Direct determination of the spin structure of ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ by means of neutron diffraction

We report on the spin structure of the pyrochlore iridate ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ that could be directly determined by means of powder neutron diffraction. Our magnetic structure refinement unravels a so-called all-in/all-out magnetic structure that appears in both the Nd and the Ir sublattice. The ordered magnetic moments at 1.8 K amount to $0.34\left(1\right){\mu }_{\mathrm{B}}/{\mathrm{Ir}}^{4+}$ and $1.27\left(1\right){\mu }_{\mathrm{B}}/{\mathrm{Nd}}^{3+}$. The ${\mathrm{Nd}}^{3+}$ moment size at 1.8 K is smaller than that expected for the ${\mathrm{Nd}}^{3+}$ ground state doublet. On the other hand, the size of the ordered moments of the ${\mathrm{Ir}}^{4+}$ ions at 1.8 K agrees very well with the value expected for a $J_{\mathrm{eff}}=1/2$ state based on the presence of strong spin-orbit coupling in this system. Finally, our measurements reveal a parallel alignment of the ${\mathrm{Nd}}^{3+}$ moments with the net moment of its six nearest neighboring ${\mathrm{Ir}}^{4+}$ ions.

Figure 1

PND pattern of ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ measured at room temperature at the D2B diffractometer together with a Rietveld fit $[a=10.3683(2) \AA ]$. Red dots: experimentally observed intensities; black curve: calculated intensities; blue curve: difference between experimentally observed and calculated intensities; green bars: Bragg peak positions. The asterisk indicates a small impurity peak from ${\mathrm{IrO}}_2$. The refinable parameter related to the oxygen position at site $48f$ ($x$, 0.125, 0.125) amounts to $x=0.3315\left(2\right)$. The final reliability factors are $R_B=3.87\%$ and ${\chi }^2=2.35$.

Figure 2

Temperature dependence of resistivity. The metal-insulator transition temperature is about 30 K. The inset shows the magnetic susceptibility measurement at $H=1000$ Oe. A small bifurcation between the field-cooled and zero-field-cooled measurements is also observed at about 30 K.

Figure 3

Rietveld refinement of the magnetic structure according to IR ${\mathrm{\Gamma }}_3^1$ for the difference pattern (a) between 1.8 and 40 K and (b) between 15 and 40 K. This data was measured at the D20 diffractometer using a neutron wavelength of $\lambda =2.41 \AA$. The red dots are experimental data; black and blue curves are the calculated pattern and the difference between experimental and calculated values. Green bars indicate the magnetic peak positions. The regions around strong Bragg peaks were excluded from the refinement due to the shift of lattice constants. The inset of (b) shows the AIAO magnetic structure of the Nd sublattice with six surrounding Ir ions. The moment size of Ir ions compared to that of Nd ions is enlarged for clearance. (c), (d) Magnetic pattern at 15 K refined by assuming the ordering only at (c) the Ir sublattice or only at (d) the Nd sublattice (according to IR ${\mathrm{\Gamma }}_3^1$).