4f-spin dynamics in La2-x-ySrxNdyCuO4

We have performed inelastic magnetic neutron scattering experiments on La2- x-ySrxNdyCuO4 (0 less than or equal to x less than or equal to 0.2 and 0.1 less than or equal to y less than or equal to 0.6) in order to study the Nd 4f-spin dynamics at low energies ((h) over tilde omega less than or simila r to 1 meV). In all samples we find at high temperatures a quasielastic lin e (Lorentzian) with a linewidth which decreases on lowering the temperature . The temperature dependence of the quasielastic linewidth Gamma/2(T) can b e explained with an Orbach process, i.e., a relaxation via the coupling bet ween crystal field excitations and phonons. At low temperatures the Nd-4f m agnetic response S(Q,omega) correlates with the electronic properties of th e CuO2 layers. In the insulator La2-yNdyCuO4 (y =0.1,0.3) the quasielastic line vanishes below 80 K and an inelastic excitation occurs. This directly indicates the splitting of the Nd3+ ground state Kramers doublet due to the static antiferromagnetic order of the Cu moments. In La1.7-xSrxNd0.3CuO4 w ith x=0.12,0.15 and La1.4-xSrxNd0.6CUO4 With X= 0.1,0.12,0.15,0.18 supercon ductivity is strongly suppressed. In these compounds we observe a temperatu re independent broad quasielastic line of Gaussian shape below T approximat e to 30 K. This suggests a distribution of various internal fields on diffe rent Nd sites and is interpreted in the frame of the stripe model. In La1.8 -ySr0.2NdyCuO4 (y=0.3,0.6) such a quasielastic broadening is not observed e ven at lowest temperature.