Study of structural, electrical and magnetic properties of Cr doped Ni–Mg ferrite nanoparticle
7
Study of structural, electrical and magnetic properties of Cr doped Ni–Mg ferrite nanoparticle Mohd. Hashim a,⇑ , S.S. Meena b , R.K. Kotnala c , Sagar E. Shirsath d , Aashis S. Roy e , Ameena Parveen f , Pramod Bhatt b , Shalendra Kumar g , R.B. Jotania h , Ravi Kumar i , Alimuddin a a Department of Applied Physics, Aligarh Muslim University, Aligarh 202 002, India b Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India c National Physical Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi 110 012, India d Spin Device Technology Centre, Department of Information Engineering, Shinshu University, Nagano 380-8553, Japan e Department of Materials Science, Gulbarga University, Gulbarga 585 106, Karnataka, India f Department of Physics, Govt. First Grade College, Gurmitkal Yadgir 585 214, Karnataka, India g School of Materials Science and Engineering, Changwon National University, Changwon, Gyeongnam 641-773, Republic of Korea h Department of Physics, University School of Sciences, Gujarat University, Ahmedabad 380 009, India i Centre for Material Science Engineering, National Institute of Technology, Hamirpur 171 005, HP, India article info Article history: Received 7 January 2014 Received in revised form 3 March 2014 Accepted 4 March 2014 Available online 12 March 2014 Keywords: Nanostructured materials Sol–gel processes Hyperfine interactions Mössbauer spectroscopy abstract We have reported the effect of Cr 3+ ion doping on the structural, ac transport and Mössbauer spectral studies of ferrite nanoparticles Ni 0.5 Mg 0.5 Fe 2x Cr x O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) prepared by sol–gel auto combustion technique. The micro-structural studies of the samples were obtained by using scanning electron microscopy (SEM). The dependence of Mössbauer parameters, viz., line width, isomer shift, quadrupole splitting and hyperfine magnetic field on Cr 3+ substitution have been analysed. Hyper- fine magnetic field and isomer shift values decreases whereas line width increases with Cr 3+ substitution. Rietveld refinement and Mössbauer spectroscopy studies reveal that the Ni 0.5 Mg 0.5 Fe 2x Cr x O 4 with various concentrations of x belongs to mixed spinel structure. Nèel’s molecular field model is used to explain the variation of hyperfine magnetic field with increasing Cr 3+ substitution. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Recent years the research on nanoscale oxides has eventually exaggerated due to its anomalous behaviour and various medical applications. Thus, researchers having a closer look at nano- mech- anisms, researchers are making significant improvements in vari- ous areas from delicate electronics to life-saving medical techniques [1–3]. Nanoparticles are quite important as they act as a bridge between bulk materials, molecules and structures at an atomic level. The name ‘cluster’ which has been extensively em- ployed in the chemical literature in previous years is now being used to designate nanoparticles with size less than 3 nm. Ferrites, with spinel type structures covers wide range of materials, such as MFe 2 O 4 (M[Fe 2 ]O 4 , here M as Zn, Cd, Mg) [4–6], and inverse spinel Fe[MFe]O 4 , where M as Ni, Co, Fe, Mn, Cu) [7–10]. Spinel ferrites usually contains a crystal structure with the space group, Fd3m, having 56 atoms; 32 are oxygen anions with a close packed cubic structure, and the rest are metal cations: 8 (of 64 available) occupying the tetrahedral (A) sites and 16 (of 32 available) occupy- ing the octahedral (B) sites. The A-site cations are present on the interstices of 2 interpenetrating FCC lattice, while the B site cations reside on the interstices of 4 interpenetrating FCC lattices. The structural formula for a generic spinel compound MFe 2 O 4 can be written as ðM 1i Fe i Þ A ½M i Fe 2i B O 4 ð1Þ where the amounts in () A and [ ] B brackets represent the average occupancy of A sites (tetrahedral) and B sites (octahedral), and i is the inversion parameter. For a normal spinel, i = 0, for an fully in- verted spinel, i = 1, and for partially inverted, 0 < i <1 [11]. The dimension and valence of the cations species decides the filling of these sites which in turn strongly affects the materials magnetic and electronic properties. Spinel ferrite materials accom- modate an array of cations at different sites enabling a wide vari- ation in properties from the last few decades. These materials are regularly exploited in numerous applications including microwave devices, magnetic resonance imaging (MRI), computer memory chips, magnetic recording media, giant magneto resistance (GMR) http://dx.doi.org/10.1016/j.jallcom.2014.03.013 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +91 9359380185. E-mail address: [email protected] (Mohd. Hashim). Journal of Alloys and Compounds 602 (2014) 150–156 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Transcript of Study of structural, electrical and magnetic properties of Cr doped Ni–Mg ferrite nanoparticle
Journal of Alloys and Compounds 602 (2014) 150–156
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier .com/locate / ja lcom
Study of structural, electrical and magnetic properties of Cr dopedNi–Mg ferrite nanoparticle
http://dx.doi.org/10.1016/j.jallcom.2014.03.0130925-8388/� 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel.: +91 9359380185.E-mail address: [email protected] (Mohd. Hashim).
