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$Powder Diffraction - Spallation Neutron Source$

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Intensity (arb. units) 4000 3000 2000 1000 0 600 400 200 (ζ,0,2) (ζ,0,0) FeGe 2 T=275 K Fe(Ge 0.96 Ga 0.04 ) 2 0 0.7 0.8 0.9 1.0 ζ 1.1 1.2 1.3 Fig. 1. Scans through the incommensurate magnetic Bragg peaks for pure (upper panel) and Ga doped (lower panel) FeGe2 at 275 K obtained on the E3 and C5 triple axis spectrometers at Chalk River. The peak in the centre arises from the two peaks displaced from the commensurate position along the b-axis, seen due to the coarse vertical collimation employed in these measurements. Resistivity (µΩ-cm) Resistivity (µΩ-cm) Resistivity (µΩ-cm) 100 50 Resistivity (µΩ-cm) 0 70 60 50 40 82 77 250 300 Temperature Tc = 263 K (K) H=0, i||[110] T c = 182 K T N = 289 K undoped T N = 285 K Ga doped 0 50 100 150 200 250 300 Temperature (K) Fig. 2. Temperature dependence of the resistivity for pure and Ga doped FeGe2 (measured on pieces of the same samples used for Figure 1). of the incommensurate wavevector (similar to what is seen in lightly doped alloys of Cr[2]). In the 4 at% Ga doped sample the ordering wavevector varies between q = 0.12 just below TN = 285K and q = 0.1 below a first order lockin transition at Tc = 182K. In the pure material the transition to the low temperature commensurate state (with q = 0) is continuous. The zero field resistivity for these two compounds is shown in Figure 2. In the doped sample (bottom panel) the two phase transitions are clearly visible and are denoted by arrows. In the pure material the anomalies are more subtle but the transitions 3
Magnetic Field (T) 30 25 20 15 10 5 H // [110] commensurate SDW incommensurate SDW FeGe 2 paramagnetic 0 250 260 270 theory 280 290 300 Temperature (K) Fig. 3. Magnetic phase diagram for FeGe2 for a magnetic field applied along the [110] direction determined by resistivity measurements and (for H=0) neutron diffraction. The line denoted theory corresponds to the approximate phase boundary expected based on the Ginzburg-Landau theory of Reference [13] Intensity (mb meV -1 sr -1 ) 0.25 0.20 0.15 0.10 0.05 E i =120 meV ∆E=77.5 meV 0.00 1.5 2.0 Q (c*) c 2.5 Fig. 4. Constant energy cut at 77.5 meV through the two counter-propagating spin wave modes obtained on HET for FeGe2 at 11 K using an incident neutron energy of 120 meV. are seen in the inset. The transition temperatures deduced from the resistive anomalies agree well with those determined by neutron scattering. We have used similar measurements, together with constant temperature, field scans which clearly show the lower phase transition, with a magnetic field applied in the basal plane to determine the magnetic phase diagram for FeGe2, shown in Figure 3, the behaviour is similar for a field applied along [100].[15] In order to fully characterise the ground state of FeGe2 we have measured the spin dynamics at low temperatures (11 K) using the direct geometry timeof-flight spectrometer HET at ISIS. This instrument allows access to much higher incident neutron energies than the thermal triple-axis measurements of Holden et. al.[14] making it possible to fully map out the c-axis dispersion. Figure 4 shows a constant energy cut through the time-of-flight trajectories of the 2.5 m detector bank on HET for an energy transfer of 77.5 meV. This was obtained for an integrated current of 2200 µA-hrs using an incident energy of 120 meV. The two spin wave modes emanating from Qc = 2 are clearly visible, 4
Page 1 and 2: The IPTS for the ARCS experiment is
Page 3 and 4: J. Phys.: Condens. Matter 12 (2000)
Page 5 and 6: High-energy magnetic excitations in
Page 11: Studies of the spin dynamics of the
Page 15: We would like to thank the technica

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