Our studies of the itinerant antiferromagnetic -Mn-based alloys are extended to investigate magnetic excitations in a Mn(13.7%Ni) alloy. The composition of the present sample being close to the part of the phase diagram where a number of structural phase transitions take place, the basic aim of this work is the search for the effect of these transitions on spin dynamics. This is the first study of the Mn - Ni alloy in the fct (c < a) phase. The parameters of the spin wave dispersion relation and their temperature dependence are determined and compared with those for Mn - Ni alloys in the fct (c > a) and fcc phases. We obtain the concentration dependence of the low-temperature parameters of the spin wave dispersion relation which differs essentially from that found for the Mn - Fe alloys. In particular the change of the spin wave parameters in the vicinity of the phase transitions in Mn - Ni alloys is remarkable.

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A systematic study of temperature dependence of magnetic inelastic neutron scattering for itinerant antiferromagnet Mn(12%Ge) is presented. At low temperatures the experimental data are well described in terms of damped spin waves. This description becomes insufficient at least 150 K below TN. In the temperature range 200-400 K the integrated intensity of distribution for energy transfer 13 THz increases by one third and the shape of the deconvoluted neutron scattering cross-section changes. These changes suggest that ca. 200 K below TN an additional contribution to the inelastic neutron scattering starts to appear and that its origin is different from spin waves.PACS numbers: 75.30.Ds, 75.50.EeThe magnetic excitations in itinerant antiferromagnets are not yet satisfactorily understood. In particular more detailed studies of their temperature dependence are needed. The aim of our experiment was to study the evolution of the neutron scattering by spin waves at low temperature into the paramagnetic scattering above TN.The crystal structure of the γ-Mn alloys with high: manganese concentration (fcc in the paramagnetic phase) becomes tetragonal with c < a in the antiferromagnetic (AF) phase. Many alloys with lower manganese concentration remain fcc in the AF phase. In some narrow concentration region (different for different diluents) two other crystal structures: orthorhombic and tetragonal with c > a appear in the AF phase. The composition of the investigated alloy is close to the composition for the tricritical point for paramagnetic fcc, AF tetragonal and AF (483)

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