  Using the spin-wave approximation elementary excitations of a semi-infinite
two-dimensional $S=\frac12$ Heisenberg antiferromagnet are considered. The
spectrum consists of bulk modes -- standing spin waves and a
quasi-one-dimensional mode of boundary spin waves. These latter excitations
eject bulk modes from two boundary rows of sites, thereby dividing the
antiferromagnet into two regions with different dominant excitations. As a
result absolute values of nearest-neighbor spin correlations on the edge exceed
the bulk value.
