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

Abstract

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)

Spin-density wave in Cr: Nesting versus low-lying thermal excitations

“…We can summarize by saying that LDA finds the experimental magnetic moment in the neighborhood of the experimental lattice constant but not at its own equilibrium, while GGA gives a lattice constant comparable with the experimental value but with a magnetic moment that is twice too large: none of both therefore gives a satisfactory description of Cr. The behavior of these two functionals is very similar to what is found in other 3d transition metal compounds as, e.g., Fe 4 N. 34 The LDA+ U functional provides an alternative. LDA+ U is a semi-ab initio method built on LDA but containing two additional free parameters: U ͑related to on-site Coulomb repulsion or electron correlation͒ and J ͑related to intra-atomic exchange͒.…”

Deposition and spin polarization study of Fe4N thin films with (111) orientation

“…This result is similar to the M s of Fe 4 N thin films reported by several other experimental works. 10,11,15 It is also consistent with our previous experiment result of Fe 4 N. 19 The M s of Fe 4 N obtained in this work shows smaller value than the theoretical calculation results that are in a range of 2.1-2.45 µ B /Fe atom [20][21][22] (1410-1650 emu/cm 3 ) at zero temperature (0K). We attribute the smaller M s of Fe 4 N thin films in our experiment to the thermal excitation on magnetization as well as the defects and grain boundaries of the thin film sample.…”

Phase formation, thermal stability and magnetic moment of cobalt nitride thin films

“…9 Theoretical studies suggest that such volume expansion enhances the itinerary of conduction band electrons and results in spreading of bandwidth of 3d electrons thereby suppression of exchange splitting leading to a larger (than pure metal) saturation magnetization. 10 Experimentally, Fe 4 N thin films have been prepared using magnetron sputtering and molecular beam epitaxy and they show higher than Fe magnetic moment. 3 However in case of Co 4 N thin films such enhanced magnetic moments have not yet been observed experimentally.…”