Giant magnetic anisotropy and quantum tunneling of the magnetization in Li$_2$(Li$_{1-x}$Fe$_x$)N*

The magnetic anisotropy of 3$d$ transition metals is usually considered to be weak, mainly due to the widely known paradigm of orbital quenching. However, a rare interplay of crystal electric field effects and spin-orbit coupling causes a large orbital contribution to the magnetic moment of iron in Li$_2$(Li$_{1-x}$Fe$_x$)N. This leads, not only to large magnetic moments of $\sim$\,5\,$\mu_{\rm B}$ per iron atom but, also, to an enormous magnetic anisotropy field that extrapolates to more than 200 Tesla. Magnetic hysteresis emerges for $T \leq 50$\,K and the coercivity fields of more than 11 Tesla exceed even the hardest 4$f$ electron based ferromagnets. Li$_2$(Li$_{1-x}$Fe$_x$)N not only has a clear and remarkable anisotropy, generally not associated with iron moments, but also shows time-dependence more consistent with molecular magnets. In particular for low iron concentrations $x \ll 1$ the spin-inversion is dominated by a macroscopic tunneling process rather than by thermal excitations. It is shown that the huge magnetic anisotropy makes Li$_2$(Li$_{1-x}$Fe$_x$)N (i) an ideal model system to study macroscopic quantum effects at elevated temperatures and (ii) a basis for novel magnetic functional materials.