Rabi N. Panda

Magnetic properties of interacting single domain Fe3O4 particles

Abstract The present study investigates magnetic properties of interacting 12.4 nm Fe3O4 spinel ferrite particles. Fe3O4 crystallizes in cubic structure with lattice parameters, a=0.839(1) nm. Ultrafine nature of the materials were ascertained by the X-ray diffraction (XRD) line broadening and field dependent magnetic analysis. The 57Fe Mossbauer spectrum is deconvoluted to two sextets indicating two different Fe sites and a central doublet indicating superparamagnetic fractions present. The saturation magnetization (at 298 K), σs=67.8 emu/g, is less than that of the bulk magnetic particles, i.e. σs (bulk)=92 emu/g. The reduction of σs in Fe3O4 particles is attributed to the presence of non-magnetic layer at the particle surface, cation distribution, superparamagnetic relaxation and spin canting because of the ultrafine nature of the material. The low field temperature dependence of magnetization below the Curie temperature, Tc, shows two distinct peaks, i.e. at 615 and 800 K. The maximization in magnetization near Tc and the decrease in Tc are attributed to a large degree of inversion of the Fe3O4 particles. A peak at 615 K in low field thermomagnetic studies on the zero field cooled (ZFC) samples confirms the dipolar interactions to be dominant over superparamagnetic blocking.

Magnetic properties of single domain ε-Fe3N synthesized by borohydride reduction route

Single domain e-Fe3N nitride particles have been synthesized by chemical reduction followed by nitridation at 823 K. The e-Fe3N phase crystallizes in hexagonal structure with unit cell parameters, a=2.70 A and c=4.39 A. There is a reduction of unpaired d electrons for intraband polarization as the nitrogen contributes to the density of states and thus results in lowering of magnetic moments. The reduction of intrinsic magnetizations and Curie temperatures with decreasing particle size is attributed to canted spin structure predominantly at the surface compared to the bulk of the particles. Chemisorption of oxygen results in the formation of oxynitride layer at the surface with a ferromagnetic coupling to the spins in the core of the particle. Mossbauer study of e-Fe3N particles exhibits the coexistence of ferromagnetic and superparamagnetic particles and corroborates the observed magnetic properties.

Electronic and magnetic properties of Fe3Mo3N

Abstract The ternary nitride Fe3Mo3N, is synthesized through an intermediate nitride having WC type structure by nitridation of an FeMoO4 precursor, at 973 K. The Fe3Mo3N compound crystallizes in a cubic structure, a = 1.1067 nm, and the particles are cubic platelets forming oblate clusters. Fe3Mo3N is thermally stable up to 633 K and shows metallic behaviour and the change in temperature coefficient of resistance (TCR) values at 120 K corroborates the magnetic phase transition. The material shows paramagnetic behaviour and orders antiferromagnetically at a Neel temperature, TN = 120 K. The spin alignments result in the coupling of magnetic moments and the localized states are distributed over the bonding and antibonding states leading to metallicity.

Mössbauer and magnetic studies of nanocrystalline γ'-Fe4-xNixN (0.2 ≤ x ≤ 0.8) compounds

Abstract Nanocrystalline γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds were synthesized by using a citrate precursor route. It was observed that lattice constants for γ′-Fe4−xNixN (0.2⩽x⩽0.8), decrease with increasing Ni atom concentration. The atmospheric oxidation of γ′-Fe4−xNixN (0.2⩽x⩽0.8) compounds result in the formation of Fe-oxide layer at the surface of the ultrafine particles in addition to pure nitride phase. The local magnetic structures of the iron atoms in the nitride materials are quite similar to those found in the case of dilute alloy systems. The role of Ni substitution in γ′-Fe4N as γ-Fe4−xNixN is investigated with regard to crystal structure and magnetic properties of the ultrafine nitride materials. The results suggest that the average magnetic moment per iron atom in ultrafine γ′-Fe4−xNixN compounds is affected by the Ni atom concentrations, superparamagnetic relaxation, presence of oxide layer and randomly canted spin structure at the particle surface. The observed larger values of coercivities in the ultrafine materials are on account of the reversal of magnetization by spin rotation mechanism.

X-Ray Diffractometry and X-Ray Photoelectron Spectroscopy Investigations of Nanocrytalline Hydroxyapatite Synthesized by a Hydroxide Gel Technique

Nanocrystalline hydroxyapatite (HA), Ca10(PO4)6(OH)2, has been synthesized by a precipitate-conversion technique using hydroxide gel. Hydroxyapatite crystallizes in a hexagonal structure (space group; P63/m) having lattice parameters; a=9.407 A and c=6.883 A, and around 37 nm in size for the 800°C-annealed sample. Substantial crystalline characteristics are observed for the material heat-treated at 80°C. With the increase of air-annealing temperature, from 80°C to 800°C, lattice expansion along all crystallographic axes occurs indicating a structural change in the HA lattice. These results have been attributed to (1) the formation of a relative Ca2+ deficiency in HA that incorporates various chemical species in the anionic sites compared to their lower temperature air-annealed counterparts, and (2) the precipitation of CaO with increasing air-annealing temperature. The X-ray photoelectron spectroscopy studies indicate a distorted anionic structure and two electronic states for the O1 s, corroborating the observed X-ray diffraction results.

