Structural, magnetic and optical characterization of Ni0.8Zn0.2Fe2O4 nano particles prepared by co-precipitation method

Physica B 502 (2016) 181–186

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Structural, magnetic and optical characterization of Ni0.8Zn0.2Fe2O4 nano particles prepared by co-precipitation method Y.B. Kannan a,n, R. Saravanan b, N. Srinivasan c, K. Praveena d,1, K. Sadhana e,2 a

Department of Physics, Arumugam Pillai Seethai Ammal College, Tiruppattur 630211, India Research Centre & PG Department of Physics, The Madura College, Madurai 625011, India c Research Centre & PG Department of Physics, Thiagarajar College, Madurai 625009, India d School of Physics, Univeristy of Hyderabad, Hyderabad 500046, India e Material Research Center, Indian Institute of Science, Bangalore 560012, India b

art ic l e i nf o

a b s t r a c t

Article history: Received 17 April 2016 Received in revised form 30 August 2016 Accepted 7 September 2016 Available online 9 September 2016

Bond strength values, between tetrahedral sites and octahedral sites atoms in the unit cell, are evaluated using maximum entropy method (MEM) for the Ni0.8Zn0.2Fe2O4 nano ferrite particles, prepared by coprecipitation method and sintered at 900 °C. The spinel structure is confirmed from the XRD analysis done using the Rietveld method. Substitution of zinc ion causes increase in lattice parameter value. Thermal behavior, morphology, magnetic properties and optical band gap energy values of the sample are determined by using thermogravimetric analysis and differential thermal analysis, scanning electron microscope, vibrating sample magnetometer and UV–VIS–NIR techniques respectively. Low value of saturation magnetization is attributed to the disorder in cation distribution. & 2016 Elsevier B.V. All rights reserved.

Keywords: Co-precipitation method X-Ray diffraction Cation distribution Mid-bond electron charge density using MEM studies Magnetic and optical properties

1. Introduction Nickel-Zinc (NZ) nano ferrite particles, belong to the category of soft magnetic material, have innumerable applications such as non-resonant devices, radio frequency circuits, high quality filters, read/write heads for high speed digital tape, electromagnetic applications that require a high permeability such as inductors, electromagnetic wave absorbers, antenna rods, suppression of electromagnetic interference, broad band transformers, etc. [1,2]. Ni-Zn ferrite is used in the high frequency region (10–500 MHz) because of its high resistivity, low eddy current loss. Microstructure and magnetic properties of Ni-Zn ferrites depend on the method of preparation, sintering conditions and the doping concentration [3]. The concentration of zinc has an important role in determining the properties of Ni–Zn ferrites because the redistribution of nickel, zinc and iron ions in the tetrahedral (A) and octahedral sites (B) as the nickel is substituted by zinc in the spinel. Zinc ferrite is known to exist as a ‘normal’ spinel, and nickel ferrite is known to exist as an ‘inverse’ spinel [4]. Co-precipitation

method is one of the simple chemical method which gives better control over crystallite size and other properties of the materials [5]. Various authors have prepared mixed Ni-Zn ferrites by using different methods, such as combustion method [6] co-precipitation [7], sol-gel [8], solid state reaction [9], microwave sintering [10], citrate precursor [11], polyacrylamide gel [12] and reported various properties of Ni-Zn ferrites. But, as far as the authors are concerned, there is not much report on ferrite materials regarding the bond strength between the atoms at tetrahedral (A) site and octahedral (B) site by employing maximum entropy method (MEM), from which numerical values for the various site interactions, namely A-A, A-B and B-B, could be derived. Hitherto, only theoretical predictions of these site interactions were published. Hence, in this communication, we evaluated mid-bond electron density values for various site interactions from maximum entropy method (MEM) and reported the same along with their structural, optical and magnetic studies of Ni0.8Zn0.2Fe2O4 nano ferrite particles.

n

Corresponding author. E-mail address: [email protected] (Y.B. Kannan). 1 Present Address: Department of Physics, National Taiwan, Normal University, Taipei 1167, Taiwan. 2 Present Address: Department of Physics, University College of Science, Osmania University, Saifabad, Hyderabad 500004, India. http://dx.doi.org/10.1016/j.physb.2016.09.006 0921-4526/& 2016 Elsevier B.V. All rights reserved.

