Abstract
In this study, multilayers [Fe/Pt]8 were alternately deposited on Si(100)//SiO2(90nm) substrates at room temperature and subsequently annealed in vacuum(＜1×10-5 Torr) at 550℃ by rapid thermal annealing(RTA) system, and the cooling of samples was natural cooling. In order to get the L10 FePt films with perfect (001) texture, we reduced the kinetic energy of sputtering atom to obtain loose atomic stack of FePt films by reducing power of dc ion source and increasing working pressure, which FePt films tend to remain tensile stress. Furthermore, stoichiometry of FePt directly affected the properties of FePt films. The FePt film with perfect (001) texture was obtained when the atomic percent of Fe is 54％. In order to know that how the annealing time affect the properties of FePt films, samples were annealed for different periods (5, 90, 150, 240, 300 second) by using an RTA system, and all samples were with the same heating rate of 40℃/s. The out of plane coercivity became larger as the annealing time increased. These features indicate that the A1-phase FePt could be converted to the L10-phase upon further annealing. However, the FePt film tends to minimize its surface energy by forming (111) texture with longer annealing time (300 second). To discuss how the heating rate of RTA system affect the properties of FePt film, samples were annealed with different heating rate (20, 40, 60, 75℃/s) for the same annealing time of 240 second. From the X-ray diffraction (XRD), the FePt film existed (111) texture with the heating rate of 20℃/s. However, there were perfect (001) texture of FePt films with increase of the heating rate (40, 60, 75℃/s). Furthermore, we analyzed uniform and non-uniform stress from XRD peak. The larger in plane tensile stress which induced by increase the heating rate of RTA system enhanced the ordering of FePt films with (001) texture. However, the larger in plane residual stress was released by forming more strain relaxation defect and the non-uniform stress induced by strain relaxation defect also increased. From Superconducting Quantum Interference Device (SQUID), the out of plane coercivities increased with increase of the heating rate. The out of plane coercivities of 40℃/s, 60℃/s and 75℃/s were 9.3kOe, 10.2kOe and 10.9kOe respectively. The increase of out of plane coercivity is because of the more dislocation induced by the strain relaxation with higher heating rate, which act as strong domain wall pinning site to obstruct domain wall motion. This conclusion was confirmed with the analysis of residual stress by XRD. The larger in plane tensile stress is induced by higher heating rate to enhance the ordering of (001) texture FePt. However, the larger in plane tensile stress apply to FePt thin film would induce more dislocation to release the stress. So we could observe the non-uniform stress which induced by dislocation also increased with the increase of heating rate. The strength of domain wall pinning is associated with domain wall width and magnetoelastic coupling. Domain wall width is inversely proportional to magnetocrystalline anisotropy (Ku). Because of the high Ku of FePt (7×107 erg/cm3), the width of FePt domain wall only several nanometer below the Curie temperature. Therefore, the pinning site with a size close to domain wall width may result in the stronger pinning effect. Furthermore, defects generate a stress field which, via magnetoelastic coupling, interact with the domain wall to obstruct domain wall motion. In order to know that what is the relationship between stress field and domain wall pinning. We got the strain mapping of HRTEM by geometrical phase analysis. By measuring the strain field of dislocation and stacking fault in the strain mapping, the strain field of dislocation was larger than that of stacking fault in FePt films. This result is confirmed with the theory of dislocation. Summarized with the effect of domain wall width and magnetoelastic coupling on pinning domain wall, the pinning strength of dislocation is stronger than stacking fault in FePt films.