Andrew Boothroyd

Abstract:

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.

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.