Abstract
Hard disk drives (HDDs) play a key role in mass data storage techniques, which are widely used in computer systems and various consumer electronics owing to their ad-vantages of low cost and high capacity. The growing demand for the HDD capacity drives the researches in increasing the areal density of the recording disk. This dissertation focuses on three main research topics to reach ultra-high density recording disks. The first topic investigates the Co-based alloy, which is currently used as the recording layer in commercial hard disks. By using first order reversal curves (FORCs), we found out distinct magnetization reversal behavior in the CoPtCr films with different oxide additives, including Ta2O5, SiO2 and their mixtures. Based on our results, increasing the Ta2O5-to-SiO2 ratio alters the inter-grain interaction from an exchange coupling (parallel) to a dipolar-field coupling (anti-parallel). During the sputtering process, the Ta2O5 additives release extra oxygen to induce the formation of CrOx. The reduced inter-grain exchange coupling strength by increasing Ta2O5 additives can be attributed to the increased volume concentration of oxides and/or the presence of the CrOx. The second topic aims at developing the advanced layer structure of composite media (or called exchange coupled composite (ECC) media) comprising magnetically hard and soft layers. We proposed a soft layer with a laminated structure (LSL, [Pt/CoPtCr-SiO2]5), which uniquely exhibits a tunable perpendicular anisotropy by modifying the Pt thickness. The incoherent reversal in the composite media with a LSL was directly observed by using X-ray magnetic circular dichroism (XMCD). With increasing Pt thickness, the anisotropy of LSL is reduced, which promotes the incoherent reversal of composite media to lower its switching field; however, further increasing the Pt thickness significantly reduced the in-terlayer coupling, and thus the assisting effect was suppressed. By optimizing the Pt thickness, the read-write test confirmed that the composite media with the proposed LSL presents superior recording performance over the conventional single layer media. The third topic focuses on characterizing the magnetization reversal and the interfacial coupling of composite media. The depth-dependent magnetization reversal in composite media was directly probed by means of XMCD technique with a magnetization marker. When the LSL thickness is increased, the transition of magnetization reversal from rigid magnet to exchange spring is observed. By using polarized neutron reflectometry (PNR), we reveal that the hard and soft layers are largely decoupled with the increase of the LSL thickness, and much more tightly coupled when the LSL thickness drops. This finding is in agreement with the fact that the change in reversal behavior of composite media can be attributed to the interfacial coupling strength.