Correlation between microstructure, electrical and optical properties of nanocrystalline NiFe_1.925Dy_0.075O₄ thin films

Abstract

Dysprosium-doped nickel-ferrite (NiFe_1.925Dy_0.075O₄) thin films were fabricated using sputter-deposition using a stoichiometric bulk target prepared by the solid state chemical reaction. The structural, electrical and optical properties of NiFe_1.925Dy_0.075O₄ thin films were studied in detail. The grain-size (L) and lattice-expansion effects are significant on the electrical and optical properties of NiFe_1.925Dy_0.075O₄ films. Air annealing (T_a) at 450–1000 °C results in the formation of nanocrystalline NiFe_1.925Dy_0.075O₄ films, which crystallize in the inverse spinel structure, with L = 5–40 nm. The lattice constant of NiFe_1.925Dy_0.075O₄ increases compared NiFe₂O₄ due to Dy-doping. Electrical conductivity of NiFe_1.925Dy_0.075O₄ films (at 300 K) decreases from 1.07 Ω⁻¹ m⁻¹ to 3.9 × 10⁻³ Ω⁻¹ m⁻¹ with increasing T_a (450 to 1000 °C). Conductivity was found to decrease exponentially with decreasing the temperature from 300 K to 120 K indicating the characteristic semiconducting nature of all the films. Band gap increases from 3.17 to 4.08 eV for NiFe_1.925Dy_0.075O₄ films with increasing T_a from 450 to 1000 °C. A correlation between grain-size, electrical conductivity and optical band gap in nanocrystalline NiFe_1.925Dy_0.075O₄ films is established.

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1. Introduction

Nickel (Ni) and cobalt (Co) based ferrites have been the subject of intense research in recent years due to their wide range of scientific and technological applications.^1–15 Ferrite materials find high frequency applications due to their high permeability and permittivity.^1–8 These materials are being used in transformer cores, antennas, radio frequency coil and radar absorbing materials (RAM).^1–6 Due to their low eddy current losses, nanostructured NiFe₂O₄ thin films are important candidates for magnetic components such as resonators, phase shifters, tunable signal filters and, most recently, for spintronics applications.^1–11 These properties also make them unique in microwave devices that require strong electromagnetic signal coupling.^1–11 Along with diverse applications, ferrites have properties which are of fundamental interest. For instance, a small change in stoichiometry and size makes the magnetic and electronic properties of ferrite tunable.^1–11 Furthermore, the effects of cationic interchange and oxygen vacancies lead to unusual magnetic properties when these materials are fabricated as thin films and nano-particles. However, these properties and the performance of ferrite thin films are very sensitive to growth conditions, method of preparation and the associated chemistry.^1–11

In Ni ferrite, the tetrahedral (A) sites are occupied by the Fe³⁺ ions and the octahedral sites (B) are occupied by the Ni²⁺ and Fe³⁺, in equal proportions.^1,2 Partial substitution of Fe³⁺ by a rare-earth ion can significantly modify the electrical and dielectric properties of Ni ferrites.^7–16 It has been reported that, inclusion of Zn, Cu, Co and Cd in ferrites,^17,18 (Ni ferrite, Ni–Zn ferrite, Ni–Cu ferrite and Li–Ni–Cd ferrite) leads to an increase in the dielectric constant and dielectric loss due to the formation of excess Fe²⁺, which leads to an increase in the hopping of electrons between Fe²⁺ and Fe³⁺. Partial substitution of Fe³⁺ by rare earth ions in the spinel structure has been reported to lead to structural distortion^7–14 that induces strain and significantly modifies the electrical and dielectric properties.^7–15

The present work was performed on the effect of Dy ion substitution at the Fe site on the structural characteristics, electrical and optical properties of Ni ferrite (NiFe_1.925Dy_0.075O₄) films. Impetus is to derive the structure–property relationship, which in turn depends on growth, composition and structure at reduced dimensions, in NiFe_1.925Dy_0.075O₄ films for their utilization in device applications. A detailed account of the crystal structure, surface morphology, electrical and optical properties of amorphous and nanocrystalline NiFe_1.925Dy_0.075O₄ films is presented and discussed in this paper.

