Weak magnetic transitions in pyrochlore Bi$_2$Ir$_2$O$_7$

The pyrochlore iridate Bi$_2$Ir$_2$O$_7$ is analogous to the rare earth pyrochlores $R$Ir$_2$O$_7$ ($R=$ Y and Pr-Lu) but has no rare earth moments or $f$ electrons to interact with the Ir subsystem. This makes it an ideal system in which to study the Ir magnetism in isolation. Bulk measurements showed that it is metallic down to 2K and no indication of magnetic ordering was found down to 50mK. The magnetic field dependence of the low-temperature specific heat shows large changes in both the linear and cubic contributions and the large Wilson ratio of 53.5 suggests proximity to a quantum critical point [1]. Our muon spin relaxation measurements find a bulk magnetic transition at $1.84(3)$K and the form of the data suggests that the low-temperature state represents ordering of exceptionally small magnetic moments with persistent weak dynamics. The relaxation rate increases further below $0.23(4)$K, coincident with a growth in the specific heat, suggesting another magnetic transition. The magnetic field experienced by muons is $\sim 0.7$T at low-temperature, around two orders of magnitude smaller than that in other pyrochlore iridates, corresponding to moments $\sim 0.01~\mu_{\rm B}$/Ir.\\[4pt] [1] T. F. Qi et al., J. Phys.: Condens. Matter 24, 345601 (2012).