Thickness Dependent Physical Property of Spray Deposited ZnFe2O4 Thin Film
Thin film of spinel ferrite has wide range of application such as magnetic sensor, reading head for magnetic recording media, switch mode power supplies, deflection yoke rings and spintronics devices. These wide applications are due to its high permeability, large resistivity, relatively high magnetization and low coercivity. In the present study spinel ZnFe2O4 with good adhesion on glass substrate have been prepared using home built spray pyrolysis unit from mixed zinc and iron nitrates of 1:2 M aqueous solutions. Photograph of the prepared film shows that as film thickness increases, the color of the film changes from golden yellow to red-brown and is due to increase in grain size. X-ray line broadening technique was adopted to obtain grain size and microstrain. It was observed that the surface morphology and optical transmittance strongly depends on film thickness. Band gap of the film varies from 2.08 to 2.70 eV as film thickness decreases. The effect of film thickness on crystalline nature, surface morphology and optical properties were analyzed and reported.
June 05, 2012; Accepted: June 18, 2012;
Published: August 09, 2012
The spinel ferrite thin films are used for various applications such as in
magnetic sensor, reading head for magnetic recording media, microwave devices,
switch mode power supplies, deflection yoke rings and spintronics devices. These
wide applications are due to its high permeability, large resistivity, relatively
high magnetization and low coercivity. It can also absorb electromagnetic radiation
in the microwave bands (Gupta et al., 2007) and
can be used as a photocatalyst under visible light irradiation (Jang
et al., 2009). The spinel type oxide is a cubic structure and consists
of tetrahedral A oxygen sites and octahedral B oxygen sites in which metal cation
distribution occurs. This spinel type oxide growth process involves the so-called
Wagners cation counter diffusion mechanism.
Among all spinel ferrite materials, ZnFe2O4 (Franklinite) has better potential application with its normal spinel structure. There are many techniques available for the synthesis of ZnFe2O4 thin films such as chemical vapour deposition, spin coating, thermal evaporation, spin spray ferrite plating, sputtering, Laser ablation techniques and spray pyrolysis. The technique of spray pyrolysis is simple and inexpensive for the preparation of homogeneous ZnFe2O4 thin films with a large surface area. In this study, spray deposited Zinc Ferrite thin film is prepared with different thickness and its correlation on physical property is analyzed and reported.
MATERIALS AND METHODS
Zinc ferrite thin films were grown on a glass substrate by spray pyrolysis
technique. It was prepared by taking a 0.02 M aqueous solution of zinc nitrate
and 0.04 M aqueous solution of ferric nitrate. The prepared solution was sprayed
onto glass substrate at a temperature of 350°C. In order to obtain homogeneous
oxide of the zinc ferrite thin films, prior to characterization, the films were
annealed at 350°C for 5 h (Wu et al., 2001).
To obtain thin films with different thickness the volume of the solutions was
varied between 20 and 80 mL in step of 20 mL. The deposited zinc ferrite films
were characterized for structural, optical and surface morphology for different
thickness. X-ray diffraction measurements were used to examine the crystalline
nature of the prepared films. CuKα; (λ = 1.54060 Å) was used
in this study. Optical studies were carried out using a UV-Vis-NIR double-beam
spectrophotometer (Model Lamda35) in the wavelength range between 250-1100 nm.
Structural studies: The X-ray diffraction of the zinc ferrite film deposited
at 350°C with different thickness is shown in Fig. 1.
|| X-ray diffractograms of the zinc ferrite thin films
|| Correlation between thickness, grain size, strain and lattice
|| Photo image of zinc ferrite film with different thickness,
(a = 276 nm, b = 329 nm, c = 413 nm, d = 428 nm)
The probability of crystallization increases as the film thickness increases
and the X-ray spectra are polycrystalline in nature (Gopalan
et al., 2009). The various thickness of zinc ferrite film showed
a preferred orientation of (311) peak at 35° (Wu et
al., 2001) which are in good agreement with the reported standard values
(JCPDS No. 22-1012). From Fig. 2, it indicates that the grain
size increases due to the grain growth in thin film as the thickness increases.
The strain in the film decreases from 0.002145 to 0.00065484 lines/m3 with
increase in film thickness. This is due to the fact that the cohesive force
between the film and the substrate decreases. The lattice parameter of ZnFe2O4
thin film is found to be wavy in nature from 8.373084613 to 8.398854788 Å.
Optical studies: The prepared film shows that as film thickness increases,
the color of the film changes from golden yellow to red brown due to increase
in grain size as shown in Fig. 3. The thickness of the film
is measured by surface profilometer and is found to be varying from 276-428
nm. The optical absorbance spectra of ZnFe2O4 film with
different thickness are shown in Fig. 3. The absorbance of
the film shown in Fig. 4 increases with increase in film thickness
because in thicker films more atoms are present so more states will be available
for the photons to be absorbed.
|| Optical absorbance vs. Wavelength (nm) at various thicknesses
|| Transmittance (T) vs. Wavelength (nm) at various thickness
Figure 5 shows that the transmittance of the film varies
from 85 to 55% with increase in film thickness. This is due to the phenomena
that as the thickness increases the scattering of light increases so that the
coherence between the primary light beam and the beam reflected between the
film boundaries is lost resulting in disappearance of the interference with
decrease in transmittance.
