  We study the exact solution of the XXZ spin-1/2 Heisenberg chain with a
boundary magnetic field in the region where the bulk excitations are gapless.
It is shown that a bound state is created near the boundary with a total spin
$S = 1/2(\pi/\gamma - 1)$ where $\cos\gamma$ is the anisotropy parameter. When
$\gamma < \pi/2$ this spin remains underscreened giving rise to non-analytic
contributions to the free energy. The IR fixed point is this case belongs to
the universality class of the underscreened Kondo model. We argue that the same
fixed point appears in the boundary sine-Gordon model when the scaling
dimension of the boundary term $1/2 < \Delta < 1$.
