Correlating defect induced ferromagnetism and gas sensing properties of undoped tin oxide sensors

A correlation between gas sensing properties and defect induced Room Temperature Ferromagnetism (RTFM) is demonstrated in non-stoichiometric SnO2 prepared by solution combustion method. The presence of oxygen vacancies (V-O), confirmed by RTFM is identified as the primary factor for enhanced gas sensing effect. The as-prepared SnO2 shows high saturation magnetization of similar to 0.018 emu/g as compared to similar to 0.002 and similar to 0.0005 emu/g in annealed samples and SnO2 prepared by precipitation respectively. The SnO2 prepared by precipitation which is an equilibrium method of synthesis shows lesser defects compared to the combustion product and hence exhibits lesser sensitivity in spite of smaller crystallite size. The study utilizes RTFM as a potential tool to characterize metal oxide gas sensors and recognizes the significance of oxygen vacancies in sensing mechanism over the microstructure. (C) 2014 AIP Publishing LLC.

Observation of Room Temperature Ferromagnetism in InN Nanostructures

The room temperature ferromagnetic behavior of InN nanosfructures grown by molecular beam epitaxy (MBE) is explored by means of magnetization measurements. The saturation magnetization and remanent magnetization are found to be strongly dependent on the size of the nanostructures. This suggests that the ferromagnetism is essentially confined to the surface of the nanostructures due to the possible defects. Raman spectroscopy shows the existence of indium vacancies which could be the source of ferromagnetic ordering in InN nanostructures.

Spin-reorientation and weak ferromagnetism in antiferromagnetic TbMn0.5Fe0.5O3

Orthorhombic single crystals of TbMn0.5Fe0.5O3 are found to exhibit spin-reorientation, magnetization reversal, and weak ferromagnetism. Strong anisotropy effects are evident in the temperature dependent magnetization measurements along the three crystallographic axes a, b, and c. A broad magnetic transition is visible at T-N(Fe/Mn) = 286K due to paramagnetic to A(x)G(y)C(z) ordering. A sharp transition is observed at T-SR(Fe/Mn) = 28 K, which is pronounced along c axis in the form of a sharp jump in magnetization where the spins reorient to G(x)A(y)F(z) configuration. The negative magnetization observed below T-SR(Fe/Mn) along c axis is explained in terms of domain wall pinning. A component of weak ferromagnetism is observed in field-scans along c-axis but below 28 K. Field-induced steps-like transitions are observed in hysteresis measurement along b axis below 28 K. It is noted that no sign of Tb-order is discernible down to 2K. TbMn0.5Fe0.5O3 could be highlighted as a potential candidate to evaluate its magneto-dielectric effects across the magnetic transitions. (C) 2015 AIP Publishing LLC.

Origin of room temperature weak-ferromagnetism in antiferromagnetic Pb(Fe2/3W1/3)O-3 ceramic

We report the origin of room temperature weak ferromagnetic behavior of polycrystalline Pb(Fe2/3W1/3)O-3 (PFW) powder. The structure and magnetic properties of the ceramic powder prepared by a Columbite method were characterized by X-ray and neutron diffraction, Mossbauer spectroscopy and magnetization measurements. Rietveld analysis of diffraction data confirm the formation of single phase PFW, without traces of any parasitic pyrochlore phase. PFW was found to crystallize in the cubic structure at room temperature. The Rietveld refinement of neutron diffraction data measured at room temperature confirmed the G-type antiferromagnetic structure of PFW in our sample. However, along with the antiferromagnetic (AFM) ordering of the Fe spins, we have observed the existence of weak ferromagnetism at room temperature through: (i) a clear opening of hysteresis (M-H) loop, (ii) bifurcation of the field cooled and zero-field cooled susceptibility; supported by Mossbauer spectroscopy results. The P-E loop measurements showed a non-linear slim hysteresis loop at room temperature due to the electronic conduction through the local inhomogeneities in the PFW crystallites and the inter-particle regions. By corroborating all the magnetic measurements, especially the spin glass nature of the sample, with the conduction behavior of the sample, we report here that the observed ferromagnetism originates at these local inhomogeneous regions in the sample, where the Fe-spins are not perfectly aligned antiferromagnetically due to the compositional disordering. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Room temperature ferromagnetism and white light emissive CdS:Cr nanoparticles synthesized by chemical co-precipitation method

