Temperature Dependent Structural and Magnetic Behaviour of Zn0.5Ni0.5Fe2O4 Nano-ferrite via Citrate Method
A. K. Srivastava*, Dikshu Bansal
Department of Physics, Lovely Professional University, Phagwara, (Punjab)- 144411, India
Keywords: Ferrite; superparamagnetism; structural properties.
In the present work, the auto-combustion citrate precursor method was used to prepare Zn doped Ni nano-ferrite. The merits of this method over conventional ceramic method was reported by E.E. Sileo et al..
Synthesis of Zn0.5Ni0.5Fe2O4 ferrite nanoparticles was done by citrate precursor method by taking nickel nitrate (Ni(NO3)2.6H2O, 99% pure), zinc nitrate (Zn(NO3)2.4H2O, 99% pure), ferric nitrate (Fe(NO3)3.9H2O, 99% pure) and citric acid (C6H8O7, 99% pure) as starting materials. The aqueous solutions of citric acid, iron and metal salts are prepared separately in stoichiometric proportions by dissolving them in double distilled water and then mixed together with constant magnetic stirring. The molar ratio of salt solutions with cations to citric acid was taken 1:1. The solutions were then heated at 800C-850C with continuous stirring for 2 hours. After evaporation of water the liquid converted into a homogeneous brown coloured gel (viscous solution). The viscous solution was dried in an oven overnight (≈ 15 hour) at 1100C to form the precursor material. The precursor material was then gently crushed with mortar and pestle to get precursor powder. Obtained precursor powder was annealed at different temperatures i.e. at 3000C, 6000C, 8000C and 10000C for 1 hour for further crystallization.
2.2. Characterization Techniques
X-ray diffraction (XRD) patterns of the samples were taken on PANALYTICAL instrument (X’pert pro) using Cu-Kα (λ= 1.54060 Å). TEM analysis of the sample was done with Hitachi (H-7500). FT-IR spectra were recorded in KBr medium in the range of 4000-375 cm-1 with a IR Prestige-21 FT-IR (Model SHIMADZU-8400S). Magnetization measurements were made on Vibrating Sample Magnetometer (VSM) (PAR 155 Model) at room temperature upto 9 KOe.
XRD patterns of Zn0.5Ni0.5Fe2O4> nano-ferrite samples at different temperatures i.e. at 1100C (i.e. precursor powder), 3000C and 6000C with cations to citric acid molar ratio of 1:1 are shown in fig.1.Experimental data obtained from XRD for (311) diffraction peak are shown in table 1. From fig.1, it is clear that all the samples have face centered cubic spinel structure due to the presence of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1),(4 4 0) reflection planes.
Fig. 1: X-ray diffraction patterns of Zn0.5Ni0.5Fe2O4 nano-ferrite at different temperatures.
where D is the crystalline size in nm, λ the radiation wavelength (1.54060 Å for Cu-Kα ), β the bandwidth at half-height, and θ is the diffraction peak angle. The particle size of these samples is found to be less than 15.56 nm i.e. varies from 10.39 nm to 15.56 nm depending on annealing temperatures. The lattice constant a0 has been calculated from eq.2  for all the samples.
where a0 is the lattice parameter of the unit cell, dhkl is the interplanar separation, and (h k l) are the Miller indices of the plane. The calculated values of lattice constant for different annealing temperatures are given in table 1. From table 1, it can be clearly seen that with increasing temperature, lattice constant remains almost constant but particle size increases sharply first and then becomes constant. The X-ray density, dx, is estimated by the eq.3 :
where M is the molecular weight, N is the Avogadro Number and a0 is the lattice constant. Each cell has 8 formula units. It can be seen from table 1 that X-ray density decreases with increasing annealing temperature.
Table 1: Peak Position, dhkl-spacing, FWHM, lattice constant, particle size and X-ray density of Zn0.5Ni0.5Fe2O4 system annealed at different temperatures.
3.2. TEM STUDY
Only XRD study seems not very sensible for close and small sizes ferrite. Since particle sizes have some size distribution and also XRD line widths are affected by factors other than the particle size broadening. In order to verify the average particle size TEM analysis are also made for one sample. The sample with x=0.5 annealed at 8000C for 1 hour yield an average particle size ~9.96 nm-16 nm from TEM analysis (fig.2).