2. Experimental methods Samples of Ni0.8Zn0.2Fe2O4 have been prepared by wet chemical co-precipitation method. The starting materials were AR grade

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nickel nitrate [Ni(NO3)2
6H2O], ferric nitrate [Fe(NO3)2
9H2O], and zinc nitrate [Zn(NO3)2
6H2O], with purity 99.99%. The desired proportions of ferric, zinc, nickel nitrate were dissolved in 50 ml of de-ionized water. Aqueous sodium hydroxide (NaOH) was added to the prepared solutions with constant stirring and controlling pH value. The precipitate was obtained at pH 4 10. The precipitations were separated by centrifugation and then washed repeatedly with de-ionized water, followed by drying in an oven overnight at 60 °C. The powders were finally sintered at 900 °C/5 h using conventional sintering method. The thermo gravimetry – differential thermal analysis (TG-DTA) in the temperature range of 40–730 °C at the rate of 5 °C/min in static air atmosphere of the sample Ni0.8Zn0.2Fe2O4 sintered at 900 °C was done using thermal analyzer (Model: Perkin Elmer, Diamond Tg/DTA). X-ray diffraction pattern of the sample was recorded at room temperature using Bruker AXS D8 advance X-ray diffractometer with Cu radiation (λ ¼1.5406 Å). The sample was exposed to the radiation with a primary beam power of 40 kV and 35 mA with step scan 0.02° in 2θ range from 25–120°. The sample was analyzed with scanning electron microscope (SEM) (Hitachi, Model: S-3400N) for morphology study. The magnetic properties of the sample were studied at room temperature by using vibrating sample magnetometer (Lakeshore VSM 7410 Model). Optical absorption spectra of the sample Ni0.8Zn0.2Fe2O4 was recorded in the UV–VIS wavelength range of 2000–7500 Å.

Fig. 1. TG-DTA of the Ni0.8Zn0.2Fe2O4 sample sintered at 900 °C.

3. Results and discussions 3.1. Thermogravimetric – differential thermal analysis Fig. 1 show the thermo gravimetry – differential thermal analysis (TG-DTA) analysis in the temperature range of 40–730 °C at the rate of 5 °C/min in static air atmosphere of the sample Ni0.8Zn0.2Fe2O4 sintered at 900 °C. From the TG curve, it can be seen that the initial weight loss is due to the evaporation of physically and chemically adsorbed water and the weight loss continues up to 650 °C and it becomes almost stable thereafter. From the TG graph it can be seen that the total weight loss in the studied temperature range is only 2%. From the thermal behavior no anomalies are observed and the behavior is as expected from already sintered samples. 3.2. X-ray diffraction analysis The raw X-ray diffractogram of the sample Ni0.8Zn0.2Fe2O4 sintered at 900 °C, is shown in Fig. 2a. The JCPDS file [13] is used to the index the (202), (311), (222), (400), (422), (511) and (440) planes in the diffraction pattern and formation of the spinel phase structure with Fd-3m (227) space group is confirmed from the indexing. A small additional peak corresponds to hematite phase (α-Fe2O3, JCPDS 89-8103) is appearing around 2θ E80°. The presence of hematite phase in nickel-zinc ferrite is also reported by [14]. When zinc ions are substituted for nickel ions in Ni1  xZnxFe2O4 nano ferrite samples, the zinc ions prefer to occupy the tetrahedral sites, hence the general molecular formula is

( Zn

(2+) Fe((13−+)x) x

)⎡⎣ Ni

(2+) (3+) ⎤ (1 − x)Fe(1 + x) ⎦

(1)

where bracket and square bracket indicates tetrahedral (A) and octahedral (B) sites respectively. The cation distribution over the available tetrahedral (A) sites and octahedral (B) sites due to the plane (hkl) in the sample Ni0.8Zn0.2Fe2O4 is evaluated by using the formula [15] 2

Ihkl = Fhkl P L p

(2)

