Dr Dharmalingam Prabhakaran

Abstract:

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La2-2xSr1+2xMn2O7 single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T c, increases under pressure (dTc/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of Tc is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below Tc at atmospheric pressure. Both peak structure in χc and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La1.25Sr1.75Mn2O7 single crystal. © 2005 Elsevier Ltd. All rights reserved.

Abstract:

An ac photopyroelectric calorimeter has been used to measure the thermal diffusivity and specific heat of the perovskite manganites RMnO3 (R ≤ La, Pr, Nd) close to their magnetic transitions. Taking into account that the inverse of the thermal diffusivity has the same critical behaviour as the specific heat, the critical exponent α of the magnetic transition has been obtained by means of both magnitudes. The results in all cases are consistent with the Heisenberg model (α ≤ -0.11), irrespective of the rare-earth ion. © 2005 IOP Publishing Ltd.

Abstract:

We present two experiments that provide information on spin anisotropy and the magnetic structure of NaxCoO2. First, we report low-energy neutron inelastic scattering measurements of the zone-center magnetic excitations in the magnetically ordered phase of Na0.75CoO2. The energy spectra suggest the existence of two gaps, and are very well fitted by a spin-wave model with both in-plane and out-of-plane anisotropy terms. The gap energies decrease with increasing temperature and both gaps are found to have closed when the temperature exceeds the magnetic ordering temperature T_m~22 K. Secondly, we present neutron diffraction studies of Na0.85CoO2 with a magnetic field applied approximately parallel to the c axis. For fields in excess of ~8T a magnetic Bragg peak was observed at the (0,0,3) position in reciprocal space. We interpret this as a spin-flop transition of the A-type antiferromagnetic structure, and we show that the spin-flop field is consistent with the size of the anisotropy gap.

Abstract:

We report muon-spin rotation, heat capacity, magnetization, and ac magnetic susceptibility measurements of the magnetic properties of the layered spin-12 antiferromagnet NaNiO2. These show the onset of long-range magnetic order below TN=19.5K. Rapid muon depolarization, persisting from TN to about 5 K above TN, is consistent with the presence of short-range magnetic order. The temperature and frequency dependence of the ac susceptibility suggests that magnetic clusters persist above 25 K and that their volume fraction decreases with increasing temperature. A frequency dependent peak in the ac magnetic susceptibility at Tsf=3K is observed, consistent with a slowing of spin fluctuations at this temperature. A partial magnetic phase diagram is deduced. © 2005 The American Physical Society.

Abstract:

Using the MAPS spectrometer at the ISIS spallation source, we have measured the magnetic excitations of single-crystal samples of stripe-ordered La2-x Srx Ni O4 with x=0.333 and 0.275. The full two-dimensional spin-wave dispersions were obtained using incident energies of 60 and 160 meV. To analyze the excitations, we have evaluated a spin-only Hamiltonian describing diagonal, site-centered stripes in the linear spin-wave approximation. Besides the superexchange energy J within antiferromagnetic domains, we have considered effective exchange couplings J1 and J2 across a charge stripe coupling second-neighbor Ni sites along Ni-O bond directions and along the plaquette diagonal, respectively. From least-squares fits of the model to the measurements on the x=1/3 sample at T=10 K, we find that the dispersions are well described by a model using just J and J1, but not J and J2. Consistent with an analysis of previous measurements, we find that J is about 90% of the superexchange energy of undoped La2 Ni O4 and J1/J 0.5. The excitations observed for x=0.275 are surprisingly similar to those for x=1/3, despite the differing magnetic-ordering wave vectors; the main difference is a broadening of the excitations for x=0.275. For both samples, we find that one spin-wave branch has a gap of ∼20 meV, confirming a previous observation for x=1/3. We discuss the possible origin of this gap. © 2005 The American Physical Society.