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Structural and magnetic properties of Mg doped cobalt ferrite nano particles prepared by sol-gel method
Accepted Manuscript Title: Structural and magnetic properties of Mg doped cobalt ferrite nano particles prepared by sol-gel method Author: H.S. Mund B.L. Ahuja PII: DOI: Reference:
S0025-5408(16)31131-X http://dx.doi.org/doi:10.1016/j.materresbull.2016.09.027 MRB 8959
To appear in:
MRB
Received date: Revised date: Accepted date:
18-3-2016 21-7-2016 23-9-2016
Please cite this article as: H.S.Mund, B.L.Ahuja, Structural and magnetic properties of Mg doped cobalt ferrite nano particles prepared by sol-gel method, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.09.027 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.
Structural and magnetic properties of Mg doped cobalt ferrite nano particles prepared by sol-gel method H. S. Mund* and B. L. Ahuja
Department of Physics, University College of Science, M. L. Sukhadia University Udaipur, Rajasthan-313001, India
*Corresponding Author E-mail: [email protected]
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Graphical abstract
Highlights
Mg doped nano cobalt ferrites with systematic Mg composition prepared by sol-gel method.
XRD, FTIR and Raman spectra confirmed the formation of single-phase cubic spinel structure.
Different ionic radii of Co/Mg and Fe ions revealed a doublet-like peak behavior at A1g and Eg modes in Raman spectra.
Coercivity data depict that hard magnetic cobalt ferrite becomes the soft magnetic material on substitution of Mg at Co site.
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Abstract Effect of Mg doping on structural and magnetic properties of nano cobalt ferrites prepared via sol-gel auto-combustion method is reported. The sintered samples (Co1-xMgxFe2O4, where x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were characterized using X-ray diffraction, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and vibrating sample magnetometer. Rietveld refinement of X-ray diffraction spectra shows the single phase cubic structure (space group Fd3m) for the prepared samples. Raman spectroscopy reveals a doubletlike peak behavior in A1g and Eg modes arising due to different ionic radii of Co/Mg and Fe ions. Two prominent vibration frequencies in FTIR spectroscopy data also confirm the cubic spinel structure in the prepared samples. The magnetic parameters such as saturation magnetization, coercivity and remanence are studied with increasing the Mg-concentration in cobalt ferrite. Systematic changes in magnetic properties while going from x=0 to 1 are discussed. KEYWORDS: A. Oxides, B. Magnetic properties, C. Raman spectroscopy, C. X-ray diffraction, D. Crystal structure.
1. Introduction Spinel ferrites are very useful in various technological devices such as switching circuits, high density magnetic storage, microwave based instruments, telecommunication equipment, magnetic fluids, gas sensors, etc. [1-10]. Generally, a normal spinel ferrite is described as (M2+)p[Fe23+]qO4, whereas an inverse spinel ferrite consists of ionic arrangement like (Fe3+)p[M2+Fe3+]qO4, where p, q and M correspond to tetrahedral site, octahedral site and 3d divalent transition element cations, respectively. It is known that the structural, electrical and 3
magnetic properties of ferrites are highly sensitive to conditions of their preparation, compositions, magnetic interactions and distribution of cations at tetrahedral and octahedral sites. The cation distribution at tetrahedral and octahedral sites is sensitive to ionic radii of Fe/Co and M ions, the type of bonding and the preparation method. Cobalt ferrite is a hard magnetic material with large coercivity and moderate magnetization whereas magnesium ferrite is a soft magnetic n-type semiconductor material. Among earlier studies on Mn doped cobalt ferrites, effect of Mn substitution on the magnetic and dielectric properties of cobalt ferrite synthesized by an auto-combustion route has been discussed by Kambale et al. [8]. Authors observed an increase in dielectric permittivity with Mn concentration. The semiconductor behavior of Mn doped cobalt ferrites (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) has been reported by Yadav et al. [9] using dc resistivity measurement. Melikhov et al. [10] have investigated the temperature variation of magnetic anisotropy and coercive field of magnetoelastic Mn substituted cobalt ferrites (CoMnxFe2-xO4 with 0 ≤ x ≤ 0.6). Raut et al. [11] have investigated the effect of Zn substitution on the structural and magnetic properties of cobalt ferrite nanoparticles prepared by sol-gel auto-combustion technique. Effect of Ni substitution on structural, electrical and magnetic properties of cobalt ferrite nanoparticles synthesized by self-combustion method without external oxidizing agents has been studied by Sontu et al. [12]. Wahba and Mohamed [13] have investigated the structural, magnetic, and electric properties of Co0.8Ni0.2CryFe2-yO4 (0.00 ≤ y ≤ 0.75; step 0.15) prepared via citrate precursor method. They found that Cr substitution improves ac electric properties of the ferrites. Wu et al. [14] have reported infrared radiation properties of CoFe2-xCexO4 (x=0, 0.01, 0.05, 0.1 and 0.15) via the sol-gel autocombustion method. Pandit et al. [15] have presented the structural, cation distribution, electrical and magnetic properties of CoAlxFe2-xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8) ferrites. The influence of the 4
Zn and Mg contents on the structural and magnetic properties of cobalt ferrites synthesized by chemical co-precipitation method was investigated by Varshney et al [16] using X-ray powder diffraction, Raman spectroscopy and vibrating sample magnetometer. Tadi et al. [17] investigated the structural and magnetic properties of Ni substituted MgFe2O4 which was synthesized using thermal decomposition of metal organic materials. Electronic structure of chemically synthesized MgFe2O4 nanoparticles has been studied by Singh et al. [18]. Aono et al. [19] have studied the surface structure and conductance of MgFe2O4 ferrite powder prepared by a chemical method. Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering has been studied by Reddy et al. [20]. Although plentiful reports are available on the magnetic and electric response of cobalt and magnesium ferrites, work related to systematic study of structure, electric and magnetic properties of Mg doped cobalt ferrites prepared by sol-gel auto combustion method is still lacking. It is worth mentioning that the sol-gel auto-combustion method has definite advantages over other methods, like variety of starting materials as cheap precursors to choose, more accurate control over phase formation, simple route of preparation with desired stoichiometry and uniformity in particle size. Therefore, in the present paper, we have attempted to investigate the structural and magnetic properties of nano Co1-xMgxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ferrites prepared by gel auto-combustion method. We have particularly studied the effect of nonmagnetic Mg2+ ions substitution in cobalt ferrite prepared via sol-gel auto-combustion method without varying the pH value of the precursor solution.
