Santhosh P N

Ni nanocrystals coated with a thin layer of NiO with a diameter of 5.0 nm show exchange bias induced ferromagnetism at room temperature. These particulates are freely dispersible in water and were obtained by annealing Ni nanoparticles coated with a thin amorphous layer of NiO. Particulates with diameters between 5.0 and 16.8 nm are studied. Detailed magnetic measurements reveal signs consistent with strong exchange bias including elevated blocking temperatures and tangible loop shifts. The structure of the particulates are characterized by high resolution transmission electron microscopy, energy dispersive x-ray analysis and x-ray diffraction.

Density functional calculations are performed to study the magnetic order of the severely distorted square planar cupric oxide (CuO) and local spin disorder in it in the presence of the transition metal impurities M (=Cr, Mn, Fe, Co and Ni). The distortion in the crystal structure, arisen to reduce the band energy by minimizing the covalent interaction, creates two crisscrossing zigzag spin-1/2 chains. From the spin dimer analysis we find that while the spin chain along (1 0 1) has strong Heisenberg type antiferromagnetic coupling (J ∼ 127 meV), along it exhibits weak, but robust, ferromagnetic coupling (J ∼ 9 meV) mediated by reminiscent p-d covalent interactions. The impurity effect on the magnetic ordering is independent of M and purely orbital driven. If the given spin-state of M is such that the dx2-y2 orbital is spin-polarized, then the original long-range ordering is maintained. However, if dx2-y2 orbital is unoccupied, the absence of corresponding covalent interaction breaks the weak ferromagnetic coupling and a spin-flip takes place at the impurity site leading to breakdown of the long range magnetic ordering.

We report first-principles study of structural, electronic, vibrational, dielectric, and elastic properties of Ba2YTaO6, a pinning material in high temperature superconductors (HTS), by using density functional theory. By using different exchange-correlation potentials, the accuracy of the calculated lattice constants of Ba2YTaO6 has been achieved with GGA-RPBE, since many important physical quantities crucially depend on change in volume. We have calculated the electronic band structure dispersion, total and partial density of states to study the band gap origin and found that Ba2YTaO6 is an insulator with a direct band gap of 3.50 eV. From Mulliken population and charge density studies, we conclude that Ba2YTaO6 have a mixed ionic-covalent character. Moreover, the vibrational properties, born effective charges, and the dielectric permittivity tensor have been calculated using linear response method. Vibrational spectrum determined through our calculations agrees well with the observed Raman spectrum, and allows assignment of symmetry labels to modes. We perform a detailed analysis of the contribution of the various infrared-active modes to the static dielectric constant to explain its anisotropy, while electronic dielectric tensor of Ba2YTaO6 is nearly isotropic, and found that static dielectric constant is in good agreement with experimental value. The six independent elastic constants were calculated and found that tetragonal Ba2YTaO6 is mechanically stable. Other elastic properties, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and elastic anisotropy ratios are also investigated and found that Poisson's ratio and Young's modulus of Ba2YTaO6 are similar to that of other pinning materials in HTS.

We present here the detailed structural, magnetic, and magnetodielectric properties of polycrystalline DyFe0.5Cr0.5O3. Rietveld refinement reveals that DyFe0.5Cr0.5O3 crystallizes in an orthorhombic structure, with Pnma space group, having a disordered arrangement of Fe and Cr ions. An anomaly observed around 250K in the temperature dependence of magnetization indicates an onset of magnetic ordering. The field dependent magnetization of DyFe0.5Cr 0.5O3 at 13K, 120K, and 240K has a small loop indicating weak ferromagnetic nature and the corresponding curve at 5K indicates a metamagnetic behavior. The small loop and the unsaturated magnetization even for a high applied magnetic field of 7T indicate a canted antiferromagnetic structure, the canting can be explained due to Dzyaloshinsky-Moriya antisymmetric exchange interaction. The temperature dependence of the dielectric permittivity shows a transition around 500K. An anomaly in the temperature dependence of magnetization (supported by differential scanning calorimetry) corresponding to this dielectric transition suggests the possibility of magnetodielectric effect in DyFe0.5Cr0.5O3. © 2014 AIP Publishing LLC.

Tungsten bronze oxides are widely studied due to their various superior physical properties. Sr6Ti2Nb8O30 (STNO) and Mn Doped STNO are synthesized and characterized by X-ray diffraction technique. The structural refinement of STNO is performed using General Structure Analysis System. Dielectric measurements indicate STNO and Mn doped STNO having diffused phase transition. © 2011 American Institute of Physics.

Magnetic properties of the double perovskite oxide, Sr 2TiMnO6, have been studied by means of bulk magnetization and powder neutron diffraction experiments. Low field magnetization data reveal transitions at ∼45 K (TC) and at ∼15 K (TN). A magnetic moment value of only ∼0.23 BF.U. is attained at 5 K in the 7 T field. Powder neutron diffraction studies suggest the possible antiferromagnetic order in this compound at 12 K with moments of ∼0.5 B at the Mn site. © 2011 American Institute of Physics.

