The magnetization of gamma '-Fe4N: theory vs. experiment

By reviewing the experimental and theoretical literature on gamma'-Fe4N, and by a systematic survey of predictions by the LIDA, PBE, WC, LDA + U (2x), PBE + U (2x,) and B3PW91 exchange-correlation functionals, the structural, magnetic and hyperfine properties of this material as well as their pressure dependencies are interpreted. The hypothesis is Put forward that gamma'-Fe4N as found in Nature is exactly at a steep transition between low-spin and high-spin behaviour. PBE + U (U = 0.4 eV) is identified as the most accurate exchange correlation functional for this material, although it is needed to fix the magnetization at the experimental value to obtain a satisfying description. Remaining disagreement between theory and experiment is pointed out. A recent experimental claim for a giant magnetic moment in gamma'-Fe4N is discussed, and is not reproduced by our calculations. We expect that the new insight obtained in the present work can lead to a consistent ab initio modeling of other materials in the iron-nitrogen binary system. In an accompanying didactic section, the physics behind some common exchange-correlation functionals is outlined. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim