Low temperature measurements of the stiffness constant D in disordered Ni:Mn alloys. Interpretation within band model (CPA-RPA)

The spin wave stiffness constant of ferromagnetic Ni:Mn alloys has been measured by inelastic neutron scattering at liquid helium temperature on samples with 1.5, 5, 7.5, 13 and 19% of Mn. As already observed in previous room temperature measurements, magnon line widths have been observed even from 7.5% of Mn. The line width is energy dependent at a given concentration and is also concentration dependent. A theoretical interpretation of the experimental D values is given with a comparison of three itinerant electron models. All the calculations have been made with a splitting value of 0.06Ry which was fitted to give the experimental stiffness constant of pure nickel and has already been used in the interpretation of stiffness constant behaviour of Ni-Fe alloy and of weak ferromagnetic nickel based alloys. The first model is the rigid band model and reproduces neither the observed variation of the stiffness constant nor the experimental total magnetization in the critical range. The second one uses the coherent potential approximation. The stiffness constant behaviour is then well described, but the results concerning the total magnetization are the same as in the rigid band model. The third one uses also the CPA but considers the alloys as pseudo-ternary, allowing the coexistence of two types of Mn. This model is built in order to take into account some local environment effects. We find that with such a kind of model we can get a quite better description of the magnetization with only minor change on the stiffness constant. (author)