Excellent GAUK project

The three-year project of Kristina Vlášková titled “Electon properties of R₂T₂O₇ pyrochlores with the strong spin-orbit coupling” (2018 – 2020; #558218) was evaluated as excellent by the Grant Agency of Charles University (GAUK). The number and quality of project outcomes – 7 publications in scientific journals with impact factor – are tightly connected with the results obtained within the MGML laboratories. MGML’s instruments were involved, among others thing, in the preparation of samples, their characterization and the measurements of their bulk properties. Selected compounds were additionally studied by means of synchrotron radiation and neutron scattering experiments at international large-scale facilities (Diamond, ISIS).

Exceptional technological ground of MGML enables to synthesize large quantity of excellent-quality samples and their proper characterization for advanced experiments, including microscopic experiments at large-scale facilities.

The project focused on pyrochlore oxides of chemical formula: A₂Ir₂O₇ with A = rare-earth element. A₂Ir₂O₇ iridates crystallize in ordered cubic structure, space group Fd-3m (227), where A and Ir cations are individually situated in the vertices of the corner-sharing tetrahedra within two interpenetrating sublattices, and oxygen anions form cubic and octagonal cages around the cations, respectively. Magnetic moments of A and Ir are hence subjected to the three-dimensional geometrical frustration resulting in a number of exotic states, including so-called spin-glass, spin-liquid, and spin-ice states. More generally, the physical properties of A₂Ir₂O₇ oxides origin from the competition and delicate balance between exchange and dipolar interactions, and single-ion spin-orbit coupling.

Phase diagram of A₂Ir₂O₇ pyrochlores: the dependence of magnetic ordering temperature of the Ir sublattice on the ionic radius of rare-earth A with the 8-coordination number. [1]

The inelastic neutron scattering E-Q maps of Er₂Ir₂O₇ measured with incident energy (a) 10 meV, (b) 30 meV, and (c) 180 meV at 5 K. The arrows mark the CEF excitations. [2]

The major outcomes of the project are: (i) establishing the preparation routes for high-quality A₂Ir₂O₇ polycrystalline samples and small-size single crystals; (ii) synthesis of heavy A (= Dy – Lu) iridates, Tm analogue for the first time; (iii) completion of the phase diagram: evolution of the Ir sublattice’s magnetic ordering with A in A₂Ir₂O₇; (iv) determination of crystal electric field (CEF) scheme of Er₂Ir₂O₇ and calculation of CEF for other iridates; and (v) mapping of the low-temperature magnetic properties of prepared compounds and their interpretation in the frame of CEF and frustration of magnetic moments on pyrochlore lattice. These results will be part of Kristina Vlášková PhD thesis, which is to be defended this year.