Thermal analysis on crystal phase synthesis of iron nitride film and its magnetic properties

Abstract

The phase transition law of Fe-N system is very important for efficiently synthesizing single-phase <i>γ</i>'-Fe₄N thin films. The <i>γ</i>"-FeN thin films are deposited on silicon wafers via DC reactive magnetron sputtering; some of them are stripped from the silicon wafers and measured by using the synchronous thermal analysis (TG-DSC) for studying the phase transition law of Fe-N system. The results of TG-DSC show that at a heating rate of 10 ℃/min, the Fe-N system has five phase transitions in a temperature range between room temperature (RT) and 800 ℃, i.e. I (330−415 ℃): <i>γ</i>''-FeN→ξ-Fe₂N with an endothermic value of 133.8 J/g; II (415−490 ℃): ξ-Fe₂N→ε-Fe₃N with no obvious latent heat of phase change; III (510−562 ℃): ε-Fe₃N→<i>γ</i>'-Fe₄N with an exotherm value of 29.3 J/g; IV (590−636 ℃): <i>γ</i>'-Fe₄N→<i>γ</i>-Fe with an exotherm value of 42.6 J/g; V (636−690 ℃): <i>γ</i>-Fe→α-Fe with an endothermic value of 14.4 J/g. According to the phase transition law of Fe-N system, the crystal phase of iron nitride thin film is effectively regulated by vacuum annealing. The x-ray diffraction pattern (XRD) results show that the iron nitride thin film obtained by direct-sputtering in pure N₂ is a single-phase <i>γ</i>"-FeN film, and it becomes a single-phase ξ-Fe₂N film after being annealed at 350 ℃ for 2 h, a single-phase ε-Fe₃N film after being annealed at 380 ℃ for 2 h, and a single-phase <i>γ</i>'-Fe₄N film after being annealed at 430 ℃ for 7 h. The annealing temperature for the phase transition of Fe-N thin film is generally lower than that predicted by the TG-DSC experimental results, because it is affected by the annealing time too, that is, prolonging the annealing time at a lower temperature is also effective for regulating the crystal phase of Fe-N thin film. The magnetic properties of the Fe-N thin film are also studied via vibrating sample magnetometer (VSM) at room temperature. The <i>γ</i>'-Fe₄N polycrystalline thin film shows an easy-magnetized hysteresis loop for the isotropic in-plane one, but a hard-magnetized hysteresis loop with a large demagnetizing field for the out-of-plane one, which belongs to the typical magnetic shape anisotropy. However, their saturation magnetizations are really the same (about 950 emu/cm³) both in the plane and out of the plane.