Kiran Dasari

We report a detailed study on the dynamical response of localized electron hopping and dipole relaxation in bulk polycrystals of Zn diluted Cuprospinel (Cu _{1 −  x} Zn _x Fe ₂ O ₄ ). The variations in the dielectric dispersion and ac -resistivity ( ρ _ac ) were analyzed over a wide temperature (77 K ≤ T ≤ 823 K) and frequency (20 Hz ≤ f ≤ 20 MHz) window for a critical composition x _c = 0.4. The variation of ϵ _R ( f , T) followed the Maxwell–Wagner type polarization mechanism in-line with the Koops phenomenological theory. Our analysis of ρ _ac (T, f ) provide strong evidence to the Mott’s variable range hopping of charge transport between the localized states at low temperatures, however, thermally-activated Arrhenius like behaviour was noticed at high temperatures with E _A = 656 meV for x _c = 0.4. Moreover, electric modulus spectroscopic studies ( M ^* ( f , T)) reveals two distinct types of relaxation phenomena: (i) the short-range oscillations of the charge carriers within the potential well of grains and (ii) the long-range movement of charge carriers across the grain boundaries. The depressed semi-circles of the Nyquist plots and lower values of non-exponential parameter extracted from M ^* ( f , T) suggest the non-Debye type relaxation process present in the system with a widespread distribution of relaxation times. The frequency exponent ( s ( x , T)) study of Jonscher’s power law reveals that the ac -conductivity follows small-polaron tunnelling followed by the correlated-barrier-hopping mechanism for x   < 0.1. However, for x ≥ x _c reorientational hopping mechanism is predominant, except for T   > 400 K, where thermally activated Arrhenius-type conduction of charge carriers is prevalent in this spinel system. Furthermore, the tetragonally ( I 4 ₁ / amd ) distorted systems ( x ≤ 0.05) exhibit less activation energy ( E _{A  −  VRH} ) values as compared to those of cubic-spinel symmetry ( Fd -3 m ) which saturates at 130 meV for 0.1 ≤ x ≤ 0.6. Compositional dependent tunability of the above discussed parameters may open a constructive approach to design low energy-loss and high-resistive electromagnetic elements for microwave devices which is the key significance of the present study.

Cadmium selenide (CdSe) thin films were deposited on indium tin oxide (ITO) coated glass substrates using pulsed laser deposition (PLD) technique under different growth temperatures. Samples were investigated for their structural, morphological, and optical properties through X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-Vis-NIR spectroscopy. AFM analysis revealed that the surface roughness of the as-grown CdSe thin films increased with the increase in deposition temperature. The optical constants and film thickness were obtained from spectroscopic ellipsometry analysis and are discussed in detail. The optical band gap of the as-grown CdSe thin films, calculated from the Tauc plot analysis, matched with the ellipsometry measurements, with a band gap of ~1.71 eV for a growth temperature range of 150 °C to 400 °C. The CdSe thin films were found to have a refractive index of ~3.0 and extinction coefficient of ~1.0, making it a suitable candidate for photovoltaics.

The engineering of materials with controlled magnetic properties by means other than a magnetic field is of great interest in nanotechnology. In this study, we report engineered magnetic graphene oxide (MGO) in the nanocomposite form of iron oxide nanoparticles (IO)-graphene oxide (GO) with tunable core magnetism and magnetic resonance transverse relaxivity (r₂). These tunable properties are obtained by varying the IO content on GO. The MGO series exhibits r₂ values analogous to those observed in conventional single core and cluster forms of IO in different size regimes—motional averaging regime (MAR), static dephasing regime (SDR), and echo-limiting regime (ELR) or slow motion regime (SMR). The maximum r₂ of 162 ± 5.703 mM⁻¹s⁻¹ is attained for MGO with 28 weight percent (wt%) content of IO on GO and hydrodynamic diameter of 414 nm, which is associated with the SDR. These findings demonstrate the clear potential of magnetic graphene oxide for magnetic resonance imaging (MRI) applications.

We report a detailed study on the nature of magnetic exchange interactions and dielectric properties of Z n 1 − x Ni _x O–NiO composites. The magnetic exchange constants | J 1 | and | J 2 | for the two sublattices have been evaluated using both molecular-field approximation ( J 1 ∼ 0.15 meV and J 2 ∼ 1.15 meV for x = 1 ) and Green’s-function theory ( J 1 ∼ 0.78 meV and J 2 ∼ 1.77 meV for x = 1 ). Magnitudes of these exchange constants determined from the Green’s-function analysis are consistent with the values obtained from the spin-wave dispersion curves ( J 1 ∼ 0.137 meV and J 2 ∼ 1.90 meV for x = 1 ) and Raman scattering ( J 2 ∼ 1.98 meV). Moreover, the carrier activation energy ϵ h (∼59.9 meV) and nearest-neighbor exchange coupling parameters J i j (∼1.61 meV) calculated from the temperature dependence of ac resistivity ρ a c ( T ) across the antiferromagnetic Néel temperature ( T _N ) of NiO using a small-polaron model involving spin-dependent activation energies are compared with the theoretical estimations from the Green’s-function theory. In addition, the role of oxygen stoichiometry in the temperature variation of J i j , ϵ R , ρ _ac and the unit-cell volume V _C have been discussed.

We report on the growth, surface, luminescence and magnetic properties of 180 nm thick Er-doped GaN thin films grown by molecular beam epitaxy (MBE) on c -sapphire substrates with no buffer layer and with different Er concentrations. In situ reflection high-energy electron diffraction (RHEED) patterns revealed crystalline and uniform growth of the films. The x-ray diffraction (XRD) pattern showed c -axis-oriented growth. Atomic force microscopy (AFM) analysis showed enhancement of surface morphology and smoothness with increasing Er doping, which could be due to minimization of surface defects because of the gettering effect of the rare earth. Scanning area-dependent surface morphology analysis showed a power law dependence indicating the fractal nature of the surface, which is confirmed by the observation of a non-integer D (fractal dimension) value. X-ray photoluminescence spectroscopy (XPS) revealed the formation of a GaN:Er phase and ruled out the presence of Ga and Er metallic and native oxide phases. The semi-quantitative elemental composition of the films was determined using N 1s, Ga 2p _3/2 and Er 4d photoemission lines. The Er concentration was estimated from the x-ray photoelectron spectra and found to be between 3.0 and 9.0 at.% (∼10 ²¹ atoms cm ⁻³ ). Photoluminescence (PL) and cathodoluminescence (CL) studies showed visible emission and concentration quenching of Er ³⁺ ions in agreement with reported results. Excitation of the Er ³⁺ ion might be affected by charge trapping due to Er-doping-induced defect complexes. The magnetic measurements carried out by a superconducting quantum interference device (SQUID) showed a ferromagnetic–paramagnetic phase transition at low temperature, contrary to the reported room temperature ferromagnetism in metalorganic chemical vapor deposition (MOCVD)-grown GaN:Er thick films of 550 nm.