L1₀ Ordering Transformation Kinetics And Controlling In FePt Thin Films

FePt thin films are potential candidates for ultrahigh density magnetic recording media since L1₀ ordered FePt crystals have a high magnetocrystalline anisotropy energy density. The high ordering temperature, however, is unfavorable for the practical application of these films as magnetic recording media. How to accelerating the ordering process and reducing the ordering temperature of FePt thin films are of great significance to promote its practical application. In this paper, the subjects about the kinetics and controlling of the ordering transformation of FePt thin films were studied by employing X ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) techniques as following:The individual annealing temperature and annealing time dependence of the nucleation and growth parameters of the L1₀ ordered domains in FePt thin films are studied by slowing down the ordering process of the films through choosing the proper annealing conditions, based on which the nucleation and growth processes of L1₀ ordered domains in FePt thin films have been studied separately. The nucleation activation energy En=0.5～0.6 eV and the growth activation energy Eg=0.9 eV are determined. En/Eg<1 indicates that the ordering process of FePt thin films is mainly controlled by the growth of the L1₀ ordered domains.An interface reaction to form a PtSi interface layer between the FePt thin films and the Si substrates was induced for the FePt thin film deposited directly on HF-cleaned Si substrates after annealing at the optimized conditions. The interface reaction accelerates the L1₀ ordering process of the FePt thin films at low annealing temperatures. Experimental results indicates that the coercivity Hc=5.4 kOe and the ordering degree parameter S=0.66 for the FePt thin film deposited on Si substrates annealed at 350 oC are much higher than that Hc=0.7 kOe and S=0.28 for the films deposited on Si/SiO₂ substrates annealed at the same temperature. The former values are close to that Hc=6.5 kOe and S=0.74 for the FePt thin films deposited on Si/SiO₂ substrates annealed at 600 oC. The accelerated L1₀ ordering of the FePt thin films at low temperatures may result predominantly from the formation of a dynamic compressive stress on the films during interface reaction. The interface reaction-induced PtSi layer can act as a diffusion barrier layer and successfully prevents the interdiffusion between the FePt thin films and the Si substrates.The grain size of the as-deposited FePt thin films was reduced by improving the sputtering pressure combined with changing the ratio of the Fe and Pt in the composite FePt target. The ordering process of the FePt thin films with the as-deposited grain size of d_as=3.4 nm and d_as=8.0 nm, which have the same compositon, are contrastively studied. Experimental results indicate that the L1₀ ordering process of the FePt thin films with the ultra-small as-deposited grain size d_as=3.4 nm has been significantly accelerated compared with the film with the as-deposited grain size of d_as=8.0 nm. The ordering parameter S close to 1 and a large coercivity H_c=14.7 kOe are obtained for the FePt thin films with the as-deposited grain size of d_as=3.4 nm after annealing at 600 oC, which are much higher than that S=0.7 and Hc=7.2 kOe for the film with the as-deposited grain size of d_as=8.0 nm. The accelerated L1₀ ordering of the FePt thin films with the ultra-small deposited grain size may be attributed to the anhanced nucleation and growth of the L1₀ ordered domains by increasing the atomic diffusion due to the increase of the grain boundaries induced by the reduction of the grain size.