Quantum magnetism in coordination polymers

Abstract

This thesis presents an experimental and theoretical examination of five polymeric quantum magnets. The first of these is Cu(pyrazine)(glycinate)ClO4, an exchange-coupled spin-dimer system that undergoes a powerful and continuous magnetocaloric effect (MCE) in a rapidly changing magnetic field H. The evolution of the sample temperature T with H must be accounted for in order to reconcile an apparent discrepancy between the results of magnetometry measurements performed in quasistatic and pulsed magnetic fields, and the MCE is likely to be an important consideration for pulsed-field experiments performed on similar insulating materials. Heat capacity measurements of Cu(pyrazine)(glycinate)ClO4 are perturbed by zero-point fluctuations for T > 400 mK, and these data further suggest that this system exhibits possible two-dimensional universal behaviour. The results of single crystal x-ray diffraction measurements of a second material [H2F]2[NiF2(3-fluoropyridine)4]3[SbF6]2 at 100 K indicate that the Ni2+ ions of this complex are arranged on the vertices of a two-dimensional kagome lattice, wherein the spin S = 1 ions are bridged via charge-assisted Ni-F· · · H-F-H· · · F-Ni linkages. However, a density-functional theory study indicates that a positional disorder of the H2F+ moieties within these bridges suppresses the intraplane spin-exchange interactions. Powder muon spin-rotation measurements imply that the system is paramagnetic for T > 19 mK, while polycrystalline electron spin-resonance (ESR), magnetization M(H), and heat capacity experiments together indicate that the unixial and rhombohedral single-ion anisotropy of the Ni2+ ions are approximately D/kb = 8.3(4) K and E/kb = 1.2(3) K respectively. Lastly, neutron powder diffraction measurements of three isotructural compounds [M(HF2)(pyrazine)2]SbF6 (M = Cu2+, Ni2+ or Co2+) reveal that each system is tetragonal (P4/nmm) and that the spin-exchange interactions facilitated by the pyrazine (Jpyz) and bifluoride (Jfhf) ligands are antiferromagnetic. The Cu2+ congener is a quasi-two-dimensional Heisenberg S = 1/2 antiferromagnet, which displays an ordered moment of 0.6(1)μb per ion that is reduced from its paramagnetic value by quantum fluctuations. For the S = 1 Ni2+ complex, powder M(H) measurements suggest that D has an easy-plane character while inelastic neutron scattering experiments determine D/kb = 13.3(3) K, Jfhf/kb = 10.4(3) K and Jpyz/kb = 1.4(2) K. The S = 3/2 Co2+ system adopts an Ising-like antiferromagnetic ground state below 7.1(1) K, and its magnetic properties are parameterized with an effective spin-1/2 Hamiltonian for T < 50 K