Evolution of structure and magnetism across the metal-insulator transition in the pyrochlore iridate ${\left({\mathrm{Nd}}_{1-x}{\mathrm{Ca}}_x\right)}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$

We report on the evolution of the thermal metal-insulator transition in polycrystalline samples of ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ upon hole doping via substitution of ${\mathrm{Ca}}^{2+}$ for ${\mathrm{Nd}}^{3+}$. Ca substitution mediates a filling-controlled 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.

Figure 1

Synchrotron XRD patterns, all taken at 300 K. Calculated curves include the pyrochlore phase and the impurity phases Ir, ${\mathrm{IrO}}_2$, and ${\mathrm{Nd}}_2{\mathrm{O}}_3$. Lines above the residual curves index only pyrochlore peaks.

Figure 2

X-ray PDF refinement of the ${\mathrm{A}}_2{\mathrm{B}}_2{\mathrm{O}}_7$ local structure: (a) 300 K PDF data in circles, offset vertically, with fits in red solid lines. (b) Metal-metal distances between A and B sites. (c) A- and (d) B-site isotropic atomic displacement parameters $U_{\text{iso}}$. Dashed lines are guides to the eye.

Figure 3

Magnetotransport measurements. (a) Relative resistivity from 2 to 300 K, all at 0 T on zero field cooling. Arrows indicate the onset of $\frac{\delta \rho }{\delta T}>0$. Resistances are normalized due to variation in pellet densities; typical values are $\rho \left(300\mathrm{K}\right)\approx 10\mathrm{m}\mathrm{\Omega }\mathrm{cm}$. (b) Relative magnetoresistance at 2 K on zero-field cooling, swept from ${\mu }_{0}H=0\to +9\to -9\to +9$ T, with arrows to indicate field sweep direction. Note the hysteretic splitting, which is largest for the $x=0$ sample. (c) Hysteretic differences of the magnetoresistance in (b), for sweeps after the initial $0\to +9$ T sweeps (virgin curves).

Figure 4

Temperature-concentration phase diagram based on resistivity measurements. Inset: Field-concentration phase diagram, indicating magnetism on the Nd sublattice. All lines are guides to the eye.

Figure 5

Magnetization measurements: (a) Field dependence of the isothermal DC magnetization, swept as in Fig. 3, with just $H>0$ shown. Arrows indicate field sweep direction. Inset: Irreversibility of the DC susceptibility (field cooling minus zero-field cooling) taken at 200 Oe. $T_{\mathrm{MIT}}$ values are indicated with arrows. (b) Solid lines are numerical derivatives of magnetization with respect to field, highlighting the hysteretic splitting. The initial $0\to +9$ T sweeps (virgin curves) are not shown. Dashed lines are the absolute differences between sweep directions, magnified for clarity.

Figure 6

X-ray spectroscopic measurements: Ir $L$-edge XAS is indicated with solid lines, and XMCD with dots. XAS lines are offset by 0.3 for clarity. XMCD was measured at 5 K and $\pm 5$ T and normalized to the corresponding XAS edge jump.

Figure 7

Irreversibility of the DC susceptibility for samples before (circles, with features at 120 K) and after vacuum annealing (triangles, denoted V).