Sudip Mukherjee

Ultrasonic properties of V2O5P2O5 amorphous materials at different temperatures

Investigations are reported of the acoustical properties of V2O5 P2O5 glass, on which other studies such as structural, spectroscopic, NMR and electrical conductivity have been reported in the literature. In the present investigation, the velocity and attenuation of longitudinal and transverse ultrasonic waves in four different compositions of (V2O5)x (P2O5)1−x glass system (x = 0.4769, 0.6020, 0.7487 and 0.8227) have been measured at temperatures ranging from 80 to 300 K with the help of an ultrasonic pulse echo apparatus operating at 8 MHz frequency. The velocity data have been used to find the elastic moduli and Debye temperature for each composition. The results indicate that for each composition the velocities decrease slowly and monotonically with increasing temperature and the attenuation as a function of temperature shows a broad peak which shifts to lower temperature as the V2O5 content increases. The temperature dependence of ultrasonic velocity and attenuation has been interpreted in terms of a thermally activated relaxation process which comes into play when the ultrasonic wave disturbs the equilibrium of an atom moving in a double-well potential in the glass network.

Size-dependent dielectric behaviour of magnetic Gd2O3 nanocrystals dispersed in a silica matrix

Magnetic nanocrystalline Gd2O3 particles have been successfully synthesized in a silica glass matrix by the sol–gel method at calcination temperatures of 700 °C and above. The optical spectra, corresponding to the transition from ground 8S7/2 to the excited 6P, 6I and 6D multiplets, show a large blue shift which increases with decreasing particle size. Calculated free ion parameters represent different electrostatic and spin–orbit interactions of nanocrystalline Gd2O3 compared with the bulk Gd2O3. The thermal behaviour of magnetization (zero-field-cooled and field-cooled) and magnetic hysteresis of Gd2O3 nanocrystals in the 5–300 K temperature interval have demonstrated that the Gd2O3 nanocrystals present in these glasses display superparamagnetic–ferromagnetic transition at low temperatures. The present Gd2O3 nanoparticles embedded in the silica glass matrix show a high (~103) dielectric constant with considerable low loss and good temperature-independent character associated with an important technological implication which promises wide applications in memory devices.

Ultrasonic investigations of V2O5-GeO2 glass

Abstract The ultrasonic velocity and attenuation in four different samples of the (V2O5)x−(GeO2)1−x glass system (x = 0.5321,0.5816,0.6635 and 0.7132) have been measured at temperatures between 77 and 300 K using the ultrasonic pulse echo technique at 8 MHz. The elastic moduli and Debye temperatures have been calculated from the velocity data for each composition. The variations in ultrasonic velocity and attenuation with temperature have been explained by a model assuming an asymmetric double-well potential having distributions of both the barrier height and the asymmetry parameter.

Size-dependent magnetic properties of VO2 nanocrystals dispersed in a silica matrix

Magnetic nanocrystalline VO2 particles have been successfully synthesized in a silica glass matrix by the sol–gel method at calcination temperatures of 700 °C and above. The presence of small quantities of diamagnetic V2O5 nanocrystals in glass samples calcined at ≥800 °C has been confirmed. The thermal behavior of magnetization (zero-field-cooled and field-cooled) and magnetic hysteresis of VO2 nanocrystals in the 10–300 K temperature interval have demonstrated that the VO2 nanocrystals present in these glasses display superparamagnetic–ferromagnetic transition at low temperatures. X-band electron paramagnetic resonance (EPR) spectra of VO2 nanoparticles have depicted broad symmetric lines [ΔHpp~104 mT at room temperature (RT) and 250 mT at liquid nitrogen temperature (LNT)]. The increase in EPR line intensity on going from RT to LNT suggests that VO2 nanocrystals have undergone ferromagnetic-like ordering in the temperature interval RT–77 K. The presence of small numbers of isolated VO2+ ions in the SiO2 matrix has also been confirmed from EPR studies. From EPR and optical spectral studies it has been shown that the VO2+ ion has square pyramidal conformation with tetragonal compression having C4v symmetry. It is established that the in-plane σ bonding and out-of-plane π bonding are predominantly ionic in nature.

Electron paramagnetic resonance and other allied studies of sol?gel derived nanocrystalline chromium oxides in a silica glass matrix

Monolithic silica gels doped with various concentrations (1, 5, 10 mol%) of chromium oxide are prepared by a sol–gel process and are subjected to calcination at different temperatures up to 1200 °C. This has principally led to the formation of Cr2O3 nanocrystals and also of CrO3 nanocrystals at the higher calcination temperatures. The sizes of the Cr2O3 nanocrystals have been determined by x-ray diffraction (XRD) and transmission electron microscopy (TEM) studies and have been found to lie in the range 5–30 nm depending upon the dopant concentration as well as the calcination temperature. The formation of nonmagnetic CrO3 crystals in 10 mol% chromium oxide doped silica glass calcined at 1200 °C as a result of oxidation of Cr5+ ions to Cr6+ ions has been confirmed from XRD studies. Optical, infrared and X-band electron paramagnetic resonance (EPR) spectra of these nanocrystals have also been studied. EPR behaviours such as the g-factor and linewidth of Cr2O3 nanocrystals investigated at room and liquid nitrogen temperatures have been found to be quite distinct from those of antiferromagnetic (AF) bulk Cr2O3 crystals and may be manifestations of superparamagnetism/ferromagnetism in these nanocrystals. A narrow anisotropic EPR line () has been detected for chromium oxide doped silica gel calcined at 800 °C and has been assigned to isolated Cr5+ ions in a tetragonally distorted tetrahedral environment. Studies of the thermal dependence of zero-field-cooled (ZFC) and field-cooled (FC) magnetization (σ) 10 mol% chromium oxide doped silica glasses calcined at 800 °C by a SQUID magnetometer in the 5–300 K temperature range have revealed the occurrence of the blocking temperature TB at about 250 K, which is a characteristic feature of superparamagnetism in Cr2O3 nanocrystals. The superparamagnetic effects are almost quenched in the samples calcined at 1000 and 1200 °C because of the presence of larger nanocrystals in these samples. Magnetic hysteresis effects in these calcined samples at 300 K as well as at 5 K in the ± 6 T magnetic field range have also been investigated. The existence of ferromagnetic order at low temperature (5 K) as well as at room temperature (300 K) (for samples calcined at higher temperatures) has been confirmed.

