Temperature-Induced Changes in the Optical and Material Characteristics of HAMR Media COC and Its Effect on Recording Performance

The carbon overcoat (COC) on heat-assisted magnetic recording (HAMR) media was deposited at increasing temperature. The optical extinction (k) of this media was measured with ellipsometry and found to increase with temperature from 0.2 to 0.9 over the temperature range from 220 °C to 360 °C. Raman spectroscopy of these COC surfaces had an increasing G-peak position and decreasing G-peak width along with an increasing I(D)/I(G). An increasing sp² ring/chain and aromatic cluster size and decreasing optical gap were inferred from these changing parameters. HAMR writing on this media was found to require incrementally less laser power (from 100% to 92%) with increasing optical extinction to attain a fixed track width. This was determined to be due to the increased imaginary component of the dielectric function in this film (2nk). The SNR of these written tracks was observed to improve by approximately 0.65 dB with deposition temperature. A finite-element method model of the head and media showed that the thermal gradient of the hot spot in the media increases with optical extinction and increases the SNR. A small decrease in the COC density with temperature was observed and was calculated to further decrease the COC thermal conductivity and thus enhance the increased thermal gradient.

I. Introduction

Heat-assisted magnetic recording (HAMR) makes use of temperature-dependent changes in the magnetic storage layer to enhance the writing process and thereby increase the ultimate areal density capacity of the storage medium. This new technology will use plasmonic structures located within the read-write head to create ultra-small sub-diffraction limited spots on the media that transiently heat the magnetic film within a well-defined area. The controlled heating of the media surface must take place for a defined time and to a fixed target temperature to allow the formation of a sufficient thermal gradient (within the media) to enhance the writing process [1]. The HAMR media films, and their response to the evanescent field emanating from the near-field transducer (NFT), must be designed with consideration, not only of its magnetic properties but also, just as important, their optical and thermal properties to obtain this significant enhancement in HAMR writing. Thus, the optical properties of the entire media film structure, at least up to the penetration depth of the evanescent field, are important for successful heat-assisted recording.