Ground States of a Disordered Frustrated Quantum Dimer Magnet*

We present results of thermodynamic measurements of the site-diluted spin-dimer magnet Ba$_3($Mn$_{1-x}$V$_x)_2$O$_8$, including magnetization, torque magnetometry, and AC susceptibility. The parent compound Ba$_3$Mn$_2$O$_8$ is a frustrated $S=1$ quantum dimer-magnet with a singlet ground state, and triplet and quintuplet excitations. A magnetic field can be used to tune the energy spectrum of this system, yielding successive triplet and quintuplet condensates at low temperatures. Site substitution with $S=0$ V breaks Mn-dimers, introducing site disorder into the high-field ordered states. This substitution also introduces unpaired $S=1$ Mn ions, and it has been an open question whether such spins order at low temperatures. Here, we present evidence of the spin-freezing of unpaired Mn ions below 240mK for all compositions measured, from $x$=0.05 to 0.85. We also present the evolution of the high field ordered state with increasing disorder.