Dr Dharmalingam Prabhakaran

Abstract:

We present ultrafast resonant soft-x-ray diffraction measurements of time-dependent orbital order in the single-layer-manganite La0.5Sr1.5MnO4. These experiments reveal the appearance of a metastable phase with reduced ordering, different from any thermal state of the system. © OSA / UP 2010.

Abstract:

We report measurements of the magnetic excitation spectrum of the layered antiferromagnet La2 CoO4 by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0-250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small (∼1meV) dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of Co2⊃+ in an axially distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbor exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum. © 2010 The American Physical Society.

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.