The spin-wave spectrum of ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ has been measured at 5.0°K and found to be extremely anisotropic with no measurable dispersion in the direction perpendicular to the Ni${\mathrm{F}}_2$ planes. A study of the temperature dependence of the two-dimensional magnons shows that even relatively long-wavelength spin waves ($\lambda \approx 110$ Å) show little renormalization or lifetime effects up to $\approx 1.1T_{\mathrm{N}}$.

• Received 4 November 1969

DOI:https://doi.org/10.1103/PhysRevLett.23.1394