTED MANUSCRIPT [41,42]. Fig. 3 shows variation of intensity ratios with nickel content for 220, 440 and 400 reflections. Mg2+ and Ni2+ ions prefer to occupy octahedral (B) sites while Zn2+ ions prefer to occupy tetrahedral (A) sites in Mg0.5Zn0.5-xNixFe2O4 [43] The Fe3+ ions have preference for both tetrahedral and octahedral sites [43]. After analyzing the X-ray diffraction results and taking preference of metal ions for lattice sites into account, the most suitable cation

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distribution has been obtained and given in Table 2. According to cation distribution, the mean ionic radii rA of tetrahedral sites (A) and rB of octahedral sites (B) have been calculated using relations [35];

rA = [CMg2+ .rMg2+ + CZn2+ .rZn2+ + CNi2+ .rNi2+ + CFe3+ .rFe3+ ]

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rB = 0.5 × [CMg2+ .rMg2+ + CZn2+ .rZn2+ + CNi2+ .rNi2+ + CFe3+ .rFe3+ ]

(1) (2)

where C and r are ionic concentration and ionic radii respectively of metal cations. Mean

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ionic radii of tetrahedral sites decrease with increase in doping of Ni2+, as Ni2+ doping replaces larger Zn2+ ions from tetrahedral sites with smaller Fe3+ (0.67 Å) ions (Table 3) . The mean ionic radii of octahedral sites increase with increase in Ni2+ doping (Table3 ). It is due the incorporation of larger Ni2+ and migration of smaller Fe3+ ions towards tetrahedral sites.

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The theoretical lattice parameter relates the ionic radii of different metal ions situated at A and B crystallographic lattice sites and can be calculated using the relation [35];

ath = (8 / 3 3)[(rA + Ro ) + 3(rB + Ro )]

(3)

where Ro is the oxygen anion radii (Ro = 1.32Å). The theoretical lattice parameter decreases (0.69Å). Both, theoretical and experimental lattice parameters are in good agreement.

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Ni2+

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with increase in Ni2+ content (Table 3), which is due to the replacement of Zn2+ (0.82Å) by

The little difference in their values may be on an account of assumption of regular arrangement of cations and anions in an ideal structure of unit cell considered for theoretical calculations [44]. The tetrahedral sites are small to accommodate metal ions and to adjust them oxygen ions move slightly. This movement of oxygen ions is termed as oxygen positional parameter. Along with this, the tetrahedral sites enlarge to a magnitude equivalent to contraction of octahedral sites, which is due to the movement of oxygen ions [43]. The oxygen positional parameter has been calculated using [35], u = (1/ ath 3)(rA + Ro ) + 1/ 4 . Oxygen positional parameter decreases with increase in Ni2+ concentration (Table 3). The substitution of Ni2+ migrates smaller Fe3+ metal ions from octahedral sites to tetrahedral sites, 6

ACCEPTED MANUSCRIPT which reduces expansion of tetrahedral sites with decrease in oxygen positional parameter (Table 3). The bond lengths at tetrahedral sites (RA) and at octahedral sites (RB) have been calculated

using

equations

1 δ 2 1/ 2 RA = a 3(δ +1/ 8) , RB =a( − +3δ ) where 16 2

[45];

δ = u − uideal ,  is the inversion parameter, which signifies departure from ideal oxygen

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parameter (uideal = 0.375 Å) and a is the experimental lattice parameter. The inversion parameter decreases with increase in Ni2+ content (Table 3). The bond length RA decreases with increase in Ni2+ content (Table 3). The substitution of Ni2+ migrates smaller Fe3+ from octahedral sites to tetrahedral sites and reduces mean ionic radii of tetrahedral sites. The

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decrease in mean ionic radii of tetrahedral sites reduces the bond length RA with increase in Ni2+ content. The bond length RB shows non-linear behaviour with increase in Ni2+ content; it

