Ferromagnetic Ising Chains In Frustrated Magnetocaloric Frameworks

Magnetic materials with strong local interactions, but lacking long range order, have long been a curiosity of physicists. Probing the magnetic interactions is crucial for understanding the unique properties they can exhibit. Framework materials have recently gathered more attention as they can produce more exotic structures, allowing for controlled design of magnetic properties not found in conventional metal-oxide materials. Probing the magnetic interactions, in functional magnetic materials, can reveal detailed insight into how to optimise the properties they possess while providing key understanding of the exotic phenomena they may host. Historically, magnetic diffuse scattering in such materials has been overlooked but has attracted greater attention recently, with advances in techniques. This thesis probes the short range magnetic order and long range magnetic structure of some highly efficient magnetocaloric frameworks containing heavier lanthanides. In particularly we aimed to identify the magneto-structural relationship that leads to an optimised magnetocaloric effect in low applied magnetic fields (<2 T) and at temperatures 2-10 K, where these materials could find use in adiabatic refrigeration applications as an energy efficient alternative for cooling to temperatures for which liquid helium has historically been used. The magnetic structure and correlations have been probed through neutron scattering, using reverse MonteCarlo refinements to establish the short range order they manifests, and Rietveld refinements to probe the long range order. A variety of physical property measurements have been usedto provide additional information about the bulk properties of these materials.