Mohd. Hashim a,⇑, S.S. Meena b, R.K. Kotnala c, Sagar E. Shirsath d, Aashis S. Roy e, Ameena Parveen f,Pramod Bhatt b, Shalendra Kumar g, R.B. Jotania h, Ravi Kumar i, Alimuddin a
a Department of Applied Physics, Aligarh Muslim University, Aligarh 202 002, Indiab Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, Indiac National Physical Laboratory (CSIR), Dr. K.S. Krishnan Road, New Delhi 110 012, Indiad Spin Device Technology Centre, Department of Information Engineering, Shinshu University, Nagano 380-8553, Japane Department of Materials Science, Gulbarga University, Gulbarga 585 106, Karnataka, Indiaf Department of Physics, Govt. First Grade College, Gurmitkal Yadgir 585 214, Karnataka, Indiag School of Materials Science and Engineering, Changwon National University, Changwon, Gyeongnam 641-773, Republic of Koreah Department of Physics, University School of Sciences, Gujarat University, Ahmedabad 380 009, Indiai Centre for Material Science Engineering, National Institute of Technology, Hamirpur 171 005, HP, India
a r t i c l e i n f o
Article history:Received 7 January 2014Received in revised form 3 March 2014Accepted 4 March 2014Available online 12 March 2014
Keywords:Nanostructured materialsSol–gel processesHyperfine interactionsMössbauer spectroscopy
a b s t r a c t
We have reported the effect of Cr3+ ion doping on the structural, ac transport and Mössbauer spectralstudies of ferrite nanoparticles Ni0.5Mg0.5Fe2�xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) prepared bysol–gel auto combustion technique. The micro-structural studies of the samples were obtained by usingscanning electron microscopy (SEM). The dependence of Mössbauer parameters, viz., line width, isomershift, quadrupole splitting and hyperfine magnetic field on Cr3+ substitution have been analysed. Hyper-fine magnetic field and isomer shift values decreases whereas line width increases with Cr3+ substitution.Rietveld refinement and Mössbauer spectroscopy studies reveal that the Ni0.5Mg0.5Fe2�xCrxO4 withvarious concentrations of x belongs to mixed spinel structure. Nèel’s molecular field model is used toexplain the variation of hyperfine magnetic field with increasing Cr3+ substitution.
� 2014 Elsevier B.V. All rights reserved.
1. Introduction structure, and the rest are metal cations: 8 (of 64 available)
Recent years the research on nanoscale oxides has eventuallyexaggerated due to its anomalous behaviour and various medicalapplications. Thus, researchers having a closer look at nano-mech-anisms, researchers are making significant improvements in vari-ous areas from delicate electronics to life-saving medicaltechniques [1–3]. Nanoparticles are quite important as they actas a bridge between bulk materials, molecules and structures atan atomic level. The name ‘cluster’ which has been extensively em-ployed in the chemical literature in previous years is now beingused to designate nanoparticles with size less than 3 nm. Ferrites,with spinel type structures covers wide range of materials, such asMFe2O4 (M[Fe2]O4, here M as Zn, Cd, Mg) [4–6], and inverse spinelFe[MFe]O4, where M as Ni, Co, Fe, Mn, Cu) [7–10]. Spinel ferritesusually contains a crystal structure with the space group, Fd3m,having 56 atoms; 32 are oxygen anions with a close packed cubic
occupying the tetrahedral (A) sites and 16 (of 32 available) occupy-ing the octahedral (B) sites. The A-site cations are present on theinterstices of 2 interpenetrating FCC lattice, while the B site cationsreside on the interstices of 4 interpenetrating FCC lattices. Thestructural formula for a generic spinel compound MFe2O4 can bewritten as
ðM1�iFeiÞA½MiFe2�i�BO4 ð1Þ
where the amounts in ()A and [ ]B brackets represent the averageoccupancy of A sites (tetrahedral) and B sites (octahedral), and i isthe inversion parameter. For a normal spinel, i = 0, for an fully in-verted spinel, i = 1, and for partially inverted, 0 < i < 1 [11].
