Dr Dharmalingam Prabhakaran

Abstract:

We have conducted the first soft x-ray diffraction experiments from a bulk single crystal, studying the bilayer manganite La2-2xSr1+2xMn2O7 with x=0.475 in which we were able to access the (002) Bragg reflection using soft x rays. The Bragg reflection displays a strong resonant enhancement at the L(III) and L(II) manganese absorption edges. We demonstrate that the resonant enhancement of the magnetic diffraction of the (001) is extremely large, indeed so large that it exceeds that of the nonresonant Bragg diffraction. Resonant soft x-ray scattering of 3d transition metal oxides is the only technique for the atomic selective measurement of spin, charge, and orbital correlations in materials, such as high temperature superconductors, colossal magnetoresistance manganites, and charge stripe nickelates.

Abstract:

Polarized- and unpolarized-neutron inelastic scattering has been used to measure the spin excitations in the striped spin-charge-ordered phase of La5/3Sr1/3NiO4. Above 30 meV, sharp magnetic modes are observed characteristic of a static stripe lattice. This part of the energy spectrum is described well by a linear spin-wave model with intrastripe and interstripe exchange interactions between neighboring Ni spins given by J = 15 ±1.5 meV and J′ = 7.5±1.5 meV, respectively. A pronounced suppression and broadening of the magnetic fluctuations in a band between 10 meV and 25 meV is suggestive of coupling to collective motions of the stripe domain walls.

Abstract:

Single crystals of ANb(2)O(6) (A = Nil Co, Fe, Mn) niobates have been grown by the floating-zone technique for the first time. The crystals were melt-grown from stoichiometric mixtures of the starting oxides. The orientation of the crystallographic axes relative to the crystal rod axis was found to be influenced by the size of the molten zone. Magnetization measurements as a function of temperature and applied field show that the magnetic properties are consistent with those found in previous studies. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

Large, high-quality crystals of undoped and Zn-doped CuO have been prepared successfully by the floating-zone technique. Formation Of Cu2O during the crystal growth of CuO has been eliminated under optimal growth conditions for the first time. The size of the grown crystals is typically 5-7 cm length and 8-9 turn in diameter, although with Zn doping the single crystal grains are found to form less readily. The Neel temperature is found to decrease with increasing Zn doping. Magnetic hysteresis measurements did not show any irreversible behaviour in either high purity ZnxCu1-xO crystals or polycrystalline CuO. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract:

We report a series of muSR measurements performed on single crystals of La1-x,SrxMnO3, (0 less than or equal to x less than or equal to 0.125) and on a polycrystalline bilayer manganite La2-2x Sr1+2xMn2O7 (v = 0.52). We find that the temperature dependence of the spinlattice relaxation rate is strongl, dependent on the hole doping and the associated changes in the magnetic structure. Y The systems have competing, anisotropic interactions, leading to a complex interplay of charge, spin and orbital order. This has a significant influence on the spin dynamics, both for perovskite and layered manganites. (C) 2002 Elsevier Science B.V. All rights reserved.