K. Kamala Bharathi

Effect of Structure and Size on the Electrical Properties of Nanocrystalline WO3 Films

Nanocrystalline WO3 films were grown by reactive magnetron sputter-deposition by varying the substrate temperature in the range of 303(RT)-673 K. The structure and electrical transport properties of WO3 films were evaluated using X-ray diffraction and dc electrical conductivity measurements. The effect of ultramicrostructure and grain-size was significant on the electrical properties of WO3 films. DC conductivity variation of the WO3 films measured in the temperature range of 120-300 K reveals their semiconducting nature. The temperature dependent electrical conductivity curves exhibit two distinct regions indicative of two different types of electrical transport mechanisms. Analysis of the conductivity indicates that the small polaron and variable-range-hopping mechanisms are operative in 180-300 K and 120-180 K temperature regions, respectively. The density of localized states at the Fermi level, N(EF), has been calculated and it was found to be ∼1×10(19) eV(-1) cm(-3) for all the films.

Enhanced optical constants of nanocrystalline yttrium oxide thin films

Yttrium oxide (Y2O3) films with an average crystallite-size (L) ranging from 5 to 40 nm were grown by sputter-deposition onto Si(100) substrates. The optical properties of grown Y2O3 films were evaluated using spectroscopic ellipsometry measurements. The size-effects were significant on the optical constants and their dispersion profiles of Y2O3 films. A significant enhancement in the index of refraction (n) is observed in well-defined Y2O3 nanocrystalline films compared to that of amorphous Y2O3. A direct, linear L-n relationship found for Y2O3 films suggests that tuning optical properties for desired applications can be achieved by controlling the size at the nanoscale dimensions.

Correlation between structural, magnetic, and dielectric properties of manganese substituted cobalt ferrite

Manganese (Mn) substituted cobalt ferrites (CoFe2−xMnxO4, referred to CFMO) were synthesized and their structural, magnetic, and dielectric properties were evaluated. X-ray diffraction measurements coupled with Rietveld refinement indicate that the CFMO materials crystallize in the inverse cubic spinel phase. Temperature (T = 300 K and 10 K) dependent magnetization (M(H)) measurements indicate the long range ferromagnetic ordering in CoFe2−xMnxO4 (x = 0.00–0.15) ferrites. The cubic anisotropy constant (K1(T)) and saturation magnetization (Ms(T)) were derived by using the “law of approach” to saturation that describes the field dependence of M(H) for magnetic fields much higher than the coercive field (Hc). Saturation magnetization (Ms), obtained from the model, decreases with increasing temperature. For CoFe2O4, Ms decreases from 3.63 μB per formula unit (f.u.) to 3.47 μB/f.u. with increasing temperature from 10 to 300 K. CFMO (0.00–0.15) exhibit the similar trend while the magnitude of Ms is dependent on...

Impedance Spectroscopic Characterization of Sm and Ho Doped Ni Ferrites

We report on the impedance spectroscopic characterization of Sm and Ho doped Ni ferrite materials, namely NiO.Fe 1.925 Sm 0.075 O 3 and NiO.Fe 1.925 Ho 0.075 O 3 , to demonstrate their improved electrical properties compared to pure NiO.Fe 2 O 3 . Sm and Ho doped Ni ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO 3 and HoFeO 3 as the additional phase, respectively. Atomic force microscopy measurements indicate that the bulk grains are approximately 2―5 μm in size while the grain boundaries are thin compared to bulk grains. Frequency variation of the dielectric constant shows the dispersion that can be modeled with a modified Debyes function, which considers the possibility of more than one ion, contributing to the relaxation. The resistivity values (at 3.5 KHz) of NiO . Fe 2 O 3 , NiO . Fe 1.925 Sm 0.075 O 3 , and NiO . Fe 1.925 Ho 0.075 O 3 compounds are found to be 0.1 × 10 4 Ω m, 0.5 × 10 4 , Ω m and 0.8 × 10 4 Ω m, respectively. Impedance spectroscopic analysis indicates the different relaxation mechanisms and their variation with temperature, bulk grain and grain-boundary contributions to the electrical conductivity (Rg), and capacitance (C g ) of these materials. While the conductivity in pure NiFeO 4 is predominantly due to intrinsic bulk contribution (R g = 213 kΩ and C g = 4.5 × 10 ―8 F), NiO.Fe 1.925 R 0.075 O 3 (R = Sm,Ho) exhibits distinct grain and grain-boundary contributions to the conductivity.

Ferroelectric and ferromagnetic properties of Gd substituted nickel ferrite

Ferromagnetic and ferroelectric characteristics of Gd substituted nickel ferrite (NiO⋅Fe1.925Gd0.075O3) were investigated. The material formed in the cubic inverse spinel phase and in addition, a small amount of GdFeO3 phase was identified. A small distortion of the cubic lattice was observed upon the substitution of Fe by Gd in the B site. Substitution of Gd for Fe lowered the saturation magnetization. However, the saturation magnetostriction is seen not to change significantly by the substitution of Gd. From the temperature variation of dielectric constant measurement, the ferroelectric transition temperature was found to be 512K. The existence of the ferroelectricity was confirmed from the ferroelectric loop. The (high) dielectric constant with frequency is seen to reveal a dispersion of relaxation times.

