Spin wave dispersion just above the magnetic order-order transition in the metallic antiferromagnet Mn$_3$Pt

Spin wave dispersion of the metallic antiferromagnet Mn$_3$Pt was investigated just above the order-order transition temperature by using the inelastic neutron scattering technique. The spin wave dispersion at $T = 400$ K along [100], [110] and [111] directions was isotropic within the measurement accuracy. The dispersion was described by $({\hbar\omega})^2 = c^2q^2 + \Delta^2$ with $c = 190$ meV{\AA} and $\Delta = 3.3$ meV. Compared with the previous reports, the result shows that large reduction of the stiffness constant with increasing temperature. This feature is similar to that in FePt$_3$, and will be an indication of the itinerancy of the magnetic moments.