LAUSR

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 polycrystalline ${\mathrm{Li}}_{2}({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\text{N}$ with $x=0$ and $x\ensuremath{\approx}0.30$. Magnetic hysteresis emerges at temperatures below $T\ensuremath{\approx}50\phantom{\rule{4pt}{0ex}}\mathrm{K}$ with coercivity fields of up to ${\ensuremath{\mu}}_{0}H=11.6\phantom{\rule{4pt}{0ex}}\mathrm{T}$ at $T=2\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and magnetic anisotropy energies of 310 K (27 meV). The ac susceptibility is strongly frequency-dependent ($f=10--10\phantom{\rule{0.16em}{0ex}}000$ Hz) and reveals an effective energy barrier for spin reversal of $\mathrm{\ensuremath{\Delta}}E\ensuremath{\approx}1100\phantom{\rule{4pt}{0ex}}\mathrm{K}$ (90 meV). The relaxation times follow Arrhenius behavior for $Tg25\phantom{\rule{4pt}{0ex}}\mathrm{K}$. For $Tl10\phantom{\rule{4pt}{0ex}}\mathrm{K}$, however, the relaxation times of $\ensuremath{\tau}\ensuremath{\approx}{10}^{10}$ 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 $25\phantom{\rule{0.28em}{0ex}}\mathrm{J} {\mathrm{mol}}_{\mathrm{Fe}}^{\ensuremath{-}1}$ ${\mathrm{K}}^{\ensuremath{-}1}$, which significantly exceeds $R\phantom{\rule{0.16em}{0ex}}\mathrm{ln}2$, the value expected for the entropy of a ground-state doublet. Thermal expansion and magnetostriction indicate a weak magnetoelastic coupling in accordance with slow relaxation of the magnetization. The classification of ${\mathrm{Li}}_{2}({\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\text{N}$ as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior.