Study Of Graphitization And Oxidation Of Amorphous Carbon Overcoat For Heat-assisted Magnetic Recording

Hard disk drive is the most important data storage hardware device.With the increase of its storage capacity and area density,the grain size of magnetic recording media has to decrease.The superparamagnetic effect restricts the decrease of grain size which reduces to a certain extent.Heat-assisted magnetic recording(HAMR)can increase the storage area density and decrease the grain size of magnetic media by using high coercivity and anisotropy magnetic recording media,such as L10 Fe Pt.The pulsed laser is used to locally heat magnetic media to lower its coercivity in the writing process.In order to improve the performance of the head disk interface,the amorphous carbon overcoat with thickness ~2 nm is deposited on the magnetic recording media.The high locally temperature irradiated by pulsed laser may damage or destroy the amorphous carbon overcoat by graphitization or oxidation.The thermal stability of amorphous carbon is essential on the reliability of HAMR disk.The doping of silicon atoms into amorphous carbon film can improve its thermal stability.But the works on the thermal stability of non-doped amorphous carbon(a-C)and silicon-doped amorphous carbon(a-C:Si)are scarce.The mechanisms of graphitization and oxidation of a-C and a-C:Si are unclear.Making the mechanisms clearly can provide technical supports for the optimization design of amorphous carbon overcoat in HAMR disk.The processes of graphitization and oxidation of a-C film induced by pulsed laser are investigated by molecular dynamics(MD)simulation and experiment in this dissertation.The evolutions of structure and oxidation of a-C film are revealed.Mechanisms of oxidation and failure are revealed.The processes of graphitization,oxidation and failure of a-C:Si film also are studied by MD simulations.The oxidation mechanism of a-C:Si film is revealed.The MD models of a-C and a-C:Si films are constructed by rapid quenching method.The effects of empirical potential functions,densities and quenching time on the structure of a-C films are analyzed by coordination number,radial distribution function and bond angle distribution.The method of selecting the potential function,density and quenching time on modeling a-C films is proposed.The effects of silicon atom content and density on the coordination number and radial distribution function of a-C:Si films are also studied.The MD model of disk is simplified as a-C film and magnetic recording media.The relationship between the temperature of a-C film with time is studied in one laser irradiation cycle.The theoretical formula of temperature evolution as function of time is fitted.The graphitization model of the a-C film is established to study graphitization process using the theoretical formula of temperature evolution.The structural evolution of a-C film in the first laser irradiation process is researched by analyzing the system potential energy,hybrid states,bond angle distribution,radial distribution function,pair distribution function,atomic strain and carbon cluster.The effects of laser irradiation cycles and density on graphitization of a-C film are investigated.The oxidation model of the a-C film is established.The effects of laser heating rates and irradiation cycles on the oxidation processes of a-C film are studied.The distribution of temperature on the oxidation surface is analyzed,and two different oxidation mechanisms of a-C film are proposed.The ultra-thin a-C film deposited on L10 Fe Pt film is prepared by filtered cathodic vacuum arc.The optical HAMR simulated setup is built.The effects of laser power and irradiation time on structure of a-C film are studied.The MD simulation results of graphitization of a-C films are verified qualitatively.The methods of MD simulation are proved accurately.The failure mechanism of non-doped a-C films is proposed.The effects of graphitization and oxidation of non-doped a-C film on the HAMR disk are analyzed.The high temperature and pressure field of surface oxidation of a-C in HAMR disk is proposed.The oxidation model of a-C:Si film is established.The oxidation of a-C:Si film in the relaxation process is studied by analyzing system potential energy and number of bonds.The evolutions of structure and oxidation of a-C:Si films in the first laser irradiation process are studied by analyzing the system potential energy,number of bonds,density,pair distribution function,atomic shear strains and carbon cluter.The effect of laser irradiation cycles on the structure and oxidation is investigated.The failure process of a-C:Si film is studied by analyzing the system potential energy,number of bonds and density.The oxidation mechanism of a-C:Si films is proposed.