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.
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