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Soutenance de thèse

Jeudi 29 septembre à 14h00,
Salle des séminaires, Bâtiment A

Oratrice : Emilie Lefrancois
"Synthesis and study of iridium oxide compounds for entangled spin-orbit physics"


This thesis focuses on the study of iridium oxides, in particular on the consequences of the strong spin-orbit coupling of the iridium. Two families of compounds have been investigated : Sr3NiM’O6, with mixed spin chains arranged on a triangular lattice, and R2Ir2O7 with interpene- trated pyrochlores networks of spins. Polycrystalline samples have been synthesized and in some instances single crystals were successfully grown. They were investigated macroscopically by magnetization measurements and probed microscopically by neutron and synchrotron X-ray scattering experiments.

Our measurements showed that in the spin chain compounds Sr3NiPtO6 and Sr3NiIrO6, the Ni2+ ions show a strong easy plane magnetocrystalline anisotropy, perpendicular to the chain axis. This stabilizes in Sr3NiPtO6 the so-called "large-D" non-magnetic phase. The planar anisotropy comes out in Sr3NiIrO6 at high temperature. The compound however orders at low temperature in a magnetic configuration with all the magnetic moments confined along the chain axis. We explain this change of anisotropy as due to the Ir4+ ions whose spin-orbit coupling produces a strong anisotropy of the intra-chain Ni-Ir magnetic interactions overwhelming the single-ion Ni2+anisotropy.

Concerning the pyrochlore iridates R2Ir2O7, magnetization measurements and neutron powder diffraction experiments are consistent with an "all-in/all-out" magnetic ordering of the Ir magnetic moments, revealed indirectly through the magnetic behavior of the rare-earth sublattice. This ordering is the only one consistent with a Weyl semi-metal phase predicted to arise from the spin-orbit coupling. The magnetic behavior of the rare-earth sublattice depends on the rare earth magnetocrystalline anisotropy. The ions with local uniaxial anisotropy are polarized by the Ir molecular field, whose direction coincides with the anisotropy axis. The ions with local planar anisotropy perpendicular to this direction show no iridium induced long-range magnetic ordering. At lower temperature, rare- earth interactions generate more complex magnetic behaviors.

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