K Sethupathi

Neutron diffraction measurement on the spin glass double perovskite Sr2FeCoO6 reveals site disorder as well as Co3+ intermediate spin state. In addition, multiple valence states of Fe and Co are confirmed through M̈ossbauer and X-ray photoelectron spectroscopy. The structural disorder and multiple valence lead to competing ferromagnetic and antiferromagnetic interactions and subsequently to a spin glass state, which is reflected in the form of an additional T-linear contribution at low temperatures in specific heat. A clear evidence of Jahn-Teller distortion at the Co3+-O6 complex is observed and incorporating the physics of Jahn-Teller effect, the presence of localized magnetic moment is shown. A large, negative and anomalous magnetoresistance of ≈63% at 14 K in 12 T applied field is observed for Sr2FeCoO6. The observed magnetoresistance could be explained by applying a semi-empirical fit consisting of a negative and a positive contribution and show that the negative magnetoresistance is due to spin scattering of carriers by localized magnetic moments in the spin glass phase. © Springer-Verlag 2012.

Structural, magnetic and magnetocaloric properties of sol-gel prepared, nanocrystalline oxides Pr 1-xA xMn1-yCo yO 3 (A = Ca, Sr) (x = 0.3; y = 0.5) (cubic, space group Fm3̄m) have been studied. From the X-ray data, the crystallite size of Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 samples is found to be ∼24 nm and ∼15 nm respectively. High resolution transmission electron microscopy image shows average particle size of ∼34 nm and ∼20 nm. Magnetization measurements indicate a Curie temperature of ∼153 K and ∼172 K in applied magnetic field of 100 Oe for Pr 0.7Ca 0.3Mn 0.5Co 0.5O 3 and Pr 0.7Sr 0.3Mn 0.5Co 0.5O 3 compounds. The magnetization versus applied magnetic field curves obtained at temperatures below 150 K show significant hysteresis and magnetization is not saturated even in a field of 7 T. The magnetocaloric effect is calculated from M versus H data obtained at various temperatures. Magnetic entropy change shows a maximum near T C for both the samples and is of the order ∼2.5 J/kg/K. © 2011 American Scientific Publishers.

This work investigates the high-pressure structure of freestanding superconducting (Tc=4.3 K) boron-doped diamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0-30 GPa range. High-pressure Raman scattering experiments revealed an abrupt change in the linear pressure coefficients, and the grain boundary components undergo an irreversible phase change at 14 GPa. We show that the blueshift in the pressure-dependent vibrational modes correlates with the negative pressure coefficient of Tc in BDD. The analysis of x-ray diffraction data determines the equation of state of the BDD film, revealing a high bulk modulus of B0=510±28 GPa. The comparative analysis of high-pressure data clarified that the sp2 carbons in the grain boundaries transform into hexagonal diamond.

Morphotropic phase boundary (MPB) Lead zirconate titanate [i.e Pb(Zr0.52Ti0.48)O3, PZT] is modified by incorporation of i) La in Pb-site, ii) simultaneously La in Pb-site and Nb in Zr/Ti site and iii) simultaneously La in Pb-site, Nb and Fe in Zr/Ti-site. Samples are prepared by alkoxide sol-gel synthesis. XRD study reveals peak shift towards lower 2á upon La addition but the shift is towards higher 2á for both La and Nb are incorporated. Addition of Fe makes peak shift towards higher angle but the amount of shift is very small with respect to that of La and Nb doping. The XRD data is further analyzed with the help of Rietveld refinement which reveals change in lattice parameters. It is observed that successive doping of La, La-Nb and La-Nb-Fe decrease both lattice parameters a and c.However, for the case of Nb and La co-doped PZT the value of c increases with respect to the c of only La doped PZT. The charge difference in Nb (5+) and Fe (3+) ions have compensating effect as they are doped in the same atomic site (i.e. B-site of the ABO3) which is having charge 4+. Raman spectroscopic study reveals more complex effect of successive multiple doping of La, Nb and Fe. Some of the observations in Raman spectroscopy are supportive to the observations made in XRD. Raman spectra are analyzed by cumulative fitting of de-convoluted peaks, which represent various individual vibrational modes: Reveal changes happening in the lattice due to the systematic successive doping of La, Nb and Fe.

Our work investigates the impurity band and the core excitonic level evolution of a series of systematically boron doped diamond (BDD) films using Raman and X-ray absorption (XAS) spectroscopies. The resistivity versus temperature data shows that Tc of the BDD films achieves a peak at a maximum gaseous phase boron to carbon ratio, [[Formula presented]]max, beyond which Tc drops gradually as a result of excess defect formation, revealing a dome-shaped behaviour. Study of lattice expansion clearly shows the unambiguous presence of substitutional boron atoms beyond [[Formula presented]]max. Direct probe of the impurity band using XAS show a systematic increase in the absorption intensity of the bandgap states which ultimately degrades as the limit of [Formula presented]>[[Formula presented]]max is exceeded. The binding energy of the BDD 1s core exciton reveal a shallow measured value of 0.11 eV. This is less than earlier found values of 0.19 - 1.2 eV, in various reports for intrinsic diamond.

