Improved lattice elongation for Fe8Nx (x > 1) thin films prepared via nitrogen ion implantation

Iron nitrides α″-Fe16N2 and α′-Fe8N have attracted significant interest due to their potential as a rare-earth-free semi-hard magnetic materials. In this work, a systematic investigation of nitrogen ion implantation fluences on iron thin films was performed to increase tetragonality beyond what is currently possible, thereby enhancing the magnetic performance. A structural characterization of samples with a different nitrogen concentration highlighted that α′-Fe8N can be formed within a fluence range from 4 to 7 × 1016 N+ ions cm−2. For increasing doses, at first crystalline Fe8Nx (x > 1) was obtained; afterward, the material went through amorphization. After annealing samples at 150 °C, the elongation of their lattice constant reached values higher than that of stoichiometric Fe8Nx (x = 1), with a largest value of 3.158(3) Å at 11.6(1) N at. % versus 3.1455 Å at 11.1 N at. %. Characterization of the magnetic properties revealed that samples with the highest magnetization also exhibit an increase in magnetic hardness.