4 f -spin dynamics in La 2 − x − y Sr x Nd y CuO 4

We have performed inelastic magnetic neutron scattering experiments on ${\mathrm{La}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{Sr}}_{x}{\mathrm{Nd}}_{y}{\mathrm{CuO}}_{4} (0l~xl~0.2$ and $0.1l~yl~0.6)$ in order to study the Nd $4f$-spin dynamics at low energies $(\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\lesssim}1 \mathrm{meV}).$ 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 $\ensuremath{\Gamma}/2(T)$ can be explained with an Orbach process, i.e., a relaxation via the coupling between crystal field excitations and phonons. At low temperatures the $\mathrm{Nd}\ensuremath{-}4f$ magnetic response $S(\mathbf{Q},\ensuremath{\omega})$ correlates with the electronic properties of the ${\mathrm{CuO}}_{2}$ layers. In the insulator ${\mathrm{La}}_{2\ensuremath{-}y}{\mathrm{Nd}}_{y}{\mathrm{CuO}}_{4} (y=0.1,0.3)$ the quasielastic line vanishes below 80 K and an inelastic excitation occurs. This directly indicates the splitting of the ${\mathrm{Nd}}^{3+}$ ground state Kramers doublet due to the static antiferromagnetic order of the Cu moments. In ${\mathrm{La}}_{1.7\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nd}}_{0.3}{\mathrm{CuO}}_{4}$ with $x=0.12,0.15$ and ${\mathrm{La}}_{1.4\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nd}}_{0.6}{\mathrm{CuO}}_{4}$ 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 $T\ensuremath{\approx}30 \mathrm{K}.$ This suggests a distribution of various internal fields on different Nd sites and is interpreted in the frame of the stripe model. In ${\mathrm{La}}_{1.8\ensuremath{-}y}{\mathrm{Sr}}_{0.2}{\mathrm{Nd}}_{y}{\mathrm{CuO}}_{4} (y=0.3,0.6)$ such a quasielastic broadening is not observed even at lowest temperature.