Synthesis And Magnetic Property Research Of Co-N Thin Films

Since the late eighties and early nineties, the magnetic thin films have drawn many scientists'attention. One important reason is that when magnetic materials are transformed into thin films, a lot of properties which are not obvious or found in the bulk materials have been observed, such as perpendicular anisotropy, magneto-optical effects, giant magneto resistive effect. These properties can be designed by changing the composition of thin film, and these properties may be used to invent a variety of special devices. We will greatly improve our lives with these technologies applied into practice. Co-N thin films are mainly used in the field of high-density magnetic recording. Firstly, magnetic recording materials were made ofγ-Fe2O3 or Fe-Co alloy, but their practical applications are confined by the disadvantages, such as limited resources, corrosion resistance, abrasion resistance, oxidation resistance. Co-N thin films with excellent ferromagnetic performance and strong perpendicular magnetic anisotropy, are expected to be used as high-density perpendicular magnetic recording materials.The first report about Co-N thin films is published by M. Matsuoka et al in 1986. Their found Co-N thin films have very strong anisotropy (Hk = 4kOe) perpendicular to the film plane, and coercivity is 800Oe. Authors proposed that these films can be used in high-density magnetic recording field; K. Oda synthesized thin films composed of Co4N phase in 1987, the films displayed moderate coercivity (300-1000Oe); K. K. Shih and J. Karasinski found that the coercivity of the Co-N thin film has a minimum (4Oe) when the nitrogen partial pressure increases, and the minimum corresponds to Co4N single phase structure. From these studies, we can conclude that the magnetic properties of Co4N phase need more research.In this paper, a series of Co-N thin films are grown on Si (111) substrates by DC magnetron sputtering, and then characterize them by XRD, SAED, SEM and SQUIDS in order to study the relationship between the properties of Co-N thin films and the parameters in this experiment. The conclusions are summarized as follows:1. The impact of nitrogen partial pressure on structure, surface morphology and magnetic properties of Co-N thin films:1.1β-Co(N) phase thin films are synthesized in low nitrogen partial pressure with N atoms arrayed in disorder. Samples N1 and N3 have lower saturation magnetization (110.5Am2/kg) and coercivity (2.71kAm-1) than N0, it means that few nitrogen reduces coercivity ofβ-Co thin films;1.2 When the nitrogen partial pressure increases to 10%, Co4N phase thin film is synthesized. This film has a strong growth orientation in (111) plane, and N atoms arrayed in body-centered position in order. Grains tend to be triangular shape. Its coercivity is 7.8kAm-1, saturation magnetization is 103.9Am2/kg, much higher than that reported by Oda of 46.5Am2/kg.1.3 When the nitrogen partial pressure increases to more than 15%, Co3N phase with less Co content is synthesize. At this time thin films are mixtures of Co3N and Co4N, and the content of Co3N phase increases with the raise of the nitrogen partial pressure. Grains shape trend to be enlarged due to the formation of Co3N phase. Co3N phase is a paramagnetic phase, so saturation magnetizations of N15 and N20 decrease further, respectively 87.3Am2/kg and 71.9Am2/kg. Coercivity of thin films increases because of the existence of Co3N phase, coercivity of N15 and N20 were 7.72kAm-1 and 9.32kAm-1 respectively.2. The impact of sputtering power on structure, surface morphology and magnetic properties of Co-N thin films:2.1 The content of Co increases when the sputtering power rises. When sputtering power is 11.2W, Sample obtained is a mixture of Co3N and Co4N, and the grain size is very small with square grain morphology. The saturation magnetization and coercivity of P1 are very low, 12.9Am2/kg and 5.57kAm-1 respectively. 2.2 When the sputtering power increases to more than 34W, samples synthesized are composed of Co4N single phase. Grains shape is still square, but the grain size increasing. The saturation magnetization and coercivity can increase to 32.0Am2/kg and13.85kAm-1, respectively.2.3 When the sputtering power increases to 200W, sample obtained is still composed of Co4N single phase, but the shape of grains changes a lot, is triangle. The saturation magnetization increases greatly to 103.9Am2/kg, while coercivity decreases to 7.80kAm-1. We can conclude that sputtering power has a great influence on soft magnetic properties of thin films.