We report on isothermal magnetization, Mossbauer 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.
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