Spin Freezing in The Frustrated Disordered Quantum Magnet Ba_3(Mn_1-xV_x)_2O_8

Ba_3Mn_2O_8 is a geometrically frustrated spin dimer compound. We investigate the effect of site disorder on the zero field phase diagram of this material by considering the solid solution Ba_3(Mn_1-xV_x)_2O_8, where nonmagnetic V^5+ ions partially substitute magnetic Mn^5+ ions. This substitution results in unpaired S=1 moments for half-substituted dimers, which are ungapped and therefore susceptible to types of magnetic order not present in the parent compound. AC susceptibility measurements of compositions between x=0.046 and x=0.84 show a sharp frequency- and composition-dependent kink at temperatures below 210mK, suggesting that unpaired spins form a spin glass. The case for a glassy state is made clearer by the absence of any sharp features in the specific heat. However, Ba_3(Mn_1-xV_x)_2O_8 is not a paradigmatic spin glass. Whereas both the freezing temperature and the Weiss temperature (determined from susceptibility above 1K) vary strongly as a function of composition, the heat capacity per unpaired spin is found to be insensitive (above the glass transition) to the density of unpaired spins for the broad regime 0.18≤ x ≤ 0.84. This surprising result is consistent with a scenario in which nearest-neighbor unpaired spins form local, possibly fluctuating, spin-singlets prior to the eventual spin freezing. The spin glass state is only found for temperatures below the energy scale of single-ion anisotropy, suggestive this plays a significant role in determining the eventual ground state. Possible ground states in the "dilute" limit (x < 0.04 and x > 0.9) are also discussed.