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2. Experimental work Nano powdered Mg doped cobalt ferrites (for all compositions x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by the sol-gel auto-combustion method under similar parameters. In the present work, citric acid was used as a fuel and complexing agent with analytical reagents of Fe(NO3)3·9H2O, Co(NO3)2·6H2O and Mg(NO3)3·6H2O were the original materials. The molar ratio of nitrates to citric acid was kept to 1:1. Firstly, depending upon the composition, the appropriate amount of metal nitrates and citric acid were dissolved in deionized water to obtain a mixed solution. During the mixing process, the solution was continuously stirred for 30 min at 50 °C using a magnetic agitator. Thereafter, the mixed solution was individually kept stirred at 100 °C for 3 h to transform into gel. The individual gel was dried at 150 °C for 2 h in order to obtain the dried gel. The dried gel was burnt in an auto-combustion manner until the entire gels was burnt out (as visible from complete extinction of redness) to form a loose powder, known as as-burnt powder. This whole process was carried out under ambient atmosphere. After that, the individual powders were sintered at the temperature 1000 °C for 6 h. The powder X-ray diffraction pattern of as-prepared samples was carried out using Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ = 0.15406 nm) to identify the phase and structure of annealed samples. To find out the vibrational modes which are sensitive to chemical and structural changes, Fourier transform infrared (FTIR) spectroscopy studies were carried out in the range 4000-400 cm-1. The FTIR spectrometer (PerkinElmer Spectrum Two) used in the present measurements was having resolution of 0.5 cm-1 in KBr medium. It is known that KBr is an alkyl halide and it does not show any absorption spectrum in the infrared region, therefore wafers for FTIR spectroscopy were made by diluting as-prepared ferrites samples in KBr. Since
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Raman spectroscopy is a novel tool to study the electronic properties of spinel ferrites, the Raman spectroscopy measurements on the as-prepared samples were carried out using STR-500 Confocal micro Raman spectrometer with 532 nm DPSS laser. The laser power was optimized to 5 mW on the surface area. Room temperature magnetic measurements were carried out using vibrating sample magnetometer (Lakeshore 7304) as a function of magnetic field in the range of ±10 kOe. 3. Results and discussion The XRD pattern of Co1-xMgxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ferrites in the 2θ range of 20-80° with step size of 0.02° are shown in Fig. 1. The XRD pattern shows that all the reflection peaks corresponding to (220), (311), (222), (400), (422), (511) and (440) planes in pure (x=0) and doped ferrites. The weak reflection peak corresponding to (533) is due to higher angle reflection index and small structure factor. Further, the moderate band in the base line 20°<2θ<30° in Fig. 2 (b) is due to diffraction by glass plate sample holder. Therefore, the XRD pattern validates the formation of cubic spinel structure with Fd3m space group. The peak position in XRD patterns are also indexed using the standard JCPDS card Nos. 79-1744 and 732410 for the end ferrites CoFe2O4 and MgFe2O4, respectively. It is unambiguously observed that the XRD pattern agree very well with the measured data with no structural phase transition from cubic to any other phase in the present ferrites with the varying Mg concentration. The FullPROF program based on the Rietveld refinement method [21] has been employed to analysis the XRD pattern of Mg doped cobalt ferrites. Fig. 2 (a-f) show the Rietveld refinement of XRD pattern of present Co1-xMgxFe2O4 ferrites. It is worth mentioning that we have employed pseudo-Voiget function for the refinement. In Fig. 2, the tetrahedral and octahedral positions were considered as 7
fixed, while the oxygen positions were approximated to be as free parameter. Present Rietveld analysis shows a good agreement between observed and calculated diffraction patterns, which is obvious from difference patterns observed from measured and computed diffraction curves. The result obtained from the Rietveld analysis, such as lattice constant, residuals for the Bragg factor, structure factor, goodness of fit, oxygen positions parameter along with crystalline size, X-ray density and variation in intensity ratio of planes (220) to (400) and (422) to (220) are collected in Table 1. The tabulated lattice constant for pure cobalt ferrite (a=8.383 Å) is in a good agreement with the reported data [11, 22]. The variation in crystalline size and lattice constant of asprepared ferrites has been discussed in terms of cations distribution over tetrahedral and octahedral sites. The intensities corresponding to (220) and (422) planes are most sensitive to cations on tetrahedral sites, while intensities of (222) plane corresponds to cations on octahedral sites [16]. Fig. 1 shows that relative intensities of (222) plane decrease more in comparison to (220) and (422) planes with substitution of Mg2+ ions, which confirm that Mg2+ ions prefer the octahedral site. The average crystalline size (D) listed in Table 1 is estimated from X-ray diffraction peak (311) using the following Debye-Scherer’s formula, D 0.94
. cos ,
(1)
where D is crystalline size, λ the wavelength of X-ray, β is the line broadening at half maximum intensity (FWHM) after subtracting the instrumental line broadening and diffraction angle is θ. The most intense peak attributed to (311) reflection occur at 2θ = 35.49˚, 35.53˚, 35.41˚, 35.37˚, 35.36˚ and 35.39˚ for x = 0, 0.2, 0.4, 0.6, 0.8 and 1, respectively. The lattice parameter (a) for spinel cubic structure was calculated using the standard relation a=d(h2+k2+l2)1/2, where h, k and l are the Miller indices of the lattice plane and d is interplanar spacing. Altough in the present Mg
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doped ferrites, there is shifting of Fe3+ ions from tetrahedral to octahedral sites by the addition of Mg2+ ions, but the exchange interaction strength due to change of Fe3+ ions in the as-prepared ferrites seems to be small leading to small variation in lattice constants while going from x=0 to 1 (Table 1). Further, a varying trend of crystalline size in Table 1 is understandable in terms of different nature of driving force for grain boundaries motion and retarding force by pores in these particular ferrites. The IR spectrum was used to determine the quality of the sample and identification of unknown materials. The FTIR spectra in the range of 4000-400 cm-1 of nano Mg-doped cobalt ferrites are shown in Fig. 3. Since in cubic ferrites the low frequency (ν2) absorption band is detected above 400 cm-1 [23], we have chosen the FTIR spectra range 4000-400 cm-1. Fig. 3 shows that two principal absorption bands in the range of 600-400 cm-1 in which the first band is between 580-568 cm-1 (high frequency ν1) corresponds to vibration of tetrahedral metal-oxygen bond and the second band in the range 424-409 cm-1 (low frequency ν2) arises due to stretching vibration of metal-oxygen bond at octahedral sites, which indicates formation of cubic spinel structure, as also observed by Modi et al. [24] in case of magnesium ferri-aluminates. It is observed that the normal mode of vibration of the tetrahedral site is higher than that at octahedral site, which is due to the shorter bond length of tetrahedral than the octahedral. In case of ferrites, Waldron [25] has reported that the bond distance of Fe-O at tetrahedral site (1.89Å) is smaller than the bond distance at octahedral site (1.99 Å), which suggest that Fe3+ ions have greater covalent bonding character at tetrahedral site than that at octahedral site. We believe that the difference between present ν1 and ν2 is due to different Fe3+-O2- bond lengths at the octahedral and tetrahedral sites as suggested by Waldron [25]. From the FTIR spectra (Fig. 3, Table 2), we observe that peak positions corresponding to absorption bands ν1 and ν2 shift from 580 to 568 9
