Using a quantum theory for an ensemble of two-level atoms driven by a field
in an optical cavity, we show that the spin associated with the atomic ens
emble can be squeezed. Two kinds of squeezing are obtained: on the one hand
, self-squeezing of the spin when the input field is a coherent one and the
atomic ensemble exhibits a large nonlinearity; and on the other hand, sque
ezing transfer from an incoming squeezed field when the atomic ensemble has
a quasilinear behavior.