Dr Dharmalingam Prabhakaran

Abstract:

We report magnetization and neutron scattering measurements of the half-doped compound La1.5Sr0.5CoO4, which exhibits a checkerboard pattern of charge ordering below not, vert, similar800 K. In the antiferromagnetically-ordered phase below not, vert, similar40 K the spins are found to be canted in the ab plane. The spin excitation spectrum includes spin-wave excitations with a maximum energy of 16 meV, and diffuse magnetic modes at energies around 30 meV.

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.