The magnetization of γ′-Fe4N: theory vs. experiment

By reviewing the experimental and theoretical literature on γ′-Fe4N, and by a systematic survey of predictions by the LDA, PBE, WC, LDA + U (2×), PBE + U (2×) 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 γ′-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 γ′-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. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)