  We study localized modes on a single magnetic impurity positioned in the bulk
or at the surface of a one-dimensional chain, in the presence of a magnetic
field $B$ acting at the impurity site. The strong on-site nonlinear interaction
$U$ between two electrons of opposite spin at the impurity site, modelled here
as a nonlinear local term, and the presence of the external field induce a
strong correlation between parallel and anti-parallel spin bound states. We
find that, for an impurity in the bulk, a localized vector mode (with up and
down spin components) is always possible for any given value of $U$ and $B$,
while for a surface impurity, a minimum value of both, $U$ and $B$ is needed to
create a vector mode. In this case, up to two localized modes are possible, but
only one of them is stable. The presence of the surface seems to destabilize
the bulk mode in the parameter region U~B, creating a "forbidden strip" region
in parameter space, bounded by U=B+V and U=B-V, approximately.