The dimension and valence of the cations species decides thefilling of these sites which in turn strongly affects the materialsmagnetic and electronic properties. Spinel ferrite materials accom-modate an array of cations at different sites enabling a wide vari-ation in properties from the last few decades. These materials areregularly exploited in numerous applications including microwavedevices, magnetic resonance imaging (MRI), computer memorychips, magnetic recording media, giant magneto resistance (GMR)
Mohd. Hashim et al. / Journal of Alloys and Compounds 602 (2014) 150–156 151
sensor, magnetically guided target-specific drug delivery systems,cells, DNAs and genes sorting and delivery, ferrofluids radio fre-quency coil fabrication, transformer cores, rod antennas, electronicengineering and many branches of telecommunication [12–15]. Anextensive amount of research is paying attention on ferrites suchas Ni ferrite, Mg ferrite and Ni–Mg based mixed ferrites becauseof their potential applications [6,16]. In Co0.5Ni0.5CrxFe2�xO4 ferritesystem synthesized by the sol–gel auto combustion method, thelattice constant and saturation magnetization decreased withincreasing Cr3+ substitution. Cation distribution suggests that Cr3+
ions have a strong preference towards the octahedral B site. [2].Therefore, in the present study we have focused to investigatestructural, AC resistivity, cation distribution and Mössbauer spec-troscopic studies of Cr3+ substituted Ni–Mg based mixed ferriteswith the chemical formula Ni0.5Mg0.5Fe2�xCrxO4 prepared by usingsol–gel auto combustion method.
0
2000
4000
6000
8000
10000
12000(4
42)
(531
)
(620
)
(622
)(5
33)
(space group: Fd3m; a=8.357(4) Å)
(deg)2θ
(511
)
(422
)
(440
)
(333
)
(331
)
(400
)
(222
)(3
11)
(220
)
x = 0.0
Inte
nsity
(arb
. uni
ts)
Observed Calculated Difference Bragg peak Positions
0
2000
4000
6000
8000
10000
12000
(442
)(5
31)
(620
)
(622
)(5
33)
(space group: Fd3m; a=8.344(1) Å)
(511
)
(422
)
(440
)
(333
)
(331
)
(400
)
(222
)(3
11)
(220
)
x = 0.4
Inte
nsity
(arb
. uni
ts)
Observed Calculated Difference Bragg peak Positions
0
2000
4000
6000
8000
10000
12000
(442
)(5
31)
(620
)
(622
)(5
33)
(space group: Fd3m; a=8.339(3) Å)
(511
)
(422
)
(440
)
(333
)
(331
)(400
)
(222
)(3
11)
(220
)
x = 0.8
Inte
nsity
(arb
. uni
ts)
Observed Calculated Difference Bragg peak Positions
20 30 40 50 60 70 80
(deg)2θ 20 30 40 50 60 70 80
(deg)2θ 20 30 40 50 60 70 80
Fig. 1. Rietveld refined X-ray diffraction patterns of Ni0.5Mg0.5Fe2�xCrxO
2. Materials and methods
All chemicals used were of analytical reagent (AR) grade. Highly pure salts ofNi(NO3)2�6H20, Mg(NO3)2�6H2O, Cr(NO3)3�9H2O, Fe(NO3)3�9H2O were used as re-ceived from Thomas Baker. Metal nitrates were dissolved double distilled waterin proper required stoichiometric ratio and stirred for 20 min till solution becomescolourless. The mixed metal nitrate solution was then added to the citric acid solu-tion in 1:1 M ratio. Analytical grade liquor ammonia (30%) was then added underconstant stirring in order to make the pH value 8. The resulting solution was con-tinuously heated on the magnetic stirrer at 70 �C to form a gel. The gel obtainedwas annealed at 200 �C for 24 h and then ground for half an hour till the fine pow-der is obtained. The powder was heated 6 h at 600 �C to remove any organic mate-rial present in it and then grounded for half an hour. The crystallinity, structure andcrystallite size of Cr doped Ni–Mg ferrite nanoparticles were obtained by powderX-ray powder diffraction (XRD) on a Rikagu Miniflex (II) using Cu Ka radiation(k = 0.15,406 nm) in 2h range from 20� to 70�. Field emission scanning electronmicroscopy (FE-SEM) images were obtained using a TESCAN, MIRA II LMH micro-scope. The ac resistivity measurements were carried out in the frequency range42-Hz to 5 MHz using LCR HI-Tester (HIOKI 3532–50).
