MAGNETIC EXCITATIONS OF ND IN ND2-XCEXCUO4 (X=0 AND 0.13)

We have studied the wave vector dependence of the magnetic excitation spectrum of Nd in Nd2-xCexCuO4 (x = 0,0.13) by inelastic neutron scatt ering experiments on single crystals. The results are analyzed with th e help of model calculations which are performed in the context of the mean field random-phase approximation (RPA). This enables us to obtai n direct information on the coupling constants between the rare earth ions. In Nd2CuO4 we observe a dependence of the coupling constants on the initial and final state that is involved in a crystal field excita tion. 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 instability of the Nd-ordering in the Ce- doped systems which might be important for the onset of high gamma-val ues.