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Muon spin rotation and relaxation experiments in the pyrochlore iridate Eu 2 Ir 2 O 7 yield a welldefined muon spin precession frequency below the metal-insulator/antiferromagnetic transition temperature T M = 120 K, indicative of long-range commensurate magnetic order and thus ruling out quantum spin liquid and spin-glass-like ground states. The dynamic muon spin relaxation rate is temperature-independent between 2 K and ∼T M and yields an anomalously long Ir 4+ spin correlation time, suggesting a singular density of low-lying spin excitations. Similar behavior is found in other pyrochlores and geometrically frustrated systems, but also in the unfrustrated iridate BaIrO 3 . Eu 2 Ir 2 O 7 may be only weakly frustrated; if so, the singularity might be associated with the small-gap insulating state rather than frustration.

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Muon relaxation experiments reveal a slowly fluctuating magnetic field in the pseudogap phase of a cuprate superconductor.