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This is the first systematic study of the spin excitations around the MBZ boundary points in -Mn alloy. The magnetic inelastic neutron scattering was measured in the antiferromagnetic Mn(13.7% Ni) alloy in a sample with unequal domain distribution in the temperature range 0.025 TN -1.15 TN . The evidence of the magnetic excitations around the MBZ boundary points equivalent to the MBZ centres in -Mn alloy is presented. At low temperatures these excitations differ from excitations observed around the MBZ centres; they are rather similar to excitations observed above TN . When the temperature increases up to TN the excitations observed around MBZ centres become more similar to those around the MBZ boundary equivalent points. The results of the analysis are consistent with the general features of spin fluctuations.

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) 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.