Fig. 2. (a) Raw XRD of Ni0.8Zn0.2Fe2O4 sintered at 900 °C/5 h (b) Rietveld refined [Jana 2006] powder profile of Ni0.8Zn0.2Fe2O4.

where Ihkl and Fhkl is the relative integrated intensity and structure factor of the (hkl) plane, P is the multiplicity factor and Lp is Lorentz polarization factor. The structure factor formulae for the (hkl) planes are calculated using the equations from [16]. The multiplicity factor and Lorentz polarization factor were taken from [17]. The scattering factor for the tetrahedral (A) and octahedral (B) sites, from Eq. (2) is assumed as

fa = (Znx )fZn + ( Fe1 − x )fFe

and

fb = ( Ni1 − x )fNi + ( Fe1 + x )fFe

(3)

where fa, and fb, is the scattering factor of tetrahedral cations, octahedral cations, The atomic scattering factor for nickel, zinc, iron and oxygen ions were taken from the international tables for X-ray crystallography [18]. It is well known that the intensity of plane (220) and (440) are sensitive to cation on A-site, whereas the (400) plane intensity is sensitive to cation on B-site [15]. Hence, X-ray intensity ratios of I220/I440, I400/I440, and I220/I400 were calculated for all the possible combinations of nickel, zinc and iron

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Table 1 Cation distribution and X-ray intensity ratio of Ni0.8Zn0.2Fe2O4 ferrite sample. Cation distribution

I220/I440

I400/I440

Table 3 Observed (Fo) and calculated (Fc) structure factor of various hkl planes obtained from Rietveld refinement method using JANA 2006 software for Ni0.8Zn0.2Fe2O4.

I220/I400 220

(Fe1)A [Ni0.8Zn0.2Fe1]B O4

183

Obs

Cal

Obs

Cal

Obs

Cal

0.43

0.24

0.98

0.82

0.44

0.30

311

222

400

422

511

440

Fo

Fc

Fo

Fc

Fo

Fc

Fo

Fc

Fo

Fc

Fo

Fc

Fo

Fc

150

152

253

253

204

191

260

256

114

106

196

201

361

367

‘A’ refers tetrahedral sites and ‘B’ refers octahedral sites.

ions, to determine the cation distribution in the present study and the combination which matches closely with that of observed intensity ratios were chosen as cation distribution, and the same is tabulated in Table 1. From Table 1, it can be seen that zinc ions are occupying octahedral against its preferred tetrahedral sites. The following most intense spinel phase hkl (311, 400 and 440) peaks are used for calculating average particle size (t) from Scherrer formula [19] and’t’ value is found to be 30 72 nm. When the particle size is in nanometer regime, the site preference of various ions does not hold good anymore. Occupation of zinc at octahedral sites is also reported by [11,20,21] The raw XRD was refined for their structural parameters, by applying the cation distribution from Table 1 as occupancy parameter in the Rietveld refinement method which is a standard tool, devised by Hugo Rietveld [22]. The Rietveld method [22] is used for refining the background, pseudo-voigt, asymmetry, preferred orientation, scale and lattice parameters using the software JANA 2006 [23] of the raw XRD data and the refined powder profile of the sample is shown in Fig. 2b. Due to the substitution of nickel (rNi2 þ ¼ 0.69 Å) ions by zinc ions (rZn2 þ ¼0.74 Å), an increase in the value of lattice parameter (a ¼8.3648 Å) in the present study is observed when compared with that of [13] (a ¼8.339 Å). This lattice parameter ‘a’ value, obtained in the present study, agrees well with that reported by [24] prepared by co-precipitation method. The X-ray density (Dx) of the unit cell is calculated using the relation from [14]. Structural parameters obtained from Rietveld refinement method [22] are listed in Table 2 along with magnetic parameters. The agreement between the observed (Fo) and calculated (Fc) structure factor, as listed in Table 3, also supports the cation distribution obtained in the present study. Table 4 tabulates the reliability indices on Rietveld refinement of the sample. The lattice parameter (ath) value is theoretically calculated using the equation [25] Table 2 Structural and magnetic parameters of the sample Ni0.8Zn0.2Fe2O4. Parameters Lattice parameter Cell volume X-ray density Crystallite size Oxygen positional parameter Radius of tetrahedral site rA(Å) Radius of octahedral site rB(Å) Tetrahedral shared edge length Octahedral shared edge length Octahedral unshared edge length Saturation magnetization Coercivity Retentivity Bohr magneton