2. Experimental details

2.1 Target (bulk) material synthesis

NiFe_1.925Dy_0.075O₄ materials were synthesized by the conventional solid state chemical reaction method. The starting materials were 99.99% pure NiO, Fe₂O₃ and Dy₂O₃ powders, which were weighed in their stoichiometric ratio. Powders were then mixed together and ground in a mortar and pestle for one hour and heat treated in air at 1200 °C for 12 h. After the heat treatment, the powders were allowed to settle down to room temperature. The powders thus obtained were made into a target of 2′′ diameter and then sintered at 1250 °C in air for 12 h. The sintered pellet was used as a target for the thin film preparation by sputter-deposition. In order to reduce the formation of secondary phase (DyFeO₃) and to get improved electrical and dielectric properties¹³, 3.5% Dy³⁺ was substituted for Fe³⁺ (NiFe_1.925Dy_0.075O₄).

2.2 Thin film fabrication

NiFe_1.925Dy_0.075O₄ films were deposited onto silicon (Si) (100) and optical grade quartz substrates by radio-frequency (RF) (13.56 MHz) magnetron-sputtering. The films grown on Si were used for XRD, AFM and electrical measurements. The films grown on quartz were used for the optical measurements. The deposition conditions were listed in Table 1. All the substrates were thoroughly cleaned, as per the procedure reported elsewhere, and dried with nitrogen before introducing them into the vacuum chamber, which was initially evacuated to a base pressure of ∼10⁻⁶ Torr. The ferrite target was placed on a 2 inch sputter gun, which is placed at a distance of 8 cm from the substrate. A sputtering power of 40 W was initially applied to the target while introducing high purity argon (Ar) into the chamber to ignite the plasma. Once the plasma was ignited the power was increased to 100 W and oxygen (O₂) was released into the chamber for reactive deposition. Deposition was made at room temperature (RT) to obtain a ∼50 nm thick film. NiFe_1.925Dy_0.075O₄ films prepared at RT were air-annealed (one hour) in the temperatures (T_a) range 450–1000 °C.

Table 1 Sputter-deposition conditions employed for NiFe_1.925Dy_0.075O₄ films

Deposition parameter	Set value
Base pressure	∼10^−6 Torr.
Sputtering pressure	∼10^−3 Torr.
Target	NiFe1.925Dy0.075O4 films
	(2′′ × 0.125′′)
Substrates	Si(100) and Quartz
Substrate temperature (Ts)	Room temperature
Target–substrate distance	8 cm
RF power	100 W
Film thickness	∼50 nm
Annealing temperature (Ta)	450–1000 °C
Ar:O2 ratio	90 : 10

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2.3 Characterization

NiFe_1.925Dy_0.075O₄ films were studied to evaluate their structural, electrical and optical properties. Crystal structure and density measurements were made using grazing incidence X-ray diffraction (GIXRD) and X-ray reflectivity (XRR) measurements, respectively. The GIXRD and XRR measurements on NiFe_1.925Dy_0.075O₄ films were performed using a Bruker D8 Advance X-ray diffractometer. XRD patterns were recorded using Cu Kα radiation (λ = 1.54056 Å) at RT. The coherently diffracting domain size (d_hkl) was calculated from the integral width of the diffraction lines using the well known Scherrer equation¹⁶ after background subtraction and correction for instrumental broadening. The Scherrer equation is:

d _hkl = 0.9λ/βcosθ (1)

where d_hkl is the size, λ is the wavelength of the X-rays used, β is the width of a peak at half of its intensity, and θ is the angle of the peak. Surface imaging was performed using an Atomic Force Microscope (AFM) (Nanoscope IV-Dimension 3100 SPM system). Interface morphology is primarily obtained from scanning electron microscopy (SEM). Imaging was performed using a high-performance and ultra high resolution scanning electron microscope (Hitachi S-4800). The secondary electron imaging was performed on NiFe_1.925Dy_0.075O₄ films grown on Si wafers. DC electrical resistivity measurements were carried out under the vacuum of 10⁻² Torr by two-probe method in the temperature range 60–300 K employing a closed cycle refrigerator (CCR). All the measurements were carried out during the heating cycle and equilibrium was reached by allowing one minute at each temperature measurement. Resistance was measured by employing a Keithley electrometer (Keithley 6517A Electrometer/High resistance meter). The temperature was measured using a silicon diode sensor and employing a Lakeshore temperature controller (Model No. 330). The film was kept on the cold head of the CCR. The point contacts were made by soldering the indium metal at the corners of the films.

