New slow and short range magnetic correlations in superconducting _a2_xSxCuO

Inelastic neutron scattering measurements have been performed on high quality single crystals of $\rm La_{ 2-{\it x}}Sr_{\it x}CuO_4$ in order to study the spin dynamics of this compound. In addition to the well-established incommensurate magnetic response, we show the existence of a new set of low energy excitations present in the whole superconducting region of the phase diagram. This new feature of the dynamical cross section is characterized, below about 10 K, by very short range ($\sim 3$ lattice spacing) antiferromagnetic correlations and by a low energy scale of $\sim 1$ meV. At higher temperatures these fluctuations become nearly Q-independent. Different possible origins of these new spin correlations are discussed