Permeability Squareness

Value Theoretical ath(Å) Experimental a(Å) (Å)3 Dx(g/cm3) t (nm) ‘u’ From cation distribution From XRD From cation distribution From XRD dAES (Å) dBES (Å) dBEU (Å) MS (emu/g)

8.3243 8.3648(8) 585.3 5.395 30(2) 0.3794(1) 0.640 0.552 0.670 0.734 3.062 2.852 2.958 50.55

( HCi ) G ( Mr ) (emu/g)

412.8

μH B (From hysteresis loop)

17.18 2.13

μN B (From cation distribution)

1.60

μ(emu/g kOe) (Mr/Ms)

3.36 0.340

Table 4 Reliability indices on Rietveld refinements of the sample Ni0.8Zn0.2Fe2O4.

a th =

Parameter

Value

Reliability index Robs(%) Weighted reliability index wRobs(%) Rall(%) wRall(%) Profile reliability index Rp(%) Weighted profile reliability index wRp(%) Goodness of fit GOF

9.45 11.27 10.72 11.54 2.28 3.27 0.59

8 (rA + R o) + 3 3

3 (rB + R o)

(4)

where Ro (1.32 Å) is the radius of oxygen ion , rA and rB are the radius of the tetrahedral (A) and octahedral site (B) ions. The value of rA and rB can be calculated from cation distribution (Table 1), by the following equations [26]

rA = (1)r 3Fe+

(5)

and

rB =

1⎡ 2+ 2+ 3 +⎤ ⎣ ( 0.8)r Ni + ( 0.2)r Zn + ( 1)r Fe ⎦. 2

(6)

For zinc, nickel and iron ions, the ionic radius values are taken as rZn2 þ ¼0.74 Å, rNi2 þ ¼ 0.69 Å and rFe3 þ ¼0.64 Å. rA and rB values can also be evaluated from XRD data using equations [26]

rA = a √3 ( u–0.25)–R o.

( )

(7)

rB = a( 5/8–u)–R o.

(8)

By substituting the value of ‘a’ and ‘u’, both obtained from the result of Rietveld refinement method [22], in Eqs. (7) and (8), rA and rB value can be determined and its comparison with that from cation distribution is shown in Table 2 and from the comparison, it can be seen that there is a small difference in the values, which may be attributed to the presence of Fe2 þ ions (rFe2 þ ¼0.78 Å) on octahedral sites and this Fe2 þ ions force the Fe3 þ ions migrate to tetrahedral sites, which in turn force the Zn2 þ ions to the octahedral sites. Since the bigger Zn2 þ ions (0.74 Å) and Fe2 þ ions (rFe2 þ ¼0.78 Å) occupy the octahedral (B) sites, the radius of B-site (rB) is greater than that of (rA) and hence, the increase in the experimental lattice parameter value [27]. Uniformly distributed particles can be seen from the morphology of the Ni0.8Zn0.2Fe2O4 sample is visualized from scanning electron microscope as shown in Fig. 3. 3.3. Analysis of electron density in the unit cell from maximum entropy method (MEM) MEM was introduced by Gull and Daniel [28] and Collins [29] formulated this method for crystallographic applications. The structure factors extracted from the Rietveld refinement method [22] are used in Maximum entropy method (MEM).

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Fig. 3. SEM image of Ni0.8Zn0.2Fe2O4.

