Investigation On Nanocrystalline Fe-N Thin Films Grown By Direct Current Magnetron Sputtering

Nanocrystalline Fe-N thin films have been widely studied. They are excellent materials due to their variety of structural and high corrosion/wear resistance and good mechanical properties. Recently, ferromagnetic iron nitride compounds, such asα?-Fe16N2,γ?-Fe4N andε-Fe3N, have attracted considerable attention by virtue of their excellent magnetic properties. In particular, among the different phases,γ?-Fe4N has been considered to be a potential candidate for a high-density recording material, which has a good resistance to corrosion than that ofα-Fe. Moreover, theγ?-Fe4N phase has a better thermal stability and chemical stability than theα?-Fe16N2 phase. In this study, synthesis and characterization of Fe-N films grown by direct current magnetron sputtering are investigated systemically. The conclusions are summarized as follows:1. The structural, morphology and magnetic properties of nanocrystalline Fe-N thin films were significantly influenced by deposition conditions.The nanocrystallineγ?-Fe4N andε-Fe3N thin films were synthesized on glass,single crystal Si (100),and NaCl (100) substrate, respectively, by DC magnetron sputtering. Effect of the substrate temperature, sputtering time and substrate bias voltage on the growth and properties of the films was investigated. The results showed: (1) With the rise of the substrate temperature, the adhesion force between Fe and N particle with substrate, the efficiency for the reaction between Fe and N, the atomic ratio Fe/N in Fe-N films increased. The structure of iron nitride compounds was influenced by these changes. (2) The adhesion force between Fe and N particle with substrate, the efficiency for the reaction between Fe and N and roughness of thin films were influenced by substrate materials. With the rise of roughness of the substrate materials, the adhesion force between Fe and N particle with substrate, the efficiency for the reaction between Fe and N, the atomic ratio Fe/N in Fe-N films and roughness of thin films increased. (3) The coercive force of thin films increased with increasing thickness and RMS when the sputtering time increased. (4) The efficiency for the reaction between Fe and N and the growth rate forγ?-Fe4N were improved when the substrate was biased. The films obtained at substrate bias voltage were smooth, and values of Ms for all Fe-N films were almost the same, but their coercivity was quite different. With the increase of substrate bias voltage, the coercivity of Fe-N films decreased.2. The structure, morphology and magnetic properties of nanocrystal Fe3N thin films were influnced by deposition conditions.Theε-Fe3N thin films with different grain sizes deposited at different substrate temperatures (150℃, 250℃, 350℃and 450℃respectively) were analyzed by XRD and SEM. experimental results showed that the spindle shapeε-Fe3N particles were arranged paralleled to surface of thin films. The growth of thinε-Fe3N film had a highly preferred orientation along (110). VSM results showed thatε-Fe3N had ferromagnetic properties, and its unsaturated magnetization measured at 5kOe field increased with increasing grain sizes. According to the relationship between coercive force Hc and grain size D, Hc could be described by Hc∝D6, when D was less than exchange length Lex. It can be deduced that with decreasing the grain size ofγ?-Fe4N thin films, Hc decreased. Nevertheless, Hc∝1/D, when D was more than exchange length Lex.3. The dependence of structural and magnetic properties on temperature at low temperature of nanocrystalγ?-Fe4N thin film was explored.Theγ?-Fe4N thin films deposited at Si substrate were analyzed by XRD and SEM. Experimental results showed that theγ?-Fe4N phase had a grain size of 55nm. The growth of thinγ?-Fe4N film had a highly preferred orientation along (111). VSM results showed that the ratio of remanence Mr/Ms at room temperature ofγ?-Fe4N was 0.528, The thin films had single easy magnetization direction. In the range of 80～350K, the saturation magnetization Ms and coercive force Hc increased with decreasing the testing temperature, whereas the Mr/Ms decreased. The relation between the coercive force Hc and temperature T accorded to T1/2 law, Hc(0) =356.87Oe, TB=667K. The relation between the saturation magnetization Ms and temperature T showed that the interaction among spin waves should be considered. By fitting, we obtained Ms(0)=202emu/g;constant B=1.0×10-4K-3/2,C=1.485×10-8K-5/2,D=3.465×10-11K-7/2,and Je=1.537×10-21。4. The structure, magnetic properties and thermal stability of nanocrystalγ?-Fe4N thin film were studied.The Fe-N samples after heat treatment were analyzed by XRD and SEM. The results showed: as the annealing temperature was in the range of 300℃～500℃, the films consisted ofγ?-Fe4N grains packed withα?-Fe8N. The transformation of packed layer fromα?-Fe8N toα-Fe appeared at annealing temperature of 600℃. When the temperatue was in the range of 300℃～600℃,γ?-Fe4N phase was dominant and its average grain size was kept 47nm. Theγ?-Fe4N andα?-Fe8N phase transformed intoα-Fe phase at annealing temperature of 700℃. The packed layer could prevent the diffusion of N atom.In summary, iron nitride films were deposited by dc magnetron sputtering using an Ar/N2 gas mixture. The structure, roughness, surface morphology and magnetism of the films were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), scaning electron microscopy (SEM), superconducting quantum interference device (SQUIDS) and vibrating sample magnetometer (VSM). The deposited conditions of nanocrystal single phaseε-Fe3N andγ?-Fe4N were optimized, and the magnetic properties at lower temperature and thermal stability for the obtained Fe-N samples were explored.