  We investigate the effect of a magnetic field supported at a single lattice
site on the low-energy spectrum of the ferromagnetic Heisenberg XXZ chain. Such
fields, caused by impurities, can modify the low-energy spectrum significantly
by pinning certain excitations, such as kink and droplet states. We distinguish
between different boundary conditions (or sectors), the direction and also the
strength of the magnetic field. E.g., with a magnetic field in the z-direction
applied at the origin and ++ boundary conditions, there is a critical field
strength B_c (which depends on the anisotropy of the Hamiltonian and the spin
value) with the following properties: for B < B_c there is a unique ground
state with a gap, at the critical value, B_c, there are infinitely many
(droplet) ground states with gapless excitations, and for B>B_c there is again
a unique ground state but now belonging to the continuous spectrum. In
contrast, any magnetic field with a non-vanishing component in the xy-plane
yields a unique ground state, which, depending on the boundary conditions, is
either an (anti)kink, or an (anti)droplet state. For such fields, i.e., not
aligned with the z-axis, excitations always have a gap and we obtain a rigorous
lower bound for that gap.
