Preparation, Microstructure And Properties Of Fept And FePt-Si-N Films For Magnetic Recording Medium

As a new generation material for the high density magnetic recording medium, L1₀-FePtordered alloys have high uniaxial magnetocrystalline anisotropy constant （107erg/cm3） andhigh thermal stability even for very small grain size （3⁵nm）. Current researches on this typeof alloys mainly focused on reducing its grain size, weakening the magnetic couplinginteraction between FePt grains, and obtaining perpendicular anisotropy. In this thesis, theeffects of the composition and ordering annealing on the magnetic properties of pure FePtcontinuous films have been systematically studied. Si-N was introduced into the FePt films toform nanogranular FePt-Si-N films. The preparation, microstructure and magnetic propertieswere systematically investigated. In addition, the preparation and structure of the magneticunderlayer and its effect on the anisotropy of the FePt films were also studied.Firstly, FePt films with various thicknesses and Fe/Pt atom ratios were fabricated by DCmagnetron sputtering, followed by annealing at500⁷00°C for1h. The influences of filmthickness, Fe/Pt atom ratio and heat treatment on the structure and magnetic properties of thefilms were investigated. It was found that the film thickness has a great effect on the orderingdegree of FePt films. The ordering degree and magnetic properties were improved byincreasing the film thickness. The high annealing temperature and long annealing time canpromote the atoms diffusion, resulting in an increase in the coercivity of the film. When Feand Pt atomic ratio is55:45, the film has the highest ordering degree, the fastest orderingprocess and the highest coercivity. The optimal magnetic property of HCis14.2kOe.Perpendicular orientation can be obtained in the film when the film thickness is small, such as20nm.Secondly, Si-N was introduced into the FePt films to form nanogranular FePt-Si-Ncomposite films. The effect of the annealing temperature, Fe/Pt atomic ratio and Si, Nadditions on the structure and magnetic properties were investigated. FePt-Si-N films have thehighest coercivity at an Fe content of55at%. The ordering temperature for FePt-Si-N films is100°C higher than that of FePt films. Doping Si and N can improve the coercivity of FePtfilm through weakening the exchange coupling and reducing the grain size. To achieve highcoercivity in FePt-Si-N film, the Si/N atomic ratio has to be well controlled to form amorphous Si-N phase. For the (Fe₆₀Pt₄₀)₉₀Si₁₀-N films prepared with various nitrogen partialpressures, the highest coercivity and lowest switching field distribution were obtained at thepartial pressure of20%. The atomic force microscopy （AFM） characterizations showed thatthe FePt particle size of the film is uniform and the surface roughness is0.8nm. Themicrostructure of (Fe₅₅Pt₄₅)₉₀Si₁₀-N film is characterized by HRTEM and EDS. Thenanocomposite granular structure consisting of FePt nanograins and Si-N amorphous matrixwas confirmed.Finally, the magnetic underlayers for anisotropic FePt films were investigated. Cr filmswere prepared by heating substrate with magnetron sputtering. The effects of substratetemperature, bias voltage, film thickness and sputtering power on the orientation of the Crunderlayer were studied. The results showed that crystallization temperature of the Cr film isabout400°C and the bias voltage is not beneficial to the formation of Cr （200） texture. Theintensity of Cr （200） peak is increased with increasing the film thickness. As the sputteringpower increases, the Cr （200） peak is slightly enhanced. The effect of Cr layer on the structureof FePt film was also discussed. Inserting Cr underlayer will reduce the ordering degree of theFePt film. However, the optimal process for fabricating anisotropic FePt films is still in needof research.The work in this thesis indicate that the FePt-Si-N nanocomposite granular films withsmall grain size, weak intergranular exchange interaction and high coercivity are promisingcandidates for the high density recording media. However, it remains a challenge forachieving high anisotropy in this type of films.