Dr Dharmalingam Prabhakaran

Abstract:

To study the static and dynamic properties of the charge stripe phases in the nickelates by neutron scattering, single crystals are needed with a large volume ( >1 cm(3)). We will report the optimization and growth of large R2NiO4+delta (R = Pr, Nd) crystals by the floating-zone technique in an image furnace. Crack-free single crystals of diameter 8-10 mm and length 8090 mm have been obtained. The phase-purity of the crystals has been checked by powder X-ray diffraction, and information on the crystal quality has been obtained by X-ray and neutron diffraction. We will give an account of our investigations into the conditions needed to obtain the highest quality crystals, and we will discuss the magnetic studies by SQUID magnetometry. (C) 2003 Kluwer Academic Publishers.

Abstract:

Large single crystals of La2-2xSr1+2xMn2O7 (x= 0.3 to 0.525) have been prepared under controlled atmospheric conditions. The crystals were grown by the floating-zone technique in an image furnace under a mixed oxygen/argon atmosphere pressurized to 6-8 x 10(5) Pa. Rectangular single crystals with sizes up to 50 x 9 x 4 mm(3) have been obtained. The phase-purity, composition, and quality of the crystals were analyzed by X-ray diffraction and electron probe microanalysis. The magnetic behavior is found to be sensitive to the composition of the atmosphere during growth. (C) 2003 Kluwer Academic Publishers.

Abstract:

In this paper we present x-ray scattering results of charge and orbital ordering in the bilayer manganite La2−2xSr1+2xMn2O7 with doping levels x = 0.5 and x = 0.475. Using high-energy x-ray scattering, the structural modulation due to the Jahn-Teller ordering and the charge ordering due to the Mn³⁺/Mn⁴⁺ pattern have been measured. Both the x = 0.5 and x = 0.475 samples are found to display charge and Jahn-Teller order. We have confirmed that the wave vectors of the Jahn-Teller order, charge order, and orbital order are (Formula presented), (Formula presented) and (Formula presented). The origin of these has been confirmed by resonant x-ray scattering in the vicinity of the Mn K edge using polarization analysis. Contrary to previous studies, the Jahn-Teller order is found to be not reentrant, but is found to reduce in intensity at temperatures below 140 K for both samples. Charge ordering was also detected in the x = 0.5 sample below this temperature. © 2003 The American Physical Society.

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.