A. M. Niraimathi

Single crystals of Nd1+xBa2−xGayO7−δ and their characterization

Abstract Large Nd1+xBa2−xCu3−yGayO7−δ single crystals measuring up to 22 × 15 × 3 mm have been obtained from an oxide flux which was stepwise cooled from 1030 to 960°C in air. In the crystals the upper solubility limit of Nd is found to be x = 0.35 and that of Ga as high as y = 0.58. The Ga content is linearly related with that present in the molten solution. X-ray single crystal diffraction studies show that the tetragonal symmetry occurs in three states, i.e., (1) x ≥ 0.18, y = 0; (2) x = 0, y > 0.10 and (3) x + y ≥ 0.16. Optical polarizing microscopy reveals the twin domain structure existing in pure Nd1+xBa2−xCu3O7−δ crystals with x = 0.15. Transmission electron microscopy studies provide the evidence of the tweed tetragonal phase in the highly Ga-doped compound. The resistivity measurements reveal superconductivity of all samples with y ≤ 0.08, metallicity with y = 0.09–0.10, and semiconduction with y ≥ 0.15.

Study of magnetic ordering and crystal field effects of Sm in a single crystal of Sm2CuO4 by specific heat and susceptibility measurements

Heat capacity and magnetic susceptibility measurements were performed on a single crystal of Sm2CuO4 in different magnetic fields up to 7 Tesla applied both in parallel and perpendicular to the c-axis, in the temperature range from 1.5K to 150 K. Sm ions order antiferromagnetically atTN=5.96K and exhibits a sharp magnetic anomaly reminiscent to three dimensional ordering in the heat capacity and as a kink in the susceptibility. The magnetic anomaly shows a meagre magnetic field dependence ofTN indicating a strong magnetic coupling of Sm ions.

Influence of Ga substitution on the magnetic ordering of Nd ions in NdBa2Cu3 − xGaxO7 − y at low temperatures

Abstract The temperature dependence of magnetic specific heat and magnetic susceptibility of Ga substituted Nd-123 compounds has been investigated at low temperatures. The magnetic ordering of the Nd 3+ ions changes remarkably with the Ga concentration. The nature of the magnetic ordering is analysed using different magnetic models. The results are compared with those reported for pure unsubstituted NdBa 2 Cu 3 O 7 − y samples with various oxygen contents. The magnetic susceptibility data are analysed in the light of crystalline electric field effect (CEF) of different symmetry. The importance of CEF effects in determining the nature of magnetic ordering is explained.

On the nature of low dimensional ordering of Nd3+ ions in superconducting NdBa2Cu3−xGaxO7−y (0<x<0.5)

Abstract We report on heat capacity experiments on Ga-doped Nd-123 superconductors, NdBa2Cu3−xGaxO7−y (x=0, 0.01 and 0.03) in the temperature range from 0.5K to 5K. Below 2K, the specific heat is dominated by anomalies resulting from magnetic ordering of the Nd3+ ions. These anomalies can be described with the one-dimensional Ising model. The sudden increase of the maximum of the specific heat anomaly from 0.53K for the undoped (x=0) compound to nearly 0.9K for the Ga-doped samples (x=0.01 and x=0.03) is mainly attributed to the changes of hole carrier density and to the anisotropic changes of Nd-O bond distances in a-b plane.

Raman study of phonons and crystal-field excitations in NdBa2Cu3-xGaxO7−δ

Abstract We have performed Raman measurements on a series of NdBa 2 Cu 3− x Ga x O 7− δ ceramics (0 x −1 with increasing Ga content between x =0.00 and 0.08, the O(2)+O(3) in-phase phonon frequency is much less influenced. At low temperature, the O(2)–O(3) out-of-phase vibration is split into two peaks by interaction with a Nd 3+ crystal-field (CF) excitation of similar energy. Using a two-level model, we derive from the spectra the energies of the uncoupled phonon and the CF excitation, as well as the coupling constant of the CF-excitation–phonon interaction. The coupling constant and the phonon frequency are almost independent of Ga doping, while the energy of the CF excitation is (5.6±1.6) cm −1 lower in NdBa 2 Cu 2.92 Ga 0.08 O 7− δ than in NdBa 2 Cu 3 O 7− δ . This decrease is attributed to changes in the positions and charges of the oxygen ions in the CuO 2 planes. Using structural data obtained by neutron diffraction, we perform a point-charge-model calculation to separate structural and charge-transfer contributions to the observed change in the CF level energy. We find that the negative oxygen charge in the CuO 2 planes is increased by about 1% in NdBa 2 Cu 2.92 Ga 0.08 O 7− δ as compared to the Ga-free sample.

MAGNETIC ORDERING IN PYROCHLORE HO2MN2O7

The magnetic susceptibility of Ho2Mn2O7 with a spontaneous rise below about 40 K and a paramagnetic Curie temperature of +39 K suggests a ferromagnetic ordering. Indeed neutron diffraction profiles show strongly enhanced Bragg peaks with a temperature dependence which indicates an apparent Tc≊35 K. Nonetheless, the magnetic diffraction pattern is not consistent with a collinear ferro or ferrimagnetic ordering of the Ho3+ and Mn4+ sublattices. Furthermore, specific heat and small angle neutron scattering (SANS) show features which are also incompatible with conventional long‐range order.

Magnetic specific heat and susceptibility measurements on NdBa2Cu3−xGaxO7−y at low temperatures☆

Abstract We report on the temperature dependence of magnetic specific heat and magnetic susceptibility results on Ga-substituted Nd-123 compounds at low temperatures. The magnetic ordering of the Nd 3+ ions changes remarkably with increasing Ga concentration. The results are similar to those reported for pure undoped NdBa 2 Cu 3 O 7− y samples when the oxygen content is reduced. The nature of the magnetic ordering is analysed using different magnetic models and the importance of crystalline electric field effects, in determining the nature of magnetic ordering, is emphasised.