Dr Dharmalingam Prabhakaran

Abstract:

We present data on the magnetic and magnetoelastic coupling in the hexagonal multiferroic manganite LuMnO3 from inelastic neutron scattering, magnetization, and thermal-expansion measurements. We measured the magnon dispersion along the main symmetry directions and used this data to determine the principal exchange parameters from a spin-wave model. An analysis of the magnetic anisotropy in terms of the crystal field acting on the Mn is presented. We compare the results for LuMnO3 with data on other hexagonal R MnO3 compounds. © 2010 The American Physical Society.

Abstract:

Polarized neutron inelastic scattering has been used to measure spin excitations in ferromagnetic La0.82Sr0.18CoO3. The magnon spectrum of these spin excitations is well defined at low energies but becomes heavily damped at higher energies and can be modeled using a quadratic dispersion. We determined a spin wave stiffness constant of D=94±3 meV Å2. Assuming a nearest-neighbor Heisenberg model we find reasonable agreement between the exchange determined from D and the bulk Curie temperature. Several possible mechanisms to account for the observed spin-wave damping are discussed. © 2010 The American Physical Society.

Abstract:

The critical behavior of La0.67 Sr0.33 CoO 3 single crystal has been investigated from the bulk magnetization measurements around the Curie temperature (TC). The detailed analysis of the magnetization indicates the occurrence of a continuous ferromagnetic to paramagnetic phase transition at 223.0 K. The critical exponents β=0.361±0.007, γ=1.31±0.001, and δ=4.64±0. 01 characterizing this second order phase transition, have been estimated using different techniques such as the Kouvel-Fisher plot, the Arrott-Noaks plot, and critical isotherm analysis. With these values of TC, β, and γ, one can scale the magnetization below and above TC following a single equation of state. The consistency in the values of the critical exponents obtained from different methods and the well-obeyed scaling behavior confirm that the calculated exponents are unambiguous and purely intrinsic to the system. These values of the exponents match well with those theoretically predicted for the three-dimensional Heisenberg model with nearest-neighbor interaction. © 2010 The American Physical Society.

Abstract:

We report the direct measurement of antiferromagnetic spin polarization at the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray magnetic scattering. This supports recent theoretical models suggesting that the oxygen spin polarization is key to the magnetoelectric coupling mechanism. The spin polarization is observed through a resonantly enhanced diffraction signal at the oxygen K edge at the commensurate antiferromagnetic wave vector. Using the fdmnes code we have accurately reproduced the experimental data. We have established that the resonance arises through the spin polarization on the oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we have discovered that the position of the Mn3+ ion directly influences the oxygen spin polarization.

Abstract:

We show that resonant scattering signals at the Bragg forbidden positions in YVO3 originate from the anisotropic nature of the atomic scattering factor. The E1-E1 scattering from the electric-quadrupole moment is explained entirely through the crystal structure and does not require sensitivity to 3d orbital order. Simulations of the resonance using the FDMNES code suggest that the Jahn-Teller distortion provides an insignificant contribution to the intensity of the forbidden reflection. Thus we show that resonant x-ray diffraction at the vanadium K -edge in YVO3 is largely insensitive, even indirectly, to 3d orbital order through Jahn-Teller distortions. © 2010 The American Physical Society.