cm-1 and 424 to 409 cm-1 while going from x= 0 to 1 (with increasing Mg-concentration), respectively. Therefore, a similar trend in shifting of peak for the bands ν1 and ν2 with increasing Mg-concentration unambiguously depicts a mixed spinel state of as prepared Mg-doped cobalt ferrites. It is worth mentioning that the observed values of absorption bands ν1 and ν2 of the base ferrites are found to be in tune with earlier reported data for CoFe2O4 [26] and MgFe2O4 [27]. The other IR bands around 3400 and 1633 cm-1 are originated due to bending and stretching vibration of H2O molecules, which indicate about remnants of hydroxyl groups during the sample preparation. A very week band near 1380 cm-1 is found due to the nitrate group which is expected to remain as small residue in such samples. Hence the present spectra on Mg doped cobalt ferrite indicates negligible amount of impurity in the as-prepared samples. The room temperature Raman spectra of present samples in the frequency range of 180900 cm-1 are shown in Fig. 4. It is worth mentioning that in inverse spinel ferrite tetrahedral sites are occupied by half of the Fe3+ cations, whereas the remaining of the Fe3+ and Co2+ cations are distributed over the octahedral sites. In this structure the factor group analysis should lead to five optic active Raman modes namely A1g, Eg and 3T2g [28]. Earlier study on other ferrites [29] showed that the higher frequency Raman modes (A1g) above 600 cm−1 reflect the local lattice effect in the tetrahedral sub-lattice, while lower frequency Raman modes (Eg and T2g) corresponds to the local lattice effect in the octahedral sub-lattice. Different Raman modes for present Mg doped cobalt ferrites are given in Table 2. We observed that five peak maxima at 210, 307, 466, 572 and 693 cm-1 are correspond to Raman modes T2g(1), Eg, T2g(2), T2g(3) and A1g, respectively for cobalt ferrite. These modes are in tune with those reported for cobalt ferrite [30]. The maximum at 707 cm-1 assigned to A1g mode in MgFe2O4 [27] is in agreement with the present maximum position 706 cm-1 for MgFe2O4. In Raman spectroscopy, we observed a 10
considerable distribution of band distance in Fe/Co/Mg-O arrangement which is expected due to differences in ionic radii of Co/Mg and Fe ions. We found that the distribution of bond distances results to double peak like structures corresponding to A1g and Eg modes. From Table 2, it is observed that increase in Mg-contents shifts Raman band to the higher frequency range (blue shift) which can be understood in term of lower atomic mass of Mg than that of Co ions. Field dependent magnetic properties of Mg doped cobalt ferrites measured at room temperature using VSM are shown in Fig. 5. Present magnetization measurements show that the saturation magnetization and coercivity decreases with increase in Mg-concentration. The magnetic parameters such as saturation magnetization (Ms), retentivity, coercivity and magnetic moment evaluated from M-H loop are given in Table 3. The magnetic moment in μB per formula unit is computed using the following relation, nB
( M wt M s )
5585
.
(2)
In Eq. 2, Mwt and Ms represent the molecular weight of the composition and saturation magnetization in g/mol and emu/g, respectively. The magnetic anisotropy (K) is calculated by the following relation [12],
K o (coercivity saturation magnetisation) / 2
(3)
where μo is vacuum permeability. From the present measurement, the saturation magnetization moments for cobalt and magnesium ferrites are found as 77.94 emu/g (=3.27 μB/f.u.) and 35.22 emu/g (=1.26 μB/f.u.), respectively. According to Neel’s model, the magnetization of the tetrahedral sub-lattice is antiparallel to the octahedral sub-lattice in spinel ferrites [31]. In the present Mg doped ferrites, non-magnetic Mg2+ ions substitute Co2+ ions which are having higher magnetic moment, because 11
the preferential occupancy of Mg2+ ions is the octahedral site. Therefore, the substitution of Mg ions affects the arrangement of cation distribution due to exchange of cobalt/magnesium and iron ions between octahedral and tetrahedral sites. This non-equilibrium distribution amends the cations ratio at octahedral and tetrahedral sites, which is expected to shrink the tetrahedraloctahedral interaction and improve the octahedral-octahedral interaction. The spin orbit coupling which is resulted from the unquenched orbital angular moment of Co2+ ions decreases due to substitution of cations within the structure. Since the magnetic anisotropy in Mg ferrite is expected to be lower than cobalt ferrite because Mg2+ ions have no unpaired electrons (zero electrons spin), the magneto crystalline anisotropy is expected to decreases with increase in Mgconcentration. The super-exchange interaction due to non magnetic ions (Mg-O-Fe) seems to be weaker than the magnetic cations i.e. Co-O-Fe and Fe-O-Fe, which also decreases the magneto crystalline anisotropy. These effects are visualized in the present measurement [Fig. 6] wherein gradual decrease in saturation magnetization and coercivity with increasing in Mg concentration (x = 0 to 1) is found. The gradual decrease in coercivity depicts that hard magnetic cobalt ferrite becomes the soft magnetic material on substitution of Mg at Co site. A comparison of present work with the earlier reported data on Mg1-xCoxFe2O4 (x=0. 0.05, 0.1, 0.15, 0.2 and 0.25) by Anis-ur-Rehman et al. [32] shows a general similarity in trend of coercivity in nano Mg-Co ferrites, although our sample preparation (particularly annealing conditions) differs from these authors. 4. Conclusions The sol-gel auto combustion method is found to be successful in preparation of the spinel ferrite Co1-xMgxFe2O4 (x= 0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoparticles. The X-ray diffraction has confirmed the formation of single-phase cubic structure without any trace of impurity, whereas 12
Rietveld refinement using FullPROOF program confirmed existence of cubic structure with space group Fd3m. The FTIR spectra confirmed the absorption band in the range 580-568 cm-1 for the tetrahedral site and at 424-409 cm-1 for octahedral sites. It is seen that the increase in Mgconcentration shifts high frequency bands ν1 and low frequency ν2 shift to lower values. Evolution of Raman spectra shows a doublet like structure for A1g and Eg modes in all the ferrites, which is attributed to spinel structure. Magnetization measurement shows that an increase in Mg2+ concentration reduces the saturation magnetization and magnetic moment. Moreover, substitution of magnesium into the cobalt ferrite alters its magnetic properties and transforms cobalt ferrite from hard to soft magnetic material. Acknowledgments We thank Head, Department of Physics, M. L. Sukhadia University, Udaipur for proving VSM facilities under DST-FIST programme. Dr. M. Gupta, UGC-DAE-CSR, Indore is thanked for XRD measurements. FTIR and Raman spectroscopy measurements were undertaken at Material Research Centre, MNIT, Jaipur. One of us (HSM) is grateful to Science and Engineering Research Board, New Delhi for proving grant under Fast Track Young Scientist Scheme (SR/FTP/PS-160/2012).
