Giant magnetic anisotropy in Li$_{3-x}$Fe$_x$N permanent magnets*

Single crystals of Li$_2$(Li$_{1-x}$Fe$_x$)N were successfully grown out of Li-flux. Fe-concentrations and lattice parameters were determined by means of single crystal and powder diffraction which also confirmed the substitution of Fe on only one of the Li sites resulting in Li$_{1-x}$Fe$_x$ layers separated by Li$_2$N layers. Magnetization measurements revealed a ferromagnetically ordered ground state with Curie temperatures of $\sim 60$\,K for Fe concentrations of $x \approx 20$\%. Large saturation moments of up to 5\,$\mu_B$ per Fe atom were found along the hexagonal crystallographic $c$-axis. These values exceed the spin-only contribution of Fe and are also reflected in correspondingly large effective moments at room temperature. The anisotropy field at $T = 2$\,K, defined as intersection of the magnetization for $M \parallel c$ and $M \perp c$, can be estimated to lie well beyond 100 Tesla. Electrical resistivity measurements show insulating behavior and raise questions about the nature of the underlying magnetic exchange mechanism.