Controllable synthesis of γ′-Fe₄N via prolonged high vacuum magnetic annealing of deposited Fe–N thin films

In order to develop a fast and low-cost method for preparing γ′-Fe₄N thin films for spintronic applications, the iron nitride (Fe–N) thin films were deposited through reactive sputtering in pure nitrogen, followed by high vacuum magnetic annealing process. The X-ray diffraction pattern (XRD) results revealed that the N content of the Fe–N thin films were decreasing continuously with the increase of annealing temperatures and times, so as to affect their crystal phase compositions, thus a single phase of γ″-FeN, ξ-Fe₂N and γ′-Fe₄N can be controllable prepared through annealing process. The phase transition characteristics for Fe–N to γ′-Fe₄N were studied carefully. It was found that the sample (Fe_1.8N) with a mixed crystal phase of γ″-FeN and ξ-Fe₂N can be totally changed into a single phase of γ′-Fe₄N by annealing it at 430 °C for 3 h, and the annealing temperatures can also be as low as 375 °C but for a prolonged time to 24 h. The stoichiometric matter of the as-prepared Fe–N thin films was confirmed by the XPS analysis. The vibrating sample magnetometer (VSM) results indicated that the magnetic properties of the Fe–N thin films are mainly depending on their crystal phase compositions. The saturation magnetization (M_s) of γ′-Fe₄N films depends on various factors, and it was to about 1080 emu/cm³ for the γ′-Fe₄N/MgO/Si.