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Figure captions Fig. 1. X-ray diffraction pattern of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites.
Fig. 2. Rietveld refined X-ray diffraction pattern of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites. Fig. 3. FTIR spectra of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites at room temperature. Fig. 4. Raman scattering spectra of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites. Fig. 5. Magnetic hysteresis curves for Mg doped CoFe2O4 nano ferrites measured at room temperature. Fig. 6. Variation of crystalline size and coercivity of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites.
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x = 1.0
(533)
(440)
(511)
(422)
(400)
(222)
(311)
(220)
Fig. 1.
Intensity (arb. unit)
x = 0.8 x = 0.6
x = 0.4
x = 0.2 Co1-xMgxFe2O4
20
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2 (degree)
18
x = 0.0 60
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80
Fig. 2. YObs
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40
50
2 (degree)
60
70
80
YObs
(f) x=1.0
YCalc
YCalc
YObs-YCalc
YObs-YCalc
Bragg position
Bragg position
Intensity (arb. unit)
Intensity (arb. unit)
(e) x=0.8
30
20
30
40
50
2 (degree)
60
70
80
19
20
30
40
50
2 (degree)
60
70
80
Fig. 3.
x=1.0
Transmission (%)
x=0.8
x=0.6 x=0.4 x=0.2 x=0.0
Co1-xMgxFe2O4 4000
3500
3000
2500
2000
1500 -1
Wave number (cm )
20
1000
500
Fig. 4.
Co1-xMgxFe2O4
Intensity (arb. unit)
x=0.0 x=0.2 x=0.4 x=0.6 x=0.8 x=1.0
200
300
400
500
600
700 -1
Raman shift (cm )
21
800
900
Fig. 5.
80
Co1-xMgxFe2O4
Magnetic moment (emu/gm)
60 40 20
0.0 0.2 0.4 0.6 0.8 1.0
0 -20 -40 -60 -80 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Magnetic field (T)
22
0.4
0.6
0.8
1.0
Fig. 6.
Coercivity (Oe) iffffffffffffffffffffffffffffffffffffff
700 600
70 500
60
400 300
50
200
40 100 0
30 0.0
0.2
0.4
0.6
Mg concentration (x)
23
0.8
1.0
Coercivity (Oe)
Saturation magnetization (emu/gm)
80
Table 1: Various Rietveld refined parameters deduced from X-ray diffraction of Co1-xMgxFe2O4 ferrites. Crystallite size is computed using 2θ for (311) plane.
0.0
Crystalline size (nm) 60.25
χ2
RF
RBragg
GOF-index
I(220)/I(400)
I(422)/I(220)
8.383
Density (g/cm3) 5.290
0.86
5.96
5.62
0.93
1.08
0.71
0.2
66.84
8.385
5.177
0.88
5.28
4.34
0.93
1.06
0.66
0.4
58.50
8.381
5.018
0.88
6.36
3.86
0.95
1.16
0.67
0.6
62.60
8.380
4.812
0.87
5.82
4.99
0.94
1.16
0.67
0.8
50.78
8.380
4.737
0.92
5.25
4.05
0.96
1.24
0.70
1.0
56.33
8.382
4.554
0.94
4.77
3.23
0.97
1.32
0.58
x
a (Å)
24
Table 2: Raman phonon modes and FTIR frequency bands of nano Co1-xMgxFe2O4.
Raman shift (cm-1)
IR bands wavelength (cm-1)
x A1g(1)
T2g(3)
T2g(2)
Eg
T2g(1)
ν1
ν2
0
692.69
572.19
465.59
306.49
209.48
580
424
0.2
694.65
571.01
466.97
310.99
209.48
578
417
0.4
696.42
557.68
462.42
312.17
211.26
578
417
0.6
698.37
556.07
466.38
321.77
211.26
572
418
0.8
702.29
551.22
474.61
328.68
215.37
568
413
1.0
706.02
533.77
481.47
331.18
215.48
568
409
25
Table 3: Variation in saturation magnetization (Ms), coercivity (Hc), remanance magnetization (Mr), Magnetic anisotropy (K) and magnetic moment (nB) with varying Mg concentration in Co1xMgxFe2O4. x
Ms (emu/g)
Hc (Oe)
Mr (emu/g)
nB (μB/f.u.)
K (erg/cm3)
0
77.94
680.20
21.42
3.27
0.033
0.2
72.96
488.92
23.19
2.97
0.022
0.4
61.61
402.53
21.02
2.44
0.016
0.6
49.41
254.44
19.18
1.89
0.008
0.8
39.10
161.87
11.97
1.45
0.004
1.0
35.22
38.28
4.90
1.26
0.001
26
S0025-5408(16)31131-X http://dx.doi.org/doi:10.1016/j.materresbull.2016.09.027 MRB 8959
To appear in:
MRB
Received date: Revised date: Accepted date:
18-3-2016 21-7-2016 23-9-2016
Please cite this article as: H.S.Mund, B.L.Ahuja, Structural and magnetic properties of Mg doped cobalt ferrite nano particles prepared by sol-gel method, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.09.027 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.
Structural and magnetic properties of Mg doped cobalt ferrite nano particles prepared by sol-gel method H. S. Mund* and B. L. Ahuja
Department of Physics, University College of Science, M. L. Sukhadia University Udaipur, Rajasthan-313001, India
*Corresponding Author E-mail: [email protected]
1
Graphical abstract
Highlights
Mg doped nano cobalt ferrites with systematic Mg composition prepared by sol-gel method.
XRD, FTIR and Raman spectra confirmed the formation of single-phase cubic spinel structure.
Different ionic radii of Co/Mg and Fe ions revealed a doublet-like peak behavior at A1g and Eg modes in Raman spectra.
Coercivity data depict that hard magnetic cobalt ferrite becomes the soft magnetic material on substitution of Mg at Co site.
2
Abstract Effect of Mg doping on structural and magnetic properties of nano cobalt ferrites prepared via sol-gel auto-combustion method is reported. The sintered samples (Co1-xMgxFe2O4, where x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were characterized using X-ray diffraction, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and vibrating sample magnetometer. Rietveld refinement of X-ray diffraction spectra shows the single phase cubic structure (space group Fd3m) for the prepared samples. Raman spectroscopy reveals a doubletlike peak behavior in A1g and Eg modes arising due to different ionic radii of Co/Mg and Fe ions. Two prominent vibration frequencies in FTIR spectroscopy data also confirm the cubic spinel structure in the prepared samples. The magnetic parameters such as saturation magnetization, coercivity and remanence are studied with increasing the Mg-concentration in cobalt ferrite. Systematic changes in magnetic properties while going from x=0 to 1 are discussed. KEYWORDS: A. Oxides, B. Magnetic properties, C. Raman spectroscopy, C. X-ray diffraction, D. Crystal structure.
