The one-magnon scattering was studied in antiferromagnetic ${\mathrm{Ni}}_c{\mathrm{Mg}}_{1-c}\mathrm{O}$ solid solutions. We observed unpredicted behavior of both temperature and composition dependences of one-magnon excitation energies. First, the abrupt change of the one-magnon frequency by $7–8{\mathrm{cm}}^{-1}$ occurs between $c=0.98$ and $c=0.9$ in the limit of $T\to 0\mathrm{K}$. Second, upon increasing temperature, the one-magnon energy for highly diluted nickel oxide vanishes significantly below the Néel temperature.

• Received 11 October 2004

DOI:https://doi.org/10.1103/PhysRevB.71.134415