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

Abstract

We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba 2 YMoO 6 , 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 Mo 5+ 4d 1 electrons in triply degenerate t 2g 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 4d 1 electrons have paired in spin-singlets dimers, and residual unpaired Mo 5+ 4d 1 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 6

Spin-orbital-lattice entangled states in cubic d1 double perovskites

“…Contrary to the Os compounds, in Ba 2 YMoO 6 no magnetic order develops down to 50 mK [11]. Despite the stronger exchange interaction [4,7,8,11,13] than in Ba 2 AOsO 6 , the absence of the ordering hinders the large Zeeman splitting of vibronic levels, and hence, the dynamical JT effect develops as supported by neutron diffraction data [8] and sustains the magnetic entropy of k B ln 4 observed by muon spin resonance [7].…”

“…On the other hand, in Ba 2 YMoO 6 , the pseudo JT coupling plays a crucial role to enhance M eff [ Fig. 3 [11,13], and it rapidly grows with T . Taking into account the covalency effect on both λ SO and l (gray triangles), M eff at T ≈ 300 K reaches the experimental values (1.3-1.5µ B [4,7,11,13]).…”

Valence Bonds in Random Quantum Magnets: Theory and Application to YbMgGaO4

“…[120] Defect spin-1/2 moments are observed at finite density within the dimerization pattern, and these defects appear to freeze into a dilute spin glass at ultralow temperatures T g ∼ 0.01θ CW with θ CW =160 K the Curie-Weiss temperature. [121] A recent theory [122] suggests that this dimer-singlet phase can arise from an unusually robust extensive degeneracy due to the orbital dimers; these properties of the orbital moments, together with a lack of observed bond randomness but apparent presence of some magnetic site dilution disorder [120,123], suggest that the physics in Ba 2 YMoO 6 may involve additional ingredients beyond those considered here.…”

Spin-Orbit Dimers and Noncollinear Phases in d1 Cubic Double Perovskites

“…Its extensive degeneracy explains the observed glassy behavior and suggest the presence of a residual entropy, that cannot be excluded based on heat capacity data [22]. Magnetic susceptibility and electronic heat capacity [22,23] suggest the presence of pseudo-gapped, rather than hard-gapped, low-energy excitations, consistent with the dimer-singlet phase. Neu- tron scattering experiments on powder samples [35] revealed excitations that are in line with the spectrum of weakly coupled spin-orbit dimers.…”