Undoped and Cr (3% and 5%) doped CdS nanoparticles were synthesized by chemical co-precipitation method. The synthesized nanocrystalline particles are characterized by energy dispersive X-ray analysis (EDAX), scanning electron microscope (SEM), X-ray Diffraction (XRD), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS), photoluminescence (PL), Electron paramagnetic resonance (EPR), vibrating sample magnetometer (VSM) and Raman spectroscopy. XRD studies indicate that Cr doping in host CdS result a structural change from Cubic phase to mixed (cubic + hexagonal) phase. Due to quantum confinement effect, widening of the band gap is observed for undoped and Cr doped CdS nanoparticles compared to bulk CdS. The average particle size calculated from band gap values is in good agreement with the TEM study calculation and it is around 4-5 nm. A strong violet emission band consisting of two emission peaks is observed for undoped CdS nanoparticles, whereas for CdS:Cr nanoparticles, a broad emission band ranging from 420 nm to 730 nm with a maximum at similar to 587 nm is observed. The broad emission band is due to the overlapped emissions from variety of defects. EPR spectra of CdS:Cr samples reveal resonance signal at g = 2.143 corresponding to interacting Cr3+ ions. VSM studies indicate that the diamagnetic CdS nanoparticles are transform to ferromagnetic for 3% Cr3+ doping and the ferromagnetic nature is diminished with increasing the doping concentration to 5%. (C) 2015 Elsevier B.V. All rights reserved.

Ferromagnetism and the effect of free charge carriers on electric polarization in the double perovskite Y2NiMnO6

The double perovskite Y2NiMnO6 displays ferromagnetic transition at T-c approximate to 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (Ms) and also validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P2(1)/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of the frequency-dependent dielectric constant and equivalent circuit analysis of impedance data take into account the bulk contribution to the total dielectric constant. This reveals an anomaly which coincides with the ferromagnetic transition temperature (T-c). Pyrocurrent measurements register a current flow with onset near T-c and a peak at 57 K that shifts with temperature ramp rate. The extrinsic nature of the observed pyrocurrent is established by employing a special protocol measurement. It is realized that the origin is due to reorientation of electric dipoles created by the free charge carriers and not by spontaneous electric polarization at variance with recently reported magnetism-driven ferroelectricity in this material.

Microemulsion-mediated synthesis of cobalt (pure fcc and hexagonal phases) and cobalt-nickel alloy nanoparticles

By choosing appropriate microemulsion systems, hexagonal cobalt (Co) and cobalt-nickel (1:1) alloy nanoparticles have been obtained with cetyltrimethylammonium bromide as a cationic surfactant at 500 degrees C. This method thus stabilizes the hcp cobalt even at sizes (<10 nm) at which normally fcc cobalt is predicted to be stable. On annealing the hcp cobalt nanoparticles in H-2 at 700 degrees C we could transform them to fcc cobalt nanoparticles. Microscopy studies show the formation of spherical nanoparticles of hexagonal and cubic forms of cobalt and Co-Ni (1:1) alloy nanoparticles with the average size of 4, 8 and 20 nm, respectively. Electrochemical studies show that the catalytic property towards oxygen evolution is dependent on the applied voltage. At low voltage (less than 0.65 V) the Co (hexagonal) nanoparticles are superior to the alloy (Co-Ni) nanoparticles while above this voltage the alloy nanoparticles are more efficient catalysts. The nanoparticles of cobalt (hcp and fcc) and alloy (Co-Ni) nanoparticles show ferromagnetism. The saturation magnetization of Co-Ni nanoparticles is reduced compared to the bulk possibly due to surface oxidation.

