ICM 2024
Thanks for checking out my page! Below you will find a digital version of my poster with some more information and a video explainer, or you can download the original poster here.
I am currently based at the Paul Scherrer Institute in Switzerland, where I am focusing on using muon-spin spectroscopy techniques to understand magnetic systems and supporting user experiments. If you would like to collaborate, or to learn more about my work, please do not hesitate to get in touch.
Substitutional disorder in the Pr2Zr2O7 family of spin-liquid candidate materials
I would like to express my gratitutude to all my collaborators on this project, including:
Phoebe Meadows (Royal Holloway University of London), Tom Northam (Royal Holloway University of London), David J Voneshen (ISIS Neutron and Muon Source, Royal Holloway University of London), Dharmalingam Prabhakaran (University of Oxford), Li Ern Chern (University of Cambridge), Siân E Dutton (University of Cambridge), C Castelnovo (University of Cambridge), and Jon P Goff (Royal Holloway University of London).
Pyrochlore magnets host exciting magnetism
Pyrochlores have the chemical formula A2B2O7, where A and B are both metals. The magnetism is driven by the crystal structure: corner-sharing tetrahedra of Ising-like moments.
The interaction between the magnetic moments is frustrated, meaning not all interactions can be simultaneously satisfied. This leads to exotic magnetic states with short-range correlations, but no long-range order. We name these states quantum spin-liquids!
Pyrochlores host magnetic moments that often point either in or out of the corner-sharing tetrahedra. The tetrahedra are made out of either the A or B atoms.
The energy dependence of inelastic neutron scattering shows how quantum fluctuations affect the spin-ice correlations in Pr2Zr2O7. The normal pinch point pattern is broadened, suggesting something else is going on. From K Kimura et al., Nat. Comm. 4, 1934 (2013).
Disorder in Pr2Zr2O7 gives rise to quantum fluctuations
Pr2Zr2O7 is reported to be a spin ice, with 2 spins pointing in and 2 pointing out of each tetrahedra. There are lots of pyrochlores that host spin ice ground states, and they are particularly interesting as they can host magnetic monopole excitations.
Intrinsic disorder in Pr2Zr2O7 is believed to give rise to quantum fluctuations, changing the Ising-like magnetic moments. This gives Pr2Zr2O7 unqiue properties when compared to other pyrochlore spin ices.
We want to understand how this works!
We introduce more disorder in Pr2ScNbO7
To understand the disorder in Pr2Zr2O7, we add more! By replacing two Zr ions (4+ valence) with one Sc (3+ valence) and one Nb (5+ valence), we introduce strains without charge imbalance or vacancies.
One might expect disorder to relieve the frustration and lead to long-range magnetic order, but theory suggests that for certain systems it may stabilise quantum spin liquid states. Our objective is therefore to be able to control the disorder to tune the magnetic proprties of our system.
As a first step, we want to build a phase diagram that will tell us what the magnetic state is for a certain level of disorder. To be able to do this, we have to understand what happens to the crystal structure.
By changing the disorder (h) in pyrochlores with non-Kramers magnetic ions, theory predicts quantum spin-liquid states are stabilised. We would like to build this phase diagram experimentally. From L Savary et al., Phys. Rev. Lett. 118, 087203 (2017).
The lowest energy configuration of Pr2ScNbO7, a charge ice with Sc-Nb chains. Each tetrahedra has exactly two Sc and Nb, and the chains aim to alternate between Sc and Nb. Both conditions can't be satisfied, so it is crystallographically frustrated!
Density functional theory explains
disorder-sensitive diffuse neutron scattering
The crytal structure can be found using density functional theory. By considering all the symmetrically inequivalent configurations, we can learn about what strucutral properties lower the energy of the system. We find there are short-range correlations for the Sc-Nb locations: they form a charge ice and prefer to be in chains of alternating Sc and Nb.
We can calculate the diffuse neutron scattering from this first-principles prediction. As diffuse scattering arises from deviations away from the perfect crystal structure, it is the perfect technique to look for short-range correlations. Our prediction matches experimental measurements very well, demonstrating we understand the disorder.
Diffuse neutron scattering, measured experimentally on SXD at ISIS (left) and theoretically predicted using SCARF at STFC (right). Different planes show different features.
The strains introduced by replacing Zr with Sc and Nb leads to two inequivalent Pr sites in Pr2ScNbO7. They have different symmetries of the nearby O atoms, changing the single-ion magnetism.
We find unexpected changes to the magnetism
We predict that Pr2ScNbO7 has two inequivalent Pr sites. Only one of these sites is magnetic at zero temperature as the doublet ground state has been disturbed.
Our model predicts all the experimentally found inelastic neutron scattering features. The peaks observed correspond to crystal field excitations from both Pr environments. This means that the calculations explain the single-ion magnetism.
Now we can keep going to predict and measure the stabilised magnetic states in this material and others in the series! Work on pristine Pr2Zr2O7, and less dramatic substitution on the Zr site are ongoing!
The density functional theory calculation well explains the features in the experimental inelastic neutron scattering from LET at ISIS. The peak positions are determined in our model using a point charge model - the slight deviation in position compared to experiment suggests that this isn't perfect, but captures all the physics!
The two Pr sites form a complex pattern in Pr2ScNbO7. By understanding these patterns, we can compare theory to experimental measurements of the collective magnetism, and begin to place our systems on the disorder phase diagram.
We can apply the same techniques to other materials in the series, as shown here for Pr2Zr2O7.