  Recent scanning tunneling microscope measurements on magnetic clusters reveal
an anomalous conductance dip at zero bias voltage, whenever the cluster
consists of an odd number of magnetic atoms [Science, {\bf 312}, 1021 (2006)].
We address the conductance anomaly within a model, and reproduce the
conductance dip with the only assumption that the magnetic cluster has a
conducting level near the Fermi level of the system. We show that the width of
the conductance dip scales quadratically with external magnetic fields, in
excellent agreement with experiments. In addition, we predict that the presence
of the impurity will be measurable as inelastic Friedel oscillation in the
substrate density of electron states.
