  We report on isothermal magnetization, M\"ossbauer spectroscopy, and
magnetostriction as well as temperature-dependent alternating-current (ac)
susceptibility, specific heat, and thermal expansion of single crystalline and
polycrstalline Li$_2$(Li$_{1-x}$Fe$_x$)N with $x = 0$ and $x \approx 0.30$.
Magnetic hysteresis emerges at temperatures below $T \approx 50\,$K with
coercivity fields of up to $\mu_0H = 11.6\,$T at $T = 2\,$K and magnetic
anisotropy energies of $310\,$K ($27\,$meV). The ac susceptibility is strongly
frequency dependent ($f\,=\,10$--$10,000\,$Hz) and reveals an effective energy
barrier for spin reversal of $\Delta E \approx 1100\,$K. The relaxation times
follow Arrhenius behavior for $T > 25\,$K. For $T < 10\,$K, however, the
relaxation times of $\tau \approx 10^{10}\,$s are only weakly
temperature-dependent indicating the relevance of a quantum tunneling process
instead of thermal excitations. The magnetic entropy amounts to more than
$25\,$J mol$^{-1}_{\rm Fe}\,$K$^{-1}$ which significantly exceeds $R$ln2, the
value expected for the entropy of a ground state doublet. Thermal expansion and
magnetostriction indicate a weak magneto-elastic coupling in accordance with
slow relaxation of the magnetization. The classification of
Li$_2$(Li$_{1-x}$Fe$_x$)N as ferromagnet is stressed and contrasted with highly
anisotropic and slowly relaxing paramagnetic behavior.
