Temperature variation of the magnetic inelastic neutron scattering from pure chromium

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Abstract
The frequency and wavevector dependence of the magnetic INS response in pure chromium has been investigated over the temperature range 4 to 450 K (∼1.5 TN). At all temperatures the Q-dependence was centred on the (100) commensurate lattice position. The frequency dependence was found to differ in the ordered and paramagnetic phases. The transverse response in the AF1 phase at 295 K showed the following features: (i) for energy transfers <1 THz two peaks centred on (1±δ, 0, 0) were observed together with a small peak at (100), (ii) at higher energy transfer the response coalesced into a Gaussian shaped peak centred on (100) and (iii) for energy transfers ≳6.5 THz the response became strongly attenuated. This critical frequency is consistent with the expected energy gap, i.e., TN = 311K (6.5 THz). Moreover measurements in the paramagnetic range above TN did not reveal any attenuation for energy transfers up to 11 THz. In the AF2 phase region at 4.2 K energy scans parallel to the SDW vector showed a similar coalescence of the response on to (100) above 1 THz. The results are discussed in terms of excitations emanating from (100). In addition we put forward the idea of a non-linear dispersion owing to the small energy gap and discuss the implications.