Investigation On The Single And Double Layer CoPt Based Perpendicular Recording Media With Small Grain Size

In order to further increase the recording density of perpendicular hard disk, the grain size of the magnetic recording media have to be reduced without sacrificing its thermal stability. This requires the application of a material with higher crystalline anisotropy (K_u) than CoCrPt and reducing its switching field. The hcp-CoPt based recording media can have higher Ku than CoCrPt based media due to the reduced density of defects that brought by Cr element. Hence, hcp-CoPt is a potential material that can replace CoCrPt to further increase the recording density of the recording media. On the other hand, the anisotropy field of hcp-CoPt can be higher than the maximum writing field the current recording head could provide (24 kOe), for example, the theoretical anisotropy field of hcp-Co₃Pt is as high as 36 kOe. Therefore, the switching field of the hcp-CoPt has to be reduced. In this thesis, the Ru-SiO₂ underlayer has been used to reduce the grain size of the hcp-CoPt based recording media. The perpendicular exchange coupled composite (ECC) has been used to reduce the switching field hcp-CoPt. The effects of soft layer thickness, anisotropy and hard/soft interface exchange coupling strength on the magnetic properties of the ECC media have been studied both experimentally and theoretically. The detailed works and results are summarized as following:Firstly, the effects of fabrication conditions (sputtering pressure, power and SiO₂ volume fraction) of the Ru-SiO₂ underlayer on the microstructures of the Ru-SiO₂ underlayer, as well as the microstructures and magnetic properties of the CoPt recording layer have been investigated. Results showed that under certain fabrication conditions, a small addition of SiO₂ into the Ru underlayer could effectively reduce the grain size of the Ru-SiO₂ underlayer and hence, the CoPt magnetic recording layer without sacrificing its magnetic properties.Secondly, the effects of the Pt content on the microstructure and magnetic properties of the Co_100-xPt_x-TiO₂ recording layer fabricated on the Ru-TiO₂ underlayer have been investigated. Results showed that the microstructures and magnetic properties of the Co_100-xPt_x-TiO₂ recording layer could be effectively adjusted by the Pt content. The Co₇₁Pt₂₉-TiO₂ film has the best hcp structure, out-of-plane anisotropy and coercivity.Thirdly, the Co_100-xPt_x-TiO₂/Co₇₁Pt₂₉-TiO₂ ECC media have been fabricated on the Ru-TiO₂ underlayer. The effects of Co_100-xPt_x-TiO₂ layer thickness and anisotropy on the magnetic properties of the ECC media have been investigated. Results showed that for the Co-TiO₂ soft layer, on the condition of strong interface exchange coupling, the magnetic reversal mechanism inside the ECC particles depended strongly on the thickness of the soft layer. With the increase of the soft layer anisotropy, the effects of soft layer thickness on the reversal mechanism of the ECC reduced. This result agreed with the coercivity and remanence ratio properties of the ECC media. On the other hand, the direction of the soft layer anisotropy axis also affected the reversal mechanisms of the ECC particles, and this influence increased with the soft layer anisotropy constant. The effects of the demagnetization field of the ECC film on the magnetic properties of the ECC media increased with a decrease in the hard/soft interface exchange strength.Fourthly, in order to find a perfectly perpendicular exchange coupled hcp-CoPt based ECC media, the CoPtRu particles have been fabricated on the Ru underlayer. The effects of Ru content on the microstructures and magnetic properties of the CoPtRu-SiO₂ single layer film, as well as the magnetic properties of the [CoPt]1-xRux-SiO₂/CoPt-SiO₂ double layer film have been studied. The magnetic properties of the CoPt-SiO₂ multi-layer film have also been studied. Results showed that doping Ru or SiO₂ into CoPt could reduce its ordering degree slightly. However, the anisotropy constant of CoPt and CoPtRu were still very high due to their low symmetry crystalline structure. Therefore, the CoPtRu-SiO₂ and high SiO₂ content CoPt-SiO₂ could not be used as the soft layers of the ECC media.Lastly, the micromagnetic simulation method has applied to investigate the effects of soft layer thickness, hard/soft interface exchange coupling strength and intergrainular exchange coupling strength on the magnetic properties of the ECC grain array. The simulation results were compared with the single ECC grain and experimental results. Results showed that with an increase in the soft layer thickness and hard/soft interface exchange coupling strength, both the coercivities of the single ECC grain and ECC array reduced greatly firstly and then approached saturation. This result agreed with the experimental results. The remanence ratio (Mr/Ms, ratio of the remanence to the saturation magnetization) of the single ECC grain was nearly constant with the changing of soft layer thickness and interface exchange coupling strength. However, the remanence ratio of the ECC array reduced with soft layer thickness and increased with the interface exchange coupling strength. The results of ECC array agreed with the experimental results. The simulation results showed that the magnetic reversal mechanism of singe ECC depended on its shape anisotropy. However, in ECC array, the inter-granular magnetic interactions affected more on the reversal mechanism of the ECC grains in the array than the shape anisotropy of the grains. Therefore, the simulation results based on single ECC grain could be used to predict certain magnetic properties like coercivity, but they would fail on the prediction on the magnetic properties such as remanence.Both the theoretical and experimental results showed that, for the application of hcp-CoPt based ECC media, the interface exchange coupling strength should be large, the soft layer thickness should be optimized, and the inter-granular exchange de-coupling should be optimized.