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may be attributed to the presence of metal ions (Mg2+, Ni2+ and Fe3+) of different ionic radii at octahedral sites. The tetrahedral edge length R, shared octahedral edge length R' and unshared octahedral edge length R" have been calculated using equations [35];

R = a 2(2u − 0.5)

(4)

R' = a 2(1 − 2u)

(5)

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R' ' = a 4u 2 − 3u +11/ 16

(6)

The variation in values of R, R', R" depend on the distribution of metal ions in tetrahedral and octahedral lattice sites [46]. The decrease in the R (Table 4) is due to reduction in mean ionic

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radii of tetrahedral sites (rA) with increase in Ni2+ content. The increase in octahedral ionic radii (rB) with Ni2+ substitution is responsible for increasing R' and decreasing R" (Table 4).

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Magnetic properties of spinel ferrites depend on the exchange interactions which further depend on the inter-ionic length and bond angles between metal ions. The strength of magnetic exchange interactions have direct dependence on bond angles and inverse dependence on inter-ionic lengths [43]. The inter-ionic lengths (cation-anion and cationcation distances) have been calculated using relations [47];

p = a{(5 / 8) − u} ; q = a(u −1/ 4) 3 ; r = a(u −1/ 4) 11 and s = a{(1/ 3u) + (1/ 8)} 3 for cation-anion distances,

b = (a / 4) 2

;

c = (a / 8) 11

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;

d = (a / 4) 3

;

e = (3a / 8) 3 and

ACCEPTED MANUSCRIPT f = (a / 4) 6 for cation–cation distances. The values of these inter-ionic lengths are given in

Table 4. The bond angles between metal ions have been calculated using values of cation-cation and cation-anion distances using equations [48];

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θ1 = cos−1[( p 2 + q 2 − c 2 ) / 2 pq] ; θ2 = cos−1[(p 2 + r 2 − e2 ) / 2 pr] ; θ3 = cos−1[(2p2 −b2 ) / 2p2 ]; θ4 = cos−1[( p 2 + s 2 − f 2 ) / 2 ps] and θ5 = cos−1[(r 2 + q2 − d 2 ) / 2rq]

The theoretical calculations of bond angles indicate increase in θ1, θ2 and θ5 while θ3 and θ4 decrease with increase in Ni2+ substitution (Table 5). The bond angles θ1, θ2 and θ5 are

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associated with A-B and A-A exchange interactions while θ3 and θ4 are related with B-B exchange interaction [48]. The increase in bond angles θ1, θ2 and θ5 with Ni2+ substitution

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signifies strengthening of A-B and A-A exchange interactions. The substitution of magnetic Ni2+ ions by replacing diamagnetic Zn2+ ions create anti-parallel arrangement of magnetic metal ions at A and B lattice sites and enhances A-B exchange interaction. The substitution of Ni2+ ions migrate Fe3+ ions from octahedral sites to tetrahedral sites and strengthen A-A exchange interaction. The decrease in value of bond angles θ3 and θ4 with Ni2+ substitution indicates weakening of B-B exchange interaction. The substitution of Ni2+ ions (2µB) migrate

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the Fe3+ ions (5µB) from octahedral sites to tetrahedral sites, which lower the overall magnetic moment at B sites and causes weakening of B-B exchange interaction. 3.3 FE-SEM and EDS

Fig. 4(a-e) shows FE-SEM micrographs of Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250,

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0.375, 0.500) ferrite samples. FE-SEM micrographs of all samples show non-uniform grain growth. The nano-particles of varying sizes and different shapes have been observed from

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FE-SEM micrographs. With increase in Ni2+ doping the agglomeration of nano-particles increases and is appreciable for x > 0.250. All prepared samples have been sintered at same temperature (1173 K), so the presence of permanent magnetic dipole moment which increases with Ni2+ content is the dominant factor which leads to agglomeration of nano-particles [46]. EDS spectra of pure and Ni2+ doped Mg-Zn ferrites are shown in Fig. 5 (a-e), it confirms the presence of Mg, Zn, Fe, Ni and O elements. In ferrites, the standard metal cation to anion ratio is 3:4. The observed atomic % of metal cations and anions (Table 6 ) confirm that the metal cation to anion ratio lies close to the standard ratio. 3.4 FT-IR studies

