Abstract
Hard disk drive (HDD) is one of the most common and widely used mass data storage devices for a long period due to low cost and high recording capacity. The flourishing application is contributable to and coexists with the era of personal computer (PC). Nowadays, HDD though is not the top choice for portable electronic devices in post-PC era because of the size dimension; wile the rise of CLOUD storage (online storage service) raises additional demands on HDDs. For decades, the increase in recording density of HDD never stops, but a limitation is foreseen along with the demands on recording capacity, which requires recording granules approaching so-called superparamagnetic limit. For next generation magnetic recording technology, L10 FePt seems promising to against the thermal fluctuation at smaller size in comparison with current Co-based commercial products for its higher magnetocrystalline anisotropy. This dissertation is about the ordering of L10 FePt and the related microstructure control to meet industrial requirements. We first established a solid and rational evaluation of ordering degree (S) for FePt from x-ray diffractometry (XRD) pattern. The most referred formula: S ≈ 0.85 (A001/A200)1/2 was modified by two considering: 1. Incorporating Nuni (number of unit cell) into integrated intensity equation. 2. The intensity can’t be directly integrated from θ-2θ patterns unless the oriental distribution is characterized. Correspondingly, the integrated intensity ratio of A(100)/A(200) becomes a function of both S and disorder-to-order volumetric ratio. The latter can be obtained alternatively by A(111)/A(222). This approach is however inapplicable based on our proposed scan geometry for poor signal-to-noise ratio and enormous time-consumption. A better solution for the data acquisition is demanded. Second, we studied the enhancement of disorder-to-order phase transition of FePt. We deposited the metastable AgPt layer adjacent to the Fe layer and addressed the importance of vacancies to the disorder-order transition of FePt at reduced temperatures on the basis of a kinetic diffusion model. The decomposition of the metastable AgPt phase creates excess vacancies during the post-deposition annealing process. This accelerates the intermixing between Fe and Pt hence the nucleation of L10 FePt. The evolution of phase transformation from AgPt-Fe to L10 FePt-Ag was monitored by in situ high temperature X-ray diffractometry (XRD) and was also validated by first-principles calculations. The intermixing between Fe and Pt and the nucleation of L10 FePt after annealing at 230˚C were directly observed by transmission electron microscopy and grazing incidence XRD, respectively. With the assistance of the decomposition of AgPt, we obtained a (001)-dominated L10 FePt film with an out-of-plane coercivity as large as 13.3 kOe after annealing at a temperature as low as 350˚C. The third is a focus on the microstructure control of L10 FePt with C as the segregants. FePt:C films with varying C concentration indicates that more C leads to better planner isolation but no size reduction. The latter is then achieved by lowering deposition rate. FePt (1 nm) at varying deposition rates was fabricated to mimic the nucleation stage. The results suggested that the higher density of nuclei at lower deposition rate leads to smaller granular size of FePt:C. We therefore constructed one growth mechanism of FePt:C based on nucleation and growth model and the hypothesis of impurity exclusion during film growth. Another effective way leading to higher nucleation density is to grow on a rougher surface. Therefore, a L10 MnPt:C seed-layer having the same structure and accommodate lattice but smaller size with L10 FePt was developed. It is successfully reduce the granular size of FePt:C. Moreover it shows potential in achieving granular FePt:C films at reduced C concentration or higher substrate temperature. Our growth model implies a spherical nature of FePt:C, we therefore developed a surface clean process by controlling a weak plasma to remove only the top surface C accumulation. Applying this treatment on the bottom layer in FePt/FePt:C bi-layers shows good potential for columnar growth.