Joseph A. M. Paddison

Liquidlike Correlations in Single-Crystalline Y2Mo2O7: An Unconventional Spin Glass

The spin glass behavior of Y2Mo2O7 has puzzled physicists for nearly three decades. Free of bulk disorder within the resolution of powder diffraction methods, it is thought that this material is a rare realization of a spin glass resulting from weak disorder such as bond disorder or local lattice distortions. Here, we report on the single crystal growth of Y2Mo2O7. Using neutron scattering, we present unique isotropic magnetic diffuse scattering arising beneath the spin glass transition despite having a well-ordered structure at the bulk level. Despite our attempts to model the diffuse scattering using a computationally exhaustive search of a class of simple spin Hamiltonians, we were unable to replicate the experimentally observed energy-integrated (diffuse) neutron scattering. A T^2-temperature dependence in the heat capacity and density functional theory calculations hint at significant frozen degeneracy in both the spin and orbital degrees of freedom resulting from spin-orbital coupling (Kugel-Khomskii type) and random fluctuations in the Mo environment at the local level.

Searching beyond Gd for magnetocaloric frameworks: magnetic properties and interactions of the Ln(HCO2)3 series

This study probes the magnetic properties and interactions of the Ln(HCO2)3 (Ln = Tb3+–Er3+) frameworks. We show that the magnetocaloric effect of Tb(HCO2)3 is significantly higher above 4 K in moderate magnetic fields compared to the promising Gd(HCO2)3. While the peak performance of Tb(HCO2)3 is lower than Gd(HCO2)3, we also find that the Gd-rich members of the solid solution Gd1−xTbx(HCO2)3 blend the advantages of both end-members. Using neutron diffraction experiments, Tb(HCO2)3 is found to be antiferromagnetic below 1.7 K with ferromagnetic face-sharing chains and antiferromagnetic coupling between them. Analysis of magnetic diffuse scattering of the paramagnetic phase indicates that ferromagnetic coupling is retained, and it is likely that this plays a role in improving its magnetocaloric performance in low fields.

SPINVERT: a program for refinement of paramagnetic diffuse scattering data.

We present a program (spinvert; http://spinvert.chem.ox.ac.uk) for refinement of magnetic diffuse scattering data for frustrated magnets, spin liquids, spin glasses, and other magnetically disordered materials. The approach uses reverse Monte Carlo refinement to fit a large configuration of spins to experimental powder neutron diffraction data. Despite fitting to spherically averaged data, this approach allows the recovery of the three-dimensional magnetic diffuse scattering pattern and the spin-pair correlation function. We illustrate the use of the spinvert program with two case studies. First, we use simulated powder data for the canonical antiferromagnetic Heisenberg model on the kagome lattice to discuss the sensitivity of spinvert refinement to both pairwise and higher-order spin correlations. The effect of limited experimental data on the results is also considered. Second, we re-analyse published experimental data on the frustrated system Y0.5Ca0.5BaCo4O7. The results from spinvert refinement indicate similarities between Y0.5Ca0.5BaCo4O7 and its parent compound YBaCo4O7, which were overlooked in previous analyses using powder data.

Hidden order in spin-liquid Gd₃Ga₅O₁₂.

Elucidating order within disorder In some materials, the geometry of the crystal lattice gets in the way of magnetic ordering. Their spins, although magnetically interacting, remain seemingly disordered and form a so-called spin liquid. Paddison et al. propose a different model for the spin-liquid compound Gd3Ga5O12. Neutron diffraction measurements and numerical techniques revealed that even though individual spins in this material were disordered, they formed 10-spin loops that were correlated with one another. The nature of this “hidden order” was such that it escaped direct detection by conventional techniques Science, this issue p. 179 Reverse Monte Carlo refinements of neutron diffraction data are used to deduce a model of ordered 10-spin loops in Gd3Ga5O12. Frustrated magnetic materials are promising candidates for new states of matter because lattice geometry suppresses conventional magnetic dipole order, potentially allowing “hidden” order to emerge in its place. A model of a hidden-order state at the atomic scale is difficult to deduce because microscopic probes are not directly sensitive to hidden order. Here, we develop such a model of the spin-liquid state in the canonical frustrated magnet gadolinium gallium garnet (Gd3Ga5O12). We show that this state exhibits a long-range hidden order in which multipoles are formed from 10-spin loops. The order is a consequence of the interplay between antiferromagnetic spin correlations and local magnetic anisotropy, which allows it to be indirectly observed in neutron-scattering experiments.