3. Results and discussion

3.1 Crystal structure and surface/interface morphology

The XRD patterns of NiFe_1.925Dy_0.075O₄ films are shown in Fig. 1. The XRD curves of NiFe_1.925Dy_0.075O₄ films grown at RT did not show any peaks indicating their characteristic amorphous nature (Fig. 1). The XRD peaks began to appear for samples annealed at T_a = 450–1000 °C indicating the crystallization of NiFe_1.925Dy_0.075O₄ films upon annealing. The peaks can be unambiguously assigned to face centered cubic (FCC) inverse spinel phase.^12–15 Comparison of the XRD data with that of pure NiFe₂O₄ films and bulk (JCPDS Card No. 86-2267) indicates the expected peaks of spinel NiFe₂O₄ for NiFe_1.925Dy_0.075O₄ films (as labeled in Fig. 1). The evolution of XRD peaks indicate that the average grain-size (L) increases with increasing annealing temperature. The grain size increases from 5 to 40 nm with increasing T_a from 450 to 1000 °C. The lattice constant (a) derived from XRD for nanocrystalline NiFe_1.925Dy_0.075O₄ films showed an interesting behavior as a T_a. For NiFe_1.925Dy_0.075O₄ films, ‘a’ increases linearly from 8.353 (± 0.001) to 8.362(± 0.001) Å with increasing T_a from 450 to 1000 °C. The lattice constant obtained for NiFe_1.925Dy_0.075O₄ films annealed at 450 °C, where the onset of crystallization and inverse spinel phase formation occurs, can be compared to that of pure NiFe₂O₄ to see the effect of Dy-substitution. The lattice constant of NiFe_1.925Dy_0.075O₄ films at T_a = 450 °C is 8.353 Å while that of NiFe₂O₄ is 8.340 Å.^12,13,17 This observation indicates the lattice expansion upon Dy-doping. Increased lattice constant upon Fe³⁺ substitution by Dy³⁺ is due to the larger ionic size of Dy³⁺ (1.05 Å) compared to that of Fe³⁺ (0.63 Å). Such behavior is also noticed in Sm and Ho substituted Ni-ferrites.¹² Further increase in the lattice constant with T_a may be due to the lattice-strain developed in the nanocrystalline NiFe_1.925Dy_0.075O₄ films at higher T_a. The AFM images of NiFe_1.925Dy_0.075O₄ films are shown in Fig. 2. The effect of T_a on the surface morphology of NiFe_1.925Dy_0.075O₄ films is remarkable. For annealed samples, presence of the small, dense particles can be noticed in AFM images. The fine microstructure and uniform distribution characteristics of the particles are evident in the micrographs (Fig. 2). The average particle size is 5–32 (± 0.3) nm, which agrees with the calculated L values from the XRD data. As-grown films exhibit featureless images (not shown) indicating the characteristic amorphous phase.

Fig. 1 XRD patterns of NiFe_1.925Dy_0.075O₄ films. It is evident from the curves that the films grown at RT are amorphous whereas films anealed at T_a ≥ 450 °C are nanocrystalline, crystallize in inverse spinel phase

Fig. 2 High resolution AFM images of NiFe_1.925Dy_0.075O₄ films annealed at different temperatures. The effect of T_a on the surface morphology of NiFe_1.925Dy_0.075O₄ films is evident.

The XRR plots of NiFe_1.925Dy_0.075O₄ films are shown in Fig. 3a. The following observations can be made from the XRR spectra. A positive shift (higher angle) of the critical edge with increasing annealing temperature or grain size is the first. The positive shift of the critical edge indicates an increase in the film density with increasing annealing temperature. Simulation of the XRR curves allows obtaining thickness and density of the NiFe_1.925Dy_0.075O₄ films. Simulation of XRR data indicates that the as-grown, amorphous NiFe_1.925Dy_0.075O₄ films at RT are ∼50 nm while thickness slightly decreases for those films annealed at 800 and 1000 °C. The density of NiFe_1.925Dy_0.075O₄ films as a function of T_a is shown in Fig. 3b. Density is seen to increase from 3.181 to 3.814 g cm⁻³ with increasing T_a due to an increase in packing. Roughness of the film is found to increase with increasing T_a. A comparison of the physical parameters obtained from XRR and AFM measurements are presented in Table 2.

