Open Access Version

Abstract:
This work explores the magnetic properties of two novel compounds BaCu2V2O8 and BaNi2V2O8 which both provide rare physical realizations of model magnetic systems that are characterized by unconventional magnetic behaviour at finite temperatures. BaNi2V2O8 has an ideal honeycomb crystal structure where S=1 nickel magnetic ions are arranged on well isolated honeycomb layers. The nearest neighbour magnetic ions within the plane interact with each other via the dominant antiferromagnetic exchange interaction while the interaction between the honeycomb planes is very weak. The spins develop long-range magnetic order in their ground state and lie within the honeycomb plane forming collinear arrangement so that each spin is antiparallel to its 3 nearest neighbours. The crystal and magnetic structure of BaNi2V2O8 imply that this compound is a 2D planar antiferromagnet. The magnetic properties of this compound were explored using DC susceptibility, powder and single crystal inelastic neutron scattering measurements as well as single crystal neutron diffraction measurements whose results were analysed using different theoretical approaches. The single crystal measurements at base temperature reveal a spin-wave magnetic excitation spectrum which disperses within the honeycomb plane but is completely dispersionless in the out-of-plane direction confirming the strongly 2D magnetic behaviour of this compound. The dispersion of the magnetic excitation spectrum at base temperature was analysed using linear spin-wave theory which allowed the Hamiltonian of BaNi2V2O8 to be solved. The critical phenomena in BaNi2V2O8 were explored by performing the neutron diffraction measurements at finite temperatures both below and above the ordering temperature TN. The extracted critical exponents reveal that BaNi2V2O8 behaves as a 2D antiferromagnet over the whole explored temperature range displaying the crossovers from 2D Ising like to 2D XY and then to 2D Heisenberg magnetic behaviour with increasing temperature. The second compound is BaCu2V2O8 which has tetragonal symmetry where the magnetic S=1/2 Cu2+ ions form screw chains along the c-axis. The DC susceptibility data reveal a non-magnetic ground state and were fitted well by the dimer-model which allows the magnetic intra- and interdimer exchange constants to be estimated. The single crystal inelastic neutron scattering measurements of BaCu2V2O8 reveal that the magnetic excitation spectrum is gapped and characterized by a high ratio of the gap to the bandwidth which equals 3.6 indicating a strong dimerization. The magnetic excitations disperse along the chain direction but are completely dispersionless within the tetragonal plane implying that BaCu2V2O8 is a strongly dimerized 1D chain where the dimers are coupled together along the c-axis. The magnetic excitation spectrum was analysed using the theoretical model for the 1D alternating chain which allowed the Hamiltonian of BaCu2V2O8 to be solved. The results were verified by comparison with theoretical simulations of the spectra using this Hamiltonian. The strong dimerization of BaCu2V2O8 makes this compound a good candidate for the observation and detailed investigation of strongly correlated phenomena at finite temperatures. The thermal behaviour of the magnetic excitations in BaCu2V2O8 were explored by analysing of the temperature dependence of their lineshape at finite temperatures over a wide temperature range. The analysis was performed using a new fitting function and revealed the extraordinary coherence of the magnetic excitations in BaCu2V2O8 at finite temperatures which manifests itself in an asymmetric thermal lineshape broadening in contrast to the Lorentzian lineshapes observed in conventional magnets. The observed results were verified by comparison with the results of theoretical simulations of the magnetic excitations at finite temperature in BaCu2V2O8 which were performed using both numerical and analytical theoretical approaches.