  The ruthenium based molecular magnet
[Ru$_2$(D(3,5-Cl$_2$Ph)F)$_4$Cl(0.5H$_2$O)$\cdotp$C$_6$H$_{14}$] (hereafter
Ru$_2$) behaves as a two-level system at sufficiently low temperatures. The
authors performed spin detection by means of single-crystal measurements and
obtained magnetic hysteresis loops around zero bias as a function of field
sweeping rate. Compared to other molecular systems, Ru$_2$ presents an enhanced
irreversibility as shown by ``valleys'' of negative differential susceptibility
in the hysteresis curves. Simulations based on phonon bottleneck model are in
good qualitative agreement and suggest an abrupt spin reversal combined with
insufficient thermal coupling between sample and cryostat phonon bath.