0
2000
4000
6000
8000
10000
12000
(442
)(5
31)
(620
)
(622
)(5
33)
(space group: Fd3m; a=8.351(5) Å)
(511
)
(422
)
(440
)
(333
)
(331
)
(400
)
(222
)(3
11)
(220
)
x = 0.2
Inte
nsity
(arb
. uni
ts)
Observed Calculated Difference Bragg peak Positions
0
2000
4000
6000
8000
10000
12000
(442
)(5
31)
(620
)
(622
)(5
33)
(space group: Fd3m; a=8.336(8) Å)
(511
)
(422
)
(440
)
(333
)
(331
)(400
)
(222
)(3
11)
(220
)
x = 0.6
Inte
nsity
(arb
. uni
ts)
Observed Calculated Difference Bragg peak Positions
(deg)2θ 20 30 40 50 60 70 80
(deg)2θ 20 30 40 50 60 70 80
(deg)2θ 20 30 40 50 60 70 80
0
2000
4000
6000
8000
10000
12000
(442
)(5
31)
(620
)
(622
)(5
33)
(space group: Fd3m; a=8.333(7) Å)
(511
)
(422
)
(440
)
(333
)
(331
)(400
)
(222
)(3
11)
(220
)
x = 1.0
Inte
nsity
(arb
. uni
ts)
Observed Calculated Difference Bragg peak Positions
4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) ferrite nanoparticles at 300 K.
152 Mohd. Hashim et al. / Journal of Alloys and Compounds 602 (2014) 150–156
Mössbauer spectra of all samples were recorded using a standard PC-basedMössbauer spectrometer operated in constant acceleration mode (triangular wave)in transmission geometry at room temperature. The source employed was Co-57 inRh matrix of strength 50 m Ci. The calibration of the velocity scale is done by usingan enriched a-57Fe metal foil. The line width (inner) of calibration spectra is0.23 mm s�1.
3. Results and discussion
Rietveld refined XRD study of Cr3+ substituted Ni0.5Mg0.5Fe2�x
CrxO4 ferrites have confirmed the formation of the single phasecubic spinel structure (Fig. 1). The lattice constant (ar) valueshave been computed by using the d-spacing values and therespective (h, k, l) parameters from Rietveld refined XRD patterns.The values of lattice constant as a function of chromium concentra-tion are shown in Table 1 for various Ni0.5Mg0.5Fe2�xCrxO4 ferrites.It can be seen that the lattice constant decreased proportionally tothe doping contents. The decrease in lattice constant value can beexplained on the basis of Vegard’s law. According to this law, thelinear variation in the lattice constant is due to the ionic radii ofthe doped and the replaced ion. In the present case we arereplacing Fe3+ ion with Cr3+ ion, as the ionic radius of Cr3+ ion beingsmaller, (0.064 nm) compared to the radius of Fe3+ ion (0.067 nm)[16,17]. Thus the substitution of Fe3+ ion with Cr3+ results in adecrease in lattice constant of these ferrite material. Rp, Rwp andv2 are reliability factors that characterise the quality of fit for theRietveld refinement and ar are tabulated in Table 1. The averagecrystallite size (DXRD) of the nano phase particles was determinedby using the Debye–Scherer formula [18]. Variation in latticeconstants (ar) obtained from Rietveld refinement and crystallitesize (DXRD) are shown in Fig. 2. The average crystallite size for allthe samples is in the range of 30 to 36 nanometres (nm).
30
31
32
33
34
35
36
0.0 0.2 0.4 0.6 0.8 1.08.33
8.34
8.35
8.36
Cry
stal
lite
size
' DXR
D' (
nm)
DXRD (nm)
Cr substitution (x)
Latti
ce c
onst
ant '
a r' (Å)
ar (Å)
Fig. 2. Variation in lattice constants obtained from Rietveld refinement (ar) andcrystallite size (DXRD) with Cr substitution (x) in Ni0.5Mg0.5Fe2�xCrxO4 (x = 0.0, 0.2,0.4, 0.6, 0.8 and 1.0) ferrite nanoparticles.