Effect of nitridation on magnetic properties of Fe53Pt47 thin film

The Fe53Pt47 thin films with varying thickness were prepared using dc magnetron sputtering at various substrate temperatures (i.e., from 250 to 600 °C) on to CrMo seeded glass substrates. The powder x-ray diffraction studies reveal that the ordered fct γ2-FePt phase begins to appear starting from the substrate temperature of 250 °C. The estimated ordering parameters show that the films prepared at 300 °C are well ordered and ordering parameters increase slowly as increasing substrate temperatures. On nitridation of the Fe53Pt47 thin films, expansion of the face centered tetragonal crystal lattice along the c axis is observed. The saturation magnetization is decreased with decreasing film thickness. This has been explained mainly on the basis of size and surface effects of nanocrystals in the films including intergranular interactions. Maximum coercivity of 8.7 kOe is observed for the film thickness of 350 nm. The drastic decrease in magnetization with the increase in nitridation time duration has been att...

FTIR, magnetic and Mössbauer investigations of nano-crystalline FexCo1−x (0.4 ≤ x ≤ 0.8) alloys synthesized via a superhydride reduction route

The present investigation reveals the synthesis of capped nano-crystalline FexCo1−x (0.2 ≤ x ≤ 0.8) alloys via a superhydride reduction route using oleic acid and oleylamine as stabilizing agents. The synthesized nano-particles are stable against oxidation in an air atmosphere (air stable) at room temperature (298 K). The structure–property correlation in FeCo alloys has been achieved using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM), magnetic and Mossbauer measurements. A FTIR study indicates the presence of organic content at the surface of the nanoparticles, which helps in the stabilization of the FeCo samples in an air atmosphere. FeCo alloys crystallize in a pure α-phase with the increase in the values of lattice parameters with the increase in Fe content, i.e. 2.836(±4) A, 2.852(±2) A, 2.859(±1) A and 2.868(±1) A for x = 0.2, 0.4, 0.6 and 0.8, respectively. Average crystallite sizes and TEM particle sizes were found to be in the range of ≈23–38 nm and 11–51 nm, respectively. The values of the saturation magnetization (Ms) for FeCo alloys range from 71.1–92.5 emu g−1 for heat treated materials and 93.1–142.2 emu g−1 after applying the corrections for organic wt% at the surface of the materials. The observed values of effective anisotropy constants (Keff) of FexCo1−x alloys from field cooled (FC) and zero field cooled (ZFC) studies (i.e. 1.5 kJ m−3, 4.6 kJ m−3 and 14.3 kJ m−3 for x = 0.4, 0.6 and 0.8, respectively) reveal the contribution from the reduced particle size and surface anisotropy. The maximum value of a hyperfine field for FexCo1−x alloys was found to be 34.9 T for Fe0.6Co0.4 composition and has been interpreted on the basis of enhancement of Fe moments in the disordered crystal lattice.

Mesoporous Vanadium Nitride Synthesized by Chemical Routes

Nanocrystalline vanadium nitride (VN) materials are synthesized by two different routes, namely, the urea route and the ammonia route, using various V2O5 precursors obtained by citric acid–based sol–gel method. The VN nanomaterials obtained are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and BET method. The XRD patterns show the presence of pure cubic phase with line broadening. The lattice parameters were found to be 4.1266 and 4.1136 Å for the urea route and 4.1125 and 4.1120 Å for the ammonia route using V2O5 precursors heat treated at 350 and 550 °C, respectively. The estimated crystallite sizes are found to be 9 and 7 nm for the urea route and 22 and 28 nm for the ammonia route using V2O5 precursors synthesized at 350 and 550 °C, respectively. The SEM images show the presence of agglomerates having particle dimensions of 115 and (300 × 15) nm for the urea route and 111 and 68 nm for the ammonia route using V2O5 precursors heat treated at 350 and 550 °C, respectively. The maximum BET surface area of 57 m2/g was obtained for VN sample synthesized by the urea route using V2O5 precursors synthesized at 350 °C. The isotherms show typical Type IV with H2 hysteresis, which is characteristic of the mesoporous particles. The production of high-surface-area nitride materials has been correlated to synthetic procedure, precursor used and formation of mesopores in the material.

Advanced nanoporous materials: synthesis, properties, and applications

1 Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India 2Department of Chemistry and the Institute of Nanotechnology, Bar-Ilan University, 52900 Ramat Gan, Israel 3 Department of Chemistry, Birla Institute of Technology and Science Pilani, K K Birla Goa Campus, Zuari Nagar, Goa 403726, India 4Distributed Power Generation and Energy Storage Group, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea 5 Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6181, USA

Synthesis of High Surface Area W2N and Co-W-N Nitrides by Chemical Routes

This study investigates the synthesis of high surface area W2N and Co–W–N nitrides by nitridation of various precursors obtained by chemical routes. For the synthesis of W2N nitride, WO3 precursors were obtained by acidifying Na2WO4·2H2O (acid route) and by thermal decomposition of the tungstate–citrate precursor. The solid-state reactivity, BET surface areas and pore structures of the nitride materials have been investigated in detail. Co–W–N nitride was obtained from CoWO4 synthesized by co-precipitation. W2N and Co–W–N nitrides crystallize in β-W2N structure. The single-point BET surface areas were estimated to be 58, 55 and 60 m2/g for the β-W2N nitride materials synthesized using commercial WO3 and WO3 obtained from acid and citrate precursor, respectively. The maximum surface areas (40 m2/g) are obtained for Co–W–N nitrides synthesized at 700 °C. We have investigated the change in pore volume and pore diameter when the synthesis conditions are changed. The thermogravimetric and differential thermal analysis studies corroborate the fine particle nature of the materials.