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ACCEPTED MANUSCRIPT Fourier transform infrared spectroscopy is an important spectroscopic technique to get information about the position of the metal ions in the crystal lattice through the presence of different vibrational modes in the crystal lattice. Fig. 6 shows FT-IR spectra of Ni2+ doped Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) ferrites from 400 cm-1 to 1000 cm -1. There is presence of prominent bands around 400 cm-1 and 600 cm-1. It has been reported by

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Waldron that the frequency band υ1 (600 cm-1) corresponds to intrinsic vibration of tetrahedral sites and frequency band υ2 (400 cm-1) corresponds to intrinsic vibration of octahedral sites [49]. So the presence of frequency bands in the range 434.23 cm-1 to 490.38 cm-1 and 522 cm-1 to 603.11cm-1 confirm the formation of spinel ferrites. The difference in Fe3+-O2- distances for tetrahedral and octahedral sites is responsible for the different positions

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of tetrahedral and octahedral frequency bands [50]. With increase in Ni2+ doping the position of most prominent frequency bands, corresponding to tetrahedral sites (Fig. 6) shifts towards

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the higher wave-number. The shift is due to Ni2+ substitution which decreases the bond length of tetrahedral sites (Table 3) and frequency of vibration varies inversely with bond length. The frequency bands corresponding to octahedral sites shifts non-linearly with increase in Ni2+content, with additional absorption band at 436.06 cm-1 for x = 0.250 and at 434.23 cm-1 for x = 0.500 which also corresponds to octahedral site vibration. The presence of

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Mg2+, Ni2+ and Fe3+ metal ions of different ionic radii at octahedral sites can be attributed to these additional absorption bands at x = 0.250 and x = 0.500. There are some other reasons which may influence the position of frequency bands such as bond length, grain size, synthesis route etc.[19].

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3.5 Magnetic properties

Fig. 7 represents room temperature M-H curves for Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) samples. With Ni2+ doping samples show a transition from superparamagnetic

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to soft ferrimagnetic behaviour. The presence of diamagnetic Mg2+ and Zn2+ metal ions, which have zero electron spin along with no contribution to magneto-crystalline anisotropy, leads to super-paramagnetic behaviour for x = 0 sample [51]. The super-paramagnetic nature for x = 0 sample indicates that the magneto-crystalline anisotropy energy has been overcome by thermal energy and show super-paramagnetic nature [47]. The replacement of diamagnetic Zn2+ by magnetic Ni2+ ions, having magnetic moment of 2µB introduces ferrimagnetic nature. The substitution of soft magnetic Ni2+ ions enhance magneto-crystalline anisotropy and samples become able to sustain magnetic ordering and show soft ferrimagnetic behavior. Similar soft magnetic behavior in nickel doped ferrites has also been reported in literature [16-18]. 9

ACCEPTED MANUSCRIPT The saturation magnetization (Ms), coercivity (Hc) and retentivity (Mr) determined from M-H curves have been presented in Table 7. According to Neels’ two sub-lattice model of ferrimagnetism, the A-B exchange interaction is dominant over A-A and B-B exchange interactions. The magnetic moment of metal ions at A and B sub-lattices are anti-parallel to each other and have a collinear spin arrangement. So the net magnetic moment per formula

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unit is calculated using [52];

Mcal. = M B − M A

(7)

where Mcal. is net calculated magnetic moment and MA and MB are the magnetic moment at the tetrahedral (A) and octahedral (B) sites.

