Shakeel Khan

Magnetic and electrical properties of In doped cobalt ferrite nanoparticles

Nanoparticles of CoFe2O4 and CoIn0.15Fe1.85O4 ferrites were prepared by citrate gel route and characterized to understand their structural, electrical, and magnetic properties. X-ray diffraction and Raman spectroscopy were used to confirm the formation of single phase cubic spinel structure. The average grain sizes from the Scherrer formula were below 50 nm. Microstructural features were obtained by scanning electron microscope and compositional analysis by energy dispersive spectroscopy. The hysteresis curve shows enhancement in coercivity while reduction in saturation magnetization with the substitution of In3+ ions. Enhancement of coercivity is attributed to the transition from multidomain to single domain nature. Electrical properties, such as dc resistivity as a function of temperature and ac conductivity as a function of frequency and temperature were studied for both the samples. The activation energy derived from the Arrhenius equation was found to increase in the doped sample. The dielectric cons...

Copper tetramers in high-temperature superconductors

Abstract Clusters of four copper ions coupled by an exchange interaction are shown by EPR in the constituents of high- T c superconductors and in an oxygen deficient 123-superconductor. The spin-Hamiltonian parameters of the EPR spectra have been reported. Copper tetramers may be at the heart of the superconductivity mechanism.

EPR study of deoxygenated La2CuO4

Abstract The EPR spectra of deoxygenated La 2 CuO 4 were studied. On deoxygenation, various species are formed as copper monomers and copper tetramers. Their spin-Hamiltonian parameters have been given. The ferromagnetic coupling of spins of the 4 Cu 2+ ions in the tetramers may be due to superexchange interactions mediated by intervening oxygen ions. The importance of this study has been discussed.

EPR spectra of deoxygenated high Tc superconductors

EPR spectra of deoxygenated Bi-2201, Bi-2212, Bi-2223, Y-123, Tl-2223 and Hg-1223 have been investigated. Different types of Cu-clusters as octamers, tetramers, dimers and monomers have been identified. EPR spectra were recorded for more than 100 samples of these materials, from different batches of preparations and from different portions of the same batch with the aim of recording all varieties of the spectra in these materials. Different samples of the same compound gave different spectra, some of which were repetitive but some spectra of the samples of different compounds were also similar. In different samples of the same compounds, different combinations of the spectra of octamers, tetramers, dimers and monomers appeared. In all 20 different types of spectra could be observed. The role of Cu-tetramers in the mechanism of high temperature superconductivity has been hinted at.

EPR study of La1.854Sr0.146CuO4

Abstract EPR spectra of La1.854Sr0.146CuO4 and its aging effect have been studied. Temperature variation of the EPR spectra of La1.854Sr0.146CuO4 (from room temperature to liquid nitrogen temperature, LNT) along with CaCuO2 for comparison has been carried out. In all the materials, after quenching the most abundant species, the ferromagnetically coupled Cu-tetramer (CuO)4 was obtained. The (CuO)4 square, being the unit cell of all important two-dimensional CuO2 planes superconductors, may reflect the properties of the superconductors. At about 100 K, in La1.854Sr0.146CuO4, the tetramer signal was destabilized, disappeared and was immediately followed by a new signal, which may be seen as a signature of formation of quantum stripes.

EPR study of La2-xMxCuO4 (M = Sr, Ba)

Abstract EPR spectra of deoxygenated La1.81Sr0.19CuO4 and La1.94Ba0.06CuO4 have been investigated. In both of them 4 Cu2+ ions are found to combine ferromagnetically to give S = 2 spectra. At low Sr Ba content the spins probably start getting polarised in the ab plane. Spin-Hamiltonian parameters for all the spectra have been determined. Dependence of Tc on the content of Sr or Ba has been attempted to be explained. It seems that holes which are generated on the substitution of La3+ by Sr2+ or Ba2+ and are current carriers first get uniformly distributed over the LaO plane but on increasing Sr2+ or Ba2+ content, they may combine together to form O22− peroxide ions and become unable to carry current. On further increasing the Sr2+ or Ba2+ content, the same process is repeated, until the substance becomes metallic or until further amounts of the substituents can bot be dissolved in the parent compound.

Guidelines regarding management of adrenal insufficiency in the Holy month of Ramadan

Adrenal insufficiency is a life-threatening event, so it is recommended for patients with known adrenal insufficiency to be properly educated regarding sick-day management. In the month of Ramadan, people refrain from eating and drinking during daylight hours. It is very important for patients with adrenal insufficiency, who wish to keep a fast, to be well aware of the disease, the suitable drug to be used for that particular period, warning signs, sick-day management, physical activity, and dietary limits. This article describes guidelines for the sick-day management of patients with adrenal insufficiency, in the month of Ramadan.

S = 12, S = 1 and S = 2 EPR spectra in copper doped KHSO4 single crystal

Abstract In the room temperature EPR investigation of copper doped KHSO 4 single crystals, different signals corresponding to monomer, dimer and tetramer of copper are obtained simultaneously. The spin-Hamiltonian parameters are reported.

Study of structural property of Co ferrite thin film grown by pulsed laser deposition technique

Thin film of Cobalt Ferrite was deposited on Si (1 0 0) substrate using Pulsed Laser Deposition (PLD) technique. The deposited film was characterized by X–ray Diffraction (XRD), X–ray reflectivity (XRR), Rutherford Backscattering Spectroscopy (RBS) and Raman Spectroscopy and was found to be single phase, textured along (1 1 1) directions and approximately matching the stoichoimetry of the target with negligible strain. The film had a very uniform and flat surface. Raman spectroscopy measurement further confirmed the formation of single phase cubic spinel structure. T2g Raman mode was missing from the spectra which may be due to cation redistribution and crystallite size effect.

TEMPERATURE PROFILE OF RESISTIVITY OF MANGANITES

In the resistivity–temperature curves of La0.7Ca0.3MnO3 and La0.75Ca0.25MnO3, five distinct regions have been identified. For example, in La0.7Ca0.3MnO3, the regions are: (1) from the lowest temperature of measurement to 200 K where resistivity is directly proportional to temperature with a small slope, (2) the reciprocal of resistivity follows Blochs T3/2 law, (3) resistivity is directly proportional to temperature with a large slope, (4) the cusp region, where there are FM clusters of reduced dimensions and varying degrees of correlated polarons or bipolarons and small polarons, (5) the semiconducting region having small polarons. The regions 1, 2 and 3 together appear to replicate the spontaneous magnetization curves of the materials. The effect of magnetic field on the resistivity–temperature curves are as follows: (a) resistivity is reduced. In particular, the resistivity peak with magnetic field occurs at a temperature μH/K downward from the Tc of the curve without magnetic field; (b) resistivity peak is displaced by μ · μH/K towards the higher temperature side compared to the resistivity peak without magnetic field; (c) cusp region is broadened; (d) magnetoresistance is appreciable only within 10% of Tp on both sides. All these observations suggest that temperature profile of resistivity of manganites is controlled by magnetic effects.