Defect induced ferromagnetism in Mn3Ga.

Ni-substituted Mn3Ga displays a weak ferromagnetism embedded in an antiferromagnetic (AF) phase. Upon field cooling, the alloy exhibits exchange bias and an open hysteresis loop, signifying kinetic arrest at room temperature. For the first time, a kinetic arrest is seen in a compound due to the first order transition of an embedded defect phase. A systematic study of crystal structure, local structure, and magnetic properties of Mn3-xNixGa (x= 0, 0.25) alloys reveal the origin of ferromagnetism in Mn2.75Ni0.25Ga is due to the segregation of a Heusler-type environment around Ni in the cubic Mn3Ga matrix. Upon temper annealing at 400∘C, these local structural defects around the Ni phase separate into a modulated ferromagnetic (FM) Ni-Mn-Ga Heusler phase. A strong interaction between the AF host and the FM defect phase gives rise to exchange bias. The first-order transition of the defect phase seems to be responsible for the observed kinetic arrest in Mn2.75Ni0.25Ga.

Kinetic arrest of the ferromagnetic state in Mn[Formula: see text]GaC and Ni[Formula: see text]MnGa composite mixtures.

The kinetics of the ferromagnetic to antiferromagnetic transition in Mn[Formula: see text]GaC can be arrested and its magnetic properties can be tuned by mixing a small amount ([Formula: see text] 10%) of Heusler Ni[Formula: see text]MnGa to Mn[Formula: see text]GaC. A detailed study of magnetic properties of composite mixtures of Mn[Formula: see text]GaC and Ni[Formula: see text]MnGa with different antiperovskite to Heusler ratio, reveals that the ferromagnetic Ni[Formula: see text]MnGa polarizes magnetic spins of the antiperovskite phase by creating a magnetic strain field in its vicinity. The Heusler phase acts as a defect centre whose influence on the magnetic properties of the majority antiperovskite phase progressively diminishes, creating a distribution of transition temperatures. Such strong interaction between the two phases of the mixture allows for tunability and control over the properties of such magneto-structurally transforming materials.

Lattice strain accommodation and absence of pre-transition phases in Ni50Mn25+xIn25-x.

The stoichiometric Ni50Mn25In25Heusler alloy transforms from a stable ferromagnetic austenitic ground state to an incommensurate modulated martensitic ground state with a progressive replacement of In with Mn without any pre-transition phases. The absence of pre-transition phases like strain glass in Ni50Mn25+xIn25-xalloys is explained to be the ability of the ferromagnetic cubic structure to accommodate the lattice strain caused by atomic size differences of In and Mn atoms. Beyond the critical value ofx= 8.75, the alloys undergo martensitic transformation despite the formation of ferromagnetic and antiferromagnetic clusters and the appearance of a super spin glass state.

Reinvestigation of growth of thiourea urea zinc sulfate crystal.

Reinvestigation of the growth of thiourea urea zinc sulfate crystal is reported. Aqueous reaction of thiourea, urea and zinc sulfate in 1:1:1 mol ratio results in the formation of the well known [Zn(tu)3(SO4)] (1) (tu=thiourea) crystal and not the 'so called' novel semiorganic nonlinear optical thiourea urea zinc sulfate (2) crystal, as claimed by Redrothu Hanumantha Rao, S. Kalainathan, Spectroscopic investigation, nucleation, growth, optical, thermal and second harmonic studies of novel semi-organic nonlinear optical crystal - Thiourea urea zinc sulfate, Spectrochim. Acta A97 (2012) 456-463. In this work, we demonstrate the usefulness of elemental analytical data, infrared and NMR spectra and X-ray powder pattern, for accurate product characterization.

Influence of local structure on magnetic properties of layered cobaltites PrBaCo2O(5+δ), δ > 0.5.

The effect of local structure on the magnetic and transport properties of the layered perovskites has been investigated. The samples PrBaCo(2)O(5+δ), (δ = 0.80 and 0.67) crystallize in the same 112 type tetragonal structure but have different magnetic ground states. Extended x-ray absorption fine structure (EXAFS) spectroscopy has been employed to explain the changes in magnetic interactions based on the rotation and tilting of CoO polyhedra in these oxygen-rich double perovskites.

