Thermomechanical, Electromagnetic and Material Issues in Heat-assisted Magnetic Recording Technology

Amorphous carbon (a-C) possessing exceptional physical, chemical and mechanical properties is one of the most important building blocks for numerous technical applications. Specifically, a-C films have widespread applications as protective coatings in mechanical parts, biomedical instruments, microelectromechanical devices and magnetic recording, because of their high hardness and excellent chemical inertness. At present, the urgent demands for higher data areal density in magnetic recording industry have generated high interest in acquiring fundamental understanding of the structure and properties of a-C ultrathin films. The most promising magnetic recording technique to achieve higher data areal density is heat-assisted magnetic recording (HAMR). HAMR solves a critical problem in high data areal density or high data stability associated with the superparamagnetic limit. In HAMR, an integrated optical system is used to focus a laser beam to a sub-diffraction-limit spot in order to locally heat the magnetic media above its Curie temperature, thus enabling data writing in the fine-grained, high-magnetic-anisotropy magnetic media that allows both high data areal density and data stability. The incorporation of laser heating in magnetic recording creates a major challenge, i.e., the durability and stability of ultrathin a-C films (a few nanometers thick) under high temperature. Thus, the principal objective of this dissertation is to address the thermomechanical, electromagnetic, and material issues relevant to a-C films used in HAMR technology. Device-level simulations using the finite element method (FEM) were performed to elucidate the heat transfer process in HAMR. The thermal protrusion and the surface temperature distribution in a typical HAMR head under various heat sources and read/write conditions were examined to understand the near-field heat transfer at the head/disk interface. The effects of material optical properties of various stack layers on the thermo-plasmonic performance of a HAMR head/disk stack were also investigated. The device-level simulations provide insight into the unique heat transfer process in HAMR and an effective means of tuning the optical properties of stacking layers in HAMR devices to optimize their energy transmission performance. After obtaining the representative temperature distribution in a HAMR device via the device-level simulations, the thermal stability and diffusion characteristics of a-C films were studied experimentally. The growth and thermal stability of ultrathin a-C films synthesized by filtered cathodic vacuum arc on pure Si and a SiNx layer formed by Si nitrogenation using radio-frequency sputtering were investigated. The samples were thermally annealed for various durations and subsequently characterized by high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). The HRTEM images confirmed the continuity and uniformity of the a-C films and the morphology of the SiNx underlayer. The EELS analysis of cross-sectional samples revealed the thermal stability of the a-C films and the efficacy of the SiNx underlayer to prevent carbon migration into the Si substrate. Despite several experimental studies of the thermal stability of a-C films, basic understanding of the structure evolution of ultrathin a-C films during film growth and heating is highly empirical, presumably due to the lack of high-resolution instruments that can probe structural changes at the atomic level in real time. Thus, molecular dynamics (MD) simulations that provided insight into changes in the structure of ultrathin a-C films during deposition and annealing were performed. Simulation results revealed a multi-layer film structure, even for a-C films as thin as ~20 Å, the existence of a deposition energy that yields a-C films with the highest sp³ content, the transient and steady-state stages of the structure evolution during annealing at different temperatures, and insight into the changes in the hybridization state encountered during annealing. The correlation between internal stress, film structure, and temperature in ultrathin a-C films was also analyzed by MD. The physical mechanisms of a-C film growth and stress built-up under energetic particle bombardment and stress relief due to thermal annealing were examined. Simulations of film growth revealed a correlation between internal stress and energy of incident carbon atoms. A significant stress relief was observed mainly in the bulk layer of the multilayered a-C film structure at a critical annealing temperature, which intensified with the further increase of the temperature. Besides a high temperature, a-C films may experience oxidation and sliding contact during operation. MD simulations of oxidation and sliding of a-C films were performed to elucidate the oxidation and tribo-oxidation processes of the films in the presence of oxygen. The simulations demonstrated oxygen kinetic energy thresholds for continuous carbon loss and for oxygen surface adsorption saturation, a surface dominated oxidation process, significant interfacial adhesion of the active film surface and significant changes of the tribological behavior and film structure during sliding. Tribo-induced a-C film oxidation was observed in sliding simulations with surface oxygen adsorption. Graphene is another carbon form that shows outstanding mechanical properties and a high potential as a protective and lubricious coating in various industrial applications. In this dissertation, the synthesis of graphene on a-C films was explored and a facile single-step synthesis method for fabricating thin a-C layers containing graphitic structures was developed. By annealing thin-film stacks of Si/NiFe/a-C, a thin layer with a hybrid a-C-planar graphene (PG)-orbicular graphitic carbon (OGC) structure was produced. Raman spectroscopy and cross-sectional TEM confirmed the transformation from amorphous to graphitic structure in the a-C film during thermal annealing. The obtained results indicated that the development of this hybrid a-C-PG-OGC microstructure is due to a metal-catalyzed PG nucleation mechanism and a mismatch-induced OGC growth mechanism.

Advisor: Komvopoulos, Kyriakos