Spin Freezing in The Frustrated Disordered Quantum Magnet Ba${}_3$(Mn${}_{1-x}$V${}_x$)${}_2$O${}_8$

Abstract

Ba${}_3$Mn${}_2$O${}_8$ 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${}_{1-x}$V${}_x$)${}_2$O${}_8$, 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${}_{1-x}$V${}_x$)${}_2$O${}_8$ 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 $0.18\le x\le 0.84$. 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.

(9 pages, 5 figures)