Table 1Results of Rietveld refinement: structural parameters for Mg0.5Ni0.5Fe2�xCrxO4 (x = 0,0.2, 0.4, 0.6, 0.8 and 1.0) ferrite nanoparticles. Rp, Rwp and v2 are reliability factors thatcharacterise the quality of fit. ar is the lattice constant obtained from Rietveldrefinement of XRD patterns recorded at room temperature.
(x) 0.0 0.2 0.4 0.6 0.8 1.0
ar 8.357 8.352 8.344 8.336 8.339 8.333Rp 2.28 2.24 2.49 2.99 2.41 2.96Rwp 3.15 3.06 3.60 3.99 3.17 4.00v2 2.09 1.79 2.19 1.99 1.52 1.89
Scanning electron micrographs of Ni0.5Mg0.5Fe2�xCrxO4 (x = 0,0.2, and 0.4) powder samples are shown in Fig. 3a–c. The averagegrain size obviously tends to decrease with Cr3+ content. It is ob-served from Fig. 3 that the SEM images of the samples show irreg-ular in shape and size highly clustered and flake like structure.
3.1. AC resistivity
The AC resistivity (qac) of prepared samples was designed fromdielectric data and loss tangent with the help of following relation[19]:
qac ¼ 1=e0e0x tan d ð2Þ
Where e0 is the dielectric constant, e0 is permittivity of free space(8.854 � 10�12 F/m), x is the angular frequency (x = 2pf) and tand is the dielectric loss factor. The analysis (Fig. 4) reveals that theac-resistivity (qac) shows the decreasing trend with increasing fre-quency. The AC conductivity of ferrites are strongly depends onthe several factors, including the method of preparation chemicalcomposition, domain alignment, grain boundaries and grain size.It is clearly evident from these figures that the ac-resistivity initiallydecreases rapidly in the low frequency region but at very high fre-quencies; its value becomes so small that it becomes independentof applied frequency. The low-frequency dispersion in ferrites is ob-served due to a space-charge effect. The observed behaviour can beexplained on the basis of interfacial polarisation suggested byKoops [20]. During the sintering process the presence of majorityFe3+ions and minority Fe2+ ions makes ferrite material dipolar, asFe2+ ions are usually formed due to the partial reduction of Fe3+ ionsduring the synthesis process. The electron exchange interactionbetween the ferrous (Fe2+) and ferric (Fe3+) ions results in the localdisplacement of the electrons in the direction of the applied fieldwhich determines the polarisation in the ferrites [21]. The decreasein ac-resistivity with increasing frequency is explained to be due tothe decrease of polarisation of the dipoles when electric field prop-agates with high frequency. In other words beyond a certain fre-quency region or electric field the electron exchange does notfollow the applied alternating field. The high value of the dielectricconstant at lower frequency region is due to the existence of Fe2+
ions, oxygen vacancies, grain boundary defects, etc. [22,23]. Whilethe decrease in dielectric constant with frequency is natural, i.e.,any species contributing to the polarizability is found to show lag-ging behind the applied field at higher frequencies [24]. It is ob-served from Fig. 4 that the ac-resistivity decreased when Cr3+
substitution increased from 0.0 to 0.2 and 0.4 to 0.8. This behaviourcan be explained by using the assumption of the mechanism ofdielectric polarisation is similar to that of conduction in ferrites.According to this, the electronic exchange interaction betweenFe2+
M Fe3+ results in a local displacement of the electrons in thedirection of an electric field which determines the polarisation offerrites. The presence of Fe2+ ions in excess amount favours thepolarisation effects [25,26]. Thus, the more dispersion observed inthe sample with low Cr3+ ion substitution and can be attributedto the presence of more Fe2+ ions in excess amount. As the Cr3+ ionssubstitution increased which replaces Fe3+ ions, as the Fe3+ ionsare reduced in the samples there is less possibility of electronicexchange interaction between Fe2+
M Fe3+, results in decrease indielectric parameters.
3.2. Impedance spectroscopy
Fig. 5 shows the Cole–Cole plot as a function of frequency atroom temperature of different compositions. Impedance spectros-copy is an important method to study the electrical properties offerrites as the impedance of grains can be separated from the otherimpedance sources, such as impedance of electrodes and grain
Fig. 3. SEM images of Ni0.5Mg0.5Fe2�xCrxO4 (a) x = 0.0, (b) x = 0.2 and (c) x = 0.4 ferrite nanoparticles.
