  The magnetic properties of dilute Li$_2$(Li$_{1-x}$Fe$_x$)N with $x \sim
0.001$ are dominated by the spin of single, isolated Fe atoms. Below $T = 10$ K
the spin-relaxation times become temperature-independent indicating a crossover
from thermal excitations to the quantum tunneling regime. We report on a strong
increase of the spin-flip probability in $\textit{transverse}$ magnetic fields
that proves the resonant character of this tunneling process.
$\textit{Longitudinal}$ fields, on the other hand, lift the ground-state
degeneracy and destroy the tunneling condition. An increase of the relaxation
time by four orders of magnitude in applied fields of only a few milliTesla
reveals exceptionally sharp tunneling resonances. Li$_2$(Li$_{1-x}$Fe$_x$)N
represents a comparatively simple and clean model system that opens the
possibility to study quantum tunneling of the magnetization at liquid helium
temperatures.
