Pitcairn, Jem
(2024)
Tuning magnetic properties in layered metal-organic frameworks.
PhD thesis, University of Nottingham.
Abstract
This thesis explores the role of chemistry and structure in the magnetic properties of metal-organic framework materials. New magnetic materials with enhanced properties are promising for next-generation information technology with higher speeds, energy efficiency and data density. Realising these new technologies calls for programmable materials where the magnetic properties can be tuned by controlling the chemical composition and crystal structure. Metal-organic magnets (MOMs), assembled from metal nodes connected by organic molecular linkers into extended networks, offer these benefits. The modularity of their construction allows for the tuning of magnetic properties by substituting the nodes and linkers while retaining network topology. The chemical and structural complexity of these materials makes predicting their magnetic properties challenging. Developing this understanding requires deeper exploration of the influence chemistry and structure have on the magnetism in new MOMs.
In this thesis I discuss six chemically distinct MOMs with closely related chemical connectivity and crystal structures, MCl2L with M = Cr2+, Fe2+ and Ni2+ ; L = pym (pyrimidine), btd (2,1,3–benzothiadiazole). In these compounds the M2+ are coordinated equatorially by four Cl-ligands and axially by two N atoms from the L ligands to form MCl4N2 octahedra. These octahedra form MCl2 chains along the crystallographic a direction, which are connected into layers by L in the b direction. These layers stack in the crystallographic c direction through van der Waals (vdW) interactions. I determined their structures through single crystal X-ray diffraction, powder X-ray diffraction and powder nuclear neutron diffraction. To characterise their bulk magnetic properties I carried out magnetisation measurements under variable temperature and field conditions. All of these compounds were found to magnetically order at low temperature. With the crystal structures and magnetic ordering temperatures established, I was able to determine their magnetic ground states through powder magnetic neutron diffraction. Finally, I was able to determine the magnetic interaction energies of the Cr analogues by characterising their excitations with inelastic neutron scattering.
I show that substituting the metal centre for one with a different spin state and/or d-electron configuration will drastically alter the magnetic properties. Substituting the ligand can further tune magnetic properties by modulating the energy of magnetic interactions and the crystal structural geometry. In Chapter 3 I report the synthesis, crystal structure, bulk magnetisation behaviour and magnetic ground state of the Fe and Ni analogues. I show that ligand substitution in these compounds can be used to control spincanting, which can introduce or tune metamagnetism and weak ferromagnetism. This effect is caused by controlling tilt angle between the MCl4N2 octahedra and demonstrates how the structure of the organic ligands can be leveraged to tune for properties. In Chapter 4 I report the synthesis, crystal structure, bulk magnetisation, magnetic ground state and magnetic excitations of CrCl2(pym). I show that the pym ligand carries a weak ferromagnetic superexchange interaction, effectively magnetically separating the CrCl2 into quasi-one-dimensional spin chains and uncovering a potential chemical design route for the S = 2 Haldane phase. In Chapter 5 I report the synthesis, crystal structure, bulk magnetisation behaviour, magnetic ground state and magnetic excitations of CrCl2(btd). In contrast to pym, the btd ligand carries a strong antiferromagnetic superexchange, effectively magnetically connecting the CrCl2 chains, raising the magnetically dimensionality to quasi-two-dimensional. I show that in the Cr compounds the ligand can be substituted to modulate the exchange interactions.
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