Temperature dependence of the magnetic excitations in γ-Mn90Cu10

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Abstract
The inelastic neutron scattering due to the magnetic excitations in a single crystal of gamma -Mn90Cu10 has been measured between 1 and 10 THz for six temperatures between 110 and 520K. The Neel temperature is 465K. Below TN, the scattering is well described by isotropic, heavily damped magnons. At room temperature, the velocity of the magnons is about 44 THz AA and the gap is 2.1 THz. Both values depend on the exact nature of the model used in the analysis. Consistent with theoretical expectation, the damping is wavevector dependent. When the temperature is raised above TN, there is no dramatic change in either the magnon scattering or the fitted parameters. However, above TN, a better description of the scattering at low energies is given by a scattering cross section derived from fluctuations, whereas this description breaks down at higher energy. Conversely, the magnon description is better at high energies but produces inconsistent parameters at lower energies. The change from fluctuation- to magnon-like behaviour occurs at k=0.12 AA-1 at 470K (1.011TN) and at 0.19 AA-1 at 549K (1.116TN). These wavevectors are interpreted as a measure of the size of the regions of fluctuating but correlated spins that can sustain magnons above TN.