Fig.2: TEM of Zn0.5Ni0.5Fe2O4 ferrite annealed at 8000C for 1 hour.
3.3. FTIR STUDY
The FT-IR spectra of Zn0.5Ni0.5Fe2O4 nano-ferrite samples with cation to citric acid molar ratio of 1:1 and at different temperatures i.e. at 1100C, 3000C, 6000C, 8000C and 10000C are shown in fig.3. The presence of the bands in the range 375-600 cm-1 in the spectra confirms the formation of spinel phase [11,12]. Some additional bands around 3400-3200 cm-1, 1200-1500 cm-1, 1500-1700cm-1 and 2100-2400cm-1 are also present in the FT-IR spectra of the samples. These bands correspond to the stretching and bending modes of –OH group, C-H bond bending in plane mode, N-H bond in bending mode and C≡C bond in stretching mode respectively. From these results it appears that hydroxyl groups are retained in the samples during preparation of the spinel ferrites by citrate precursor method and is not completely removed even after annealed at 3000C for 1 hour. It can be noticed from fig.3 that the amount of hydroxyl group in the sample annealed at 8000C for 1 hour is much less than in the samples annealed below this temperature. This suggests that hydroxyl group is gradually removed as the annealing temperature is increased. The bands around 3400-3200 cm-1 are absent in the spectra of the sample annealed at 10000C, implying that hydroxyl is completely removed when the sample is annealed at temperature greater than 8000C. The bands around 1200-1500cm-1 and 1500-1700cm-1 are decreased in the spectra of the sample annealed at 10000C for 1 hour, implying that N-H and C-H groups are not completely removed even though the sample is annealed at temperature 10000C. It is observed that the band around 2100-2400cm-1 i.e. C≡C remains as such in the sample even after annealing at 10000C for 1 hour.
Fig.3. FT-IR spectra of Zn0.5Ni0.5Fe2O4 nano-ferrite prepared at different temperatures.
It is found that as the temperature is increasing the lower frequency goes on increasing from 378.908 cm-1 to 394.773 cm-1 and the higher frequency goes on increasing from 567.428 cm-1 to 583.430 cm-1. The little change in the values of frequencies is observed which may be due to the distribution of cations at tetrahedral and octahedral sites. In other words the change of frequency takes place due to dimensional changes in the bond lengths of the corresponding sites.
In fig.4, we present the magnetization measurements as a function of applied magnetic field for the Zn0.5Ni0.5Fe2O4 nano-ferrite at different temperatures i.e. 8000C and 1100C. It can be visualized from this figure that the sample annealed at 8000C exhibits a saturation magnetization (MS) of 77.4275emu/g, which is much higher compared with that of the sample obtained at 1100C (26.4542emu/g) which is due to improved crystallinity. Thus the particle size has been found to influence the magnetic properties of materials. Saturation magnetization decreases with decreasing crystallite size of nano particles which is due to thermal fluctuations. Magnetic properties such as saturation magnetization of the nano-ferrite obtained after overnight heating at 1100C are much lower than that of bulk materials and the values are also varies from method to method. Since the nanoparticles possess a large surface to volume ratio, the surface disorder phase and non-magnetic layer would reduce the magnetization behavior of the material. It can be seen from fig.6 that magnetic properties of nano sized particles are largely depended on their crystallinity which is consistent to the work of Wang . The samples show zero value of retentivity and coercivity at room temperature which confirms the presence of superpara-magnetism in the prepared samples. It can be seen that saturation magnetization increases with increasing temperature.
In the present study, the increase in magnetization with the increase in sintering temperature could be understood as a result of the increase in particle size and thereby the change in degree of inversion parameter, i.e. there are more Ni2+ ions occupying A-sites and also more Zn2+ ions occupying B-sites in smaller Ni-Zn ferrites resulting in lower magnetization for those samples. The retentivity and coercivity of Zn0.5Ni0.5Fe2O4 ferrite annealed at 8000C for 1 hour are found 21.88emu/g and 0.67KOe respectively.
Fig.4. Magnetic hysteresis loop of Zn0.5Ni0.5Fe2O4 ferrite annealed at different temperatures.
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