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We carried out AC magnetic susceptibility measurements and muon spinrelaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK.Below ~1 K the muon relaxation is typical of a magnetic insulator with aspin-liquid type ground state, i.e. without broken symmetries or frozenmoments. However, the AC susceptibility revealed a dilute-spin-glass liketransition below ~ 1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons intriply degenerate t2g orbitals are in this material arranged in a geometricallyfrustrated fcc lattice. Bulk magnetic susceptibility data has previously beeninterpreted in terms of a freezing to a heterogeneous state with non-magneticsites where 4d^1 electrons have paired in spin-singlets dimers, and residualunpaired Mo5+ 4d1 electrons. Based on the magnetic heat capacity data it hasbeen suggested that this heterogeneity is the result of kinetic constraintsintrinsic to the physics of the pure system (possibly due to topologicaloverprotection), leading to a self-induced glass of valence bonds betweenneighbouring 4d1 electrons. The muSR relaxation unambiguously points to astatic heterogeneous state with a static arrangement of unpaired electronsisolated by spin-singlet (valence bond) dimers between the majority of Mo5+ 4delectrons. The AC susceptibility data indicate that the residual magneticmoments freeze into a dilute-spin-glass-like state. This is in apparentcontradiction with the muon-spin decoupling at 50 mK in fields up to 200 mT,which indicates that, remarkably, the time scale of the field fluctuations fromthe residual moments is ~ 5 ns. Comparable behaviour has been observed in othergeometrically frustrated magnets with spin-liquid-like behaviour and theimplications of our observations on Ba2YMoO6 are discussed in this context.

Low-temperature spin dynamics of a valence bond glass in Ba2YMoO6