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摘要
The phase transition law of Fe-N system is very important for efficiently synthesizing single-phase gamma'-Fe4N thin films. The gamma ''-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 degrees C/min, the Fe-N system has five phase transitions in a temperature range between room temperature (RT) and 800 degrees C, i.e. I (330-415 degrees C): gamma ''-FeN ->xi-Fe2N with an endothermic value of 133.8 J/g; II (415-490 degrees C): xi-Fe2N ->epsilon-Fe3N with no obvious latent heat of phase change; III (510-562 degrees C): epsilon-Fe3N ->gamma'-Fe4N with an exotherm value of 29.3 J/g; IV (590-636 degrees C): gamma'-Fe4N ->gamma-Fe with an exotherm value of 42.6 J/g; V (636-690 degrees C): gamma-Fe ->alpha-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 N-2 is a single-phase gamma ''-FeN film, and it becomes a single-phase xi-Fe2N film after being annealed at 350 degrees C for 2 h, a single-phase epsilon-Fe3N film after being annealed at 380 degrees C for 2 h, and a single-phase gamma'-Fe4N film after being annealed at 430 degrees C 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 gamma'-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/cm(3)) both in the plane and out of the plane.
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单位中国科学院兰州化学物理研究所; 电子科技大学; 甘肃省科学院
