Quantifying uncertainties in crystal electric field Hamiltonian fits to neutron data

We systematically examine uncertainties from fitting rare earth single-ion crystal electric field (CEF) Hamiltonians to inelastic neutron scattering data. Using pyrochlore and delafossite structures as test cases, we find that uncertainty in CEF parameters can be large despite visually excellent fits. These results show Yb$^{3+}$ compounds have particularly large $g$-tensor uncertainty because of the few available peaks. In such cases, additional constraints are necessary for meaningful fits.

Dirac magnons, nodal lines, and nodal plane in elemental gadolinium

We investigate the magnetic excitations of elemental gadolinium (Gd) using inelastic neutron scattering, showing that Gd is a Dirac magnon material with nodal lines at $K$ and nodal planes at half integer $ell$. We find an anisotropic intensity winding around the $K$-point Dirac magnon cone, which is interpreted to indicate Berry phase physics. Using linear spin wave theory calculations, we show the nodal lines have non-trivial Berry phases, and topological surface modes. Together, these results indicate a highly nontrivial topology, which is generic to hexagonal close packed ferromagnets. We discuss potential implications for other such systems.

Spin exchange Hamiltonian and topological degeneracies in elemental gadolinium

We present a comprehensive study of the magnetic exchange Hamiltonian of elemental Gadolinium. We use neutron scattering to measure the magnon spectrum over the entire Brillouin zone, and fit the excitations to a spin wave model to extract the first 30 nearest neighbor magnetic exchange interactions with rigorously defined uncertainty. We find these exchange interactions to follow RKKY behavior, oscillating from ferromagnetic to antiferromagnetic as a function of distance. Finally, we discuss the topological features and degeneracies in Gd, and HCP ferromagnets in general. We show theoretically how, with asymmetric exchange, topological properties could be tuned with a magnetic field.

The pyrochlore Ho2Ti2O7: Synthesis, crystal growth and stoichiometry

We have investigated the effect of synthesis and growth conditions on the magnetic, structural, and compositional properties of pyrochlore oxide holmium titanate and demonstrate a method for growing high quality stoichiometric single crystals. A series of polycrystalline samples with various contents of Ti (-0.08 leqslant x leqslant 0.08, and nominal compositions of Ho2Ti2+xO7) were synthesized at different temperatures, and characterized using powder X-ray diffraction. The results show that synthesizing powders at a higher temperature of 1500 {deg}C yield single phase compounds. Ti deficient powders showed an increase of lattice constant due to stuffing (Ho into Ti positions), while Ti rich powders showed a decrease in lattice constant due to anti-stuffing (Ti into Ho positions). A post annealing in O2 was found to be necessary to accomplish the anti-stuffing process. Use of the conventional floating zone (FZ) technique introduced Ti deficiency, stuffing, and oxygen vacancies in the grown crystal. Growth of high structural quality and stoichiometric single crystals of Ho2Ti2O7 by the traveling solvent floating zone (TSFZ) is reported. AC susceptibility measurements revealed that the stoichiometric crystal shows a higher ice freezing temperature, indicating that crystal quality and stoichiometry play a key role on low temperature spin ice properties of this compound.

Ni$_2$Mo$_3$O$_8$: zig-zag antiferromagnetic order an integer spin non-centrosymmetric honeycomb lattice

Theoretical studies have predicted the existence of topological magnons in honeycomb compounds with zig-zag antiferromagnetic (AFM) order. Here we report the discovery of zig-zag AFM order in the layered and non-centrosymmetric honeycomb nickelate Ni$_2$Mo$_3$O$_8$ through a combination of magnetization, specific heat, x-ray and neutron diffraction and electron paramagnetic resonance measurements. It is the first example of such order in an integer-spin non-centrosymmetric structure ($P$$_6$3$mc$). Further, each of the two distinct sites of the bipartite honeycomb lattice has a unique crystal field environment, octahedral and tetrahedral Ni$^{2+}$ respectively, enabling independent substitution on each sublattice. Replacement of Ni by Mg on the octahedral site suppresses the long range magnetic order and results in a weakly ferromagnetic state. Conversely, substitution of Fe for Ni enhances the AFM ordering temperature. Thus Ni$_2$Mo$_3$O$_8$ provides a platform on which to explore the rich physics of $S = 1$ on the honeycomb in the presence of competing magnetic interactions with a non-centrosymmetric, formally piezeo-polar, crystal structure.