The dispersion of the low-energy magnetic excitations of the Pr sublattice in ${\mathrm{PrBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.2}$ is determined by inelastic neutron scattering measurements on a single crystal. The dispersion, which shows the effect of interactions with the Cu spin waves, is well described by a model of the coupled Cu-Pr magnetic system. This model enables values for the principal exchange constants to be determined. The results suggest that both Pr-Pr and Cu-Pr interactions are important in producing the anomalously high ordering temperature of the Pr sublattice. Measurements of the Cu optic spin wave mode show that the interlayer Cu-Cu exchange is significantly lower than in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.2}$.

• Received 4 February 2001

DOI:https://doi.org/10.1103/PhysRevLett.86.5994