Pressure effects on the electronic properties and the magnetic ground state of γ′-Fe 4N

Abstract

The effects of pressure on the electronic properties and the magnetic ground state of γ′-Fe 4N have been investigated using both the 57Fe Mössbauer effect (up to 6.1 GPa at 4.2 K) and X-ray diffraction (up to 8.0 GPa at 300 K) techniques. The analysis of the X-ray diffraction data under pressure shows a linear decrease of the volume with pressure with a compressibility of 5.1(1) × 10 −3 GPa −1. From the measured difference ( ΔS = 0.06 mm/s) between the isomer shifts of the Fe(I) and Fe(II) sites at ambient pressure, a charge transfer from N-2p to Fe(II)-3d is proposed, which is supported by recent band structure calculations. The pressure-induced decrease of S of Fe(I) and Fe(II) is found to be nearly the same, indicating that the increase of the s-electron density at the 57Fe nucleus is governed by the compression of the 4s-electrons rather than by charge transfers. Regarding the pressure dependence of the magnetic moments of the Fe(I) and Fe(II) sites, we find in contrast to the results of very recent high-pressure NMR experiments on γ′-Fe 4N, which reveal almost a comparable pressure-induced decrease of the 57Fe NMR frequency of the two Fe sites, that the magnetic hyperfine (hf) field ( B eff) at the Fe(I) site decreases more rapidly with pressure than that at the Fe(II) site. This finding is inconsistent with recent band structure calculations on γ′-Fe 4N which show that the magnetic moment of the Fe(I) site is more localized than that of the Fe(II) site. We suggest that this discrepancy between theory and our experimental results is due to the contribution of a transferred hf field to the pressure dependence of B eff, which is sensitive to the variation of Fe-N interactions with pressure.