1 2 3 4 5 6 7-5.0x105
0.05.0x1051.0x1061.5x1062.0x1062.5x1063.0x1063.5x1064.0x1064.5x1065.0x106
ac r
esis
tivity
Log f (Hz)
0.00.20.40.8
x =
Fig. 4. Variation of ac resistivity as a function of log f for Ni0.5Mg0.5Fe2�xCrxO4
(x = 0.0, 0.2, 0.4, 0.8) ferrite nanoparticles.
Fig. 5. Cole–Cole plot of Ni0.5Mg0.5Fe2�xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0)ferrite nanoparticles at room temperature.
Mohd. Hashim et al. / Journal of Alloys and Compounds 602 (2014) 150–156 153
boundaries. One of the important factors, which influence theimpedance properties of ferrites, is the microstructural effect.The impedance measurement gives us information about the resis-tive (real part) and reactive (imaginary part) components of amaterial. A ferrite material is assumed to be consisting of piledup crystalline plates. From the microstructural point of view, asample is assumed as a microstructure made up of parallel con-ducting plates (grains) separated by resistive plates (grain bound-aries). All the samples, as evident in Fig. 5, shows semicircle that is
due to the conduction of the grain boundary, suggesting that a pre-dominant conduction is through the grain boundary volume. Fur-thermore, contribution from the grain is not well resolved in allthe samples. It is therefore, concluded that the conductivity forall the samples is mainly due to the grain boundary contribution.
0.0 0.2 0.4 0.6 0.8 1.0
0
100
200
300
400
Rel
axat
ion
time
( τ)i
n Se
c
Cocentration x
Fig. 6. Variation of relaxation time (s) with Cr substitution (x) in Ni0.5Mg0.5Fe2�-
xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) ferrite nanoparticles.
154 Mohd. Hashim et al. / Journal of Alloys and Compounds 602 (2014) 150–156
The relaxation time (s) is calculated using the Cole–Cole plots [27]and their variation is presented in Fig. 6. It is observed that relax-ation time decreased with increase in Cr3+ content.
0.95
0.96
0.97
0.98
0.99
1.00
x = 0.0
Exp. data Fitted data Sextet A Sextet B
Rel
ativ
e C
ount
s
Velocity (mm/s)
0.97
0.98
0.99
1.00
x = 0.2
Rel
ativ
e C
ount
s
0.97
0.98
0.99
1.00
x = 0.4
Rel
ativ
e C
ount
s
-10 -5 0 5 10
Velocity (mm/s)-10 -5 0 5 10
Velocity (mm/s)-10 -5 0 5 10
Fig. 7. Mössbauer spectra of ferrite nanoparticles Ni0.5
3.3. Mössbauer study
The 57Fe Mössbauer spectra recorded at room temperature(300 K) for Ni0.5Mg0.5Fe2�xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0)ferrite nanoparticles as depict in Fig. 7. The data is analysed witha WinNormos Site fit program and as obtained parameters are withrespect to natural iron at room temperature. The spectra of compo-sitions (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) are shown in Fig. 7, spectracould be resolved into two well defined sextets for each composi-tion. One of the sextets is attributed to Fe3+ ions at tetrahedral (A)site due to its smaller isomer shift and smaller hyperfine field ofthe Zeeman pattern and the second sextet has been attributed toFe3+ ions at octahedral [B] site due to its larger isomer shift and lar-ger hyperfine field of the Zeeman pattern [28]. The presence of wellresolved six finger patterns in the spectra of all the samples showsthat the samples are magnetically ordered. The variation of theLine width (s), isomer shift (d), quadrupole splitting (D), hyperfinefield (Hhf) and relative area (RA) with Cr substitution (x),corresponding to tetrahedral (A) and octahedral (B) sites for allcompositions are shown in Fig. 8a–e respectively). The line widthof the tetrahedral sites becomes broader than that of the octahe-dral site with the increase of the Cr contents at the octahedral site(Fig. 8a). Such broadening is, in fact, expected due to a distribution
0.97
0.98
0.99
1.00
x = 0.6
Rel
ativ
e C
ount
s
0.99
1.00
x = 0.8
Rel
ativ
e C
ount
s
Velocity (mm/s)-10 -5 0 5 10
Velocity (mm/s)-10 -5 0 5 10
Velocity (mm/s)-10 -5 0 5 10
0.985
0.990
0.995
1.000
x = 1.0
Rel
ativ
e C
ount
s
Mg0.5Fe2�xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0).
