Thesis--Ph. D., University of Hong Kong, 1987.

No description available.

No description available.

No description available.

No description available.

We have studied magnetic properties of the intermetallic NdPd5Al2 com- pound by means of specific heat and magnetization measurements and neutron scattering. The compound crystallizes in the tetragonal I4/mmm space group with lattice parameters a =4.147 ˚A and c =14.865 ˚A, orders antiferromagnet- ically below TN =1.3 K and presents large magnetocrystalline anisotropy due to the crystal-field effects. The obtained magnetic phase diagram is charac- terized by two distinct magnetically ordered phases similarly to structurally re- lated tetragonal RTX5 and R2TX8 compounds. The zero-field antiferromagnetic phase is characterized by the propagation vector k = (1 2 00) and antiferromag- netic coupling of Nd moments along the teragonal c-axis with the amplitude of magnetic moments of 2.22 µB/Nd as was revealed by neutron diffraction. The transition from the paramagnetic to magnetically ordered in zero field is the first-order phase transition. The CF excitations in NdPd5Al2 were detected by means of INS at 3.0 meV, 7.4 meV, 8.6 meV and 17.1 meV. We further compare our findings about CF in NdPd5Al2 obtained from INS, susceptibility analysis and first-principles calculations and confront them with the experimental mag- netization and magnetic specific heat data. Our results will be also discussed with respect to related...

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

A combination of 169Tm-Mo??ssbauer spectroscopy and inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) interaction at the rare earth site for the tetragonal series RT2Si2 (where R = rare earth) and the orthorhombic series RNiAl4. Single phase specimens were prepared in an argon arc furnace and characterised using x-ray powder diffraction, specific heat and magnetic measurements. For the RT2Si2 series previous investigations were extended to include T = Mn and Cr whose sub-lattices are antiferromagnetic well above room temperature. However, the rare earth sub-lattices were confirmed to order close to liquid helium temperature. With the assistance of the lattice electric field gradient (EFG) for isostructural GdCr2Si2 (determined elsewhere using 155Gd-Mo??ssbauer spectroscopy) and the within-rank CF parameter ratios for HoCr2Si2 (determined elsewhere using INS), the experimental 169Tm quadrupole interaction data were analysed to arrive at CF parameters for the Tm3+ site in TmCr2Si2. The final CF parameters match well with the trend observed for other members of the series RT2Si2 (T = Fe, Co, Ni, Cu). CF schemes were also determined for Tm3+ in TmMn2Si2 and Er3+ in both ErCr2Si2 and ErMn2Si2. For the RNiAl4 series, TmNiAl4 was determined to be antiferromagnetic below TN = 4.7 K with the 169Tm-Mo??ssbauer spectra retaining a magnetic appearance up to 80 K due to the effect of slow spin-lattice relaxation. The relaxation data are consistent with a well-isolated ground state doublet (or pseudo-doublet) whose eigenfunctions have high components of mJ = ??6 states and with the relaxation proceeding via an excited state at 350 K. Because of the relaxation effect, the 169Tm-Mo??ssbauer data could not be interpreted in terms of CF parameters in the usual way. Instead, the INS spectra recorded for ErNiAl4 were analysed using a novel semi-empirical approach that coupled rank 2 CF parameters converted from the EFG tensor for isostructural GdCr2Si2 (determined elsewhere using 155Gd-Mo??ssbauer spectroscopy) with simple point-charge model calculations of the rank 4 and 6 within-rank CF parameter ratios. The first full set of CF parameters for this series was then determined for the Er3+ site in ErNiAl4.

Thesis (M. Phil.)--University of Hong Kong, 1993.