  Correlation functions of ferromagnetic spin systems satisfying a Lee-Yang
property are studied. It is shown that, for classical systems in a
non-vanishing uniform external magnetic field $h$, the connected correlation
functions decay exponentially in the distances between the spins, i.e., the
inverse correlation length ("mass gap"), $m(h)$, is strictly positive. Our
proof is very short and transparent and is valid for complex values of the
external magnetic field $h$, provided that $\text{Re} \, h \not= 0$. It implies
a mean-field lower bound on $m(h)$, as $h \searrow 0$, first established by
Lebowitz and Penrose for the Ising model. Our arguments also apply to some
quantum spin systems.