1. Introduction Spinel ferrites are very useful in various technological devices such as switching circuits, high density magnetic storage, microwave based instruments, telecommunication equipment, magnetic fluids, gas sensors, etc. [1-10]. Generally, a normal spinel ferrite is described as (M2+)p[Fe23+]qO4, whereas an inverse spinel ferrite consists of ionic arrangement like (Fe3+)p[M2+Fe3+]qO4, where p, q and M correspond to tetrahedral site, octahedral site and 3d divalent transition element cations, respectively. It is known that the structural, electrical and 3
magnetic properties of ferrites are highly sensitive to conditions of their preparation, compositions, magnetic interactions and distribution of cations at tetrahedral and octahedral sites. The cation distribution at tetrahedral and octahedral sites is sensitive to ionic radii of Fe/Co and M ions, the type of bonding and the preparation method. Cobalt ferrite is a hard magnetic material with large coercivity and moderate magnetization whereas magnesium ferrite is a soft magnetic n-type semiconductor material. Among earlier studies on Mn doped cobalt ferrites, effect of Mn substitution on the magnetic and dielectric properties of cobalt ferrite synthesized by an auto-combustion route has been discussed by Kambale et al. [8]. Authors observed an increase in dielectric permittivity with Mn concentration. The semiconductor behavior of Mn doped cobalt ferrites (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) has been reported by Yadav et al. [9] using dc resistivity measurement. Melikhov et al. [10] have investigated the temperature variation of magnetic anisotropy and coercive field of magnetoelastic Mn substituted cobalt ferrites (CoMnxFe2-xO4 with 0 ≤ x ≤ 0.6). Raut et al. [11] have investigated the effect of Zn substitution on the structural and magnetic properties of cobalt ferrite nanoparticles prepared by sol-gel auto-combustion technique. Effect of Ni substitution on structural, electrical and magnetic properties of cobalt ferrite nanoparticles synthesized by self-combustion method without external oxidizing agents has been studied by Sontu et al. [12]. Wahba and Mohamed [13] have investigated the structural, magnetic, and electric properties of Co0.8Ni0.2CryFe2-yO4 (0.00 ≤ y ≤ 0.75; step 0.15) prepared via citrate precursor method. They found that Cr substitution improves ac electric properties of the ferrites. Wu et al. [14] have reported infrared radiation properties of CoFe2-xCexO4 (x=0, 0.01, 0.05, 0.1 and 0.15) via the sol-gel autocombustion method. Pandit et al. [15] have presented the structural, cation distribution, electrical and magnetic properties of CoAlxFe2-xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8) ferrites. The influence of the 4
Zn and Mg contents on the structural and magnetic properties of cobalt ferrites synthesized by chemical co-precipitation method was investigated by Varshney et al [16] using X-ray powder diffraction, Raman spectroscopy and vibrating sample magnetometer. Tadi et al. [17] investigated the structural and magnetic properties of Ni substituted MgFe2O4 which was synthesized using thermal decomposition of metal organic materials. Electronic structure of chemically synthesized MgFe2O4 nanoparticles has been studied by Singh et al. [18]. Aono et al. [19] have studied the surface structure and conductance of MgFe2O4 ferrite powder prepared by a chemical method. Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering has been studied by Reddy et al. [20]. Although plentiful reports are available on the magnetic and electric response of cobalt and magnesium ferrites, work related to systematic study of structure, electric and magnetic properties of Mg doped cobalt ferrites prepared by sol-gel auto combustion method is still lacking. It is worth mentioning that the sol-gel auto-combustion method has definite advantages over other methods, like variety of starting materials as cheap precursors to choose, more accurate control over phase formation, simple route of preparation with desired stoichiometry and uniformity in particle size. Therefore, in the present paper, we have attempted to investigate the structural and magnetic properties of nano Co1-xMgxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ferrites prepared by gel auto-combustion method. We have particularly studied the effect of nonmagnetic Mg2+ ions substitution in cobalt ferrite prepared via sol-gel auto-combustion method without varying the pH value of the precursor solution.
5
2. Experimental work Nano powdered Mg doped cobalt ferrites (for all compositions x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by the sol-gel auto-combustion method under similar parameters. In the present work, citric acid was used as a fuel and complexing agent with analytical reagents of Fe(NO3)3·9H2O, Co(NO3)2·6H2O and Mg(NO3)3·6H2O were the original materials. The molar ratio of nitrates to citric acid was kept to 1:1. Firstly, depending upon the composition, the appropriate amount of metal nitrates and citric acid were dissolved in deionized water to obtain a mixed solution. During the mixing process, the solution was continuously stirred for 30 min at 50 °C using a magnetic agitator. Thereafter, the mixed solution was individually kept stirred at 100 °C for 3 h to transform into gel. The individual gel was dried at 150 °C for 2 h in order to obtain the dried gel. The dried gel was burnt in an auto-combustion manner until the entire gels was burnt out (as visible from complete extinction of redness) to form a loose powder, known as as-burnt powder. This whole process was carried out under ambient atmosphere. After that, the individual powders were sintered at the temperature 1000 °C for 6 h. The powder X-ray diffraction pattern of as-prepared samples was carried out using Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ = 0.15406 nm) to identify the phase and structure of annealed samples. To find out the vibrational modes which are sensitive to chemical and structural changes, Fourier transform infrared (FTIR) spectroscopy studies were carried out in the range 4000-400 cm-1. The FTIR spectrometer (PerkinElmer Spectrum Two) used in the present measurements was having resolution of 0.5 cm-1 in KBr medium. It is known that KBr is an alkyl halide and it does not show any absorption spectrum in the infrared region, therefore wafers for FTIR spectroscopy were made by diluting as-prepared ferrites samples in KBr. Since
6
Raman spectroscopy is a novel tool to study the electronic properties of spinel ferrites, the Raman spectroscopy measurements on the as-prepared samples were carried out using STR-500 Confocal micro Raman spectrometer with 532 nm DPSS laser. The laser power was optimized to 5 mW on the surface area. Room temperature magnetic measurements were carried out using vibrating sample magnetometer (Lakeshore 7304) as a function of magnetic field in the range of ±10 kOe. 3. Results and discussion The XRD pattern of Co1-xMgxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ferrites in the 2θ range of 20-80° with step size of 0.02° are shown in Fig. 1. The XRD pattern shows that all the reflection peaks corresponding to (220), (311), (222), (400), (422), (511) and (440) planes in pure (x=0) and doped ferrites. The weak reflection peak corresponding to (533) is due to higher angle reflection index and small structure factor. Further, the moderate band in the base line 20°<2θ<30° in Fig. 2 (b) is due to diffraction by glass plate sample holder. Therefore, the XRD pattern validates the formation of cubic spinel structure with Fd3m space group. The peak position in XRD patterns are also indexed using the standard JCPDS card Nos. 79-1744 and 732410 for the end ferrites CoFe2O4 and MgFe2O4, respectively. It is unambiguously observed that the XRD pattern agree very well with the measured data with no structural phase transition from cubic to any other phase in the present ferrites with the varying Mg concentration. The FullPROF program based on the Rietveld refinement method [21] has been employed to analysis the XRD pattern of Mg doped cobalt ferrites. Fig. 2 (a-f) show the Rietveld refinement of XRD pattern of present Co1-xMgxFe2O4 ferrites. It is worth mentioning that we have employed pseudo-Voiget function for the refinement. In Fig. 2, the tetrahedral and octahedral positions were considered as 7
