Structure, Magnetic Properties And Magnetoresistance Of γ′ Fe₄ N Films

Iron nitride films were extensively investigated due to the excellent magnetic, resistance to wear, corrosion, oxidation resistance properties those make them suitable for the practical applications in the field of magnetic devices, coating and steel industries. With the rapid development of spintronics, the iron nitrides as potential candidates for the spin injection source in the spintronics devices have been paid a lot of attention. The iron nitride films are one of the hot topics in the field of the condensed matter physics and materials science.In this work, we have performed a systematic study on theγ′–Fe₄N films due to its simple crystalline structure, high chemical inertness, high thermal stability, high Curie temperature and excellent magnetic properties. The as-deposited iron nitride films, post-annealed iron nitride films, polycrystalline and epitaxialγ′–Fe₄N films were fabricated using a facing-target reactive sputtering method. Their phase structure, microstructure, magnetic and magnetoresistance effects were measured and analyzed systematically.It is found that the nitrogen concentration in the as-deposited and post-annealed iron nitride films increases with the increase of nitrogen flow rates and the dominant phase of the iron nitride films evolve fromα–Fe intoξ–Fe2N with the increase of nitrogen flow rates in the as-deposited and post-annealed iron nitride films. The saturation magnetization of the as-deposited iron nitride films decreases with the increase of the nitrogen flow rates. After being annealed, theγ′–Fe₄N phase appears in the post-annealed iron nitride films and other iron nitride phases coexist in the post-annealed iron nitride films. The saturation magnetization of the post-annealed films is higher than that of the as-deposited iron nitride films.It is found that the single-phase polycrystallineγ'?Fe₄N film was obtained at a nitrogen flow rate of 20 sccm and a substrate temperature of 450°C. The magnetic and electrical transport properties of the iron nitride films vary with the change of nitrogen flow rate and substrate temperature.It is found that the saturation magnetization of the polycrystalline and epitaxialγ'–Fe₄N films with different film thicknesses increases with the decrease of measuring temperature. The polycrystalline and epitaxialγ'–Fe₄N films show the metallic conductive behavior. Many kinds of mechanisms influence the magnetoresistance of the polycrystalline and epitaxialγ'–Fe₄N films. The magnetoresistance presents a complex behavior. The anisotropic magnetoresistance of the epitaxialγ'–Fe₄N film is attributed to spin-orbit interaction, which lowers the symmetry of the wavefunctions. The symmetry reduction makes the s–d scattering anisotropic, and ultimately leads to the anisotropic magnetoresistance. The negative anisotropic magnetoresistance can be attributed to the minority spin conduction inγ'–Fe₄N films.