Fe16N2 Structure Research Articles

Facile synthesis of single-phase spherical α″-Fe16N2/Al2O3 core-shell nanoparticles via a gas-phase method

When nitrogen was inserted into the spherical α-Fe/Al2O3 core shell of 45 nm nanoparticles, the XRD pattern showed a clear change in the crystal modification from a body-centered cubic crystal to that of a single-phase α″-Fe16N2 structure. SEM, TEM, and energy-dispersive X-ray spectroscopy mapping analysis gave the particle size distributions, the shell thickness, and the Fe and Al elements. An examination of the total electron yield (surface sensitive) and fluorescence yield (bulk sensitive) of X-ray absorption fine structure on Fe and N atoms of these core shell nanoparticles confirmed the nitriding of the core iron and showed iron oxide formations on the core surface, indicating stability and resistivity performance. The nitriding process also changed the magnetic properties from paramagnetic to ferromagnetic with a coercivity above 3000 Oe, indicating a promising material for a “rear-earth-free” giant magnet.

First-principles study on electronic structure and elastic properties of Fe16N2

We have performed first-principles study on structural stability, elastic properties and electronic structure of Fe16N2 by applying LSDA+U method. The calculated values of formation energy and reaction enthalpy for decomposition reaction indicate that Fe16N2 is a thermodynamically stable phase at the ground state. The six independent elastic constants are derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio are determined as 180GPa, 199GPa, 76GPa and 0.32, respectively. The elastic constants meet all the mechanical stability criteria. The ductility of Fe16N2 is predicted by Pugh's criterion. The strong bonding between Fe and N atoms results in high values of elastic constants C11 and C33, and contributes to the strengthening of the Fe16N2 structural stability. The total and partial densities of states (DOS) suggest the existence of hybridization between N-p and Fe-d bands. The position of the Fermi level in DOS curve implies that Fe16N2 is a metastable phase.

磁石・磁性材料の化学 ＩＩ α鉄のアンモニア窒化による窒化鉄ＦｅｘＮ（ｘ：２，２〜３，４，１６／２）の合成とＦｅ１６Ｎ２バルク試料の磁性

Iron nitrides were prepared by nitrogenizing α-iron in ammonia gas at temperatures from 100°C to 700°C. In the case using iron foils as a starting material, Fe2-3N and Fe4N were formed as a main crystal phase at the temperature ranges of 450-700°C and 250-400°C, respectively. A crystal phase like to Fe16N2 was formed at 210°C, but its XRD and Mossbauer data were too insufficient to identify as Fe16N2. While bulk Fe16N2 was formed by nitrogenizing iron fine powder with the diameter of about 20nm at 110°C for 10days, but small amounts of Fe3N coexisted as a second phase with Fe16N2 at 120°C for 10days. Crystal structure of Fe16N2 was refined by Rietveld method. The average magnetic moment of three kinds of Fe atoms in Fe16N2 was evaluated to be 2.52μB on the basis of its Mossbauer data, the value of which was larger by 15% than the magnetic moment of α-Fe. The saturation magnetization of the Fe16N2 powder was also larger by about 14% than that of the α-Fe powder before nitrogenizing.

Magnetic and physical microstructure of Fe16N2 films grown epitaxially on Si(001)

Epitaxial Fe16N2 films were grown on Si(001) substrates with an Ag underlayer by reactive sputtering in nitrogen. Pure α′-Fe8N films were obtained which on subsequent annealing resulted in mixtures of α′-Fe8N (54%) and α′′-Fe16N2 (46%). An average moment of 1780 emu/cc, considerably larger than that of pure α-Fe (1714 emu/cc), was measured for both samples. Plan-view transmission electron microscopy of the films confirms the orientation relationship Fe16N2(001)‖Ag(001)‖Si(001) and Fe16N2[100]‖Ag[110]‖Si[100], and a small grain size (∼100 Å), while electron energy-loss spectroscopy confirms an average composition of Fe8N for both samples. Electron diffraction patterns indicate that the as-deposited α′ films already contain very small regions of ordered α′′ which were not previously detected by x-ray diffraction measurements. Mössbauer spectroscopy performed at both 300 and 16 K gave three hyperfine fields corresponding to three different iron sites for both the unannealed α′ and the annealed α′/α′′ mixtures. Lorentzian fitting of the three iron components for the α′/α′′ spectrum obtained at room temperature gave an intensity ratio of 1:2:1 (FeI:FeII:FeIII) corresponding to the expected occupancy for the three Fe sites in the Fe16N2 structure. Moreover, the pure α′ film at 300 K and both samples at 16 K showed deviation from this distribution. The three components show notable differences in the temperature dependence of their occupancies; however, all three magnetic components deviate similarly from the surface normal.

The calculated electronic and magnetic structure of martensite iron nitrides

The electronic structure and local magnetic moment of BCT-FeN compounds were calculated by the first-principles self-consistent cluster method. For the alpha '-martensite, the author found that the magnetic moment of the first-neighbour Fe atoms of N was reduced while that of the second-neighbour Fe atoms was increased compared with those of pure BCC-iron; this was attributed to the concomitant effects of p-d interactions and tetragonal elongation of BCT-FeN, and was in good agreement with experiment. For the Fe16N2 structure, the calculated average magnetic moment is smaller than that of experiment.

Structure and magnetic moment of α"-Fe16N2 sputtered films

α”-Fe16N2 with different lattice constants a and c were synthesized on MgO single crystal substrate under various fabrication conditions (total gas presure, deposition rate and N2 flow ratio). Magnetic moment of α”-Fe16N2 compound films was discussed in connection with the change of the structural properties. As a result, no strong correlation was found between saturation magnetization, σs, and structural properties (1) degree of order of N atoms in b.c.t. Fe16N2 structure, (2) the magnitude of unit cell volume of α”-Fe16N2 and (3) the value of axial ratio of α”-Fe16N2.

Mössbauer study on Fe16N2 films prepared by ion-implant nitrification of iron films

Mössbauer spectroscopy was carried out to elucidate the magnetic structure of Fe16N2 with the giant magnetization. Samples were prepared by ion-implant nitrification of single-crystal iron films, followed by an annealing at 150 °C for 60 h. The nitrification developed three Fe atom environments, attributable to three different Fe sites in Fe16N2 structure. The magnetic moments of the first, second, and third nitrogen neighbor Fe atoms were determined to be 1.3, 2.5, and 3.8 μB, respectively. The quadrupole effect in Fe16N2 was interpreted consistently on the basis of the consideration that the magnetization is directed along [001] and nitrogen atoms had a negative charge.