4f-spin dynamics in La_2-x-ySr_xNd_yCuO₄

We have performed inelastic magnetic neutron scattering experiments on La_2-x-ySr_xNd_yCuO₄ (0<~x<~0.2 and 0.1<~y<~0.6) in order to study the Nd 4f-spin dynamics at low energies (ħω≲1 meV). In all samples we find at high temperatures a quasielastic line (Lorentzian) with a linewidth which decreases on lowering the temperature. The temperature dependence of the quasielastic linewidth Γ/2(T) can be explained with an Orbach process, i.e., a relaxation via the coupling between crystal field excitations and phonons. At low temperatures the Nd-4f magnetic response S(Q,ω) correlates with the electronic properties of the CuO₂ layers. In the insulator La_2-yNd_yCuO₄ (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 La_1.7-xSr_xNd_0.3CuO₄ with x=0.12,0.15 and La_1.4-xSr_xNd_0.6CuO₄ with x=0.1,0.12,0.15,0.18 superconductivity is strongly suppressed. In these compounds we observe a temperature independent broad quasielastic line of Gaussian shape below T≈30 K. This suggests a distribution of various internal fields on different Nd sites and is interpreted in the frame of the stripe model. In La_1.8-ySr_0.2Nd_yCuO₄ (y=0.3,0.6) such a quasielastic broadening is not observed even at lowest temperature.