N: stable but manipulable states.*

Li₂(Li_1-xFe_x)N shows extremely large magnetic anisotropy and coercivity that are based on the presence of unquenched orbital moments [1,2]. For low Fe-concentrations the spin-relaxation becomes temperature-independent indicating a crossover from thermal excitations to the quantum tunneling regime at T = 10 K. The resonant character of this tunneling process is proven by a strong increase of the spin-flip probability in transverse magnetic fields. Longitudinal fields, on the other hand, lift the ground state degeneracy and destroy the tunneling condition. An increase of the relaxation time by four orders of magnitude in external fields of only a few millitesla reveals exceptionally sharp tunneling resonances. Accordingly the system can be set from stable to manipulable by tuning from off- to on-resonant using small applied fields. Li₂(Li_1-xFe_x)N represents a comparatively simple and clean model system that opens the possibility to study and utilize magnetic quantum tunneling at liquid helium temperatures.
[1] Nat. Commun. 5:3333 (2014)
[2] Nanoscale 9, 10596 (2017)