Investigation of temperature dependence of Raman shift of amorphous carbon coatings used in heat assisted magnetic recording

Heat assisted magnetic recording (HAMR) can overcome the super-paramagnetic limit and increase areal density beyond 1 Tb/in² in hard disk drives [1]. In HAMR, a pulsed laser beam is first used to heat the magnetic medium beyond its Curie temperature and then data is magnetically recorded during the cool-down process while the coercivity of the medium is still low. High temperatures at the head disk interface are caused by the laser heating process which leads to lubricant depletion, magnetic degradation, and carbon overcoat damage of the head/disk interface [2]. The carbon overcoat can also be graphitized or oxidized leading to poor corrosion protection and inferior tribological performance [2]. The heated spot size achieved by the pulsed laser is very small, approximately 25nm in diameter. In addition, the duration of a typical laser heating cycle, including the cooling process, is very short, on the order of one ns. The measurement of the temperatures in the heated laser spot is of great interest for HAMR technology since reliability is strongly affected by the temperature rise in the heated laser spot. It is very difficult, if not impossible, to measure the temperature rise in the laser heated spot using conventional methods such as thermocouple measurements. To simulate and investigate the near field effects in heat assisted magnetic recording, tip enhanced Raman spectroscopy can be used (Fig. 1). Tip Enhanced Raman Spectroscopy (TERS) combines an atomic force microscope (AFM) and a Raman spectrometer to investigate the near field effects between the tip of an AFM and the adjacent surface. In TERS, a laser is focused on the tip of a metallized AFM tip, mounted in a scanning probing microscope. The laser illumination causes localized heating of the probe tip and the disk surface. Plasmon interactions at the interface between the probe tip and the surface enhance Raman scattering from the laser illumination. The scattered light is measured with a spect- ometer and then analyzed in order to quantify the temperature rise in the disk due to the near-field heating by the AFM tip.