Abstract
We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK. Below ∼1 K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the ac susceptibility revealed a dilute-spin-glass-like transition below ∼1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d1 electrons have paired in spin-singlets dimers, and residual unpaired Mo5+ 4d1 electron spins. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics of the pure system (possibly due to topological overprotection) leading to a self-induced glass of valence bonds between neighbouring 4d1 electrons. The muon spin relaxation (μSR) unambiguously points to a heterogeneous state with a static arrangement of unpaired electrons in a background of (valence bond) dimers between the majority of Mo5+ 4d electrons. The ac susceptibility data indicate that the residual magnetic moments freeze into a dilute-spin-glass-like state. This is in apparent contradiction 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 from the residual moments is ∼5 ns. Comparable behaviour has been observed in other geometrically frustrated magnets with spin-liquid-like behaviour and the implications of our observations on Ba2YMoO6 are discussed in this context.
Highlights
Geometrical frustration of the exchange interactions in antiferromagnetic (Mott) insulators with magnetic topologies based on triangles and tetrahedra can fully suppress the spontaneous symmetry breaking to the common long-range ordered antiferromagnetic (Neel) state [1, 2]
This has been termed “topological overprotection” [12, 15]. Within this context we studied the cubic fcc lattice antiferromagnet Ba2YMoO6, which has previously been suggested [22, 23] to freeze into a valence bond glass (VBG) state [24]
An intriguing question that remains is whether the thermodynamic state would be a topologically ordered state, in which case the kinetic constraints stabilising the VBG are due to topological overprotection, or an orbitally ordered state leading to a structural distortion from cubic to tetragonal symmetry at low temperature
Summary
Geometrical frustration of the exchange interactions in antiferromagnetic (Mott) insulators with magnetic topologies based on triangles and tetrahedra can fully suppress the spontaneous symmetry breaking to the common long-range ordered antiferromagnetic (Neel) state [1, 2]. A number of materials with a spin-liquid-like absence of frozen moments have been identified [5, 6, 7, 8, 9] These are without exception gapless, often still display some weak hysteretic behaviour below 2 K, and have only short-ranged dynamic magnetic correlations [7, 10, 8] which is contrary to theoretical expectations of gapless spin liquids. In Mott insulators this concept can be turned on its head; here the physics is local [21] and the only route to access any topologically-ordered states in the first place, is via an infinite number of local operations This has been termed “topological overprotection” [12, 15]. The Hamiltonian of this cubic perovskite with unquenched orbital degrees of freedom is significantly more complex than that of the Heisenberg Hamiltonian, as was recently pointed out [25]
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