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The magnetic moment (MCal.) per formula unit has been calculated according to the proposed cation distribution (Table 2) and using magnetic moment values of Mg2+ (0 µB), Zn2+(0 µB),

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Fe3+ (5 µB) and Ni2+ (2 µB) metal ions positioned at A and B lattice sites [43,52]. For x = 0, the Mcal.= 1.5 × 5 µB – 0.5 × 5 µB = 5 µB/ formula unit, where 1.5 × 5 µB = MB, 0.5 × 5 µB = MA and 0.5 and 1.5 are molar ratio of Fe3+ ions situated at A sites and B sites respectively. Similarly for other compositions i.e. 0.125, 0.250, 0.375 and 0.500, the magnetic moment have been calculated (Table 7). The increase in the Ni2+ content decreases the calculated magnetic moment. The calculated magnetic moment decreases due to preference of Ni2+

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metal ions for octahedral sites and their substitution migrates Fe3+ metal ions from octahedral sites to tetrahedral sites. This increases the magnetic moment at tetrahedral sites and simultaneously decreases the magnetic moment at octahedral sites. The observed magnetic moment (ηB) has been calculated using [42,51]; η B = ( M w × M s ) / 5585 where Mw is the

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molecular weight of the sample and Ms is saturation magnetization. The magnetic moment values are reported in Table 7. The observed and calculated magnetic moment decrease with

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Ni2+ content (x ≥ 0.125). The saturation magnetization decreases (Table 7) with increase in Ni2+ content (For x ≥ 0.125). Saturation magnetization decreases for smaller crystallites and may be attributed to the surface distortions due to the interaction of transition metal ions with oxygen atoms of the spinel lattice. This reduces the net magnetic moment. The disorders at the surface reduces the magnetic interaction between bonds and this disorder and frustration at the surface increases with size reduction, due to large surface to volume ratio [53,54]. Coercivity and retentivity increase with increase in Ni2+content (Table 7). In the present work, higher values of coercivity has been obtained in comparison to Padmapriya et al.[16] on replacing Zn2+ ions with Ni2+ ions. The increase in Hc values is attributed to large magneto-crystalline anisotropy of Ni2+ [55]. Magneto-crystalline anisotropy is intrinsic 10

ACCEPTED MANUSCRIPT property of materials and it plays major role in coercivity [56]. The anisotropy constant (Ka), according to Stoner-Wohlfarth theory, has been calculated using [52]; H c = ( 0 . 98 × K a ) / M

s

. The anisotropy constant increases (Table 7) from 1.103 × 103 erg/cm3 for x = 0.125 to 3.242 × 103 erg/cm3 for x = 0.500. The saturation magnetization has inverse dependence with coercivity and the same behaviour has been observed. The remanence ratio has been

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calculated using [57];  =  ⁄ . The remanence ratio increases from 0.0283 for x = 0.125 to 0.2035 for x = 0.500 (Table 7), the small value for remanence ratio reflect the isotropic nature of samples [57].

The variation in magnetic moment can also be explained with the help of Yafet-Kittel

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three sub-lattice model. According to Yafet-Kittel model the B sub-lattice can be divided into two sub-lattices B1 and B2 having triangular spin arrangement. Yafet-Kittels model gives insight about the non-collinear or triangular spin arrangement of spins [58]. The Yafet-Kittel

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(Y-K) angles have been calculated using relation [59]; η B = M B cos α YK − M A

(8)

where ηB is observed magnetic moment, MA and MB are value of magnetic moments at tetrahedral and octahedral sites and αY-K is Yafet-Kittel angle. The Y-K angles decrease with increase in Ni2+ substitution (Table 7) . The decrease in Y-K angles with Ni2+ substitution

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confirm increase in super-exchange interaction (A-B) and decrease in non-collinear arrangement of spins [58]. The decrease in Y-K angles suggest increase in overlap of wave functions between nearby neighboring magnetic ions, which leads to increase in A-B superexchange interactions and decrease in B-B interaction [58, 60]. The same behavior has been

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observed in magnetic interactions, calculated using proposed theoretical cation distribution. For high density recording media applications the nano-particles must have low values of

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coercivity which make reading and writing process possible [61,62]. In present work the lower values of coercivity have been obtained indicating usage of prepared nano-particles for reading and writing process in high density recording media or information storage. El Maalam et al. have also reported that nickel doped ferrites are suitable in memory storage devices [19].

