Evolution of structure and magnetism across the metal-insulator transition in the pyrochlore iridate (Nd1−xCax)2Ir

Abstract

We report on the evolution of the thermal metal-insulator transition in polycrystalline samples of Nd2Ir2O7 upon hole-doping via substitution of Ca 2+ for Nd 3+ . Ca substitution mediates a fillingcontrolled Mott-like transition with minimal resolvable structural changes and without altering site symmetry. Local structure confirms that Ca substitution does not result in local chemical phase separation, and absorption spectroscopy establishes that Ir cations maintain a spin-orbit entangled electronic configuration. The metal-insulator transition coincides with antiferromagnetic ordering on the Ir sublattice for all measured samples, and both decrease in onset temperature with Ca content. Weak low-temperature upturns in susceptibility and resistivity for samples with high Ca content suggest that Nd sublattice antiferromagnetism continues to couple to carriers in the metallic regime.arXiv:1908.04874v1 [cond-mat.mtrl-sci]

Mapping the structural, magnetic and electronic behavior of (Eu1− x Ca x )2Ir2O7 across a metal–insulator transition

“…That the measured Ir moment is consistent with other pyrochlore iridates with non-magnetic A-sites (A = Y, Lu) points to a common basis for the Ir XMCD signal. We also note that a parallel XMCD measurement on Nd2Ir2O7 yielded a very similar value [18]. The source of this signal remains an open question, and XMCD measurements of the remanence (i.e.…”

“…Calculations based on application of the sum rules reveal a small Ir moment of ≈ .005 µB/Ir. Details on this calculation and estimation of the <TZ> term can be found in Porter et al [18]. A previous measurement for Sr2IrO4, which also contains Ir 4+ in a cubic CEF environment, showed a L3 XMCD signal of 3%, with a corresponding net moment of ≈ .05 µB/Ir in the weakly ferromagnetic state [40].…”

“…The XAS nh/valence is calculated by multiplying the valence in the parent (4+) by the ratio of Idoped/Iparent , where I = IL2 + IL3. BR = branching ratio, <L•S> = expectation value of the spin-orbit operator, mtot = sum of the spin (ms) and orbital (ml) moments at H = 5 T, LZ/SZ = ml/ms description of the calculation can be found in Porter et al [18], following Clancy et al [34]). This increase is quantified by the branching ratio, BR = IL3/IL2, which shows a significant increase over the "statistical" value of BR = 2, though it decreases with Ca-doping.…”

“…However, a more Mott-like state emerges at lower temperatures, possibly due to the ordering of the Nd moments [14]. Detailed magneto-resistance studies on Nd2Ir2O7 samples further show a complex interplay of the Nd/Ir magnetic orders with the charge transport due to the presence of fd exchange coupling [12,18], furthering the motivation to study compounds with a nonmagnetic A-site ion.…”

Non-Fermi liquid regime in metallic pyrochlore iridates: Quantum Griffiths singularities

“…Similar to other strongly correlated systems, a direct route to investigating the evolution of the quantum phases in pyrochlore iridates is by chemical substitution. Several such studies have already been reported, for example, in Bi doped Eu 2 Ir 2 O 7 17 ; chemical doping of Ca 2+ for Nd 3+ in Nd 2 Ir 2 O 7 18 , etc. In PIO, the local moment comes from Pr 3+ ion and conduction electrons are mainly contributed by Ir 4+11,19 .…”