Critical dynamics of ferromagnets

The crossover in the dynamics from isotropic to dipolar critical behaviour has been a matter of debate over many years. We review a mode coupling theory for dipolar ferromagnets which gives a unified explanation of the seemingly contradictory experimental situation. The shape functions, the scaling functions for the damping coefficients and the precise position of the crossover are computed. Below Tc only the exchange interaction is taken into account

Nuclear techniques in studies of condensed matter

Nuclear techniques have played an important role in the studies of materials over the past several decades. For example, X-ray diffraction, neutron diffraction, neutron activation, and particle- or photon-induced X-ray emission techniques have been used extensively for the elucidation of structural and compositional details of materials. Several new techniques have been developed recently. Four such techniques are briefly reviewed which have great potential in the study and development of new materials. Of these four, Mossbauer spectroscopy, muon spin rotation, and positron annihilation spectroscopy techniques exploit their great sensitivity to the local atomic environments in the test materials. Interest in synchrotron radiation, on the other hand, stems from its special properties, such as high intensity, high degree of polarization, and high monochromaticity. It is hoped that this brief review will stimulate interest in the exploitation of these newer techniques for the development of improved materials

Low temperature Mössbauer spectroscopic studies on Sm3+ doped Zn-Mn ferrites

For the first time, we report on the low temperature Mössbauer spectroscopic study of Zn2+ 0.5Mn2+ 0.5Sm3+ xFe3+ 2�xO4 (where x = 0.01�0.05) prepared by the modified solution combustion method using a mixture of urea and glucose as a fuel. The Mössbauer spectroscopy at room and low temperatures was applied to understand the magnetic properties of the samples. The room temperature Mössbauer spectroscopy results suggest that the occupation of the octahedral sites by Sm3+ ions leads to the distortion enhancement of 57Fe nuclei environments, which leads to an increase in quadrupole splitting � values of D2 and D3 doublets. The low temperature Mössbauer spectroscopy results indicate that the presence of Sm3+ ions in the octahedron sites causes the decrease in the number of Fe�O�Fe chains. The transformation of Mössbauer spectra doublets into Zeeman sextets is accompanied by a significant decrease in the magnitude IM of Mössbauer spectra intensity within the 0�1.2 mm/s velocity range normalized to its value at 300 K. This drop in the temperature dependence of IM allows one to obtain the magnetic phase transition temperature TM from the Mössbauer experiment. © 2017 Elsevier B.V

Magnetic phase transitions in SmCoAsO

Magnetization, x-ray diffraction and specific-heat measurements reveal that SmCoAsO undergoes three magnetic phase transitions. A ferromagnetic transition attributed to the Co ions, emerges at TC=57 K with a small saturation moment of 0.15muB/Co. Reorientation of the Co moment to an antiferromagnetic state is obtained at TN2=45 K. The relative high paramagnetic effective moment Peff=1.57 MuB/Co indicates an itinerant ferromagnetic state of the Co sublattice. The third magnetic transition at TN1=5 K is observed clearly in the specific-heat study only. Both magnetic and 57Fe Mossbauer studies show that substitution of small quantities of Fe for Co was unsuccessful.Comment: 10pages text+Figures: comments welcome ([email protected]

Microstructural and Mössbauer properties of low temperature synthesized Ni-Cd-Al ferrite nanoparticles

We report the influence of Al3+ doping on the microstructural and Mössbauer properties of ferrite nanoparticles of basic composition Ni0.2Cd0.3Fe2.5 - xAlxO4 (0.0 ≤ x ≤ 0.5) prepared through simple sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray, transmission electron microscopy (TEM), Fourier transformation infrared (FTIR), and Mössbauer spectroscopy techniques were used to investigate the structural, chemical, and Mössbauer properties of the grown nanoparticles. XRD results confirm that all the samples are single-phase cubic spinel in structure excluding the presence of any secondary phase corresponding to any structure. SEM micrographs show the synthesized nanoparticles are agglomerated but spherical in shape. The average crystallite size of the grown nanoparticles was calculated through Scherrer formula and confirmed by TEM and was found between 2 and 8 nm (± 1). FTIR results show the presence of two vibrational bands corresponding to tetrahedral and octahedral sites. Mössbauer spectroscopy shows that all the samples exhibit superparamagnetism, and the quadrupole interaction increases with the substitution of Al3+ ions

Characterization of Iron-Ruthenium Bimetallic Catalyst Systems (Methanation, Moessbauer, Zeolite).