3.6 Optical studies UV-visible absorption spectrum for Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) samples has been used to calculate the absorption coefficient α using relation [63]; α = 2 .303

log( A) t

11

(9)

ACCEPTED MANUSCRIPT where A is absorbance and t is the thickness of the sample. The relation between α and optical band gap (Eg) is given by Tauc’s equation [64]; α hv = B ( hv − E g ) m

(10) where B is a constant, hv is incident photon energy and m is a constant which depends on the nature of transition between valence band and conduction band. For indirect band gap

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materials m = 2 and for direct band gap materials m = ½ [64]. The optical band gap (Eg) of Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) samples near the absorption edge for indirect band transition have been calculated by extrapolating the linear portion of the plot

α hv 1 / 2 vs . hv (Tauc Plot) to zero absorption making intercept at photon energy axis (Fig.8).

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The values of optical band gap shows blue shift with increase in Ni2+content. The optical band gap increases from 4.50 eV for x = 0 to 5.60 eV for x = 0.500 with increase in

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Ni2+content. The optical band gap increases with decrease in crystallite size, which confirms quantum confinement for all prepared samples [65]. This may be due to the polycrystalline nature of samples, having crystallites of the order of few nanometers. Fig. 9 shows room temperature photoluminescence spectra of Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) at an excitation wavelength of 220 nm. Photoluminescence (PL) spectroscopy has been regarded as an important spectroscopy tool for exploring optical

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properties. PL spectrum provides information about the band gap and defects present in the material [66]. The band edge emission varies from 297.18 nm for x = 0 to 305.77 nm for x = 0.500 with increase in Ni2+ content. Near band emission is due to the radiative re-

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combination of hole in the valence band with the electron in the conduction band [67]. The band edge emission is at lower energy (higher wavelength) than optical band gap energy for all samples. Along with near band edge emission, peaks at higher wavelengths have also been

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observed for all samples. The presence of defects creates donor and acceptor levels in the band gap and provides possible paths for electron-hole combination. So the presence of emission peaks at higher wavelengths correspond to presence of defect states [66,67]. 4. Conclusions

The spinel Ni2+ doped Mg0.5Zn0.5-xNixFe2O4 (x = 0, 0.125, 0.250, 0.375, 0.500) ferrites has been synthesized by co-precipitation route and confirmed from X-ray diffraction and FTIR study. The crystallite size and lattice constant have been found to decrease with increase in Ni2+ content. The crystallite size decreases from 59.40 nm to 43.58 nm with Ni2+ content. Various theoretical parameters have been calculated using proposed cation distribution. The variation in theoretical parameters show dependence on doping concentration of Ni2+, its 12

ACCEPTED MANUSCRIPT preference for various lattice sites and its intrinsic properties. FE-SEM micrographs show agglomeration of nano-particles with increase in Ni2+ content. M-H curves indicate transition from super-paramagnetic behaviour to ferrimagnetic behaviour with Ni2+ substitution. Moderate values of coercivity (for x = 0.500 the maximum coercivity is 125.58 Oe) and saturation magnetization (decreases from 57.84 emu/gm to 25.30 emu/gm with increase in

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Ni2+ content) indicate that these materials may find applications in high density information storage devices. An increase in remanence ratio with Ni content indicates that these spinel nanoferrites may be used in magnetic recording and memory devices. The optical band gap calculated with the help of Tauc plot increases with Ni2+content. Photo-luminescence spectra

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show band edge emission at lower energy than optical band gap energy confirms presence of defect states for all of the sample.

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Acknowledgement

Authors acknowledge Dr. R.R. Singh, JUIT Waknaghat for providing experimental facility from SERB-DST project SR/FTP/PS-032/2013 for photoluminescence measurements.

