Direct Conversion Analysis from Near-Field Optics to Thermal Distribution for Heat Assisted Magnetic Recording Media and Head

다국어 초록 (Multilingual Abstract)

Heat assisted magnetic recording (HAMR) is a promising candidate to achieve high recording density over 1 Tb/in^2. For the HAMR approach, near-field light has to be generated to heat the recording layer in the nanoscale, where thermal distribution strongly depends on the near-field optical characteristics. Thus this paper focuses on the development of direct conversion process of thermal distribution in the HAMR media and head using their optical data. First, near-field optical characteristics simulated by a finite difference time domain (FDTD) method was calculated to the corresponding optical energy by Poynting vector, based on the electric and magnetic field data from the Yee-cells inside the media. Then the difference between the incoming and outgoing energy at each cell was considered as an absorbed energy by each cell. If there is additional absorbed optical energy through the side of each cell, it is also considered in this conversion method. In this process, the FDTD data in the surface cell shows the unacceptable high field values due to some optical interactions. Thus the optical energy in the surface cells was obtained from the numerical curve fitting with the absorption curve of the materials. Finally the thermal distribution was calculated by finite volume method (FVM) simulation. For this FVM simulation, the absorbed optical energy in each cell was considered as a heating source value of each cell volume.
Direct conversion process was applied to evaluated the crosstalk issue and optimize the input power for different media structures, including continuous media, discrete track media (DTM) and surface plasmon assisted HAMR (SPAH) media. For nanoscale thermal simulation, we used the thermal properties in thin film value. The required input power for the continuous media, DTM and SPAH media were calculated as 80 mW, 60 mW and 25 mW, respectively to increase the temperature around 560 K. The large difference for SPAH media comes from the surface plasmon coupling effect occurred at the interface of metal and dielectric layer. Actually, more optical energy was absorbed to the recording layer due to a strong polarization of near-field light at the interface of magnetic and dielectric layers for SPAH media. This effect also occurs at the interface of magnetic layer and dielectric layer for the DTM. In addition, the relatively lower thermal conductivity of the dielectric layer may help to increase the temperature of recording track for both DTM and SPAH media.
To confirm the crosstalk from the adjacent tracks, the temperature distribution was calculated through the media at the surface and the inside of media. The temperature of 395 K and 450 K were obtained for DTM and SPAH media at surface of adjacent tracks which means much lower temperature than that of central track without any crosstalk problem. However, the continuous media results in a severe crosstalk problem if it is used for the HAMR application. From the temperature distribution analysis in the media, we can conclude that both DTM and SPAH media can be used as a HAMR media without any crosstalk problem. In addition, the SPAH media requires much lower input power to realize the HAMR recording compared to those of other media structures, which means it is a best selection for the HAMR media.
Direct conversion process developed in this research was also applied for HAMR head structure. For the grating near-field transducer (NFT) in the HAMR head, absorbed optical energy due to a surface plasmon enhancement at the interface of metal grating NFT and dielectric optical path was considered in this simulation. In addition, optimized input powers for the different media were used to thermal simulation of HAMR head. Compared to other analysis based on the assumed absorbed energy, our new analysis based on the optical energy distribution can lead the more realistic results. From the temperature distribution analysis in the HAMR head, we can conclude there is no thermal effect to the magnetic pole with temperature increasing under 328 K, and temperature-induced protrusion will be occurred under 0.3 nm at the bottom of grating NFT and there is no contact between grating NFT and media surface.