2 Jahn-Teller distortion [24, 27, 28], while the exchange interaction is of the order of 100 K [24]. The other com- pounds were found to be paramagnetic and with cubic symmetry at all temperatures. Ba$_{2}$YMoO$_{6}$ is the sim- plest of these compounds because the Y 3+ ion does not carry a magnetic moment. The magnetic exchange is mainly via the 90 ◦ B’-O-O-B’ π − π bonds [21, 24], giv- ing rise to 12 near-neighbour AF J$_{1}$ bonds for each spin, across the edges of the tetrahedra (Fig. 1). Polycrystalline Ba$_{2}$YMoO$_{6}$ was prepared by the solid state reaction of stoichiometric oxides of Y$_{2}$O$_{3}$, MoO$_{3}$ and BaCO$_{3}$ powders of at least 99.99% purity. These were ground, die-pressed into a pellet and heated under flowing 5% H$_{2}$/N$_{2}$. The final synthesis temperature was 1200-1250 $^{◦}$C with three intermediate regrinding steps to ensure phase homogeneity. It was found that a first heat- ing step of ∼ 2 hr at 900 $^{◦}$C in air and thorough homoge- nization helps to prevent the formation of BaMoO$_{4}$ and Y$_{2}$O$_{3}$ impurities. Phase purity was confirmed by labo- ratory X-ray powder diffraction. In a related paper [29] neutron powder diffraction results are discussed, which show that the Y/Mo site disorder is less than 1%. The diamagnetic analog, Ba$_{2}$YNbO$_{6}$, was prepared at 1200 $^{◦}$C in air from YNbO$_{4}$ and BaCO$_{3}$. The sample magnetisa- tion was measured on a Quantum Design magnetic prop- erty measurement system (MPMS) in fields up to 5 T. The heat capacity was measured on a Quantum Design physical property measurement system (PPMS), using 7 . 0 mg of a sintered pellet. The µ SR experiment was carried out at MUSR at ISIS, UK. is shown in Fig. 2. A Curie-Weiss fit to the high temper- ature susceptibility yields a Weiss temperature of -160 K and a Curie constant of 0.25 emu mol $_{−}$1 K $^{$_{−}$1}$, small com- pared to the 0.38 emu mol $_{−}$1 K $_{−}$1 expected for s = 1 / 2. This difference is attributed to strong quantum fluctu- ations common in low-spin antiferromagnets and previ- ously observed in double perovskites [26]. Below 25 K a second linear regime is observed in χ $^{$_{−}$1}$, corresponding (for a 1 T field) to a ∼ 10% fraction of all the s = 1 / 2 moments (or ∼ 5% if they have the full j = 3 / 2 where g$_{J}$ = 4 / 3) and a Weiss temperature of − 2 . 3 K indicat- ing weak AF exchange. This fraction is too large to be ascribed directly to either structural disorder or an im- purity phase in the sample. Furthermore, fits to M ( H ) measured at 2.3 and 5 K (inset of Fig. 2) with Brillouin functions lead to estimates of respectively 2 and 7% of all spins, compared to 10% for fits to the M ( T ) curve. This suggests that the apparently quasi-free spins are an emergent property of the (disorder free) system. The AC susceptibility measured with a field amplitude of 5 Oe and zero DC offset field is shown in Fig. 3. The dispersive part of the AC susceptibility ( χ $^{′}$) is almost frequency independent and is comparable to the diverg- ing low temperature DC susceptibility. The dissipative part ( χ $^{′′}$) shows a frequency dependent maximum be- tween 26 and 70 K. Remarkably, the maximum gradually gets sharper as the frequency increases, instead of weaker as expected for a spin-glass transition. The agreement between the DC susceptibility and χ ′ below 20 K is a strong indication that the Curie-term can be ascribed to the weakly coupled spins. The heat capacity associated with the single Mo 5+ 4 d electron in Ba$_{2}$YMoO$_{6}$ (as shown in Fig. 4) was ob- tained from comparison with the heat capacity of the dia- magnetic analogue Ba$_{2}$YNbO$_{6}$. The heat capacity from phonons of Ba$_{2}$YMoO$_{6}$ is expected to be lower than for Ba$_{2}$YNbO$_{6}$ by a factor 0.991 due to the mass difference The DC magnetic susceptibility measured in a 1 T field FIG. 2: (color online) The DC magnetic susceptibility χ ( x, left axis) and χ $_{−}$1 (x, right axis) of Ba 2 YMoO 6 measured in 1 T. Curie-Weiss fits in the two linear regimes in χ $_{−}$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 5K but only for 2% at 2.3 K. FIG. 3: (Color online) 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. 0 50 100 150 200 250 300 T (K) 3 6 9 12  10 3  (emu/mol) Curie-Weiss fits "bulk" magnetism (T) in 1 T 0 2 4 6 8 H (T) 0 1 2 3 M 10 -2 (emu/mol) M(H), 5 K M(H), 2.3 K fit to 5 K data fit to 2.3 K data 0 0.4 0.8 1.2 1.6 -1  10 -3  (mol/emu) 0 50 100 150 T (K) -0.4 -0.2 0 0.2 0.4 0.6 0.8 ’’ 10 3 (emu/mol) 1 kHz 3 5 kHz 9 kHz 1 2 3 4 5 6 ’ 10 3  (emu/mol) dc