Enhanced ionic conductivity in an otherwise poorly conducting Ce0.90Ca0.10O(2-δ) system.

A poorly conducting ionic material Ce(0.90)Ca(0.10)O(2-δ) was converted to a highly conducting composition by a codoping strategy with Sm(3+) and Gd(3+). A 50% replacement of Ca with either Sm or Gd has increased the conductivity at 550 °C of Ce(0.90)Ca(0.10)O(2-δ) from 0.0040 to 0.0169 S/cm for the Ce(0.90)Ca(0.05)Sm(0.05)O(2-δ) composition and to 0.0184 S/cm for the Ce(0.90)Ca(0.05)Gd(0.05)O(2-δ) composition. The enhancement in the oxide ion conductivity of these codoped samples has been related to the low ionic radii mismatch and the elastic strain. The extended X-ray absorption fine structure measurements on these systems confirmed that Gd, when coupled with Ca, introduced more disorder in the system, leading to lower activation energy and higher conductivity. In addition, a reduction in the Ce-O bond distance and coordination number has also been observed with codoping.

Temperature dependent spin momentum densities in Ni-Mn-In alloys.

The spin-dependent electron momentum densities in Ni(2)MnIn and Ni(2)Mn(1.4)In(0.6) shape memory alloy using magnetic Compton scattering with 182.2 keV circularly polarized synchrotron radiation are reported. The magnetic Compton profiles were measured at different temperatures ranging between 10 and 300 K. The profiles have been analyzed mainly in terms of Mn 3d electrons to determine their role in the formation of the total spin moment. We have also computed the spin polarized energy bands, partial and total density of states, Fermi surfaces and spin moments using full potential linearized augmented plane wave and spin polarized relativistic Korringa-Kohn-Rostoker methods. The total spin moments obtained from our magnetic Compton profile data are explained using both the band structure models. The present Compton scattering investigations are also compared with magnetization measurements.

Local structure around Mn and Co in LaMn(1-x)Co(x)O(3 ± δ): an EXAFS study.

Detailed investigations of extended x-ray absorption fine structure (EXAFS) associated with the K-edges of Mn and Co have been carried out for LaMn(1-x)Co(x)O(3 ± δ) (0.3≤x≤1) compounds. It is found that the local structure around Mn is different from that around Co. The distortion in MnO(6) octahedra decreases with the increasing Co content whereas CoO(6) octahedra are undistorted over the entire range of x. The Co-O bond length has been found to decrease with the increase in x. Based on the information about the first shell, the structural anomalies observed in these compounds have been discussed. Complementary information obtained from the XRD and EXAFS studies have been used to explain the behaviour of resistivity in these compounds.

Local structure of Pt and Pd ions in Ce(1-x)TixO2: x-ray diffraction, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure.

Ce(1-x-y)Ti(x)Pt(y)O(2-delta) (x=0.15; y=0.01) and Ce(1-x-y)Ti(x)Pd(y)O(2-delta) (x=0.25; y=0.02 and 0.05) are found to be good CO oxidation catalysts [T. Baidya et al., J. Phys. Chem. B 110, 5262 (2006); T. Baidya et al., J. Phys. Chem. C 111, 830 (2007)]. A detailed structural study of these compounds has been carried out by extended x-ray absorption fine structure along with x-ray diffraction and x-ray photoelectron spectroscopy. The gross cubic fluorite structure of CeO(2) is retained in the mixed oxides. Oxide ion sublattice around Ti as well as Pt and Pd ions is destabilized in the solid solution. Instead of ideal eight coordinations, Ti, Pd, and Pt ions have 4+3, 4+3, and 3+4 coordinations creating long and short bonds. The long Ti-O, Pd-O, and Pt-O bonds are approximately 2.47 A (2.63 A for Pt-O) which are much higher than average Ce-O bonds of 2.34 A.