Research Highlights

New Quantum Spin Liquid

The quantum spin liquid (QSL) is a complex magnetic state, whereby quantum fluctuations and magnetic frustration lead to a highly quantum entangled, superpositional ground state. Experimental realisations of these materials are important, given the drive for new technologies to meet the ever growing demand of increased storage, efficiency and computational power. The pursuit of these exotic states remains a challenge, and for that reason a compelling direction to focus on in modern materials research.

TbInO3 is a layered hexagonal antiferromagnet comprised of distorted triangular layers of Tb3+ ions, of which some form a frustrated honeycomb sublattice. We have explored this material using a combination of neutron powder diffraction, magnetic susceptibility, μSR and inelastic neutron scattering.

The absence of conventional long-range magnetic order can be measured in a number of ways. In our study, numerous measurements indicated characteristic short-ranged correlations. For instance, inelastic neutron scattering data collected on SEQUOIA and CNCS at ORNL, revealed broad low-lying inelastic features up to 2 meV that are dispersionless and may be attributed to CEF excitations of Tb3+ ions. Additionally, the intensity of these features increased below the Curie-Weiss temperature, suggesting they are magnetic in nature and their broad, diffuse character reflect that the correlations are likely short-ranged.

Artwork courtesy of friend of the group, Lizzie Driscoll (@EHDriscoll), PhD student at the University of Birmingham.

Muon Over Zn-Doped Barlowite

What do ancient Japanese basket weavers and state-of-the-art quantum materials research have in common? The kagome of course! A kagome network is comprised of corner sharing triangles, and materials that realise this arrangement, like herbertsmithite, have garnered much attention as QSL candidates. A related material, barlowite, shares the same kagome arrangement of Cu2+ ions as herbertsmithite, but instead of the layers being separated by diamagnetic Zn2+ ions, are entirely replaced by Cu2+ ions. Doping this interlayer site in barlowite with ions such as Zn2+, effectively decouples the frustrated kagome layers – giving us an ideal opportunity to study the intrinsic properties of an S=1/2 kagome antiferromagnet.

Our previous studies have explored the chemical and magnetic structures of barlowite, but the question remains – how do the dynamics of the magnetic ground state evolve across the Zn-barlowite series? To answer this, we combine muon spin rotation and relaxation studies (μSR) with density functional theory (DFT) calculations.

μSR is a powerful technique which allows us to measure a materials internal magnetic field by implanting muons into a material and observing how they decay. The signal arises from the location of where the muon stops – and from our measurements we were able to identify two stopping sites within the Zn-barlowite series; μ-F and μ-OH. We have verified this by examining the asymmetry of the decay, extracting characteristic frequencies from Fourier transforms and DFT calculations. We also find that increasing the zinc content leads to a suppression of magnetic order, culminating in a dynamically fluctuating state at all temperatures when the interlayer zinc content is larger than 66 %.

Thinking Outside the Triangle

Novel spin liquid states may be achieved through the experimental realisation of new materials. Although geometric representations of frustration are easiest to visualise, many magnetic structures have the potential to become frustrated given the right conditions. Two such systems are the two-dimensional S = 1/2 Heisenberg square lattice and three-dimensional diamond lattice, where competition between neighbouring exchange interactions leads to novel spin-spiral structures and rich magnetic phase diagrams.

Hybrid coordination frameworks offer a controlled way to create a range of versatile structures, and as such we have synthesised two polymorphs of the S=1/2 Ti3+ hybrid coordination framework. Through a combination of single crystal X-ray and neutron powder diffraction, magnetisation and specific heat measurements and DFT calculations we explored the chemical and magnetic structures of our oxolate materials.

We were able to create α and β phases that adopted the square and diamond structures respectively. Magnetisation data reveal both polymorphs generate slight magnetic frustration, but dominant antiferromagnetic interactions lead them to order. Perhaps most striking was that the exchange coupling in the β phase was found to be an order of magnitude larger than the α phase, which we addressed through examining the degree of orbital overlap in DFT calculations.

Honeycomb Layered Spin Glass

Being flat isn’t always boring – materials often take on a new life when reduced to low dimensions. This can include enhanced conductivity and unique electrochemical properties. Additionally, combining with the application of pressure may result in novel electronic and magnetic states that are rarely encountered in their bulk counterparts. The MPS3 compounds represent a diverse family of honeycomb layered van der Waals materials, where the physical properties are heavily reliant on the choice of transition-metal ion (M).

Whilst a spin glass state is known to exist in the mixed Mn0.5Fe0.5PS3 compound, relatively little is known of the local nuclear and magnetic correlations that form this glassy state. Here, we combine neutron scattering techniques and magnetisation measurements to explore the nature of this glassy state.

Spin glasses are frustrated magnets with an additional ingredient – randomness – in Mn0.5Fe0.5PS3, this randomness arises from the chemically disordered magnetic ions within the honeycomb layers. The result is a mixture of satisfied and unsatisfied correlations, which induce the spin glass state. Using xyz-polarisation analysis on powder data collected on D7, we were able to explore the nature of these correlations and accurately reconstruct our single crystal diffraction patterns with reverse Monte Carlo methods on the program SPINVERT.

A full list of our publications can be found here.

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