Ce Sn O Crystals Achieve Structured Neutron Scattering, Revealing Quantum Spin Ice Correlations

Scientists are increasingly focused on understanding the exotic magnetic behaviour of quantum spin ice materials, and a new study published today sheds light on the puzzling correlations within the candidate compound cerium tin oxide. Bo Yuan, M. Powell, and X. Liu, from their respective institutions, alongside colleagues, present compelling neutron scattering data from exceptionally high-quality single crystals of CeSnO , a significant leap forward in this field. Their findings reveal a strikingly similar scattering pattern to that of classical dipolar spin ice, despite expectations based on current theoretical models, challenging established understandings of magnetic interactions within these materials. This research is vital as it suggests that longer-range interactions play a crucial role in defining the low-energy physics of cerium pyrochlores and necessitates a re-evaluation of existing theoretical frameworks.

This research overcomes longstanding challenges in studying these complex materials, specifically poor sample quality and weak signals in Neutron scattering experiments due to the small magnetic moment of cerium. The team successfully grew single crystals using a hydrothermal method, enabling detailed neutron scattering analysis that reveals unprecedented structural insights into the material’s magnetic behaviour. Experiments show highly structured scattering from Ce₂Sn₂O₇, with strong intensities concentrated along the boundaries of the Brillouin zone, a stark contrast to the broad, diffuse scattering observed in related compounds like Ce₂Hf₂O₇ and Ce₂Zr₂O₇.

This observed scattering pattern deviates significantly from predictions based on the commonly used nearest neighbour XYZ model for these materials, but strikingly resembles the scattering observed in classical dipolar spin ice. The study establishes that further neighbour interactions play a crucial role in determining the low energy physics of cerium pyrochlores, necessitating a revision of current theoretical frameworks to accurately capture their behaviour. The research team employed single-crystal neutron scattering to probe the magnetic correlations within Ce₂Sn₂O₇, a technique sensitive to the arrangement of magnetic moments at the atomic level. By meticulously growing high-quality crystals, they were able to obtain data with significantly improved signal-to-noise, allowing for a detailed mapping of the diffuse scattering intensity.
This detailed mapping revealed the unexpected concentration of scattering along the Brillouin zone boundaries, indicating a complex interplay of magnetic interactions beyond simple nearest neighbour exchange. The findings challenge the prevailing theoretical understanding of these materials, suggesting that longer-range interactions are essential for describing their magnetic properties. This breakthrough highlights the importance of high-quality materials in unlocking the secrets of quantum spin ice candidates. The observed similarities between Ce₂Sn₂O₇ and classical dipolar spin ice suggest that the material may exhibit similar emergent phenomena, potentially paving the way for the realization of a quantum spin liquid state. The work opens new avenues for exploring exotic quantum phases and excitations in these materials, with potential applications in future quantum technologies and materials science, a significant step towards understanding and harnessing the unique properties of these fascinating compounds.

Hydrothermal Growth and Neutron Scattering of CeSn2O7

Scientists have long sought to understand the complex magnetic behaviour of cerium pyrochlore oxides, materials exhibiting potential as Quantum Spin Ice candidates. Researchers addressed longstanding challenges in this field by performing single-crystal neutron scattering on hydrothermally grown CeSn₂O₇, achieving the highest quality crystals to date within the CeXO family. This work overcame limitations imposed by varying sample quality and weak signals in previous neutron data, attributable to the small magnetic dipole moment of cerium. The team engineered a hydrothermal growth process, reducing synthesis temperatures from over 2000°C/1000°C, typical of conventional methods, to just 700°C, substantially improving crystal quality and reducing Ce³⁺ oxidation to the non-magnetic Ce⁴⁺ state.
Experiments employed the CORELLI spectrometer at the Spallation Neutron Source (SNS) to conduct diffuse scattering measurements on a co-aligned array of these hydrothermally grown single crystals. The study pioneered a methodology utilising large reciprocal space coverage at CORELLI, enabled by thermal neutrons and a large detector array, to probe all symmetry-equivalent directions in three-dimensional reciprocal space. This approach dramatically improved the signal-to-noise ratio, revealing a highly structured 3-dimensional diffuse scattering pattern concentrated along the Brillouin zone boundaries of the underlying face-centred cubic lattice. Data was collected at 50 mK, with a 12 K background subtracted, and confirmed the magnetic origin of the scattering as the pattern disappeared at 800 mK.