fixed, while the oxygen positions were approximated to be as free parameter. Present Rietveld analysis shows a good agreement between observed and calculated diffraction patterns, which is obvious from difference patterns observed from measured and computed diffraction curves. The result obtained from the Rietveld analysis, such as lattice constant, residuals for the Bragg factor, structure factor, goodness of fit, oxygen positions parameter along with crystalline size, X-ray density and variation in intensity ratio of planes (220) to (400) and (422) to (220) are collected in Table 1. The tabulated lattice constant for pure cobalt ferrite (a=8.383 Å) is in a good agreement with the reported data [11, 22]. The variation in crystalline size and lattice constant of asprepared ferrites has been discussed in terms of cations distribution over tetrahedral and octahedral sites. The intensities corresponding to (220) and (422) planes are most sensitive to cations on tetrahedral sites, while intensities of (222) plane corresponds to cations on octahedral sites [16]. Fig. 1 shows that relative intensities of (222) plane decrease more in comparison to (220) and (422) planes with substitution of Mg2+ ions, which confirm that Mg2+ ions prefer the octahedral site. The average crystalline size (D) listed in Table 1 is estimated from X-ray diffraction peak (311) using the following Debye-Scherer’s formula, D 0.94
. cos ,
(1)
where D is crystalline size, λ the wavelength of X-ray, β is the line broadening at half maximum intensity (FWHM) after subtracting the instrumental line broadening and diffraction angle is θ. The most intense peak attributed to (311) reflection occur at 2θ = 35.49˚, 35.53˚, 35.41˚, 35.37˚, 35.36˚ and 35.39˚ for x = 0, 0.2, 0.4, 0.6, 0.8 and 1, respectively. The lattice parameter (a) for spinel cubic structure was calculated using the standard relation a=d(h2+k2+l2)1/2, where h, k and l are the Miller indices of the lattice plane and d is interplanar spacing. Altough in the present Mg
8
doped ferrites, there is shifting of Fe3+ ions from tetrahedral to octahedral sites by the addition of Mg2+ ions, but the exchange interaction strength due to change of Fe3+ ions in the as-prepared ferrites seems to be small leading to small variation in lattice constants while going from x=0 to 1 (Table 1). Further, a varying trend of crystalline size in Table 1 is understandable in terms of different nature of driving force for grain boundaries motion and retarding force by pores in these particular ferrites. The IR spectrum was used to determine the quality of the sample and identification of unknown materials. The FTIR spectra in the range of 4000-400 cm-1 of nano Mg-doped cobalt ferrites are shown in Fig. 3. Since in cubic ferrites the low frequency (ν2) absorption band is detected above 400 cm-1 [23], we have chosen the FTIR spectra range 4000-400 cm-1. Fig. 3 shows that two principal absorption bands in the range of 600-400 cm-1 in which the first band is between 580-568 cm-1 (high frequency ν1) corresponds to vibration of tetrahedral metal-oxygen bond and the second band in the range 424-409 cm-1 (low frequency ν2) arises due to stretching vibration of metal-oxygen bond at octahedral sites, which indicates formation of cubic spinel structure, as also observed by Modi et al. [24] in case of magnesium ferri-aluminates. It is observed that the normal mode of vibration of the tetrahedral site is higher than that at octahedral site, which is due to the shorter bond length of tetrahedral than the octahedral. In case of ferrites, Waldron [25] has reported that the bond distance of Fe-O at tetrahedral site (1.89Å) is smaller than the bond distance at octahedral site (1.99 Å), which suggest that Fe3+ ions have greater covalent bonding character at tetrahedral site than that at octahedral site. We believe that the difference between present ν1 and ν2 is due to different Fe3+-O2- bond lengths at the octahedral and tetrahedral sites as suggested by Waldron [25]. From the FTIR spectra (Fig. 3, Table 2), we observe that peak positions corresponding to absorption bands ν1 and ν2 shift from 580 to 568 9
cm-1 and 424 to 409 cm-1 while going from x= 0 to 1 (with increasing Mg-concentration), respectively. Therefore, a similar trend in shifting of peak for the bands ν1 and ν2 with increasing Mg-concentration unambiguously depicts a mixed spinel state of as prepared Mg-doped cobalt ferrites. It is worth mentioning that the observed values of absorption bands ν1 and ν2 of the base ferrites are found to be in tune with earlier reported data for CoFe2O4 [26] and MgFe2O4 [27]. The other IR bands around 3400 and 1633 cm-1 are originated due to bending and stretching vibration of H2O molecules, which indicate about remnants of hydroxyl groups during the sample preparation. A very week band near 1380 cm-1 is found due to the nitrate group which is expected to remain as small residue in such samples. Hence the present spectra on Mg doped cobalt ferrite indicates negligible amount of impurity in the as-prepared samples. The room temperature Raman spectra of present samples in the frequency range of 180900 cm-1 are shown in Fig. 4. It is worth mentioning that in inverse spinel ferrite tetrahedral sites are occupied by half of the Fe3+ cations, whereas the remaining of the Fe3+ and Co2+ cations are distributed over the octahedral sites. In this structure the factor group analysis should lead to five optic active Raman modes namely A1g, Eg and 3T2g [28]. Earlier study on other ferrites [29] showed that the higher frequency Raman modes (A1g) above 600 cm−1 reflect the local lattice effect in the tetrahedral sub-lattice, while lower frequency Raman modes (Eg and T2g) corresponds to the local lattice effect in the octahedral sub-lattice. Different Raman modes for present Mg doped cobalt ferrites are given in Table 2. We observed that five peak maxima at 210, 307, 466, 572 and 693 cm-1 are correspond to Raman modes T2g(1), Eg, T2g(2), T2g(3) and A1g, respectively for cobalt ferrite. These modes are in tune with those reported for cobalt ferrite [30]. The maximum at 707 cm-1 assigned to A1g mode in MgFe2O4 [27] is in agreement with the present maximum position 706 cm-1 for MgFe2O4. In Raman spectroscopy, we observed a 10
considerable distribution of band distance in Fe/Co/Mg-O arrangement which is expected due to differences in ionic radii of Co/Mg and Fe ions. We found that the distribution of bond distances results to double peak like structures corresponding to A1g and Eg modes. From Table 2, it is observed that increase in Mg-contents shifts Raman band to the higher frequency range (blue shift) which can be understood in term of lower atomic mass of Mg than that of Co ions. Field dependent magnetic properties of Mg doped cobalt ferrites measured at room temperature using VSM are shown in Fig. 5. Present magnetization measurements show that the saturation magnetization and coercivity decreases with increase in Mg-concentration. The magnetic parameters such as saturation magnetization (Ms), retentivity, coercivity and magnetic moment evaluated from M-H loop are given in Table 3. The magnetic moment in μB per formula unit is computed using the following relation, nB
( M wt M s )
5585
.
