  Ramsey et al. [Nature Phys. 4, 277-281 (2008)] report the observation of
quantum interference associated with tunnelling trajectories between states of
different total spin length in a dimeric molecular nanomagnet. They argue that
the interference is a consequence of the unique characteristics of a molecular
Mn12 wheel, which behaves as a molecular dimer with weak ferromagnetic exchange
coupling.
  We show here that the data published by Ramsey et al. are not consistent and
unfortunately mostly wrong. We show further that the Landau-Zener (LZ) formula,
which links the tunnel probability with the tunnel splitting, can only be
applied in a well-defined experimental region, which lays outside the region
accessed by Ramsey and colleagues. Only a lower-limit estimate of the tunnel
splitting can be obtained, showing that the observed transition cannot be
explained with the dimer model. We also present other shortcomings of the paper
questioning the dimer model, and that the alignment of the magnetic field is
crucial for observing quantum interference.
