  FeGe_2, and lightly doped compounds based on it, have a Fermi surface driven
instability which drive them into an incommensurate spin density wave state.
Studies of the temperature and magnetic field dependence of the resistivity
have been used to determine the magnetic phase diagram of the pure material
which displays an incommensurate phase at high temperatures and a commensurate
structure below 263 K in zero field. Application of a magnetic field in the
tetragonal basal plane decreases the range of temperatures over which the
incommensurate phase is stable. We have used inelastic neutron scattering to
measure the spin dynamics of FeGe_2. Despite the relatively isotropic transport
the magnetic dynamics is quasi-one dimensional in nature. Measurements carried
out on HET at ISIS have been used to map out the spin wave dispersion along the
c-axis up the 400 meV, more than an order of magnitude higher than the zone
boundary magnon for wavevectors in the basal plane.
