Studies Of Fe-Pt Thin Films For Ultra-high Density Magnetic Recording Media

The areal density of hard disk drive using traditional CoCr-based thin films media can not be elevated further because of the super-paramagnetism effect. In order to increase the areal density in magnetic recording system, the studies of new magnetic recording media materials are necessary. This dissertation has studied the possibility of FePt thin films as ultra-high density perpendicular magnetic recording media. Due to its high uniaxial magnetocrystalline anisotropy (7 × 10⁷erg/cm³), fct-FePt grains could be fined to nanometer-sized while remaining enough thermal stability. Therefore, lager areal density could be realized on these tiny grains.This dissertation focuses on two aspects. The first is the reduction of transforming temperature of FePt from fcc to fct phase in FePt or FePt-Al₂O₃ thin films. The actual hard disk manufacture line do not endure the temperature as higher as 500℃⁶00℃, which is necessary for FePt phase transforming from fcc to fct phase. Hence, the study on reducing the ordering temperature of FePt is very important. In this dissertation, the transforming temperature was reduced to a certain extent by the addition of N₂ during sputtering and in-situ ordering. The main results are as follows:1) 15% N₂ addition during sputtering can reduce the transforming temperature of FePt phase from fcc to fct phase about 100℃ in FePt-Al₂O₃(15vol.%) thin films. The texture of FePt phase changes from (111) dominant texture to a more random texture due to the N₂ addition. Structural analysis reveals that the release of N atoms during the post annealing induces a large number of vacancies in the films, which benefits to the transformation of FePt phase from fcc to fct.2) The transforming temperature of the FePt films prepared by in-situ ordering is only 250℃, while 400℃ for FePt-Al₂O₃(15vol.%) thin films. Both of the temperatures are 200 ℃ lower than that of the films prepared by post-annealing.3) Fine FePt grains with 6nm average diameter were obtained in FePt-Al₂O₃(15vol.%) films deposited at 400 ℃, which accords with the requirement for ultra-high density magnetic recording media. However, the grain size distribution needs to be improved further.4) The lower ordering temperature of FePt phase relates to the near distance between target and substrate in our sputtering system. Hence, at lower sputtering pressure, the sputtered Fe or Pt atoms possess higher kinetic energy before they arrive at substrates. These energy benefits to thetransformation for FePt from fee to fct phase at lower temperature. The second part of this dissertation focus on preparing (001) textured FePt thin films and the possibility of FePt films as perpendicular magnetic recording media. The easy axes of magnetic grains are required to be in-plane for magnetic thin films as perpendicular magnetic recording media. For FePt thin films, (001) texture is necessary. In this dissertation, two ways (the usage of Cr(W) underlayer and alternate sputtering) were developed to prepare (001) textured FePt thin films. The main conclusions are as follows:1) The FePt or FePtCu thin films deposited on (200) textured Cr underlayer show (200) texture.2) A 5nm Mo intermediate layer suppresses the diffusion between FePt layer and Cr underlayer. The FePt(20nm)/Mo(5nm)/Cr(80nm) film exhibit a coercivity as higher as 9kOe with good (001) texture in FePt layer.3) Appropriate addition of W element in Cr underlayer results in a shift from (200) texture to (001) texture of FePt films. A good FePt (001) texture is obtained with W content up to W=15at.% in the Crioo-xWx underlayer.4) The FePt films deposited on glass substrate with temperature of 500°C by alternate sputtering exhibit dominant (001) orientation. The strongest (001) peak intensity is obtained in Fe/Pt multilayer films with single layer thickness of 5nm, the total thickness above 50nm, the composition of 55:45. The (001) dominant texture in the FePt films can be enhanced further by post-annealing. The formation of (001) texture in FePt films prepared by alternate sputtering with higher substrate temperature and post-annealing relates to the texture of initial layer and the growth of FePt grain during post-annealing.5) In the case of [Pt(2.6nm)/Fe(2.4nm)]n films, the reversal process is controlled by domain wall motion. The pinning effect of grain boundary to domain wall motion was reduced by the growth of FePt grain caused by post-annealing. Correspondingly, the reversal process of the film exhibits an obvious nucleation field mechanism.We have made a distinct progress in reducing the ordering temperature of FePt phase and preparing FePt films with perpendicular anisotropy in this dissertation. The FePt(20nm)/Pt(5nm)/ Cr85Wi5(8Onm) film shows certain potential as the candidate of next generation ultra-high density perpendicular magnetic recording media.