0.0 0.2 0.4 0.6 0.8 1.00.3
0.4
0.5
0.6
Line
wid
th 'Γ
' (m
m/s
)
Cr Substitution (x)
0.0 0.2 0.4 0.6 0.8 1.0Cr Substitution (x)
0.0 0.2 0.4 0.6 0.8 1.0Cr Substitution (x)
0.0 0.2 0.4 0.6 0.8 1.0Cr Substitution (x)
0.0 0.2 0.4 0.6 0.8 1.0Cr Substitution (x)
ΓAΓB
(a)
0.25
0.30
0.35(b)
Isom
er S
hift
'δ' (m
m/s
)
δB
δA
-0.06
-0.04
-0.02
0.00
0.02
0.04
(c)
Δ B
Δ A
35
40
45
50
(d)
Hyp
erfin
e fie
ld 'H
hf' (
Tesl
a)
Hhf-B
Hhf-A
30
40
50
60
70(e)
Rel
ativ
e Ar
ea 'R
A' (
%)
RA A site
RB B site
'Δ' (
mm
/s)
Qua
drup
ole
split
ting
Fig. 8. The variation of the (a) Line width (s), (b) isomer shift (d), (c) quadrupole splitting (D), (d) hyperfine field (Hhf) and (e) relative area (RA) with Cr substitution (x),corresponding to tetrahedral (A) and octahedral (B) sites for ferrite nanoparticles Ni0.5Mg0.5Fe2�xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0).
Mohd. Hashim et al. / Journal of Alloys and Compounds 602 (2014) 150–156 155
of the hyperfine fields caused principally by a random distributionof Ni2+, Fe3+ and Cr3+ ions at the A and B-site (as evidenced byTable 2), since the Fe3+ A-sites would feel different configurationsof Ni2+, Fe3+ and Cr3+ ions occupying the 12 nearest neighboursB-sites [29]. The observed values of isomer shift show (Fig. 8b),within experimental error, an insignificant change with increasingsubstitution of Cr3+ ions for tetrahedral A site. It implies that thes-electron charge density of Fe3+ ions is not influenced fortetrahedral A site by increased Cr3+ substitution in Ni–Mg ferrites.It is observed from Fig. 8(b) that isomer shift value for octahedralsites (dB) is greater than that for tetrahedral sites (dA), and it
Table 2Cation distribution and inversion parameter (i) estimated from Mössbauer spectroscopy sComp.: composition, ()A: tetrahedral site, [ ]B: octahedral site.
Comp. (x) A-site (%) of Fe ions B-site (%) of Fe ions
0.0 69.7 30.30.2 66.2 33.80.4 62.6 37.40.6 30.6 69.40.8 26.1 73.81.0 51.6 48.4
decreases with increasing the Cr3+ substitution. In cubic spinelferrites, the bond separation Fe3+–O2� is larger for octahedral siteswhen compared to that for tetrahedral sites. Due to this, overlap-ping of orbital of Fe3+ ions is small at B-sites and thus a largerisomer shift at B-sites was expected [30,31]. The range of valuesof isomer shift indicates that iron exists in Fe3+ valence state withhigh spin configuration in the prepared samples [32]. The values ofquadrupole splitting for hyperfine spectra of all compositions arefound to be negligibly small and attributed to the fact that overallcubic symmetry is maintained between Fe3+ ions and theirsurroundings with the substituted Cr3+ ions in the present Ni–Mg
tudy of Ni0.5Mg0.5Fe2�xCrxO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) ferrite nanoparticles.
