Magnetic Properties of High Temperature Superconductors: a Spin Liquid Approach. Talk given at Stanford Spectroscopy Meeting, 1995.

We argue that strongly correlated two dimensional electrons form a spin liquid in some regimes of density and temperature and give the theory of the magnetic properties of this spin liquid using the representation in terms of femions interacting with a gauge field. This state is characterized by anomalous power law spin correlations and discuss the implications of these correlations for the temperature dependence of NMR relaxation rates $1/T_1$ and $1/T_2$ and for the uniform susceptibility. We also discuss the transition from the spin liquid to antiferromagnet and the critical behavior of these properties at the transition. We compare these theoretical results with the data on high $T_c$ superconductors. Finally, we discuss the formation of the spin gap due to the spin exhange interaction between adjacent layers in bilayered materials.