Quest for new materials: Inorganic chemistry plays a crucial role

There is an endless quest for new materials to meet the demands of advancing technology. Thus, we need new magnetic and metallic/semiconducting materials for spintronics, new low-loss dielectrics for telecommunication, new multi-ferroic materials that combine both ferroelectricity and ferromagnetism for memory devices, new piezoelectrics that do not contain lead, new lithium containing solids for application as cathode/anode/electrolyte in lithium batteries, hydrogen storage materials for mobile/transport applications and catalyst materials that can convert, for example, methane to higher hydrocarbons, and the list is endless! Fortunately for us, chemistry - inorganic chemistry in particular - plays a crucial role in this quest. Most of the functional materials mentioned above are inorganic non-molecular solids, while much of the conventional inorganic chemistry deals with isolated molecules or molecular solids. Even so, the basic concepts that we learn in inorganic chemistry, for example, acidity/basicity, oxidation/reduction (potentials), crystal field theory, low spin-high spin/inner sphere-outer sphere complexes, role of d-electrons in transition metal chemistry, electron-transfer reactions, coordination geometries around metal atoms, Jahn-Teller distortion, metal-metal bonds, cation-anion (metal-nonmetal) redox competition in the stabilization of oxidation states - all find crucial application in the design and synthesis of inorganic solids possessing technologically important properties. An attempt has been made here to illustrate the role of inorganic chemistry in this endeavour, drawing examples from the literature its well as from the research work of my group.

Process-dependent magnetic properties of Co-doped ZnO in bulk and thin film form

Structural, optical and magnetic studies of Co-doped ZnO have been carried out for bulk as well as thin films. The magnetic studies revealed the superparamagnetic nature for low-temperature synthesized samples, indicating the presence of cobalt metallic clusters, and this is supported by the optical studies. For the high-temperature sintered samples one obtains paramagnetism. The optical studies reveal the presence of Co2+ ions in the tetrahedral sites indicating proper doping. Interestingly, the films deposited by laser ablation from the paramagnetic target showed room temperature ferromagnetism. It appears that the magnetic nature of this system is process dependent.

Charge Ordering Induced Ferromagnetic Insulator: K2Cr8O16

Usually metallicity accompanies ferromagnetism. K2Cr8O16 is one of the less common examples of magnetic materials, exhibiting ferromagnetism in the insulating state. Analyzing the electronic and magnetic properties within first principles electronic structure calculations, we find that the doped electrons due to K induce a charge-ordered and insulating ground state and interestingly also introduce a ferromagnetic coupling between the Cr ions. The primary considerations driving the charge ordering are found to be electrostatic ones with the charge being localized on two Cr atoms that minimize the electrostatic energy. The structural distortion that accompanies the ordering gives rise to a rare example of a charge-order driven ferromagnetic insulator.

Magnetism of Ti4+ diluted manganites La1-xPbxMn1-yTiyO3 (y <= x)

The effect of nonmagnetic Ti4+ substitution for Mn4+ on magnetic ordering of La1-xPbxMn1-yTiyO3 (x = 0.15,0.26, and 0.4; 0 less than or equal to y less than or equal to x )has been studied. The ferromagnetic transition temperature and the magnetization decrease with increasing amount of titanium. Complete substitution of Mn4+ by Ti4+, for x = y, excludes the Mn3+-O-Mn4+ double exchange. However, these compounds still show ferromagnetism if the dilution of the Mn sublattice by Ti is small enough (y less than or equal to 0.2). This ferromagnetism probably originates from a ferromagnetic Mn3+-O-Mn3+ superexchange. A thorough study of magnetic properties including AC magnetic susceptibility, magnetization, temperature dependence of coercivity and relaxation of remanent magnetization has been carried out and gives evidence of cluster spin glass behaviour for La0.6Pb0.4Mn0.6Ti0.4O3. (C) 2000 Elsevier Science B.V. All rights reserved.