The researchers meticulously folded the data using the symmetry operations of the cubic space group, further enhancing the signal-to-noise ratio and allowing detailed analysis at large |Q| values up to approximately 5 Å⁻¹. Analysis of the [H+0.5, H-0.5, L] plane, devoid of nuclear Bragg peaks, revealed no significant magnetic scattering beyond |Q| ∼3 Å⁻¹, demonstrating a clear distinction from previous observations on Ce₂Zr₂O₇ and Ce₂Hf₂O₇. This high-quality data, coupled with the increased power of the SNS source, uncovered a Q-dependence of the diffuse scattering intensity that contradicts predictions based on the nearest neighbour XYZ model commonly used for CeXO compounds. The observed scattering pattern closely resembles that of classical Dipolar Spin Ice, suggesting the importance of further neighbour interactions in governing the low-energy physics of these cerium pyrochlores and necessitating a revision of current theoretical frameworks.

CeSn2O7 exhibits unique magnetic neutron scattering

Scientists have achieved a breakthrough in understanding quantum spin ice candidates with the successful growth of high-quality single crystals of CeSn₂O₇. The research team reports the first single-crystal neutron scattering data from this material, overcoming previous limitations caused by sample quality and weak signals. Experiments revealed a highly structured scattering pattern, markedly different from those observed in CeHf₂O₇ and CeZr₂O₇, featuring strong intensities concentrated along the Brillouin zone boundaries. Measurements confirm that the observed scattering at 50 mK displays a highly structured 3-dimensional pattern in Ce₂Sn₂O₇, which essentially disappears at 800 mK, definitively establishing its magnetic origin.

Data shows the intensity is maximized within the first Brillouin zone and weakens at higher |Q| values, consistent with the expected behaviour of the dipolar form factor. The team’s analysis, utilising the CORELLI spectrometer at the Spallation Neutron Source, uncovered a Q-dependence of the diffuse scattering intensity that contradicts predictions from the nearest neighbour XYZ model previously proposed for Ce₂X₂O₇ systems. Results demonstrate that the diffuse scattering pattern closely resembles that observed in classical dipolar spin ice, providing compelling evidence for the importance of further neighbour interactions beyond the nearest neighbour model. Specifically, the study highlights a strong concentration of scattering along the FCC lattice Brillouin zone boundaries, even extending into the second and third Brillouin zones around (222).

The large reciprocal space coverage achieved with CORELLI, probing all symmetry equivalent directions, significantly improved the signal-to-noise ratio and enabled detailed analysis of magnetic correlations at large |Q| values up to approximately 5 Å⁻¹. Tests prove that the powder-averaged diffuse scattering spectrum exhibits a strong peak at |Q| ≈ 0.6 Å⁻¹ and a temperature-independent flat background at |Q| ≳ 3 Å⁻¹, aligning perfectly with previous neutron powder diffraction results on hydrothermally grown Ce₂Sn₂O₇ powder. Crucially, the team observed no evidence of octupolar scattering, a phenomenon previously reported in samples grown using solid-state methods, suggesting the high-quality crystals are free from this confounding effect. This breakthrough delivers a revised understanding of the low-energy physics of Ce-pyrochlores and calls for a refinement of current theoretical frameworks to incorporate the effects of further neighbour interactions.

CeSnO exhibits classical spin ice behaviour

Scientists have successfully grown high-quality single crystals of cerium tin oxide (CeSnO), a material belonging to the pyrochlore family and considered a promising candidate for exhibiting quantum spin ice behaviour. Detailed neutron scattering experiments on these crystals revealed highly structured scattering patterns along the Brillouin zone boundaries, a significant departure from previously observed broad scattering in related cerium pyrochlores like CeHfO and CeZrO. The observed scattering closely resembles that of classical dipolar spin ice, suggesting that interactions beyond nearest-neighbour exchanges play a crucial role in determining the material’s magnetic properties. These findings challenge the prevailing theoretical framework used to describe cerium pyrochlores, which typically relies on a nearest-neighbour XYZ model, a model that failed to accurately predict the observed scattering.

The research demonstrates the importance of considering further-neighbour interactions and/or dipolar interactions to fully understand the low-energy physics of these materials, potentially necessitating a revision of the current exchange Hamiltonian. While acknowledging limitations in modelling diffuse scattering due to potential structural defects in other cerium pyrochlores, the authors highlight the superior quality of the CeSnO crystals as a key factor in obtaining these clear results. Future work should focus on developing theoretical models that incorporate these longer-range interactions to better predict and explain the behaviour of cerium pyrochlores, and potentially unlock their quantum spin ice potential.