The variation of optical bandgap as a function of thickness is shown in Fig.
6. The optical bandgap of the film varies from 2.08 to 2.70 eV as film thickness
decreases (Wu et al., 2001; Zhou
et al., 2002). The grain size increase with increase in film thickness.
Since the grain size influences the energy level of electrons, the band gap
depends on the thickness of the film. This is due to the effect of quantum size
seen in thin films of semiconductor. It is evident from the graph that the band
gap widens towards blue shift with decrease in the thickness (Kislov
et al., 2008). This kind of broadening effect can be studied based
on Burstein effect (Burstein, 1954).
Surface morphology: The SEM image with different thickness is shown
in Fig. 7. It is observed that the surface morphology depends
on the thickness of the film. The grain size is found to be ranging from 20
to 30 nm in ZnFe2O4 film. The film with thickness of 276
nm shows grain nucleation sites. As the thickness increases to 329 nm the grain
growth in the film increases. Further increase in film thickness of about 413
nm leads to layers growth of film. It is seen that the grain size increases
and the crystallization increases with the film thickness (Tian
et al., 2010).
|| (αhυ) 2 vs. hυ at various thicknesses
|| SEM image of ZnFe2O4 thin film with
different thickness; (a) 276 nm, (b) 329 nm, (c) 413 nm and (d) 428 nm
|| B-H curve of ZnFe2O4 thin film with
different thickness; (a) 276 nm, (b) 329 nm, (c) 413 nm and (d) 428 nm
Thus the final film of thickness 428 nm is observed with larger grains of
Magnetic studies: The first graph indicates diamagnetic nature which
is due to the presence of zinc content in large amount as shown in Fig.
8 Then, the dip in the Curve represents the transition of diamagnetic nature
to ferromagnetic nature of the material with increase in thickness as shown
in Fig. 5. In soft magnetic materials, the domain walls moves
easily back and forth resulting in easy magnetization and demagnetization. Thus
the coercivity and retentivity decreases with increases in thickness of zinc
ferrite film resulting in soft ferromagnetic material (Hofmann
et al., 2004). Finally, Magnetic study reveals the enhancement of
magnetic nature of the film as thickness increases.
Zinc ferrite thin films were prepared by spray deposition. With varying thickness the properties of thin films were studied. It had been observed that the films show spectacular shifts in physical properties with increase in thickness. The magnetic property of the film had shifted from diamagnetic nature to soft paramagnetic nature which finds many applications in high frequency devices. The crystallinity of the films increased with increase in thickness of the films. The study of optical properties reveals that the films can be best suited as gas sensors and for photocatalytic applications.
Burstein, E., 1954. Anomalous optical absorption limit in InSb. Phys. Rev., 93: 632-633.
CrossRef | Direct Link |
Gopalan, E.V., I.A. Al-Omari, K.A. Malini, P.A. Joy, D.S. Kumar, Y. Yoshida and M.R. Anantharaman, 2009. Impact of zinc substitution on the structural and magnetic properties of chemically derived nanosized manganese zinc mixed ferrites. J. Magn. Magn. Mater, 321: 1092-1099.
CrossRef | Direct Link |
Gupta, N., A. Verma, S.C. Kashyap and D.C. Dube, 2007. Microstructural, dielectric and magnetic behavior of spin deposited nanocrystalline nickel-zinc ferrite thin films for microwave applications. J. Magn. Magn. Mater., 308: 137-142.
Hofmann, M., S.J. Campbell, H. Ehrhardt and R. Feyerherm, 2004. The magnetic behaviour of nanostructured zinc ferrite. J. Mater. Sci., 39: 5057-5065.
Jang, J.S., S.J. Hong, J.S. Lee, P.H. Borse and O.S. Jung et al., 2009. Synthesis of zinc ferrite and its photocatalytic application under visible light. J. Korean Phys. Soc., 54: 204-208.
Kislov, N., S.S. Srinivasan, Y. Emirov and E.K. Stefanakos, 2008. Optical absorption red and blue shifts in ZnFe2O4 nanoparticles. Mater. Sci. Eng. B, 153: 70-77.
Tian, Q., Q. Wang, Q. Xie and J. Li, 2010. Aqueous solution preparation, structure and magnetic properties of nano-granular ZnxFe3-xO4 ferrite films. Nanoscale Res. Lett., 5: 1518-1523.
Wu, Z., M. Okuya and S. Kaneko, 2001. Spray pyrolysis deposition of zinc ferrite films from metal nitrates solutions. Thin Solid Films, 385: 109-114.
Direct Link |
Zhou, Z.H., J.M. Xue, H.S.O. Chan and J. Wang, 2002. Nanocomposites of ZnFe2O4 in silica: Synthesis, magnetic and optical properties. Mater. Chem. Phys., 75: 181-185.