Valence Bond Glass on an fcc Lattice in the Double Perovskite ${\mathrm{Ba}}_2{\mathrm{YMoO}}_6$

We report on the unconventional magnetism in the cubic $B$-site ordered double perovskite ${\mathrm{Ba}}_2{\mathrm{YMoO}}_6$, using ac and dc magnetic susceptibility, heat capacity and muon spin rotation. No magnetic order is observed down to 2 K while the Weiss temperature is $\sim -160\mathrm{K}$. This is ascribed to the geometric frustration in the lattice of edge-sharing tetrahedra with orbitally degenerate ${\mathrm{Mo}}^{5+}$ $s=1/2$ spins. Our experimental results point to a gradual freezing of the spins into a disordered pattern of spin singlets, quenching the orbital degeneracy while leaving the global cubic symmetry unaffected, and providing a rare example of a valence bond glass.

Figure 1

Four ${\mathrm{MoO}}_6$ octahedra (shaded grey) and Y ions (large spheres) in the cubic unit cell of ${\mathrm{Ba}}_2{\mathrm{YMoO}}_6$ (left). The ${\mathrm{Mo}}^{5+}$ ions form a lattice of edge-sharing tetrahedra (right). The cubic lattice constant is 8.389 Å.

Figure 2

The dc magnetic susceptibility $\chi$ ($\mathsf{x}$, left axis) and ${\chi }^{-1}$ ($\mathsf{x}$, right axis) of ${\mathrm{Ba}}_2{\mathrm{YMoO}}_6$ measured in 1 T. Curie-Weiss fits in the two linear regimes in ${\chi }^{-1}$ are indicated in grey (red) and the black line gives the difference between the total susceptibility and the low-temperature Curie term. The inset shows $M(H)$ at 2.3 ($+$) and 5 K (○) and fits to the data with Brillouin functions, accounting for 7% of the Mo $s=1/2$ spins at 5 K but only for 2% at 2.3 K.

Figure 3

The temperature dependence of the dispersive (left axis, open symbols) and dissipative (right axis, filled symbols) components of the ac susceptibility. The solid lines are guides to the eye.

Figure 4

The magnetic heat capacity obtained by subtracting the heat capacity of the diamagnetic analogue ${\mathrm{Ba}}_2{\mathrm{YNbO}}_2$ in zero field (black dots) and in 9 Tesla (open squares). The experimental error for the 9 T data is comparable to that indicated for the zero-field data (grey area). The inset shows the total entropy release as a function of temperature in zero field (black line) and in 9 T (red line).

Figure 5

The muon spin relaxation at 120, 5 and 1.4 K. The relaxation follows an exponential decay ($P(t)=\exp [-t/\lambda {]}^{\beta }$, solid lines) with $\beta =1$ for all but the 1.4 K data, where $\beta =0.7$. The temperature dependence of the relaxation rate $\lambda$ is shown in the inset.