First-principles Study On Fe-N Compounds

Fe-N compounds have received considerable attentions since they have excellent ferromagnetic properties, good oxidation resistance, corrosion resistance and abrasion resistance due to its special composition and structure. They are the ideal candidate materials of magnetic recording medium and magnetic head in the form of thin film, and also can be used as important surface modified materials.In this dissertation we study the structure and properties of three typical Fe-N compounds for Fe16N2, Fe4N and Fe3N based on density functional theory by using first-principles. The discussions focus on phase stability, mechanical and magnetic properties of Fe16N2, slab structure and surface characteristics of Fe4N, electronic structure and mechanical properties of Fe3N. Finally, the correlation between structure and physical properties of Fe-N compounds was systematically studied. The main conclusions are as follows:(1) The result of formation energy indicates that Fe16N2is a thermodynamically stable phase at the ground state. The analysis of electronic structure shows that it exists strong covalent bond between N atom and the nearest neighbor Fe1and Fe2atoms, which results from hybridization between p electrons of N atom and d electrons of Fe atom. The strength of bond is significantly higher than other bonding between atoms. The calculated results of elastic constants show that Fe16N2possess toughness, and it is mechanical stable. The elastic anisotropy is very significant in the a-c crystal plane. The calculations of magnetic properties for Fe16N2indicate that the average magnetic moment of Fe atoms increases while considering the Coulomb interaction between3d electrons of Fe atoms. The differences of magnetic moments for non-equivalent Fe atoms in Fe16N2mainly due to interstitial N atoms that cause distortions of lattice, and then induce different strength of hybridization between non-equivalent Fe atoms and N atoms. The interaction among different d electron orbits of non-equivalent Fe atoms themselves changes, which leads to variations of numbers of3d spin electron and distribution of state of Fe atoms, and causes differences of magnetic moments in non-equivalent Fe atoms.(2) Calculation of surface stability indicates that only the Fe2N-terminated surface can exist on the (001) surface of cubic Fe4N. The analysis of electronic structure indicates that valence and conduction bands overlap considerably at the Fermi level, suggesting that slabs of Fe4N would also exhibit metallic conductivity. Moreover, electronic structure of the (001) surface of cubic Fe4N have more localized feature than bulk Fe4N due to structure relaxation. The correlation between slab thickness of Fe4N and magnetic properties indicates that the average magnetic moments of Fe atoms decrease with decreasing slab thickness, and the difference of magnetic moments is mainly due to the smaller magnetic moment of Fe2in the second layer of slab structures.(3) Calculation of density of states for Fe3N indicate that the value of density of states at Fermi level N(EF) decreases and system is more stable after considering the Coulomb interaction between3d electrons of Fe atoms. The values of magnetic properties show that experimental measurements of magnetic moment generally less than the theoretical values, because that the samples of Fe3N for experimental measurements are not pure phase but a mixed phase for s-FexN(2<x≤3). The calculated results of mechanical properties indicate that Fe3N is mechanical stable. It is better to reflect the nature of bonding between atoms in system after considering the Coulomb interaction between3d electrons of Fe atoms, and elastic moduli and Debye temperature of material are in better agreement with experimental data. Calculated results of elastic anisotropy indicate that there is very little compressibility anisotropy and slightly anisotropy in shear.(4) The variation of crystal structures, mechanical properties and magnetic properties for three typical Fe-N compounds such as Fe16N2, Fe4N and Fe3N is studied. The correlation between structure and physical properties of three compounds is discussed. The crystal structures of this three ferromagnetic compounds can be respectively considered as resulting from the insertion of nitrogen within the metal lattice of bcc α-Fe, fcc γ-Fe and hcp ε-Fe. The increase in the amount of nitrogen schematically translates the transformation of the symmetry of the iron sublattice from bcc to fcc, and eventually to hcp. The bulk modulus of three compounds increases with increasing nitrogen content. Debye temperature rises with enhanced hybridization between Fe and N atoms in compounds. The average magnetic moment of compounds decreases with increasing nitrogen content.