Results and Discussion of Coi ZnFe2O4 Ferrites
5.1 X-ray Diffraction Analysis .1 Phase Analysis
5.1.2 Lattice Parameter
The lattice constant of all the samples have been precisely determined considering the reflections with the Cu-k0 radiation using the extrapolated Nelson-Relay function F (0) = 0 at 0
= 900. The least square linear fitting gives the precise lattice constant as an intercept of the Y- axis. The lattice constant as a function of Zn content have been plotted in Fig. 5.2 and are tabulated in Table -5.1. From Fig. 5.2 it is observed that the lattice constant increases linearly with increasing Zn content obeying Vegard's law [5.19]. The lattice constant increases with increasing x content, because the ionic radius of CO2 (0.72A) [5.20] is smaller than that of Zn2 (0.82A) [5.21]. Since the radius of the substituted ions is larger than that of the displaced ions it is expected that the lattice should expand increase the lattice constant.
A.
8.46 8.45 8.44
cc 8.43
I-
8.42
. 8.41
C)
8.40 8.39 8.38
2
- Co Zn FeO
-x x 2 4 .
- T = 1100°C/2hrs
• I • I • I • I •
0.0 0.2 0.4 0.6 0.8 1.0
Zn content (x)
Fig. 5.2 Variation of lattice constant 'a' as a function of Zn content (x) of Co1 ZnFe204 ferrites.
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The increase in lattice constant with x is similar in nature to that has been reported in the Zn-Mg [5.22 - 5.24], Cu-Zn [5.25] and Co-Zn [5.26, 5.27] ferrite system. Lattice constant of ZnFe204 ferrite is 8.633A [5.28] and 8.47 IA [5.29]. Our value of lattice constant is 8.455A.
The difference in the values of the lattice constant may be due to different sintering atmosphere, technique and the possibility of experi mental error.
5.1.3 Density
The dependence of bulk density dB and X-ray density p, upon Zn content x is presented in Fig. 5.3. X- ray density Px was calculated from the molecular weight and the volume of unit cell for each sample where as the bulk density pa was measured usual mass and dimensional consideration. The bulk density increases significantly with increasing Zn content indicating improved densification by the substitution of Zn for Co in the ferrite. The increase of density with Zn content can be attributed to the atomic weight and density of Zn (65.37 and 7.14gm /cm3) which is higher than those of Co ( 58.9 and 8.6 gm/cm3) respectively.
5.4
5.2 U E
5.0
4.8
4.6
—0---
Co Zn FeO p
- l-x x 2 4 PB
T = I I00°C/2hrs
1/
0.0 0.2 0.4 0.6 0.8 1.0
Zn content (x)
Fig. 5.3 Variation of density with Zn content(x) of Coi.ZnFe204 ferrites.
The replacement of Co2 by Zn 21 ions in the spinel leads to a variation in bonding and consequently interatomic distance and density. The oxygen ions which diffuse through the material during sintering also accelerate the densification of the material. The apparent density of the same composition reflects the same general behavior of the X-ray density Px. The X-ray
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density is higher than the apparent density value due to the existence of pores which depends on the sintering condition.
The percentage of porosity was also calculated using the equation (3.7). Porosity changes slightly with Zn content (x). It is understood from the data of Table - 5.1, porosity values are found to decrease significantly with increasing Zn concentration, thereby giving an impression that zinc might be helping in the densification of the materials. The composition x = 0.8 has the highest density and lowest porosity may be due to the increase of oxygen vacancies [5.30].
Table- 5.1 Lattice parameter (a), X-ray density (Px), bulk density (pn), porosity (P%) of Coi.ZnFe2O4 ferrite with different Zn content sintered at 1100°C/2hrs.