Field-induced spin-flop transitions of interacting nanosized α-Fe2O3 particles dispersed in a silica glass matrix

Nanoparticles of 10 mol% Fe2O3 doped in silica glass (Fe10) samples prepared by a sol–gel method followed by calcination at various temperatures in the range 700–1000 °C are studied by x-ray, transmission electron microscopy and magnetic methods, including electron paramagnetic resonance (EPR). X-ray studies reveal the presence of both γ-Fe2O3 and α-Fe2O3 nanocrystals in varying proportion, the latter being more abundant in samples subject to calcination at higher temperature. Nanocrystals have mean sizes in the range 10–40 nm and are larger in the samples calcined at higher temperatures. The relatively narrow EPR line having its origin in superparamagnetism is observed at room temperature and is transformed to an asymmetrically broad ferromagnetic resonance signal at 77 K. Mossbauer spectra of the 700 °C calcined sample at room temperature show two doublet structures due to γ-Fe2O3 and α-Fe2O3 nanoparticles signifying their superparamagnetic character, whereas those of samples calcined at higher temperatures (≤1000 °C) display two sextets indicating that the nanoparticles are magnetically ordered. At higher calcination temperatures (≥900 °C) the hyperfine lines become asymmetrically broadened and the average hyperfine field diminishes in a way typical for interacting magnetic nanoparticles. Zero-field-cooled magnetization and hysteresis studies of Fe10 samples calcined at higher temperatures (≥800 °C) in the temperature range 5–300 K reveal unusual ferromagnetic behaviour with substantial magnetization and large coercivities at low temperature (5 K). The presence of a high irreversibility field and shifted hysteresis loop in the Fe10 sample calcined at 1000 °C at 5 K has been verified from field-cooled magnetization versus the magnetic field curve. Only α-Fe2O3 nanoparticles generated in the Fe2O3:SiO2 sample calcined at 1000 °C exhibit a Morin transition like bulk α-Fe2O3 crystals but at a much lower temperature ~100 K. Spin–flop like transitions have been observed for the first time at temperatures above the Morin temperature, possibly induced by an external dc magnetic field of appropriate magnitude in conjunction with inter-nanoparticle exchange.

Magnetic properties of GdT0.17Sn2 (T = Co, Cu) compounds

We report here the powder X-ray diffraction and magnetic measurements of two ternary compounds GdCo0.17Sn2 and GdCu0.17Sn2 for the first time. Both the compounds crystallize in the defect CeNiSi2-type structure. The magnetic measurement reveals that GdCo0.17Sn2 and GdCu0.17Sn2 order antiferromagnetically below 16.5 K and 25 K, respectively.

Antiferromagnetic ordering in GdCo0.17Sn2 and GdCu0.17Sn2

We report here the results of room temperature powder X-ray diffraction and magnetic measurements of two ternary compounds, GdCo0.17Sn2 and GdCu0.17Sn2. The solubility limits of transition metal ions in these compounds are quite limited (< 1). The crystal structures of both the compounds are of orthorhombic, CeNiSi2–type, space group cmcm. The magnetic measurement reveals that paramagnetic to antiferromagnetic phase change occur for GdCo0.17Sn2 and GdCu0.17Sn2 below 16.5 K and 25 K, respectively.

Magnetic properties of GdTxSn (T = Fe, Ni) compounds

The powder X-ray diffraction and magnetic measurements of two ternary compounds GdFe0.17Sn2 and GdNi0.32Sn2 are reported for the first time. Both the compounds are isotopic with the defect CeNiSi2-type structure (space group Cmcm). The compounds GdFe0.17Sn2 and GdNi0.32Sn2 show antiferromagnetic ordering of Gd-sublattice below 17 K and 14 K, respectively. The effective magnetic moment of Gd in these compounds is lower than the free ion value (7.94 μB). An attempt has been made to explain this feature using RKKY theory.

Magnetic Properties Of Er2O3 Nanocrystals Dispersed In Silica Matrix

Nanoparticles of Er2O3 doped in silica gels with 0.5% concentration has been prepared by sol‐gel method. Sample is calcined at 800 ° and 900 °C and studied by X‐ray, TEM and different magnetic methods. X‐ray studies reveal the presence of single phase Er2O3 nanocrystals, the mean sizes lying in the range 5–10 nm. From zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetization and magnetic hysteresis measurements in the temperature range 10–300 K it has been established that Er2O3 nanocrystals present in the calcined samples exhibit ferromagnetic‐like ordering. Small‐sized nanocrystals are observed to have larger uncompensated magnetic moments. This is consistent with the postulate that uncompensated spins are present in a greater number on the surface of the antiferromagnetic inner cores of the smaller‐sized nanomagnets.