Andrew Boothroyd

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.

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.