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Polarised neutron study of the magnetic excitations in single-Q chromium
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Fig. 1. Elastic scattering scan with polarised neutrons through the magnetic satellites on the [loo] axis near (1, 0,O). SF = spin-flip scattering (open circles), NSF = non-spin-flip scattering (full circles). The lines are guides to the eye. ZOO-
monochromator and analyser. The instrument was operated with a fixed final energy of 14.7 meV and a pyrolytic graphite filter removed higher-order neutrons from the scattered beam. A spin flipper was placed in the incident beam and guide fields were mounted to preserve the neutron polarisation. With a small field on the sample directed parallel to the scattering vector, a “flipping ratio” in excess of 20 was obtained as is seen from fig. 1, where an elastic scan through a pair of satellite reflections in the TSDW phase is displayed. 3. Results and discussion Although a series of different magnetic field configurations was employed in this work we shall deal here only with those in which the magnetic field was applied parallel to the scattering vector. In this configuration the “flipper-on” intensity at (1, 0,O) is the sum of (010) and (001) direction spin fluctuation scattering plus background; we denote this as spin-flip (SF). With the spin flipper “off’ we measure coherent nuclear scattering plus background, denoted nonspin-flip (NSF). Fig. 2 demonstrates clearly the spin-flip nature of the low energy spin-wave peaks in the TSDW phase, as expected. Only
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F’(Q) [Q - 4’ 5
(1)
where F(Q) is the magnetic form factor and e the phonon eigenvector; here Q is the wave vector transfer. Between (1, 0,O) and (3,0,0) this term implies a reduction in intensity of about 60 from F’(Q), somewhat compensated by the factor of 9
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