Fig. 3 (a) The XRR plots of NiFe_1.925Dy_0.075O₄ films. The positive shift of the critical edge indicates an increase in the film density with increasing annealing temperature. (b) Variation of density of NiFe_1.925Dy_0.075O₄ films with increasing annealing temperature. Density of NiFe_1.925Dy_0.075O₄ films are found to increase with increasing T_a.

Table 2 Thickness and roughness values of NiFe_1.925Dy_0.075O₄ films calculated and observed from XRR and AFM measurements

Ta (°C)	Thickness (nm)	Roughness
		AFM	XRR (nm)
RT	49	—	1.299
800 °C	40	1.26	1.34
1000 °C	40	3.01	1.86

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The SEM cross-sectional images of the representative NiFe_1.925Dy_0.075O₄ films are shown in Fig. 4. Images shown are obtained for NiFe_1.925Dy_0.075O₄ films grown at room temperature and NiFe_1.925Dy_0.075O₄ films air-annealed at 1000 °C. The Si-substrate and NiFe_1.925Dy_0.075O₄ film regions are as indicated in the micrographs. While slightly reduced film thickness is obviously seen for NiFe_1.925Dy_0.075O₄ films annealed at the highest temperature of 1000 °C, no evidence of significant reaction with the Si-substrate at the interface is seen in either for annealed or as-grown samples. This observation indicates that the NiFe_1.925Dy_0.075O₄ films are stable with the Si substrate within the given set of conditions employed in this work.

Fig. 4 The SEM cross-sectional images of NiFe_1.925Dy_0.075O₄ films. (a) RT and (b) T_a = 1000 °C. The Si-substrate and the NiFe_1.925Dy_0.075O₄ film regions are as indicated in the micrographs. No significant interfacial reactions or compound formation occurs at the Si–NiFe_1.925Dy_0.075O₄ interface.

3.2 DC electrical properties

Fig. 5 shows the room temperature dc electrical conductivity variation of NiFe_1.925Dy_0.075O₄ films with annealing temperature. Electrical conductivity of NiFe_1.925Dy_0.075O₄ film is seen to decrease from 1.2 Ω⁻¹ m⁻¹ to 3.9 × 10⁻³ (± 10⁻⁶) Ω⁻¹ m⁻¹ with increasing T_a from RT to 1000 °C. Decrease in conductivity with increasing T_a of NiFe_1.925Dy_0.075O₄ can be attributed to the combined effect of Dy incorporation and the strain induced in the films. Substitution of small amounts of Dy³⁺ ions for Fe³⁺ ions in B site increases the inter-ionic distances and distorts the lattice due to larger ionic size of Dy³⁺ compared to that of Fe³⁺, leading to additional scattering and causing the decrease in electrical conductivity. The conductivity is reported¹⁹ to change with grain size reduction due to the increasing grain boundary volume and associated impedance to the flow of charge carriers. If the crystallite size is smaller than the electron mean free path, grain boundary scattering dominates and hence the electrical conductivity decreases. In addition, the contributions to the electrical conductivity from lattice imperfections, such as vacancies and dislocations which were reported to be present in thin films and nanocrystalline materials.^18,19

Fig. 5 Room temperature DC conductivity values of the NiFe_1.925Dy_0.075O₄ films annealed at different temperature.

The variation of DC electrical conductivity of NiFe_1.925Dy_0.075O₄ films is shown in Fig. 6. Electrical conductivity decreases exponentially with decreasing the temperature from 300 K to 120 K which indicates the semiconducting nature of the films. Conductivity in semiconductors is due to both hopping of electrons and charge transportvia excited states and it can be expressed as:^19,20

Fig. 6 Temperature variation of DC conductivity of the NiFe_1.925Dy_0.075O₄ films. Temperature variation of conductivity (from 300 K to 120 K) indicates the semiconducting nature of the films.

where E₁ is the activation energy for intrinsic conduction and E₂, E₃,…are the activation energies needed for hopping conduction. A₁, A₂, A₃ are constants and k_B is the Boltzmann constant.

