Effect of site disorder on the ground state of a frustrated spin dimer quantum magnet

Ba$_{3}$Mn$_{2}$O$_{8}$ is a geometrically frustrated spin dimer quantum magnet. Pairs of Mn $^{5+}$ ($S=1$) ions are strongly coupled via antiferromagnetic exchange to yield a singlet ground state, with excited triplet and quintuplet states. Isovalent substitution of V$^{5+}$ ($S=0$) for Mn “breaks” dimers, resulting in unpaired $S = 1$ spins, the ground state of which is investigated here for compositions spanning the range $0 \leq x\leq 1$ of Ba$_{3}$(Mn$_{1-x}$V$_{x}$)$_{2}$O$_{8}$. From a theoretical perspective, for dimers occupying an unfrustrated bipartite lattice, such site disorder is anticipated to yield long range magnetism for unpaired Mn spins both in the dilute limit where $x$ is small, a phenomena known as order-by-disorder, and in the proximity of $x=1/2$ where the system is maximally disordered and close to a percolation threshold. In this frustrated system, however, our experiments find evidence of spin freezing for six compositions $0.05 \leq x \leq 0.85$. In this regime, we find entropy removed at an energy scale independent of the freezing temperature. We discuss the possibility of a spin-glass to random singlet transition for critical compositions in the two dilute limits $x\to 0$ and $x\to 1$.