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19

ACCEPTED MANUSCRIPT List of Tables and Figure captions Table 1: Structural parameters calculated from X-ray diffraction studies of Mg0.5Zn0.5xNixFe2O4 soft

ferrites.

Table 2: Proposed cation distribution for Mg0.5Zn0.5-xNixFe2O4 soft ferrites. Table 3: Theoretically calculated ionic radii at A site (rA) and B site (rB), theoretical lattice

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parameter (ath), oxygen positional parameter (u), inversion parameter ( ), bond length (RA) and bond length (RB) from proposed cation distribution.

Table 4: Tetrahedral edge length (R), shared octahedral length (R'), unshared octahedral length (R''), cation-cation (b,c,d,e,f) and cation-anion (p, q, r, s) distances for Mg0.5Zn0.5ferrites.

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xNixFe2O4 soft

Table 5: Bond angles using cation distribution between ions pairs in spinel ferrites.

(x = 0, 0.125, 0.250, 0.375, 0.500).

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Table 6: Cation- anion atomic % and cation-anion ratio for Mg0.5Zn0.5-xNixFe2O4 soft ferrites Table 7: Magnetic parameters calculated for Mg0.5Zn0.5-xNixFe2O4 soft ferrites. Fig. 1 Schematic diagram for the synthesis of Mg0.5Zn0.5-xNixFe2O4 soft ferrites by coprecipitation method.

Fig. 2 The room temperature X-ray diffractograms for Mg0.5Zn0.5-xNixFe2O4 soft ferrites and

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inset shows shift of most prominent (311) peak with Ni2+ substitution. Fig. 3 Intensity ratios for 220, 440 and 400 reflections with increase in nickel content. Fig. 4 Micrographs (a-e) for Mg0.5Zn0.5-xNixFe2O4 soft ferrites. Fig. 5 EDS spectra (a-e) for Mg0.5Zn0.5-xNixFe2O4 soft ferrites.

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Fig. 6 FT-IR spectra for Mg0.5Zn0.5-xNixFe2O4 soft ferrites. Fig. 7 M-H curves for Mg0.5Zn0.5-xNixFe2O4 soft ferrites.

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Fig. 8 Tauc’s Plot (a-e) for optical band gap calculation of Mg0.5Zn0.5-xNixFe2O4 soft ferrites. Fig. 9 Photo-luminescence spectra (a-e) for Mg0.5Zn0.5-xNixFe2O4 soft ferrites.

20

ACCEPTED MANUSCRIPT Table 1 D

a

(nm)

(Å)

59.40

8.4116

0.125

47.21

8.3847

186.74

2.5280

-3.233 ×10-3

4.48×10-4

0.250

46.89

8.3846

185.48

2.5280

-3.233 ×10-3

4.54×10-4

0.375

46.04

8.3755

182.32

2.5253

-4.297×10-3

4.71×10-4

0.500

43.58

8.3392

173.33

2.5143

-8.710× 10-3

5.26×10-4

0

d

234.23

(nm-2)

2.5362

2.83×10-4

--

B site

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A site

content(x)

Zn0.5 Fe0.5

Mg0.5 Fe1.5

0.125

Zn0.375 Fe0.625

Mg0.5 Ni0.125 Fe1.375

0.250

Zn0.250 Fe0.750

Mg0.5Ni0.250Fe1.250

0.375

Zn0.125 Fe0.875

Mg0.5 Ni0.375 Fe1.125

0.500

Fe1.0

Mg0.5Ni0.5 Fe1.0

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0

Table 3

δ

RA

RB

(Å)

(Å)

(Å)

(Å)

rA

rB

ath

u

content(x)

(Å)

(Å)

(Å)

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Nickel

0

ρ

(Å)