The MEM refinements were carried out by dividing the unit cell into 64  64  64 pixels. The uniform prior density (F000/V¼1.536 e/Å3) was initially used for the construction of structure factors, which would then be compared with the observed structure factors. The error in the difference in the structure factors was again added to the calculated ones and thus the entropy was increased. The process of increasing the entropy in the structure factors as well as in the charge density was done iteratively. The iteration would stop when the soft criterion became ‘1’. At this situation the observed structure and MEM refined structure would match. The charge density constructed at this juncture, using the structure factors was called the desired MEM charge density. The unique feature of this charge density is that it is real and positive. This charge density gives an accurate picture of the distribution of charges in the unit cell, which is then analyzed for bond related properties [30]. The software package PRIMA [31] is employed in analyzing the MEM parameters in the present study and the same is tabulated in Table 5. The software program VESTA [32] has been used for the 3D, 2D and 1D electron density calculations. The three dimensional MEM electron density distribution of the sample under investigation is shown in Fig. 4 in which ‘A’ represents tetrahedral sites, ‘B’ represents octahedral sites and ‘O’ represents oxygen atoms. The iso-surface level is suppressed for better view in Fig. 4. 2D electron density distribution on the miller plane (110) of the sample in density range of 0–1.0 e/Å3 with contour interval of 0.1 e/Å3 is drawn using the software program VESTA [32] and shown in Fig. 5. In Ferrites, the A-B interaction takes place through super exchange interaction i.e., the atoms at A-sites and at B-sites interact through oxygen atoms. The interactions between A-site atoms and oxygen atom and similarly interactions between oxygen atom and B-site atoms can be seen clearly in the Fig. 5, by the presence of residual electron cloud around the A-site, B-site and oxygen atoms in (110) plane and A-A sites interactions can also be Table 5 MEM refinement parameters. Parameters

Ni0.8Zn0.2Fe2O4

Number of cycles Number of electrons in the unit cell Number of pixels in the unit cell Lagrange parameters (λ) MEM reliability index RMEM Weighted MEM reliability indexw RMEM

1469 899 64  64  64 0.0123 0.0172 0.0167

Fig. 4. Three dimensional electron density of Ni0.8Zn0.2Fe2O4.

Fig. 5. 2D electron density view of Ni0.8Zn0.2Fe2O4 sample sintered at 900 °C. Contours lines are drawn between 0 to 1 e/Å3 with 0.1 e/Å3 interval.

seen by the presence of residual electron cloud between two nearest A-A sites atoms in (110) plane. The mid bond electron density values give the strength of the A-B, A-A and B-B interactions of the sample, hence 1D electron density view is drawn and the same is shown in Fig. 6 and the values are tabulated in Table 6. Close agreement between Bohr magneton values calculated from, both hysteresis loop and cation distribution (Table 1) indicates absence of Yaffet-Kittel angle i.e., there is no canting and collinear spin ordering exist which means A-B interaction is the strongest among the three (A-A, A-B, B-B) interactions and the same is confirmed from MEM study (Table 6). From Table 6, it can be seen that the value of A-A interactions is higher than that of B-B interactions which is attributed to occupation of Fe3 þ ions at A-sites which strengthens A-A interactions [33]. The configuration of ion pairs in spinel ferrites with favorable distances and angles for magnetic interactions, given by Gorter [34]. The interionic distances and interionic bond angles were calculated using the equations from [35] and tabulated in Table 7. The interionic distances values between the cations (Me–Me), between the cation and anion (Me–O) and the interionic bond

Y.B. Kannan et al. / Physica B 502 (2016) 181–186

185

Fig. 6. 1D electron density view of A-A, A-B and B-B interactions in Ni0.8Zn0.2Fe2O4 sample.

Fig. 7. Variation of magnetization with applied field for Ni0.8Zn0.2Fe2O4. Inset shows the magnified view at lower applied field.

Table 6 Mid-bond electron density values between tetrahedral (A) and octahedral (B) sites of Ni0.8Zn0.2Fe2O4.

values of the magnetic parameter extracted from the measurements, which is shown in Fig. 7, are listed in the Table 2. The saturation magnetization value of Ni0.8Zn0.2Fe2O4 sample. is 50.55 emu/g. A value of 64.03 emu/g for saturation magnetization is reported by some investigators [40] prepared by coprecipitation method. This decreased value of saturation magnetization in the present case is attributed to the migration of zinc ions to the B-site, forcing iron ions to the A-site. Thus the magnetic moment value of B-site is decreased and that of A-site is increased hence a decrease in the saturation magnetization value. The Bohr Magneton value (saturation magnetization per formula unit in Bohr magneton at absolute temperature) evaluated from the hysteresis loop is given by