It is evident from the electrical conductivity plot (Fig. 6) that two different slopes exists for both the compounds indicating that the conduction is through an activated process having two difference conduction processes. Activation energy values at different temperature ranges (300–250 K and 250–120 K) were calculated from the lnσ vs 1000/T plot and are given in Table 3. Activation energy values were found to be higher at the 300–250 K region (0.44 eV for the film annealed at 450 °C) and lower at the 250–60 K region (0.1 eV). Decreasing activation energy with decreasing temperature has been explained by small-polaron theory.^19,20 The VRH^19,20 model of small polarons also predicts continuously decreasing activation energy with decreasing temperature.

Table 3 Activation energy values of NiFe_1.925Dy_0.075O₄ films at different temperature regions

Ta (°C)	Activation Energy (eV)
	300–250 K	250–120 K
450 °C	0.44	0.10
800 °C	0.47	0.12
1000 °C	0.48	0.13

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3.3 AC electrical properties

The AC electrical resistivity data of NiFe_1.925Dy_0.075O₄ films are shown in Fig. 7. The variation of resistivity, measured at room temperature, with annealing temperature is shown in Fig. 7(a) while the frequency (20 Hz–1 MHz) dependent resistivity curves are shown in Fig. 7 (b). While the resistivity values are seen to be in the order of Ω m at low frequencies (up to 1 kHz), they are found to decrease to ∼10⁻² Ω m at higher frequencies (1 MHz). It is known that in ferrites, the AC conduction is due to the hopping of electrons between ions having different valance (Fe²⁺/Fe³⁺, Co²⁺/Co³⁺, Ni²⁺/Ni³⁺) states. With increasing in frequency, hopping increases and correspondingly the resistivity decreases.

Fig. 7 Room temperature AC resistivity values of NiFe_1.925Dy_0.075O₄ films (a). Frequency dependent conductivity of NiFe_1.925Dy_0.075O₄. The dispersion relation is shown with increasing T_a (b).

The resistivity data was fit to the following equation ^2,10

where ρ_∞ is the resistivity value at 1 MHz, ρ₀ is the resistivity value at 20 Hz, τ is the mean relaxation time and α is the spreading factor about the mean relaxation time. The calculated mean relaxation time values are found to increase from 2.9 μs to 3.6 μs with increasing T_a from RT to 1000 °C due to the increase in resistivity.

3.4 Optical properties

Optical transmittance spectra of NiFe_1.925Dy_0.075O₄ films are shown in Fig. 8. A marked behavior in the spectral transmittance behavior can be seen in NiFe_1.925Dy_0.075O₄ films annealed at 450 °C when compared to that of films grown at RT. Perhaps, a large number of defects resulting from amorphous phase may be affecting the optical quality of the films. In addition, the spectral characteristics of NiFe_1.925Dy_0.075O₄ films are also sensitive to the annealing temperature. Increasing T_a is seen to exhibit higher transparency in the spectral region except where the incident radiation is absorbed across the band gap (E_g). This observation indicates the high-quality and transparent nature of NiFe_1.925Dy_0.075O₄ films with increasing T_a.

Fig. 8 Transmittance curves of NiFe_1.925Dy_0.075O₄ films. The curves indicate that the films are highly transparent.

A further analysis of the spectral transmittance curves is performed in order to better understand the electronic structure changes as a result of Dy-substitution and variations in film-microstructure upon annealing. It is well known that the optical absorption below E_g follows an exponential behavior.^21–24 The absorption, therefore, is exponentially dependent on the energy (hυ) of incident photon in that region. Ni-ferrite is a direct band gap material, which follows the power law of the form:^23,24

(αhυ) = B(hυ − E_g)^1/2 (4)

where hυ is the energy of the incident photon, α the absorption coefficient, B the absorption edge width parameter, E_g the band gap. The optical absorption coefficient, α, of the films is evaluated using the relation:^21–24

where T is the transmittance, R is the reflectance and t is the film thickness. The absorption data and the plots obtained for NiFe_1.925Dy_0.075O₄ films are shown in Fig. 9 (a). The (αhν)²versus hν plots exhibit a better fit. Linear regression fit and the extrapolation (Fig. 8 (b)) of the plot to zero absorption i.e., α (hν) = 0 provide the E_g value. Variation of E_g with T_a is shown in Fig. 10. It is evident that E_g is sensitive to T_a (Fig. 8). E_g = 3.17 eV for amorphous NiFe_1.925Dy_0.075O₄ films grown at RT. E_g increases to 4.08 eV with increasing T_a to 1000 °C. E_g values of NiFe_1.925Dy_0.075O₄ films are larger than those reported for either bulk or thin-film NiFe₂O₄.^23,24

Fig. 9 (αhν)²versus E for NiFe_1.925Dy_0.075O₄ films (a). Linear regression fit and the extrapolation of the plot to zero absorption i.e., α (hν) = 0 provide the E_g value (b).