The structural, magnetic, and transport properties of double perovskites Ba2MnRuO6 (BMRO) and LaBaMnRuO6 (LBMRO) have been investigated. Rietveld analysis of powder x-ray diffraction data show that BMRO adopts 9R rhombohedral structure with space group R3̄m while the partial doping of Ba by La leads the structure to orthorhombic with Pbnm space group. Structural as well as magnetic studies revealed that Ru ions are tetravalent in both compounds. The superexchange interactions between Mn 3+ (t2g3 eg1)-O-Ru 4+ (t2g4) lead to a ferromagnetic ordering in LBMRO. The direct metal-metal interactions are to be invoked to understand the weak magnetic nature of BMRO. Transport studies reveal that both samples have insulating behavior with variable range hopping type of conduction. Magnetoresistance (MR) measurements show a positive magnetoresistance (∼ 5 % for 1 T) in BMRO, which is not significantly varying within the temperature range measured while LBMRO exhibit a negative magneto resistance (∼ 25% for 1 T at 80 K), which increases while decreasing the temperature. © 2007 IEEE.

The perovskites Sr2MnRuO6 and LaSrMnRuO6 have been investigated and their properties compared with those of the broader family of A2MRuO6 double perovskites (A=Sr, La; M=Cr, Mn, Fe, Co, Ni). Neutron powder diffraction shows that both phases lack long-range Mn/Ru order. Sr2MnRuO6 exhibits a cooperative Jahn-Teller distortion, conductivity that proceeds via variable-range hopping, and antiferromagnetic (AFM) order (C-type structure, TN≅200K, 2.05(1) μB per transition metal). LaSrMnRuO6 exhibits nearly undistorted octahedra, conductivity that proceeds via diffusion-assisted small-polaron hopping, and ferromagnetic (FM) order (TC≅220K, 1.60(4) μB transition metal). Band structure calculations show that AFM Mn-O-Ru coupling optimizes polarization of the Ru t2g orbitals, while FM coupling favors delocalization of the Ru t2g electrons. The transition from antiferromagnetism to ferromagnetism is linked to a loss of orbital order and stabilized over competing glassy states by delocalization of Ru t2g electrons. © 2008 The American Ceramic Society.

The effect of W co-doping on the optical, magnetic and electrical properties of Fe-doped BaSnO 3 has been studied. Polycrystalline BaSnO 3, BaSn 0.96Fe 0.04O 3 and BaSn 0.95Fe 0.04W 0.01O 3 samples were prepared using solid state reaction. In the analysis of powder X-ray diffraction patterns, the samples were found to be free of secondary phases. Diffuse reflectance spectra evidenced the substitution of Fe and W for Sn in the host BaSnO 3. Micro-Raman spectra confirmed the existence of oxygen vacancies in the samples. Upon W-1% co-doping, the ferromagnetic character of Fe-4% doped BaSnO 3 is suppressed drastically and its Curie temperature is reduced to 310 K from 462 K. The existence of F-centers and ferromagnetic interactions at room temperature is evidenced by the electron paramagnetic resonance and ferromagnetic resonance signals observed in the electron spin resonance spectra of the undoped and Fe-4% doped, (Fe-4% and W-1%) co-doped BaSnO 3 samples respectively. Suppression of ferromagnetism upon W co-doping is due to the fact that each W 6 ion donates two electrons to the host lattice and it reduces the number of oxygen vacancies that are essential for ferromagnetism to exist in the Fe-doped BaSnO 3 samples. © 2012 Elsevier B.V. All rights reserved.

Compositional tuning of magnetic properties to provide technologically useful results is in the forefront of research in complex oxides. Taking advantage of the highly sensitive nature of magnetism in 6H barium ruthenates (Ba3MRu2O9, M = transition metals or rare earth ions) to composition, we have carried out M-site dual doping by 50% each of Co and Mn. The high pressure and high temperature synthesized sample of 6H Ba3Co0.5Mn0.5Ru2O9 shows giant exchange bias (5.71 kOe at 5 K and 50 kOe cooling field) and huge coercivity (9.42 kOe at 5 K) at low temperature along with Griffiths phase like behavior in the 136 K < T < 260 K temperature range. The presence of ferromagnetic (FM) clusters in a non-collinear antiferromagnetic (AFM) background for temperature below 136 K is assumed. These FM clusters extend above 136 K to persist until 260 K, and the system finally reaches the paramagnetic (PM) state closer to room temperature. The exchange bias phenomena diminish above 20 K and this could be correlated with the peculiar nature of magnetic isotherms above and below this temperature. The transition at 136 K is regarded as AFM short-range ordering from ac and dc magnetization studies and heat capacity measurement. The presence of short-range interactions at temperatures until the PM region forms the basis of Griffiths phase like behavior in this sample.