Grain size and strain effects on the optical and electrical properties of hafnium oxide nanocrystalline thin films

Nanocrystalline hafnium oxide (HfO2) thin films with an average crystal size ∼5–50 nm have been produced under controlled temperature and pressure and their structural, optical and electrical properties have been evaluated. Structural investigations indicate that the monoclinic HfO2 nanocrystals are highly oriented along the (1¯11) direction. The lattice expansion increases with a reduction in size while minimum strain energy occurs at maximum lattice expansion. The strain factor increases linearly from 2.4% to 4% with increasing HfO2 crystal dimensions from 10 to 40 nm. The optical absorption measurements indicate that the band gap (Eg) changes are not very significant with size reduction. The interband absorption corresponding to O 2p→Hf 5d transitions dominate the optical absorption edge at E∼6.25 eV; Eg varied in the range 5.42–5.60 eV for a variation in crystal dimension in the range 10–40 nm. Room temperature electrical conductivity increases from 1.08×10−3 to 1.7×10−3 (Ω cm)−1 with increasing size ...

Enhanced Dielectric Property of Ni Ferrite by Sm and Ho Substitution

The enhancement in the dielectric constant of NiO·Fe 1.925 Sm 0.075 O 3 and NiO·Fe 1.925 Ho 0.075 O 3 when compared to pure NiO·Fe 2 O 3 is reported. Sm- and Ho-substituted Ni ferrites crystallize in the cubic inverse spinel phase. Frequency variation in the dielectric constant shows the dispersion that can be modeled with a modified Debyes function, which considers the possibility of more than one ion, contributing to the relaxation. Temperature-dependent electrical conductivity curves exhibit two distinct regions indicative of two different types of conduction mechanisms, the small polaron and variable range hopping, at 220-300 and 160-220 K regions, respectively.

Coexistence of spin glass behavior and long-range ferrimagnetic ordering in La- and Dy-doped Co ferrite

The structure, dc magnetization and ac susceptibility characteristics of the rare-earth (R = La,Dy) ionsubstituted cobalt-ferrites (CoO.Fe1.925La0.075O3 and CoO.Fe1.925Dy0.075O3) are evaluated. R-substituted Co-ferrites crystallize in the cubic inverse spinel phase. The irreversible temperature (Tirr) between zero field cooled (ZFC) and field cooled (FC) magnetization for CoO.Fe1.925La0.075O3 and CoO.Fe1.925Dy0.075O3 determined from the temperature variation of magnetization measurements are 283 and 292 K, respectively. The broadening of ZFC magnetization and more than one maximum indicates the coexistence of short-range ferrimagnetic clusters of different size with a long-range ferrimagnetic phase. Magnetization curves indicate no saturation up to 30 kOe suggesting the canted spin structure inside the clusters. The relaxation times of spin clusters calculated using theVogel−Fulcher law for the frequency-dependent ac susceptibility measurements are on the order of ∼10−6 s.

Detection of electrically formed photosensitive area in Ca-doped BiFeO3 thin films

We report on the visualization of n-p junctions formed by oxygen vacancy movement under the application of an electric field in a Ca-doped BiFeO3 thin film through spatially resolved scanning photocurrent mapping. The photocurrent mapping, in conjunction with the spectroscopic approach, provides clues to local electronic structures and defect levels associated with oxygen vacancies. These observations provide insights into the spatial redistribution of oxygen vacancies in an electric field.

Epitaxial LiCoO2 Films as a Model System for Fundamental Electrochemical Studies of Positive Electrodes

Epitaxial LiCoO2 (LCO) thin films of different orientations were fabricated by pulsed laser deposition (PLD) in order to model single-crystal behavior during electrochemical reaction. This paper demonstrates that deposition of conductive SrRuO3 between a SrTiO3 (STO) substrate and an LCO film allows (1) epitaxial growth of LCO with orientation determined by STO and (2) electrochemical measurements, such as cyclic voltammetry and impedance spectroscopy. Scanning transmission electron microscopy (S/TEM and SEM) has demonstrated an orientation relationship between LCO and STO of three orientations, (111), (110) and (100), and identified a LCO/electrolyte surface as consisting of two crystallographic facets of LCO, (001) and {104}. The difference in the orientation of LCO accounts for the difference in the exposed area of {104} planes to the electrolyte, where lithium ions have easy access to fast diffusion planes. The resistance for lithium ion transfer measured by electrochemical impedance spectroscopy had inverse correlation with exposed area of {104} plane measured by TEM. Chemical diffusivity of lithium ions in LCO was measured by fitting electrochemical impedance spectroscopy data to a modified Randles equivalent circuit and allowed us to determine its dependence on film orientation.