A–B bond

A–A bond

B–B bond

Mid bond electron density

Distance Mid bond electron density

Distance Mid bond electron density

0.91 e/Å3

1.80 Å

0.59 e/Å3

1.81 Å

0.07 e/Å3

Distance

1.47 Å

angles ( θ1 − θ5) values agrees well with that reported by [21,36]. 3.4. Band gap energy value from optical absorption spectra The Energy of the incident photon (hν) can be related to the band gap (Eg) according to the Tauc relation [37,38]

(

α = A hν–Eg

m/2hν

)

(9)

where m ¼1 for direct band gap materials and m ¼4 for indirect band gap materials, α is the absorption coefficient, A is a constant, h is the Planck's constant, ν is the frequency of the incident photon (Hz) and Eg is the band gap energy (eV). In this present study, m ¼1, hence 2

( αhν)

(

= A hν–Eg

)

(10)

The value of the band gap energy can be calculated from the linear interpolation of the photon energy against (αhν)2 and the value of band gap energy of the sample at room temperature is found out as 2.46 eV. Author [39] has reported 2.5 eV band gap energy for NiFe2O4 nano particles. 3.5. Magnetic measurement The magnetic measurements of the sample were measured by using vibrating sample magnetometer at room temperature. The

⎧ molecularweight ⎫ ⎛ emu ⎞ ⎬Ms⎜ μBH (Bohr Magneton)=⎨ ⎟ ⎩ ⎭ ⎝ g ⎠ 5585

(11)

The value of Bohr Magneton calculated on the basis of the cation distribution and the Neel's two sublattice model, i.e. Neel's moment μBN ¼ MB − MA (where MB and MA are sublattice magnetizations) for the system is listed in Table 2. The agreement between the calculated and observed value of Bohr magneton justifies the cation arrangement. In the spherical particle model the critical size from single domain to multi domain can be calculated with the help of following relation [41]

Dcr = 9εp /2Ms2

(12)

where εp ¼ (2kBTC |K1|/a) is known as surface energy of the domain wall, Ms is saturation magnetization, kB is the Boltzmann constant, TC is the Curie temperature, |K1| is the absolute value of magnetocrystalline anisotropy constant and a is the lattice parameter. For nickel zinc ferrite particles, Ms ¼310 G, K1 ¼ 10.5  103 J/ m3 and a¼ 8.4 Å [42] and TC ¼593 K [43]. The value of Dcr is found out to be 25 nm whereas the ‘t’ value in the present study is 30 (2) nm indicating the particles are in multi domain state and the same is confirmed from the small value of Mr/Ms and with this small range of value it could be useful in applications such as in permanent magnets and recording media of high density. The 1/2

Table 7 Interatomic distances between cations (Me–Me), between anions – cations (Me–O) and interatomic angles (θ) of the sample. Me–Me (Å)

Me–O (Å)

θ (°)

b

c

d

e

f

p

q

r

S

θ1

θ2

θ3

θ4

θ5

2.96

3.47

3.62

5.43

5.12

2.05

1.88

3.59

3.64

123.8

147.2

92.1

125.7

75.9

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shape of magnetization curve in Fig. 7 indicates the ferromagnetic nature of the sample.

4. Conclusion Ni-Zn mixed nano ferrite with composition Ni0.8Zn0.2Fe2O4 was prepared using co-precipitation method. Spinel phase, along with a small additional peak corresponds to hematite phase, is confirmed from XRD data. The agreement between the (i) theoretical and experimental lattice parameter value, (ii) observed and calculated X-ray intensity ratio, and (iii) experimental and theoretical Bohr magneton values confirms the cation distribution in the present study. Morphology study reveals the homogeneous nature of the sample. Absence of Yaffet-Kittel angle confirms the existence of collinear spin, which is also confirmed by the mid bond electron density values between various atoms at A and B sites evaluated from maximum entropy method. The ferromagnetic nature of the sample is confirmed from the magnetic measurement.

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