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.
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