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Instruments / Technical Area	Publication Details	Published
	Role of defects in determining the magnetic ground state of ytterbium titanate D. F. Bowman , E. Cemal , T. Lehner , A. R. Wildes , L. Mangin-Thro , G. J. Nilsen , M. J. Gutmann , D. J. Voneshen , D. Prabhakaran , A. T. Boothroyd , D. G. Porter , C. Castelnovo , K. Refson , J. P. Goff Nature Communications 10 DOI: 10.1038/s41467-019-08598-z Open Access Show Abstract ▼ Abstract: Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.	Feb 2019
	Diffusion mechanism in the sodium-ion battery material sodium cobaltate T. J. Willis , D. G. Porter , D. J. Voneshen , S. Uthayakumar , F. Demmel , M. J. Gutmann , M. Roger , K. Refson , J. P. Goff Scientific Reports 8 DOI: 10.1038/s41598-018-21354-5 Open Access Show Abstract ▼ Abstract: High performance batteries based on the movement of Li ions in Li x CoO2 have made possible a revolution in mobile electronic technology, from laptops to mobile phones. However, the scarcity of Li and the demand for energy storage for renewables has led to intense interest in Na-ion batteries, including structurally-related Na x CoO2. Here we have determined the diffusion mechanism for Na0.8CoO2 using diffuse x-ray scattering, quasi-elastic neutron scattering and ab-initio molecular dynamics simulations, and we find that the sodium ordering provides diffusion pathways and governs the diffusion rate. Above T ~ 290 K the so-called partially disordered stripe superstructure provides channels for quasi-1D diffusion, and melting of the sodium ordering leads to 2D superionic diffusion above T ~ 370 K. We obtain quantitative agreement between our microscopic study of the hopping mechanism and bulk self-diffusion measurements. Our approach can be applied widely to other Na- or Li-ion battery materials.	Feb 2018
I16-Materials and Magnetism	Two-dimensional Cs-vacancy superstructure in iron-based superconductor Daniel Porter , Eron Cemal , David Voneshen , Keith Refson , Matthias Gutmann , Alessandro Bombardi , Andrew Boothroyd , A. Krzton-Maziopa , E. Pomjakushina , K. Conder , Jon Goff Physical Review B 91 (14) DOI: 10.1103/PhysRevB.91.144114 Diamond Proposal Number(s): [8686, 10377] Show Abstract ▼ Abstract: Single crystal neutron diffraction is combined with synchrotron x-ray scattering to identify the different superlattice phases present in Cs 0.8 Fe 1.6 Se 2 . A combination of single crystal refinements and first principles modeling are used to provide structural solutions for the √ 5 × √ 5 and √ 2 × √ 2 superlattice phases. The √ 5 × √ 5 superlattice structure is predominantly composed of ordered Fe vacancies and Fe distortions, whereas the √ 2 × √ 2 superlattice is composed of ordered Cs vacancies. The Cs vacancies only order within the plane, causing Bragg rods in reciprocal space. By mapping x-ray diffraction measurements with narrow spatial resolution over the surface of the sample, the structural domain pattern was determined, consistent with the notion of a majority antiferromagnetic √ 5 × √ 5 phase and a superconducting √ 2 × √ 2 phase.	Apr 2015
I10-Beamline for Advanced Dichroism - scattering	Crystal growth and neutron diffraction studies of LixCoO2 bulk single crystals S. Uthayakumar , M. Pandiyan , D. G. Porter , M. J. Gutmann , R. Fan , J. P. Goff Journal Of Crystal Growth 401, 169 - 172 DOI: 10.1016/j.jcrysgro.2013.11.043 Show Abstract ▼ Abstract: The first successful growth of neutron-size single-crystal LixCoO2 by the optical floating-zone technique is reported. Structural properties have been studied using the time-of-flight neutron Laue diffraction technique. Our experiment is the first report of the LixCoO2 single-crystal neutron study. The neutron diffraction profile yields sharp, strong Bragg reflections, indicating a single grain of high crystalline quality. The structural refinement from the single-crystal neutron diffraction data indicated a trigonal structure of space group R (3) over barm, and a Li concentration x=0.87. No superlattice reflections were detected. The surface morphology analysed by scanning electron microscopy revealed the absence of cracks. The magnetic susceptibility was measured in a field of 1 T with H parallel to c and H perpendicular to c, and an antiferromagnetic transition was observed at similar to 10 K, with no magnetic impurities. (C) 2013 Published by Elsevier B.V.	Sep 2014
	Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate D. J, Voneshen , K. Refson , E. Borissenko , M. Krisch , A. Bosak , A. Piovano , E. Cemal , M. Enderle , M. J. Gutmann , M. Hoesch , M. Roger , L. Gannon , A. T. Boothroyd , S. Uthayakumar , D. G. Porter , J. P. Goff Nature Materials 12, 1028–1032 DOI: 10.1038/nmat3739 Show Abstract ▼ Abstract: The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures1. Rattling of ions in cages results in low thermal conductivity2, 3, 4, 5, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells6. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na0.8CoO2, which has a large-period superstructure7. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.	Aug 2013
	The antiferromagnetic structures of IrMn3 and their influence on exchange-bias A. Kohn , A. Kovács , R. Fan , G. J. Mcintyre , R. C. C. Ward , J. P. Goff Scientific Reports 3 DOI: 10.1038/srep02412 PMID: 23934541 Open Access Show Abstract ▼ Abstract: Information storage Magnetic properties and materials Characterization and analytical techniques Transmission electron microscopy	Aug 2013
I16-Materials and Magnetism	The influence of interfacial roughness on the coherence of structure and magnetic coupling across barriers in Fe/MgO multilayers R. Fan , S. J. Lee , J. P. Goff , R. C. C. Ward , S. G. Wang , A. Kohn , C. Wang , A. R. Wildes , S. P. Collins Journal Of Physics: Condensed Matter 22 (2), 226004 DOI: 10.1088/0953-8984/22/22/226004 Show Abstract ▼ Abstract: Single-crystal Fe/MgO multilayers are model systems in which to study magnetic tunnel junctions. We find that the interfacial roughness leads to the loss of coherence of the crystal structure. For thick MgO layers ferromagnetic (FM) ordering is found using polarized neutron reflectivity (PNR). For thin MgO layers magnetization measurements reveal the presence of antiferromagnetic (AF) interactions, but no long-range AF order is found using PNR. After cycling in a hysteresis loop, FM correlations are found at the coercive point, and this will limit the maximum tunnelling magnetoresistance.	May 2010

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