Long-Range Magnetic Ordering in Pyrochlore Iridate Eu$_2$Ir$_2$O$_7$

In the pyrochlore iridate~Eu$_2$Ir$_2$O$_7$ [1,\,2] Eu$^{3+}$ is nonmagnetic ($L = S$, $J = 0$) and $S(\mbox{Ir$^{4+}$}) = 1/2$~[1], so that it is a rare example of a frustrated $S=1/2$ pyrochlore. Spin-glass-like behavior at the metal-insulator transition (MIT) and no magnetic ordering down to 0.3~K have been reported for this compound~[2,\,3]. We discuss $\mu$SR measurements on Eu$_2$Ir$_2$O$_7$ polycrystalline samples that yield strong evidence for long-range magnetic ordering. We observe well-defined muon spin precession frequencies below $T_M \approx 120$~K, consistent with the MIT temperature~[2] but indicating long-range ordering instead of a spin-glass like transition. Significant dynamic muon spin relaxation persists to low temperatures, as is often the case in frustrated antiferromagnets. \par \noindent Work supported by NSF (U.S.), Grants~0801407 (UCR) and 0604105 (CSULA), and MEXT (Japan), Grants-in-Aid Nos.~17071003 and 19052003. \frenchspacing \par \noindent [1] B. J. Kim et al., Phys. Rev.Lett. 101, 076402 (2008). \par \noindent [2] N. Taira et al., J. Phys.: Condens. Matter 13, 5527 (2001). \par \noindent [3] C. L. Chien and A. W. Sleight, Phys. Rev. B 18, 2031 (1978).