A series of transition metal-azido extended complexes with various anionic and neutral co-ligands: synthesis, structure and their distinct magnetic behavior

The crystal structures and magnetic properties of five new transition metal-azido complexes with two anionic [pyrazine-2-carboxylate (pyzc) and p-aminobenzoate (paba)] and two neutral [pyrazine (pyz) and pyridine (py)] coligands are reported All five complexes were synthesized bysolvothermal methods The complex [Co-2(pyzc)(2)(N-3)(2)(H2O)(2)](n) (1) is 1D and exhibit canted antiferromagnetism, while the 3D complex [MnNa(pyzc)(N-3)(2)(H2O)(2)](n) (2) has a complicated structure and is weakly ferromagnetic in nature [Mn-2(paba)(2)(N-3)(2)(H2O)(2)](n) (3). is a 2D sheet and the Mn-II ions are found to be antiferromagnetically coupled The isostructural 2D complexes [Cu-3(pyz)(2)(N-3)(6)](n) (4) and [Cu-3(py)(2)(N-3)(6)](n) (5) resemble remarkably in their magnetic properties exhibiting moderately strong ferromagnetism. Density functional theory calculations (B3LYP functional) have been performed to provide a qualitative theoretical interpietation of the overall magnetic behavior shown by these complexes.

Magnetoresistance of FexNi80-xCr20 (50

The authors have measured longitudinal and transverse magnetoresistance (MR) of crystalline pseudo-binary alloys FexNi80-xCr20 (50ferromagnetism. While the Fe-Ni-Cr alloys have a FCC gamma -phase, the binary Fe-Cr alloy has a BCC alpha -phase. This gives an opportunity to compare magnetoresistances for gamma -phase and alpha -phase Fe alloys when they lie close to the critical composition. The experiments were conducted at 4.2 K with magnetic fields up to 7 T. The authors observed that all the alloys show negative magnetoresistance at 4.2 K in fields up to 7 T. However, for the gamma -phase alloys the typical maximum MR ( Delta rho / rho ) is about 1%, while for the alpha -phase alloy it is 10%. In the gamma -phase alloys there is a small but finite anisotropy of MR in the phases with long-range magnetic order which gradually vanishes near the critical region (x=xc approximately=59-63) when it becomes a spin glass. In the range x approximately=xc, Delta rho / rho varies as Mn (M=magnetization) with n approximately=2.

Metal-Insulator Transitions in Anion-Excess LaMnO3+? Controlled by the Mn4+ Content

LaMnO3+? samples with Mn4+ content up to 50% have been prepared by different methods. The structure of LaMnO3+? changes from orthorhombic to cubic (via rhombohedral) with increase in the Mn4+ content. LaMnO3+? samples containing greater than 20% Mn4+ are ferromagnetic and show resistivity maxima at a temperature Tt which is close to the ferromagnetic Curie temperature. The resistivity maximum is due to the occurrence of a metal-insulator transition. In samples heated to the same temperature, the value of Tt increases with % Mn4+. For a given sample, Tt increases with the temperature of heat treatment due to the increase in particle size. The onset of ferromagnetism in LaMnO3+? accompanied by an insulator-metal transition is similar to that found in La1-xCaxMnO3 and La1-xSrxCoO3.

Giant magnetoresistance phenomenon in manganates

The discovery of giant magnetoresistance (GMR) in rare earth manganates of the general formula Ln(1-x)A(x)MnO(3) (Ln = rare earth, A = divalent cation) has aroused much interest not only because of its technological implications, but also due to the fascinating features and mechanism of the phenomemon in these oxides. GMR is observed in these manganates when they become ferromagnetic and transform from an insulating state to a metallic state close to the Curie temperature. The essential features of magnetoresistance in the manganates can be understood on the basis of the double-exchange mechanism, but this is too simplistic to account for all the observed data. The most curious property of the manganates relates to the high resistivity exhibited in the so-called metallic state. Charge ordering competes with the double-exchange interaction responsible for ferromagnetism and GMR in these materials. The charge-ordered (charge-crystal) insulating state in the rare earth manganates can be melted into a metallic and ferromagnetic charge-liquid state by applying a magnetic field, thus providing a unique case of charge and spin separation in solids. The observation of GMR in Tl2Mn2O7 shows that there can be causes other than double-exchange for the phenomenon.