Zn content
(x) a (A)
Px
(gm/cm3)
PB (gm/cm3)
0 8.380 5.278 4.589 13.0
0.1 8.398 5.261 4.572 13.1
0.2 8.400 5.273 4.629 12.2
0.3 8.409 5.261 4.725 10.2
0.4 8.415 5.290 4.834 8.6
0.5 8.417 5.291 4.923 6.9
0.6 8.422 5.297 4.956 6.7
0.7 8.430 5.297 4.946 6.6
0.8 8.431 5.322 4.975 1 6.5
0.9 8.442 5.309 4.920 7.3
1.0 8.455 5.300 4.827 8.9
5.2
Electromagnetic Properties5.2.1 Temperature Dependence of Initial Permeability
Curie temperature, T is a basic quantity in the study of magnetic materials. It corresponds to the temperature at which a magnetically ordered material becomes magnetically disordered. Fig. 5.4 shows the Curie temperature, Tc is obtained from the initial permeability, f versus temperature profiles of different composition at x = 0.0 - 0.5. It was found that the initial permeability increases with the increase of temperature, while it falls abruptly close to the Curie temperature. It is observed from the Fig. 5.4 that the initial permeability increases with increasing Zn content. The abrupt fall of permeability indicates the homogeneity and the single phase of the studied samples, which have also been confirmed by X-ray diffraction as evidenced from the XRD patterns for each sample. Curie temperature,
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T of the sample with x = 0.6 and above has been determined from the temperature dependence of magnetization, M using SQUID magnetometer.
Fig. 5.5 represents the variation of the complex permeability, ji', jt" and temperature derivative of permeability, djtYdT as a function of temperature of Co1ZnFe204 with x = 0.4.
At the Curie temperature, where complete spin disorder takes place, corresponds to maximum of imaginary part and the temperature derivative of the real permeability and sharp fall of the real part of permeability towards zero. From this figure it is observed that the imaginary part of permeability and temperature derivatives of permeability show peaks at value a temperature, I which excellently matches with the sharp fall of permeability at T = T.
400 350 300 250
200 150
100 50 0
Co1 ,Zn,Fe2O4 x = 0.0
1' = 1100°C /2hrs 0 X = 0.1
—A—x0.2
—*—
x = 0.5
300 400 500 600 700 800
T(K)
Fig. 5.4 Temperature dependence of permeability, f of Co1 ZnFe204 ferrites with different Zn content.
In Fig. 5.6 the variation of Curie temperature I as a function of Zn content of Co,Zn Fe201 system. It is observed that I decreases continuously with the increase of Zn 21 content.
The linear decrease of Tr with increasing Zn content may be explained by modification of the A-B exchange interaction strength due to the change of the iron distribution between A and B sites when nonmagnetic Zn is substituted for Co. The basic magnetic properties of CoFe204 system originate from CO2 ions only in the octahedral B-sites since Fe 31 ions are distributed A-sites replacing an equal amount of Fe3 to the octahedral B-sites. In such a situation JAA
becomes weaker. Therefore decrease of I is due to the weakening of A-B exchange 116
interaction and this weakening becomes more pronounced when more Zn replaces more tetrahedral Fe3 to octahedral B-sites. The similar trend in the variation of I has been observed by many researches in Zn substituted Mg-Zn and Ni-Zn [5.7, 5.31] and other ferrite systems.
320 370 370
320 270
220
170 x=O.4 120 o
70
F
---20 270
95 85 75 65 55
45
35 25 15
1
-5 420 470 520 TK
-4
Fig. 5.5 Determination of Curie temperature from the temperature dependence of p', and djf/dT as a function of temperature of Co1 Zn.,Fe204 ferrites with x = 0.4.
..,.
700
g
600 500DIs
300 ....
0 0.2 0.4 0.6 0.8
Zn content (x)
Fig. 5.6 Variation of Curie temperature, Twith Zn content(x) of Co1ZnFe204 ferrites.
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Fe,04 )°C/2hr
±735.14
A linear dependence of T with Zn content is observed with x 0.7 beyond which well defined Tc could not be determined due to complex magnetic structures and competing interactions of highly diluted composition. A linear least square fitting of the Curie temperature with x, gives an empirical relation for the whole Coi.xZn.rFe204 system as follows, T (x) = T (0) —640 x, where T (0) is the Curie temperature of the pure Co-ferrite and T (x) corresponds to the Curie temperature of any composition having Zn concentration x.
From this empirical relation, Curie temperature of Co-ferrite is found to be 735K. Our experimental result is 728K listed in Table - 5.2.