Dr Dharmalingam Prabhakaran

Abstract:

We have investigated the optimum conditions for the growth of Na xCoO2 single crystals by the optical floating-zone technique. A significant reduction in Na loss, was achieved by using a high-pressure growth atmosphere. A high-pressure oxygen-rich atmosphere during crystal growth was found to reduce the presence of CO3O4 impurity phase in the grown crystal. The magnetic properties of powder and single crystal samples were measured by superconducting quantum interference device (SQUID) magnetometry and found to vary according to the preparation conditions. © 2004 Elsevier B.V. All rights reserved.

Abstract:

In this paper we report studies of the charge stripe ordering using high resolution x-ray scattering in the nickelate system La2-xSr xNiO4 with doping levels of x=0.33, x=0.30, and x=0.275. The charge stripes for all doping levels were found to be two dimensional in nature with a high degree of correlation in the a-b plane. The in-plane inverse correlation length in the lower doped systems was greater than that in the x=0.33 system and is greatest for x=0.275, consistent with the stripes becoming less correlated as the doping level is decreased from x=0.33. However, the charge ordering in the x=0.33 system was observed to be more two dimensional in nature with a greater inverse correlation length between planes. The interaction between the lattice and charge order was observed to stabilize the charge ordering, even in the absence of spin ordering. In the x=0.30 and x=0.275 systems no long-range charge order could exist without the presence of the magnetic order. In both systems the charge stripes were incommensurate and on heating the incommensurability increased towards the stable ε=0.33 value as the stripes gained thermal energy to overcome the Coulombic repulsion. In all these systems the integrated intensity of the stripe reflections showed a clear difference in behavior from previous neutron studies with an initial increase in the integrated intensity as the temperature was increased, which was not found in the neutron studies.

Abstract:

Magnetic order has been investigated in stripe-ordered La 2-xSrxNiO4 (x=0.275,0.37,0.4) by dc magnetization and by polarized- and unpolarized-neutron diffraction. In the magnetically ordered phase, all three compositions exhibit a magnetic transition consistent with a spin reorientation in the ab plane. For x=0.37, the spin axes rotate from an angle of 37.7°±0.3° to the stripe direction at 71 K, to 52.3°±0.20 at 2 K. The x=0.275 and 0.4 compounds were found to undergo a similar spin reorientation. A spin reorientation has now been observed to occur for five different doping levels in the range of 0.275≤x≤0.5, suggesting that this spin transition is an intrinsic property of the stripe phase.

Abstract:

Perovskite manganites are interesting because of their colossal magnetoresistance. In this work high resolution thermal diffusivity measurements of La1-xSrxMnO3 (0 ≤ x ≤ 0.3) single crystals in the temperature range from 250 to 400 K are presented. A photopyroelectric device in the standard back configuration has been used. The thermal diffusivity through second-order magnetic phase transitions, as well as through first- and second-order structural phase transitions has been measured. The critical parameters of the sample with x = 0.3 at the ferromagnetic-to- paramagnetic transition have been obtained, and are close to the values predicted by the Ising model.

Abstract:

The thermal diffusivity in the vicinity of the magnetic transition of a set of La1-xSrxMnO3 (0less than or equal toxless than or equal to0.3) single crystals has been measured using the photopyroelectric technique. Taking into account that the inverse of the thermal diffusivity behaves as the specific heat, the critical exponent alpha has been obtained. In the pure and lightly doped samples (x=0,0.05) where the transition is antiferromagnetic-paramagnetic, the critical exponent is consistent with the Heisenberg model (alpha=-0.115). For Sr concentration higher than 0.20, where the low temperature phase is ferromagnetic, a drastic change in the critical exponent has been observed, approaching Ising behavior (alpha=+0.11) at the highest concentration. (C) 2004 American Institute of Physics.