Dr Dharmalingam Prabhakaran

Abstract:

We investigated the effect of uniaxial and hydrostatic pressure on resistivity and ac-magnetic susceptibility of two-dimensional layered manganite, La1.25Sr1.75Mn2O7 (LSMO125) to investigate the lattice effect on magnetic and electronic properties. Asymmetric role of uniaxial pressure, || and ⊥ to c-axis on the spin flop and charge transport has been revealed while comparing hydrostatic pressure. Uniaxial pressure along c-axis increases metal-insulator transition temperature (TMI) and ferromagnetic ordering temperature (TC), whereas it decreases the resistivity along ab-plane (ρab). In contrast to pressure along c-axis, TMI and TC decrease, whereas the resistivity along c-axis (ρc) increases with pressure || to ab-plane. ρc/ρab is quite large, increasing with pressure and shows a peak at around TMI. Uniaxial pressure behaviour is strongly related to the Mn-O-Mn linkage between MnO2 layers and the spin reorientation from the apical axis to the basal plane and vice versa with pressure. Both ρab and ρc decrease whereas TMI and TC increases under hydrostatic pressure. Influence of spin and charge on magnetic and electrical properties under hydrostatic pressure are explained by pressure-induced cant between the MnO2 bilayers and variation in bond lengths. The different pressure driving rates of TMI while measuring ρab and ρc confirms that there is a strong competition between the in and out plane components under hydrostatic pressure. © 2008 Elsevier B.V. All rights reserved.

Abstract:

We present resonant x-ray scattering experimental data from YVO3. By scattering at the vanadium K edge we are able to observe diffraction from the anisotropic tensor of susceptibility at the Bragg forbidden (010). The resonant energy spectra from these reflections are unusually complex, giving an indication of the crystal field distortions around the vanadium site.

Abstract:

The magnetism of charge-stripe-ordered La1.55Sr 0.45NiO4 was studied by a combination of neutron diffraction, muon-spin relaxation (μSR), and bulk susceptibility. Long-range magnetic ordering was observed at a lower temperature by μSR than by neutron diffraction, consistent with a glassy transition to the ordered phase. A second magnetic transition is detected by all techniques and is consistent with a spin reorientation. On cooling below TSR=42 K the spins reorientate from lying 32.9±0.6° away from the stripe direction at 70 K to 56.8±0.4° at 10 K. The magnetic order was observed by neutron diffraction to have both three-dimensional and two-dimensional (without any correlation along the c axis) characters. μSR measurements confirmed this and are consistent with a single magnetically ordered spin-stripe phase. The effects of checkerboard charge order on the ordered phase and the characteristics of the phase diagram of the spin reorientation in charge-ordered La2-xSrxNiO4 are commented on. © 2008 The American Physical Society.

Abstract:

Low-temperature transmission electron microscopy and x-ray diffraction (XRD) studies have been carried out on pellet and powder samples of Pr0.5 Ca0.5 MnO3. These studies have revealed appearance of a different type of charge-orbital ordered (COO) phase, resulting due to flipping of eg orbitals from d3 x2 - r2 / d3 y2 - r2 to d3 x2 - r2 / d3 z2 - r2 configuration. This orbital flip results in a changeover of the COO superlattice-ordering vector from (1/2,0,0) to (1/4,1/2,1/4) in the Pnma phase. This COO phase coexists with the conventional COO phase. Low-temperature XRD studies show that the COO phase appears only in pellet sample and not in the corresponding powder sample. The powder sample shows only conventional COO phase. Volume fractions of conventional and the other type COO phases in pellet sample of Pr0.5 Ca0.5 MnO3 is estimated to be ∼55% and 45%, respectively. The occurrence of orbital flip has been attributed to local strain building up in the pellet sample. The strain builds up during cooling because manganite has anisotropic thermal expansion coefficients. © 2008 The American Physical Society.