Neutron spectroscopy of 4f collective magnetic excitations in ${\mathrm{R}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{CuO}}_4$ (R=Nd,Pr)

We have studied the wave-vector dependence of the magnetic excitation spectrum of Nd in ${\mathrm{Nd}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$n(x=0,0.13) and of Pr in ${\mathrm{Pr}}_{1.86}$ ${\mathrm{Ce}}_{0.14}$ ${\mathrm{CuO}}_4$ by inelastic neutron-scattering experiments on single crystals. The results are analyzed with the help of model calculations which are performed in the context of the mean-field random-phase approximation. This enables us to obtain direct information on the coupling constants between the rare-earth ions. The exchange couplings between the Pr ions in ${\mathrm{Pr}}_{1.86}$ ${\mathrm{Ce}}_{0.14}$ ${\mathrm{CuO}}_4$ turn out to be slightly reduced compared to the ones in the undoped parent compound ${\mathrm{Pr}}_2$ ${\mathrm{CuO}}_4$. In ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$ we observe a dependence of the coupling constants on the initial and final state that is involved in a crystal-field excitation. Furthermore our results indicate that the interaction of the Nd spins is opposite to the ordering enforced by the Nd-Cu exchange. We argue that this will lead to an instability of the Nd ordering in the Ce-doped systems which might be important for the onset of high γ values.