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

TL;DR

This study used neutron diffraction to determine the all-in/all-out spin structure of Nd$_2$Ir$_2$O$_7$, revealing magnetic moments consistent with strong spin-orbit coupling and a specific Nd-Ir alignment.

Contribution

First direct determination of the spin structure in Nd$_2$Ir$_2$O$_7$ using neutron diffraction, confirming the all-in/all-out magnetic order and detailed magnetic moment sizes.

Findings

Nd and Ir sublattices exhibit all-in/all-out magnetic order.

Ir moments align with the $J_{eff}=1/2$ state expectations.

Nd moments are smaller than expected for the ground state doublet.

Abstract

We report on the spin structure of the pyrochlore iridate Nd$_2$Ir$_2$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(1) $\mu_\mathrm{B}$/Ir$^{4+}$ and 1.27(1) $\mu_\mathrm{B}$/Nd$^{3+}$. The Nd$^{3+}$ moment size at 1.8 K is smaller than that expected for the Nd$^{3+}$ ground state doublet. On the other hand, the size of the ordered moments of the 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 Nd$^{3+}$ moments with the net moment of its six nearest neighboring Ir$^{4+}$ ions.