Table 2 Nickel

ɛu

RI PT

content(x)

p

SC

Nickel

0.745

0.682

8.5179

0.3899

0.0149

2.0382

1.9854

0.125

0.726

0.683

8.4913

0.3891

0.0141

2.0201

1.9850

0.250

0.707

0.685

8.4674

0.3882

0.0132

2.0070

1.9916

0.375

0.688

0.686

8.4408

0.3873

0.0123

1.9917

1.9961

0.500

0.670

0.687

8.4158

0.3865

0.0115

1.9715

1.9935

21

ACCEPTED MANUSCRIPT Table 4

Nickel

R

content(x) (Å)

R'

R''

p

q

r

s

b

c

d

e

f

(Å)

(Å)

(Å)

(Å)

(Å)

(Å)

(Å)

(Å)

(Å)

(Å)

(Å)

3.328 2.619 2.984 1.9775 2.0382 3.9029 3.7146 2.9739 3.4872 3.6423 5.4634 5.1510

0.125

3.298 2.630 2.973 1.9779 2.0201 3.8682 3.6989 2.9644 3.4761 3.6306 5.4460 5.1345

0.250

3.277 2.651 2.972 1.9854 2.0070 3.8431 3.6945 2.9644 3.4760 3.6306 5.4459 5.1344

0.375

3.252 2.669 2.968 1.9908 1.9917 3.8139 3.6861 2.9611 3.4722 3.6266 5.4400 5.1289

0.500

3.219 2.677 2.954 1.9888 1.9715 3.7753 3.6663 2.9483 3.4572 3.6109 5.4164 5.1066

SC

RI PT

0

Nickel

θ1

θ2

0

120.53

133.91

0.125

120.78

134.80

0.250

121.06

135.83

0.375

121.35

0.500

121.60

θ3

θ4

θ5

97.51

126.91

67.37

97.07

126.82

67.96

96.58

126.71

68.64

136.88

96.09

126.62

69.33

137.82

95.67

126.52

69.95

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content(x)

Table 6

AC C

Nickel

content(x)

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Table 5

Cations

Anion

(At.%)

(At.%)

Ratio Cation:Anion

0

41.86

58.14

2.9302 : 4.0698

0.125

44.95

55.05

3.1465 : 3.8535

0.250

47.64

52.36

3.3348 : 3.6652

0.375

46.66

53.34

3.2662 : 3.7338

0.500

42.19

57.81

2.9533 : 4.0467

22

ACCEPTED MANUSCRIPT Table 7 Nickel

Mr

Hc

content(x) (emu/g) (Oe) 0

-

-

Ms

R

(emu/g) 36.18

ηB

Mcal.

(µB) (µB) -

1.42

Ka

αYK

(erg/cm3)

5.000 -

(degree) 58.48

3

1.64

18.69

57.84

0.0283 2.26

4.000 1.103×10

0.250

3.78

48.05

50.75

0.0744 1.98

3.000 2.488×103 31.90

0.375

5.11

75.13

42.18

0.1211 1.64

2.000 3.233×103 19.34

0.500

5.15

125.58 25.30

0.2035 0.99

1.000 3.242×103 3.30

SC M AN U TE D EP AC C 23

40.90

RI PT

0.125

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SC

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Fig. 1

24

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Fig. 2

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Fig. 3

26

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AC C

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Fig. 4

27

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AC C

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Fig. 5

28

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AC C

EP

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SC

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Fig. 6

29

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AC C

EP

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SC

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Fig. 7

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AC C

EP

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SC

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Fig.8

31

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Fig. 9

32

ACCEPTED MANUSCRIPT

Highlights:  Ni2+ doped Mg0.5Zn0.5-xNixFe2O4 ferrite samples prepared by co-precipitation route.  A transition from superparamagnetic (x = 0) to soft ferrimagnetic nature (x > 0) is observed.

RI PT

 With Ni2+ content an increase in coercivity with lower saturation magnetization is observed.

 Optical band gap increases with increase in Ni2+content showing quantum confinement.

SC

 Proposed cation distribution and theoretical parameters supports our experimental

AC C

EP

TE D

M AN U

results.