Fig. 10 L–E_g relationship in NiFe_1.925Dy_0.075O₄ films (a). A direct relationship is evident.

A model can be formulated to account for the observed L–E_g and a–E_g relationship and associated chemistry, which in turn establishes the functional structure–property relationship in NiFe_1.925Dy_0.075O₄ films. NiFe₂O₄ is a semiconductor with E_g = 2.1 eV. The E_g values obtained in the present work are significantly higher compared to this value. The effect of D_y incorporation is clearly the E_g enhancement compared to NiFe₂O₄. The changes in electronic structure and, hence, E_g within the NiFe_1.925Dy_0.075O₄ films can be explained as follows. It must be emphasized that the E_g value is sensitive to the microstructure; it is influenced by various factors such as surface/interface structure, grain size, crystal quality, lattice parameters, lattice strain, carrier concentrations, defect structure, and chemical composition. XRD measurements clearly indicate that the as grown NiFe_1.925Dy_0.075O₄ films are amorphous. E_g shift to 3.17 eV in NiFe_1.925Dy_0.075O₄ compared to that of pure Ni-ferrite can be attributed to the combined effect of Dy incorporation, amorphous structure and presence of oxygen vacancies. The poor quality of the films due to amorphous nature and oxygen vacancies is also evidenced by the spectral transmittance, which is significantly lower compared to the annealed films. A direct, linear relationship of T_a–E_g, L–E_g and a–E_g is evident from this work (Fig. 10). Structural transformation in NiFe_1.925Dy_0.075O₄ films from amorphous to nanocrystalline at 450 °C coupled with filling up of oxygen vacancies is responsible for the observed increase in E_g. This structural transformation is evidenced by XRD while the improved spectral transmittance accounts for fulfilment of oxygen vacancies due air annealing. A further increase in T_a from 450 to 1000 °C is seen to increase the density, grain size, and lattice parameter. It has been reported^23,24 that the band gap (E_g) enhancement in nanostructured materials is due to two factors, quantum confinement and strain. As a result, although the approximation was proposed for spherical nanocrystals, the variation in band gap energy can be written as:

E_g = E_g (Bulk) + ΔE_g (I) + ΔE_g (II) (6)

The ΔE_g (I) and ΔE_g (II) terms account for the quantum and strain induced effects, respectively. Clearly, the E_g enhancement in the present case with increasing T_a is not due to ΔE_g (I) term since the average grain size increases as noted from XRD studies. Since all the films were air-annealed for same amount of time, E_g enhancement with increasing T_a must be associated with the lattice parameter expansion, which leads to the strain in the films. In other words, the second term (ΔE_g (II)) of the above equation is at play to induce the observed changes. This accounts for the observed increase in E_g from 3.17 to 4.08 eV for NiFe_1.925Dy_0.075O₄ films with increasing T_a. Such behavior and E_g enhancement is also noticed in transition-metal doped NiFe₂O₄ films ^23–25

4. Conclusions

NiFe_1.925Dy_0.075O₄ films were fabricated using sputter-deposition and their structural and optical properties were investigated. The effect of partial substitution of Dy³⁺ ions for Fe³⁺ ions is, in general, lattice expansion associated with lattice strain and enhanced E_g. NiFe_1.925Dy_0.075O₄ films grown at RT were amorphous and oxygen deficient. Air annealing at T_a = 450–1000 °C induces structural transformation from amorphous nanocrystalline. The nanocrystalline NiFe_1.925Dy_0.075O₄ films exhibit an increase in density, grain size, lattice constant, and lattice strain. These increased factors coupled with filling up of oxygen vacancies results in changes in the electronic structure leading to E_g enhancement. Conductivity value of NiFe_1.925Dy_0.075O₄ films at 300 K for the films annealed at 450, 800 and 1000 °C was found to be 1.07 Ω⁻¹ m⁻¹, 1.39 × 10⁻¹ Ω⁻¹ m⁻¹ and 3.9 × 10⁻³ Ω⁻¹ m⁻¹ respectively. Conductivity was found to decrease exponentially with decreasing the temperature from 300 K to 120 K which indicates the semiconducting nature of the films. The functional structure–property relationship as found in this work for nanocrystalline NiFe_1.925Dy_0.075O₄ films suggests that tuning electrical and optical properties can be achieved by controlling the microstructure.

