Investigation of quasi-one dimensional antiferromagnets by neutron scattering

In this dissertation, experimental results on three quasi-one dimensional Heisenberg antiferromagnets are reported.; CuCl₂ · 2DMSO (CDC) has previously been reported to be a spin-1/2 one dimensional antiferromagnetic Heisenberg chain with J/k_B ≈ 17K in zero field. We have found an interesting phase diagram when the system is placed in an applied field. The 3D ordered phase is suppressed when the field is parallel to either the crystalline a axis or c axis, while the 3D ordered phase is enhanced when the field is parallel to the b axis. A field-induced gap is also observed when the external field is parallel to the a and c axis and the magnitude of the gap changes with orientation and magnitude of field.; Ni(C₅D₁₄N₂)₂N₃(PF ₆)(NDMAP) is a quasi-one-dimensional antiferromagnetic spin-1 chain. In an applied magnetic field we found commensurate transverse magnetic ordering by means of neutron diffraction. Depending on the direction of the applied field, the high field phase has either three dimensional long range order or quasi-two-dimensional short range order. We also performed inelastic neutron scattering experiments in a magnetic field below and above the critical field H_c at which the gap closes to probe dynamics spin correlations. For H < H_c, the behavior of NDMAP is well described by first order perturbation theory and is similar to that previously found in the extensively studied Haldane gap compound Ni(C ₂H₈N₂)₂NO₂(ClO₄) (NENP). For H > H_c, we have observed quasi-elastic excitations with broadened q-dependence.; Y₂BaNiO₅ is a quasi-one-dimensional spin-1 antiferromagnet, in which holes can be added by substituting Ca2+ for off-chain Y3+. Inelastic neutron scattering experiments were performed to study the magnetic excitations in the pure, x = 0.04 and x = 0.1 samples. The parent compound is a spin liquid with macroscopically coherent quantum ground state and no static order. The main feature of the excitation spectrum is an ∼8 meV Haldane gap separating a triplet band from the singlet ground state. At energies above the spin gap, the triplet excitations of the parent compound persist with doping. Below the gap, new excitations appear with an incommensurate structure factor. The experiment provides a quantitative view of the magnetic polarization cloud associated with holes in a doped transition metal oxide. (Abstract shortened by UMI.)...