Nanocrystalline [formula omitted] sample has been prepared by sol-gel method. The room temperature powder x-ray diffraction data show single phase nature of the sample and confirm the cubic crystal structure with [formula omitted] space group. The average crystallite size is calculated using Scherrer formula, and it is found to be [formula omitted]. Transmission electron microscopy image shows that the particles are spherical in shape and the average particle size is [formula omitted]. The sample undergoes ferromagnetic ordering at 235 K [formula omitted] and obeys the Curie–Weiss law in the paramagnetic region. The maximum value of the magnetic entropy change [formula omitted] is [formula omitted], and the relative cooling power is [formula omitted] for a field change of 50 kOe. The Arrott plot confirms that the magnetic ordering is of second order nature. The experimentally observed magnetic entropy change of the sample obeys Landau theory of phase transition well. © 2010, American Institute of Physics. All rights reserved.

Unusual features in magnetization resembling the kinetic arrest of a magnetic glass state are observed in the La-doped double perovskite, SrLaFeCoO6. Neutron powder diffraction experiments confirm the presence of antisite disorder as well as a lack of long-range magnetic order down to 4 K in this double perovskite which displays spin glass-like features in dc and ac susceptibilities. Magnetic relaxation observed through cooling and heating under unequal fields (CHUF) point towards unusual domain dynamics which is supported by a broad memory effect. Among the two anomalies that are observed at 75 K and at 250 K in the magnetic measurements, the former is associated with a spin-freezing temperature below which the magnetic glass state is experimentally verified. The magnetometric experiments detailed in the paper bring out the non-equilibrium metastable magnetic states in this disordered magnetic system. The magnetic glass state described above manifests in the electrical resistivity through the formation of a 'hard gap' because of the spin-exchange energy following the formation of magnetic glass. It is observed that the combination of disorder and magnetic glass state leads to a large, negative magnetoresistance (MR) of ≈47 at 5 K in 8 T.

Magnetoresistance and magnetocaloric effect of a double perovskite oxide PrSrMnCoO6 (cubic, Fm 3 m) has been studied in fields up to 7 T. This compound is semiconductor-like and its electrical resistivity increases by 5 orders while going from 300 to 50 K. Giant magnetoresistance of ∼40 is observed at 200 K in 7 T field. PrSrMnCoO6 orders ferromagnetically at ∼150 K and shows a maximum magnetic entropy change of ∼4.6 J/kg/K for 5 T field change in the temperature range of 110-190 K. This nearly constant magnetocaloric effect over a broad temperature span is highly suitable for Ericsson-cycle magnetic refrigeration. © 2011 American Institute of Physics.

Carbon based nanomaterials are ideal media for hydrogen storage due to their highly porous structure, low density, and large surface area. The hydrogen uptake capacity of different carbon nanomaterials can be enhanced by spill over mechanism from a supported transition metal catalyst. In the present work graphene nanoplatelets (GNP) were prepared by thermal exfoliation method and decorated with palladium nanoparticles (Pd/GNP) by ethylene glycol reduction method. The high pressure hydrogen adsorption/desorption measurements were carried out for GNP and Pd/GNP using Sieverts apparatus in the temperature and pressure ranges of 25-100°C and 0.1-4 MPa, respectively. The hydrogen storage capacity of GNP and Pd/GNP were found to be 0.28 wt% and 1.21 wt% respectively at 25°C and 3.2 MPa pressure. Uniform dispersion of palladium nanoparticles over the surface of GNP enhances the hydrogen storage capacity of GNP by 70% due to spill over mechanism. © 2011 Indian Institute of Metals.

A spin glass state is observed in the double perovskite oxide Sr 2FeCoO 6 prepared through sol-gel technique. Initial structural studies using x rays reveal that the compound crystallizes in tetragonal I4/m structure with lattice parameters, a 5.4609(2) Å and c 7.7113(7) Å. The temperature dependent powder x ray diffraction data reveal no structural phase transition in the temperature range 10-300 K. However, the unit cell volume shows an anomaly coinciding with the magnetic transition temperature thereby suggesting a close connection between lattice and magnetism. Neutron diffraction studies and subsequent bond valence sums analysis show that in Sr 2FeCoO 6, the B site is randomly occupied by Fe and Co in the mixed valence states of Fe 3+/Fe 4+ and Co 3+/Co 4+. The random occupancy and mixed valence sets the stage for inhomogeneous magnetic exchange interactions and in turn, for the spin glass state in this double perovskite, which is observed as an irreversibility in temperature dependent dc magnetization at T f ∼ 75 K. Dynamical scaling analysis of ′(T) yields a critical temperature T ct 75.14(8) K and an exponent z 6.2(2) typical for spin glasses. The signature of presence of mixed magnetic interactions is obtained from the thermal hysteresis in magnetization of Sr 2FeCoO 6. Combining the neutron and magnetization results of Sr 2FeCoO 6, we deduce that Fe is in low spin state while Co is in both low spin and intermediate spin states. © 2012 American Institute of Physics.