Distinct magnetic ground states of $R_2\mathrm{Zn}\mathrm{Ir}{\mathrm{O}}_6$ $(R=\mathrm{La},\mathrm{Nd})$ determined by neutron powder diffraction

Double-perovskite iridates A${}_2{\mathrm{ZnIrO}}_6$ (A=alkaline or lanthanide) show complex magnetic behaviors ranging from weak ferromagnetism to successive antiferromagnetic transitions. Here we report the static (dc) and dynamic (ac) magnetic susceptibility and neutron powder diffraction measurements for A=La and Nd compounds to elucidate the magnetic ground state. Below 10 K, the A=La compound is best described as canted iridium moments in an antiferromagnet arrangement with a propagation vector k = 0 and a net ferromagnetic component along the c axis. On the other hand, ${\mathrm{Nd}}_2{\mathrm{ZnIrO}}_6$ is described well as an antiferromagnet with a propagation vector k = ($\frac{1}{2}\frac{1}{2}0$) below $T_{\mathrm{N}}\sim$ 17 K. Scattering from both the Nd and Ir magnetic sublattices was required to describe the data, and both were found to lie almost completely within the ab plane. The dc susceptibility revealed a bifurcation between the zero-field-cooled and field-cooled curves below $\sim 13$ K in ${\mathrm{Nd}}_2{\mathrm{ZnIrO}}_6$. A glassy state was ruled out by ac susceptibility but detailed magnetic isotherms revealed the opening of the loop below 13 K. These results suggest a delicate balance exists between the Dzyaloshinskii-Moriya, crystal field schemes, and d-f interaction in this series of compounds.

Figure 1

Rietveld refined patterns from $R_2{\mathrm{ZnIrO}}_6$. These high-resolution data were collected at room temperature on the D2B diffractometer. The lattice parameters are a = 5.5906(2) Å, b = 5.6871(2) Å, c = 7.9312(2) Å, $\beta$ = 90.00(1)${}^{\circ }$ for (a) the R = La sample and a = 5.4697(2) Å, b = 5.7313(2) Å, c = 7.8058(2) Å, $\beta$ = 89.97(1)${}^{\circ }$ for (b) the R = Nd sample. A small peak originating from the cryofurnace has been excluded from the refinement in (b), and the asterisk marks the peak originating from the ${\mathrm{Nd}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ phase.

Figure 2

Temperature dependence of the magnetic susceptibility and isothermal magnetization measurements for (a1) and (a2) ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and (b1) and (b2) ${\mathrm{Nd}}_2{\mathrm{ZnIrO}}_6$. The susceptibility was measured with a field of 1000 Oe. The temperature dependence of the magnetic peak intensity, measured on D20, is also shown in (b1), where the 13 K transition is absent. The inset in (b1) shows the real component ${\chi }^{\prime }$ of the zero-field ac susceptibility with frequencies of 13, 113, 1333, 5330, and 9918 Hz. The top inset in (b2) is an enlargement of the low-field region measured at 2 K (blue), 8 K (cyan), 10 K (magenta), 12 K (wine), and 14 K (green). The bottom inset shows the temperature dependence of the coercive field $H_c$.

Figure 3

Rietveld refinement of the magnetic diffraction from ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ at 1.8 K. A paramagnetic data set, taken at 20 K, has been subtracted to remove the scattering from the crystal lattice. The calculated patterns are according to the IR ${\mathrm{\Gamma }}_3$ with (a) a perfectly ordered B-site model and (b) a B-site disordered model where the best model found 13(4)% site substitution between Zn and Ir ions. Red dots: experimental data; black curve: calculated data; vertical bars: magnetic peak position; blue curve: difference between the experimental and calculated data.

Figure 4

Magnetic structure of (a) and (b) ${\mathrm{La}}_2{\mathrm{ZnIrO}}_6$ and (c) and (d) ${\mathrm{Nd}}_2{\mathrm{ZnIrO}}_6$ viewed along different directions. The Ir moments have been enlarged for clarity. Note that the smaller Ir2 moments in (a) and (b) are visualized taking into account the partial occupation effect.

Figure 5

Rietveld magnetic refinement of the difference pattern at various temperatures for ${\mathrm{Nd}}_2{\mathrm{ZnIrO}}_6$ according to IR ${\mathrm{\Gamma }}_1+{\mathrm{\Gamma }}_3$. The left panels show the refinement results with perfectly ordered Nd and Ir sublattices, and the right panels highlight an expanded region of low peak intensities, comparing our best model and that with only the Nd sublattice ordering. Red dots: experimental data; black curve: calculated data; vertical bars: magnetic peak position; blue curve: difference between the experimental and calculated data. A small angular range around $2\mathrm{\Theta }={62}^{\circ }$ was excluded from the fits due to contamination from the atomic scattering that did not subtract well.