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

Spin wave dispersion in 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\mathring{A}and$\Delta = 3.3$ meV. Compared with the dispersion at $T=419$ K previously reported, the result demonstrates a large reduction of the stiffness constant $c$ with increasing temperature. This is similar to that observed in the metallic antiferromagnet FePt${}_3$, and is an indication of the itinerancy of the magnetic moments.