(2)
In Eq. 2, Mwt and Ms represent the molecular weight of the composition and saturation magnetization in g/mol and emu/g, respectively. The magnetic anisotropy (K) is calculated by the following relation [12],
K o (coercivity saturation magnetisation) / 2
(3)
where μo is vacuum permeability. From the present measurement, the saturation magnetization moments for cobalt and magnesium ferrites are found as 77.94 emu/g (=3.27 μB/f.u.) and 35.22 emu/g (=1.26 μB/f.u.), respectively. According to Neel’s model, the magnetization of the tetrahedral sub-lattice is antiparallel to the octahedral sub-lattice in spinel ferrites [31]. In the present Mg doped ferrites, non-magnetic Mg2+ ions substitute Co2+ ions which are having higher magnetic moment, because 11
the preferential occupancy of Mg2+ ions is the octahedral site. Therefore, the substitution of Mg ions affects the arrangement of cation distribution due to exchange of cobalt/magnesium and iron ions between octahedral and tetrahedral sites. This non-equilibrium distribution amends the cations ratio at octahedral and tetrahedral sites, which is expected to shrink the tetrahedraloctahedral interaction and improve the octahedral-octahedral interaction. The spin orbit coupling which is resulted from the unquenched orbital angular moment of Co2+ ions decreases due to substitution of cations within the structure. Since the magnetic anisotropy in Mg ferrite is expected to be lower than cobalt ferrite because Mg2+ ions have no unpaired electrons (zero electrons spin), the magneto crystalline anisotropy is expected to decreases with increase in Mgconcentration. The super-exchange interaction due to non magnetic ions (Mg-O-Fe) seems to be weaker than the magnetic cations i.e. Co-O-Fe and Fe-O-Fe, which also decreases the magneto crystalline anisotropy. These effects are visualized in the present measurement [Fig. 6] wherein gradual decrease in saturation magnetization and coercivity with increasing in Mg concentration (x = 0 to 1) is found. The gradual decrease in coercivity depicts that hard magnetic cobalt ferrite becomes the soft magnetic material on substitution of Mg at Co site. A comparison of present work with the earlier reported data on Mg1-xCoxFe2O4 (x=0. 0.05, 0.1, 0.15, 0.2 and 0.25) by Anis-ur-Rehman et al. [32] shows a general similarity in trend of coercivity in nano Mg-Co ferrites, although our sample preparation (particularly annealing conditions) differs from these authors. 4. Conclusions The sol-gel auto combustion method is found to be successful in preparation of the spinel ferrite Co1-xMgxFe2O4 (x= 0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoparticles. The X-ray diffraction has confirmed the formation of single-phase cubic structure without any trace of impurity, whereas 12
Rietveld refinement using FullPROOF program confirmed existence of cubic structure with space group Fd3m. The FTIR spectra confirmed the absorption band in the range 580-568 cm-1 for the tetrahedral site and at 424-409 cm-1 for octahedral sites. It is seen that the increase in Mgconcentration shifts high frequency bands ν1 and low frequency ν2 shift to lower values. Evolution of Raman spectra shows a doublet like structure for A1g and Eg modes in all the ferrites, which is attributed to spinel structure. Magnetization measurement shows that an increase in Mg2+ concentration reduces the saturation magnetization and magnetic moment. Moreover, substitution of magnesium into the cobalt ferrite alters its magnetic properties and transforms cobalt ferrite from hard to soft magnetic material. Acknowledgments We thank Head, Department of Physics, M. L. Sukhadia University, Udaipur for proving VSM facilities under DST-FIST programme. Dr. M. Gupta, UGC-DAE-CSR, Indore is thanked for XRD measurements. FTIR and Raman spectroscopy measurements were undertaken at Material Research Centre, MNIT, Jaipur. One of us (HSM) is grateful to Science and Engineering Research Board, New Delhi for proving grant under Fast Track Young Scientist Scheme (SR/FTP/PS-160/2012).
13
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13. A. M. Wahba, M. B. Mohamed, Structural, magnetic, and dielectric properties of nanocrystalline Cr-substituted Co0.8Ni0.2Fe2O4 ferrite, Ceram. Int. 40 (2014) 6127. 14. X. Wu, H. Yun, H. Dongn, L. Geng, Enhanced infrared radiation properties of CoFe2O4 by single Ce3+-doping with energy-efficient preparation, Ceram. Int. 40 (2014) 5905. 15. R. Pandit, K. K. Sharma, P. Kaur, R. K. Kotnala, J. Shah, R. Kumar, Effect of Al3+ substitution on structural, cation distribution, electrical and magnetic properties of CoFe2O4, J. Phys. Chem. Solids 75 (2014) 558. 16. D. Varshney, K. Verma, A. Kumar, Substitutional effect on structural and magnetic properties of AxCo1-xFe2O4 (A = Zn, Mg and x = 0.0, 0.5) ferrites, J. Mol. Struct. 1006 (2011) 447. 17. R. Tadi, Y. Kim, A. K. Sarella, C. G. Kim, K. S. Ryu, Relation of structure and magnetic properties in nickel substituted MgFe2O4, J. Magn. Magn. Mater. 322 (2010) 3372. 18. J. P. Singh, S. O. Won, W. C. Lim, I. J. Lee, K. H. Chae, Electronic structure studies of chemically synthesized MgFe2O4 nanoparticles, J. Mol. Struct. 1108 (2016) 444. 19. H. Aono, H. Hirazawa, T. Naohara, T. Maehara, Surface study of fine MgFe2O4 ferrite powder prepared by chemical methods, Appl. Surf. Sci. 254 (2008) 2319. 20. M. P. Reddy, R. A. Shakoora, A. M. A. Mohamed, M. Gupta, Q. Huang, Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering, Ceram. Int. 42 (2016) 4221. 21. J. R. Carvajal, Recent Developments of the Program FULLPROF, in Commission on Powder Diffraction (IUCr), Physica B 192 (1993) 55. 22. D. M. Jnaneshwara, D. N. Avadhani, B. D. Prasad, B. M. Nagabhushana, H. Nagabhushana, S. C. Sharma, S. C. Prashantha, C. Shivakumara, Effect of zinc substitution on the nanocobalt ferrite powders for nanoelectronic devices, J. Alloys Compd. 587 (2014) 50. 23. V. A. M. Brabers, Infrared spectra of cubic and tetragonal manganese ferrites, Phys. Status Solidi (b) 33 (1969) 563. 24. K. B. Modi, M. C. Chhantbar, H. H. Joshi, Study of elastic behaviour of magnesium ferri aluminates, Ceram. Int. 32 (2006) 111. 25. R. D. Waldron, Infrared spectra of ferrites, Phys. Res. 99 (1955) 8. 26. M. P. Reddy, X. Zhou, A. Yann, S. Du, Q. Huang, A. M. A. Mohamed, Low temperature hydrothermal synthesis, structural investigation and functional properties of CoxMn1-xFe2O4 (0 ≤ x ≤ 1) nanoferrites, Superlattices Microstruct. 81 (2015) 233.
