Abstract.We present a comparative study of the magnetic and electrical properties of polycrystalline and nanocrystalline Y 2 Ir 2 O 7 , the latter prepared using a new chemical route. We find that reduction in particle size leads to enhanced ferromagnetism and orders of magnitude enhancement of electrical conductivity in the nanocrystalline sample. Based on X-ray photoelectron spectroscopy and X-ray absorption near edge structure spectroscopy results, the phenomenon is attributed to the increased fraction of Ir 5+ and the resulting increase in double exchange interaction along with the weakening of spin-orbit coupling strength in the nanocrystalline sample compared to the bulk.

We report the structural, magnetic and electrical transport properties of Y2Ir2−xCrxO7 pyrochlore iridates. The chemical doping leads to order of magnitude enhancement of electrical conductivity. The introduction of Cr 3+ at Ir 4+ -site tends to distort the Ir − O6 octahedra and suppresses antiferromagnetic correlation. The X-ray photoemission spectroscopy measurements suggest the coexistence of Ir 4+ and Ir 5+ valence states in the Y2Ir2−xCrxO7 compounds. The concentration of Ir 5+ is enhanced with Cr doping, leading to weak ferromagnetism and enhanced electrical conductivity. A cluster-glass like transition is also observed at low temperature with Cr doping, possibly due to competing ferromagnetic and antiferromagnetic interaction. arXiv:1904.05541v1 [cond-mat.mtrl-sci]

We investigate the effect of partial replacement of extended 5d Ir 4+ sites by localized 3d Cr 3+ moments on the magnetocaloric properties of Y 2 Ir 2 O 7 (YIO) pyrochlore iridates. We find that Y 2 Ir 2−x Cr x O 7 (YICO) undergoes a cluster glass transition, possibly due to a Ruderman−Kittel−Kasuya−Yosida (RKKY)-like interaction between localized Cr 3+ moments occupying random sites in the pyrochlore network, mediated by 5d Ir conduction electrons. The coexistence of ferromagnetic and antiferromagnetic clusters gives rise to the conventional and inverse magnetocaloric effect (MCE). We observe significant enhancement of conventional as well as inverse MCE with substitution. Although the values of the conventional MCE and inverse MCE in substituted iridates are not large, the effect spans a giant working temperature window, thus leading to orders of magnitude enhancement of cooling power, the value being comparable to that of standard magnetocaloric materials.

We investigate the interplay of Kondo and RKKY coupling in presence of disorder by chemically substituting local moment Cr 3+ at the Ir sublattice in the metallic Pr2Ir2O7. We find evidence of non-Fermi liquid (NFL) behaviour in the transport and thermodynamic measurements at low temperature. Specifically, the magnetic susceptibility exhibits power law divergence at T = 0. The nonanalytic temperature and magnetic-field dependence of magnetic susceptibility and the associated scaling suggest existence of a two-fluid system consisting of Kondo-screened paramagnetic metal coexisting with magnetically ordered rare regions dominated by inter-impurity interaction, similar to quantum critical Griffiths phase.

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