Emergent order in the kagome Ising magnet Dy 3 Mg 2 Sb 3 O 14

The Ising model—in which degrees of freedom (spins) are binary valued (up/down)—is a cornerstone of statistical physics that shows rich behaviour when spins occupy a highly frustrated lattice such as kagome. Here we show that the layered Ising magnet Dy3Mg2Sb3O14 hosts an emergent order predicted theoretically for individual kagome layers of in-plane Ising spins. Neutron-scattering and bulk thermomagnetic measurements reveal a phase transition at ∼0.3 K from a disordered spin-ice-like regime to an emergent charge ordered state, in which emergent magnetic charge degrees of freedom exhibit three-dimensional order while spins remain partially disordered. Monte Carlo simulations show that an interplay of inter-layer interactions, spin canting and chemical disorder stabilizes this state. Our results establish Dy3Mg2Sb3O14 as a tuneable system to study interacting emergent charges arising from kagome Ising frustration.

Spin correlations in Ca3Co2O6: Polarized-neutron diffraction and Monte Carlo study

We present polarized-neutron diffraction measurements of the Ising-type spin-chain compound Ca3Co2O6 above and below the magnetic ordering temperature T-N. Below T-N, a clear evolution from a pure spin-density wave (SDW) structure to a mixture of SDW and commensurate antiferromagnet (CAFM) structures is observed on cooling. For a rapidly cooled sample, the majority phase at low temperature is the SDW, while if the cooling is performed sufficiently slowly, then the SDW and the CAFM structure coexist between 1.5 and 10 K. Above T-N, we use Monte Carlo methods to analyze the magnetic diffuse scattering data. We show that both intrachain and interchain correlations persist above T-N, but are essentially decoupled. Intrachain correlations resemble the one-dimensional ferromagnetic Ising model, while interchain correlations resemble the frustrated triangular-lattice antiferromagnet. Using previously published bulk property measurements and our neutron diffraction data, we obtain values of the ferromagnetic and antiferromagnetic exchange interactions and the single-ion anisotropy. (Less)

Spin order and dynamics in the diamond-lattice Heisenberg antiferromagnets CuRh 2 O 4 and CoRh 2 O 4

Antiferromagnetic insulators on the diamond lattice are candidate materials to host exotic magnetic phenomena ranging from spin-orbital entanglement to degenerate spiral ground-states and topological paramagnetism. Compared to other three-dimensional networks of magnetic ions, such as the geometrically frustrated pyrochlore lattice, the investigation of diamond-lattice magnetism in real materials is less mature. In this work, we characterize the magnetic properties of model A-site spinels CoRh2O4 (cobalt rhodite) and CuRh2O4 (copper rhodite) by means of thermo-magnetic and neutron scattering measurements and perform group theory analysis, Rietveld refinement, mean-field theory, and spin wave theory calculations to analyze the experimental results. Our investigation reveals that cubic CoRh2O4 is a canonical S=3/2 diamond-lattice Heisenberg antiferromagnet with a nearest neighbor exchange J = 0.63 meV and a Neel ordered ground-state below a temperature of 25 K. In tetragonally distorted CuRh2O4, competiting exchange interactions between up to third nearest-neighbor spins lead to the development of an incommensurate spin helix at 24 K with a magnetic propagation vector k = (0,0,0.79). Strong reduction of the ordered moment is observed for the S=1/2 spins in CuRh2O4 and captured by our 1/S corrections to the staggered magnetization. Our work identifies CoRh2O4 and CuRh2O4 as reference materials to guide future work searching for exotic quantum behavior in diamond-lattice antiferromagnets.

Complexity in supramolecular analogues of frustrated magnets at high pressure

Andrew Brian Cairns1, Matthew J. Cliffe2, Joseph A. M. Paddison3, Dominik Daisenberger4, Matthew G. Tucker5, François-Xavier Coudert6, Andrew L. Goodwin7, Mohamed Mezouar1 1ESRF The European Synchrotron, Grenoble, France, 2Department of Chemistry, University of Cambridge, Cambridge, United Kingdom, 3Department of Physics, University of Cambridge, Cambridge, United Kingdom, 4Diamond Light Source, Didcot, United Kingdom, 5Spallation Neutron Source, Oak Ridge, United States, 6CNRS & Chimie ParisTech, Paris, France, 7Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford, United Kingdom E-mail: andrew.cairns@esrf.fr