Due to the potential of iron-ruthenium bimetallic heterogeneous catalysis in a number of high demand, energy related processes, this study was undertaken to synthesis and characterize a series of bimetallic Iron-Ruthenium alloy and clusters. A material science approach was taken which relies on the physical and chemical characterization of catalyst material itself rather than the classical chemical kinetics investigation of the catalytic reaction. Various methods have been employed for the determination of surface and bulk properties of catalysts during dehydration-hydration, and oxidation-reduction treatments. In the case of the bimetallic systems, the use of (\u2799)Ru and (\u2757)Fe double labelled Mossbauer experiments was most useful. Additional information was obtained from ESCA, X-ray diffraction, and infrared spectroscopy. This array of physical techniques provided complementary details for the complete characterization of the systems of interest. It is known that iron cannot be reduced beyond the ferrous state when ion exchanged in Y-zeolite. Thus, the reduction behavior of iron on zeolite surfaces is considered a classical problem in catalytic research. Several attempts were made to modify the reduction behavior of iron. One approach was to introduce ruthenium into the system to encourage hydrogen spillover which should provide the reactive hydrogen atom as the reducing agent. Utilizing the preparation method designated as Homogeneous-Deposition , the iron was reduced and the formation of hcp iron-ruthenium clusters on the surface of Y-zeolite was confirmed. To elucidate the nature of the interaction between iron and ruthenium and to evaluate the chemical differences of the solid state reaction during the preparation of bimetallic iron-ruthenium, the products formed when ruthenium trichloride is mixed with a variety of iron salts and oxides were investigated. The anions associated with the metal in the initial material influenced the formation and the nature of bimetallic particles. A unique preparative method based on the reaction of a cation exchanged zeolite with a metal-containing coordination complex anion was extended to synthesize the mixed iron-ruthenium cyanide polynuclear complexes directly on the zeolite. Under reduction conditions hcp iron-ruthenium bimetallic alloy was detected. This preparative method could provide an alternative way to stabilize polymetallic particles on zeolites

Enhancement In Electrochemical Performance Of Advanced Battery Electrodes Using Carbon- Nanomaterial Composites

LiFePO4 has attracted great interest as a cathode material for lithium ion batteries due to its reasonably high theoretical capacity (170mAh/g), thermal stability, high Li ion reversibility and low cost. However, prohibitively low electronic conductivity (~10-9 S/cm) of LiFePO4 leads to high impedance, low capacity and low rate capability. To overcome this bottleneck, we have developed multiple approaches to improve the conductivity of LiFePO4. Motivated by the outstanding electronic and mechanical properties as well as high surface area of graphene, we prepared LiFePO4/graphene nano-composites by a sol-gel method. The phase purity of the nano LiFePO4/Graphene composite was confirmed by X-Ray diffraction. Addition of graphene improved the electronic conductivity of LiFePO4 by six orders of magnitude. Scanning electron microscopy and transmission electron microscope images show LiFePO4 particles being covered uniformly by graphene sheets throughout the material forming a three-dimensional conducting network. At low currents and charging rate of C/3, the capacity of the composite cathode reaches 160 mAh/g, which is very close to the theoretical limit. More significantly, the LiFePO4-graphene composite shows a dramatically improved rate capability up to 27C and excellent charge-discharge cycle stability over 500 stable cycles. To further improve the conductivity of LiFePO4 and thus its rate capability, we optimized the concentration of the Fe2P metallic impurity phase by tuning the annealing temperature. X-ray diffraction shows that samples annealed at 600o C are nearly phase pure while those treated at higher temperatures contain Fe2P and Li3PO4 impurity phases, which increase with increasing annealing temperature. MÖssbauer spectroscopy and magnetic measurements were used to quantify the amount of Fe2P impurity phase. Scanning electron microscopy measurement reveals a noticeable increase in particle size as the annealing temperature increases from 700 oC to 900 oC. Optimal results are obtained in LiFePO4/C samples annealed at 700 oC, which show the lowest charge transfer resistance, highest Li-ion diffusion coefficient, the highest specific capacity of 166 mAh/g at a rate of 1C and the best rate capability among all samples. In addition, we have also studied the effect of doping In3+ on the Fe site and found that the addition of indium significantly improves the electronic conductivity leading to further improvement in capacity and rate capability