Exploring The Topological Magnetic Excitation Spectrum Of S=half MOFs With Inelastic Neutron Scattering

Metal-Organic Frameworks (MOFs) are a hot topic in the research community across a range of disciplines including solid state chemistry, condensed matter physics and materials science. In my project, kagomé MOFs offer a valuable pathway to synthetically realise two-dimensional magnetic models whilst simultaneously remaining advantageous over purely inorganic systems due to the ability to avoid the atomic site disorder. In our kagomé systems, the magnetism arises from Cu2+ ions forming the corners of the kagomé lattice, making our systems S=half. Such systems are ferromagnetic (FM) when entire layers align parallel with one another. Antiferromagnetism (AFM) is typically when moments align antiparallel to one another, although the structure of the kagomé net renders this difficult due to magnetic frustration. Materials that are magnetically frustrated in their ground state, with S=1/2 kagomé AFMs as an example, are known as quantum spin liquids (QSLs). The in-plane FM or AFM interactions can provide the system with a characteristic topological magnetic excitation spectrum, giving rise to interesting properties such as the magnon Hall effect and chiral edge modes. However, this spectrum can be affected by interplanar interactions and therefore it is particularly useful for us to deal with MOFs as varying the linker can influence the nature and strength and presence of these interplanar interactions. Currently, research in this area remains in its 'blue skies' phase, although it is thought that these systems can be used for electronic and spintronic devices in quantum computing for low energy data storage.
Although the first S= half kagomé FM has been synthesised in the past decade, fully understanding the topological magnetic excitation spectrum is yet to be achieved and remains one of the aims of my current research. QSLs are particularly elusive to synthesise, and no system of this type has been experimentally realised to date. The Clark group has synthesised a S=half kagomé AFM MOF which we believe is a good QSL candidate, although several complimentary techniques are required to unambiguously determine this, which will form part of my research in due course. Such techniques include x-ray and neutron diffraction, muon spectroscopy, inelastic neutron scattering (INS) and magnetometry measurements.
The major component of my research so far has included analysing preliminary data sets from INS measurements taken at the international facility, ISIS. INS can reveal information about the magnetic band structure of a material. This is analogous to how electronic band structure can influence the properties of a material. For example, a band gap in the electronic band structure gives rise to a material having insulating properties as electrons cannot hop across this distance to conduct the charge carried by these electrons. Furthermore, the relative size and nature (FM or AFM) of the magnetic interactions with one another can impact the appearance of this spectrum and therefore tell us about the nature of the exhibited magnetism. This analysis has required constructing a Python code to fit our data to a given model and comparing to simulations using a MATLAB library known as SpinW. I will also be directly involved in the synthesis of these systems to take them on beamtime experiments at these international facilities.