References

1. S. Chikazumi, Physics of Ferromagnetism, 1997, Oxford University Press, New York.
2. J. Smith and H. P. J. WijnFerrites, 1965, Philips Technical Library, Eindhoven-Holland.
3. Z. H. Zhou, J. Wang, J. M. Xue and H. S. O. Chan, J. Mater. Chem., 2001, 11, 3110.
4. C. Nimai Pramanik, T. Fujii, M. Nakanishi and T. Jun, J. Mater. Chem., 2004, 14, 3328.
5. D. Carta, M. F. Casula, A. Falqui, D. Loche, G. Mountjoy, C. Sangregorio and A. Corrias, J. Phys. Chem. C, 2009, 113, 8606.
6. V. Sepelak, I. Bergmann, A. Feldhoff, P. Heitjans, F. Krumeioch, D. Menzel, F. Litterst, S. J. Campbell and K. D. Becker, J. Phys. Chem. C, 2007, 111, 5026.
7. P. Gao, E. V. Rebrov, M. Tiny, W. G. M. Verhoeven, J. C. Schouten, R. Kleismit, G. Kozlowski, J. Cetnar, Z. Turgut and G. Subramanyam, J. Appl. Phys., 2010, 107, 044317.
8. S. Jie, W. Lixi, X. Naicen and Z. J. Qitu, J. Rare Earths, 2010, 28, 469.
9. A. B. Gadkari, T. J. Shinde and P. N. Vasambekar, J. Alloys Compd., 2011, 509, 966.
10. K. Kamala Bharathi, G. Markandeyulu and C. V. Ramana, J. Electrochem. Soc., 2011, 158(3), G71.
11. K. Kamala Bharathi and C. V. Ramana, J. Mater. Res., 2011, 26, 584.
12. K. Kamala Bharathi, G. Markandeyulu and C. V. Ramana, Electrochem. Solid-State Lett., 2010, 13, G98.
13. K. Kamala Bharathi, K. Balamurugan, P. N. Santhosh, M. Pattabiraman and G. Markandeyulu, Phys. Rev. B: Condens. Matter Mater. Phys., 2008, 77, 172401.
14. N. Rezlescu, E. Rezlescu, C. Pasnicu and M. L. J. Craus, J. Phys.: Condens. Matter, 1994, 6, 5707.
15. K. Kamala Bharathi, R. S. Vemuri and C. V. Ramana, Chem. Phys. Lett., 2011, 505, 202.
16. B. D. Cullity Elements of X ray diffraction (Addison-Wesley Publishing Company, Inc., 1956).
17. M. Ajmal and A. Maqsood, Mater. Sci. Eng., B, 2007, 139, 164.
18. K. Ajai Gupta, V. Kumar and N. Khare, Solid State Sci., 2007, 9, 817.
19. R. S. Vemuri, K. Kamala Bharathi, S. K. Gullapalli and C. V. Ramana, ACS Appl. Mater. Interfaces, 2010, 2, 2623.
20. N. F. Mott and E. A. DavisElectronic Processes in Non-CrystallineMaterials, 1979, 2nd ed.; Clarendon: Oxford, U.K.
21. S. K. Gullapalli, R. S. Vemuri and C. V. Ramana, Appl. Phys. Lett., 2010, 96, 171903.
22. C. V. Ramana, R. J. Smith and C. M. Julien, J. Vac. Sci. Technol., A, 2004, 22, 2453.
23. S. N. Dolia, R. Sharma, M. P. Sharma and N. S. Saxena, Ind. J. Pure and Appl. Phys., 2006, 44, 774.
24. S. Balaji, R. Kalai Selvan, L. John Berchmans, S. Angappan, K. Subramanian and C. O. Augustin, Mater. Sci. Eng., B, 2005, 119, 119.
25. S. M. Chavan, M. K. Babrekar, S. S. More and K. M. Jadhav, J. Alloys Compd., 2010, 507, 21.

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