  The decay rate of an interstitial two-level spin impurity, located in the
center of a unit cell of an anisotropic ferromagnetic lattice subjected to an
external magnetic field is derived. The impurity is coupled to nearest-neighbor
spins through Heisenberg $XY$ interaction. By mapping the lattice spin
operators using the Holstein-Primakoff transformation, we establish the
similarity with the Fano-Anderson model at low temperatures, and we calculate
the retarded Green's function in one and two dimensions analytically for
arbitrary coupling strength. It is shown that the reduced density matrix of the
impurity satisfies an exact master equation in Lindblad form, from which the
decay rate and the Lamb shift are deduced. The evolution in time of the latter
together with the excited state occupation probability is investigated and its
dependence on the applied magnetic field is discussed. It is found that there
exists a critical resonance-like value of the magnetic field around which the
behavior of the decay rate and the density matrix changes drastically. The
Markovian decay law, as given by the Fermi golden rule, does not hold in the
weak-coupling regime unless the magnetic field is weak, typically less than the
critical value. The weak-coupling regime is further treated perturbatively up
to second order, and the obtained results are compared with the exact solution.
We also discuss the Zeno regime of the dynamics, where it is shown that at
short times, the effective decay rate is twice as small as the exact decay
rate, and that when the impurity energy lies outside the lattice continuum, the
measurement speeds up the decay of the survival probability.
