Magnetic and transport properties of FeGe2

We have measured the electrical resistivity and the elastic and inelastic neutron scattering cross-sections of the itinerant antiferromagnets FeGe2 and Fe(Ge.96Ga.04)2. FeGe2 has a tetragonal structure with a collinear antiferromagnetic phase below TC-IC = 263 K, and an incommensurate spin density wave (SDW) phase between TC-IC and the Néel temperature (289 K). The ordering wavevector is along the a-axis and the moments are ferromagnetically aligned along the c-axis. We determined the phase diagram of FeGe2 for basal plane fields up to 16 T using resistivity measurements. The incommensurate phase is much more stable (up to ~30 T) in a magnetic field than theory predicts (1 T). Quantum oscillations appeared in the resistivity below 4 K for a c-axis magnetic field above 7 T. The resulting effective carrier mass is 0.45 ± .05 me, and the frequency is 190 ± 10 T. We used elastic neutron scattering to study the magnetic phases of Fe(Ge .96Ga.04)2. TC -IC = 183 K and this transition is clearly first-order. The low-temperature phase consists of a commensurate SDW with a long-wavelength modulation along the a-axis with weaker magnetic scattering reflections which are not displaced along the a-axis. Resistivity measurements show that the low-field commensurate phase is destroyed at temperatures below 20 K by a 12 T basal-plane magnetic field. Inelastic neutron scattering on FeGe2 at 11 K up to energy transfer (E = 540 meV shows magnon dispersion that is well described by a classical Heisenberg spin-wave model. The exchange constant along the c-axis, SJFM = 68 ± 5 meV, is 15 times higher than along the a-axis, ' SJAFM' = 4.4 ± 1.0 meV, making FeGe2 a unique example of a quasi-one-dimensional itinerant magnet. Up to 350 meV the c-axis magnon linewidth can be described by Γ( E) = (2.7 ± .6) * 10-3 MeV -1E2 (E2 dependence of a hydrodynamic model). We measure Γ values several orders of magnitude larger than localised models predict, presumably due to itinerant effects which also lead to a continuum of scattering above ~350 meV. At higher temperatures the peak height and lifetime of the c-axis magnons decrease, changing rapidly between TC-IC and TN. Magnon-like excitations persist to 600 or 700K as might be expected for a 1-D system without long-range order.Ph.D