Cation distribution Inversion parameter (i)
(Fe1.39Mg0.5Ni0.11)A[Ni0.39Fe0.61]BO4 0.697(Fe1.19Mg0.5Ni0.31)A[Ni0.19Fe0.61Cr0.2]BO4 0.662(Fe1.0)A[Ni0.5Mg0.5Fe0.6Cr0.4]BO4 0.626(Fe0.43Mg0.5Ni0.07)A[Ni0.43Fe0.97Cr0.6]BO4 0.306(Fe0.31Mg0.5Ni0.19)A[Ni0.31Fe0.89Cr0.8]BO4 0.261(Fe0.52Mg0.48)A[Mg0.02Ni0.5Fe0.48Cr1.0]BO4 0.516
156 Mohd. Hashim et al. / Journal of Alloys and Compounds 602 (2014) 150–156
ferrites (Fig. 8c). The variations in hyperfine magnetic field at tet-rahedral (A) and octahedral [B] sites as a function of Cr3+ substitu-tions in Ni–Mg ferrites are shown in Fig. 8(d). In most of theferrites, B-site hyperfine magnetic field is generally larger than thatof A-site, which is attributed to the dipolar field resulting due todeviation from cubic symmetry and covalent nature of tetrahedralbonds. The observed variations in hyperfine magnetic field atA- and B-sites with increasing Cr3+ substitutions in Ni–Mg ferritescan be qualitatively explained using Nèel’s super-exchange inter-actions [33]. According to Nèel’s model, inter sublattice exchangeinteractions, i.e., AB exchange interactions are stronger than intrasublattice exchange interactions, i.e., AA or BB exchange interac-tions. As both A and B sites are occupied by Fe3+ ions, so the inter-action of Fe3+ is possible with Ni2+, Mg2+ and Cr3+ ions. The netmagnetic field is mainly due to strong Fe3þ
A � O� Fe3þB interactions.
In the present series Fe3+ ions with larger magnetic moment (i.e.5lB) are replaced my Cr3+ ions with smaller magnetic moment(3lB). This results in decrease of magnetic linkages inFe3þ
A � O� Fe3þB ; Fe3þ
A � O�Mg2þA and Fe3þ
A � O�Ni2þB and conse-
quently Fe3+ ion experience a decrease in the magnetic field at Aand B sites. Therefore, hyperfine magnetic field is expected to de-crease with increase of Cr3+ concentration. In general, the decreasein hyperfine fields is due to decrease in the super-transfer field thatarises from the magnetic ions (Fe and Cr) surrounding a given Fe3+
ion [34] and in Fermi contact field that is proportional to the mag-netic moment of Fe ions [35]. The relative area of B-site increaseswith increasing the Cr3+ substitution (Fig. 8e).
The cation distribution for each concentration and the site pref-erences of cations distributed among the tetrahedral A-sites andoctahedral B-sites and inversion parameter (i) calculated fromMössbauer spectra are presented in Table 2. In this table, the frac-tion of Fe3+ ions in either site is shown. The results demonstratethat Ni2+ and Mg2+ ions mostly occupy B sites. The Mg2+ ions havethe probability to occupy the tetrahedral site (A) or octahedral site[B], with high preference for the A-sites. Cr3+ preferentially re-places Fe3+ from octahedral sites because of favourable crystal-field effects (Cr3+6/5D0, Cr3+0D0) [16]. It is observed from Table 2that the Cr3+ ions predominately occupy the octahedral sites,which is consistent with the preference for large octahedral-siteenergy. With increasing Cr3+ content, the fraction Cr3+ ions in octa-hedral sites increases, whereas the fraction of Fe3+ ions in octahe-dral sites decreases linearly.
4. Conclusions
Structural analysis with the Rietveld refined XRD patternsreveals that the system confirms the formation of single phaseNi–Mg cubic spinel ferrite. The lattice constant decreases propor-tionally to the chromium content. The ac resistivity decreases withincreasing the frequency for all samples. The Mössbauer measure-ments showed well resolved magnetic spectra for the tetrahedraland octahedral sites. Line width of both the sublattices increaseswith Cr3+ substitution. Relative area of B-site is also increase (upto x = 0.8) with Cr3+ substitution. The hyperfine magnetic field atboth the sub lattices decreases with the introduction of increasingCr3+ ions by breaking the coupling bonds between Fe3þ
A � O� Fe3þB
interactions.
Acknowledgments
Mohd. Hashim is grateful to University grant commission for pro-viding financial assistance through grant MANF-MUS-UTT-1092.
This work was also supported by Centre of Excellence in MaterialScience (Nanomaterials), Department of Applied Physics, AligarhMuslim University, Aligarh. Shalendra Kumar is thankful for thefinancial assistant support from the Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology (MEST;2012-0009457) and by the Priority Research Centers Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(MEST; 2012-045424).
Mohd. Hashim thanks M.D. Mukadam, SSPD, BARC for helpingto improve the manuscript.
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