The spin-wave stiffness of dilute iron-palladium alloys

The spin-wave stiffness D of iron-palladium alloys containing 10, 22, 40 and 100 at.% iron has been measured by the neutron small-angle-scattering technique. The values of D are consistent with those derived from low-temperature saturation magnetization and specific heat measurements on alloys with iron concentrations ranging from 015 to 15 at.%. In the range up to 1 at.% D varies linearly with fractional iron concentration c, with an equation of best fit of the form D = (5040±290)c mev A2. At higher concentrations of iron D increases more slowly. The constant of proportionality for D in the linear region is greater by a factor of about 4 than that obtained by Doniach and Wohlfarth in a calculation based on the s-d interaction model. The results are also discussed in terms of a simple molecular field model of a dilute iron-palladium alloy, in which the giant moments are assumed to interact through a long-range Heisenberg exchange interaction J(r). The exchange field due to a giant moment is assumed to have the same spatial variation as the polarization distribution in the palladium matrix, and values of the stiffness are obtained which agree within a factor of less than 2 with the measured values. It is suggested that the non-linear behaviour of D at higher concentrations arises from close overlap between three or more giant moments.