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Figure captions Fig. 1. X-ray diffraction pattern of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites.
Fig. 2. Rietveld refined X-ray diffraction pattern of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites. Fig. 3. FTIR spectra of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites at room temperature. Fig. 4. Raman scattering spectra of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites. Fig. 5. Magnetic hysteresis curves for Mg doped CoFe2O4 nano ferrites measured at room temperature. Fig. 6. Variation of crystalline size and coercivity of nano Co1-xMgxFe2O4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ferrites.
17
x = 1.0
(533)
(440)
(511)
(422)
(400)
(222)
(311)
(220)
Fig. 1.
Intensity (arb. unit)
x = 0.8 x = 0.6
x = 0.4
x = 0.2 Co1-xMgxFe2O4
20
30
40
50
2 (degree)
18
x = 0.0 60
70
80
Fig. 2. YObs
YObs
(b) x=0.2
YCalc
YCalc
YObs-YCalc
YObs-YCalc
Bragg position
Bragg position
Intensity (arb. unit)
Intensity (arb. unit)
(a) x=0.0
20
30
40
50
2 (degree)
60
70
80
20
YObs
40
50
2 (degree)
60
70
80
YObs
(d) x=0.6
YCalc
YCalc
YObs-YCalc
YObs-YCalc
Bragg position
Bragg position
Intensity (arb. unit)
Intensity (arb. unit)
(c) x=0.4
30
20
30
40
50
2 (degree)
60
70
80
20
YObs
40
50
2 (degree)
60
70
80
YObs
(f) x=1.0
YCalc
YCalc
YObs-YCalc
YObs-YCalc
Bragg position
Bragg position
Intensity (arb. unit)
Intensity (arb. unit)
(e) x=0.8
30
20
30
40
50
2 (degree)
60
70
80
19
20
30
40
50
2 (degree)
60
70
80
Fig. 3.
x=1.0
Transmission (%)
x=0.8
x=0.6 x=0.4 x=0.2 x=0.0
Co1-xMgxFe2O4 4000
3500
3000
2500
2000
1500 -1
Wave number (cm )
20
1000
500
Fig. 4.
Co1-xMgxFe2O4
Intensity (arb. unit)
x=0.0 x=0.2 x=0.4 x=0.6 x=0.8 x=1.0
200
300
400
500
600
700 -1
Raman shift (cm )
21
800
900
Fig. 5.
80
Co1-xMgxFe2O4
Magnetic moment (emu/gm)
60 40 20
0.0 0.2 0.4 0.6 0.8 1.0
0 -20 -40 -60 -80 -1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Magnetic field (T)
22
0.4
0.6
0.8
1.0
Fig. 6.
Coercivity (Oe) iffffffffffffffffffffffffffffffffffffff
700 600
70 500
60
400 300
50
200
40 100 0
30 0.0
0.2
0.4
0.6
Mg concentration (x)
23
0.8
1.0
Coercivity (Oe)
Saturation magnetization (emu/gm)
80
Table 1: Various Rietveld refined parameters deduced from X-ray diffraction of Co1-xMgxFe2O4 ferrites. Crystallite size is computed using 2θ for (311) plane.
0.0
Crystalline size (nm) 60.25
χ2
RF
RBragg
GOF-index
I(220)/I(400)
I(422)/I(220)
8.383
Density (g/cm3) 5.290
0.86
5.96
5.62
0.93
1.08
0.71
0.2
66.84
8.385
5.177
0.88
5.28
4.34
0.93
1.06
0.66
0.4
58.50
8.381
5.018
0.88
6.36
3.86
0.95
1.16
0.67
0.6
62.60
8.380
4.812
0.87
5.82
4.99
0.94
1.16
0.67
0.8
50.78
8.380
4.737
0.92
5.25
4.05
0.96
1.24
0.70
1.0
56.33
8.382
4.554
0.94
4.77
3.23
0.97
1.32
0.58
x
a (Å)
24
Table 2: Raman phonon modes and FTIR frequency bands of nano Co1-xMgxFe2O4.
Raman shift (cm-1)
IR bands wavelength (cm-1)
x A1g(1)
T2g(3)
T2g(2)
Eg
T2g(1)
ν1
ν2
0
692.69
572.19
465.59
306.49
209.48
580
424
0.2
694.65
571.01
466.97
310.99
209.48
578
417
0.4
696.42
557.68
462.42
312.17
211.26
578
417
0.6
698.37
556.07
466.38
321.77
211.26
572
418
0.8
702.29
551.22
474.61
328.68
215.37
568
413
1.0
706.02
533.77
481.47
331.18
215.48
568
409
25
Table 3: Variation in saturation magnetization (Ms), coercivity (Hc), remanance magnetization (Mr), Magnetic anisotropy (K) and magnetic moment (nB) with varying Mg concentration in Co1xMgxFe2O4. x
Ms (emu/g)
Hc (Oe)
Mr (emu/g)
nB (μB/f.u.)
K (erg/cm3)
0
77.94
680.20
21.42
3.27
0.033
0.2
72.96
488.92
23.19
2.97
0.022
0.4
61.61
402.53
21.02
2.44
0.016
0.6
49.41
254.44
19.18
1.89
0.008
0.8
39.10
161.87
11.97
1.45
0.004
1.0
35.22
38.28
4.90
1.26
0.001
26