  Although the precise mechanism of high-Tc superconductivity in the layered
cuprates remains unknown, it is generally thought that strong 2D Heisenberg
antiferromagnetism combined with disruptive hole doping is an essential aspect
of the phenomenon. Intensive studies of other layered 3d transition metal
systems have greatly extended our understanding of strongly correlated electron
states, but to date have failed to show strong 2D antiferromagnetism or high-Tc
superconductivity. For this reason the largely unexplored 4d^9 Ag^II fluorides,
which are structurally and perhaps magnetically similar to the 3d^9 Cu^II
cuprates, merit close study. Here we present a comprehensive study of magnetism
in the layered Ag^II fluoride Cs_2AgF_4, using magnetic susceptometry, neutron
diffraction and inelastic neutron scattering techniques. We find that this
material is well described as a 2D Heisenberg ferromagnet, in sharp contrast to
the high-Tc cuprates. The exchange constant J is the largest known for any
material of this type. We suggest that orbital ordering may be the origin of
the ferromagnetism we observe in this material.
