# Thermal Effect of the Back Radiation from Disk to Head after Laser Heating in HAMR
## 1 Introduction  
One of the effective methods to increase the storage density in a magnetic recording is to decrease the size of the magnetic particle. When the magnetic particle gets smaller, the coercivity of which gets higher and causes difficulty when writing data [1]. Heat-assisted magnetic recording (HAMR) is a new technique in which the magnetic recording media is heated by a focused laser to reduce the coercivity; hence, a high storage density can be achieved. It has great potential to increase the areal density in HAMR. The main challenge of HAMR is how to quantify the nanoscale near-field heat transfer and increase the reliability of the head/disk interface due to the back heating of the disk [2, 3]. As we know, the disk, more specifically the magnetic recording layer, is heated by the focused laser to above the Curie temperature during the writing process. When the writing process is over, the disk is at a high temperature; meanwhile, the magnetic head and the air gap between the head and disk, i.e., the air bearing, are at a lower temperature. Thermal energy will transfer back from the disk to the magnetic head and the air bearing by conduction, convection, and radiation, though mainly by radiation. After absorbing the thermal energy from the hot disk, the slider can generate thermal deformation, which can change the flying height as well as the flying stability of the slider. Hence, it is very important to investigate the back heating of the HAMR disk. However, current thermal analysis in HAMR mostly focuses on the heating process [4]. Research on the energy transfer in the head/disk interface is relatively rare. There are three layers in the head/disk interface, which are the following: (a) the lubricant layer [5]; (b) the near-field transducer [6]; and (c) the air bearing [7, 8, 9]. As discussed before, the thermal effects on the magnetic head have a great influence on the air bearing indirectly. Research on the thermal effects of the slider on the head is mainly focused on the results considering multiple factors rather than a single factor [10]. The net temperature rise of the slider caused by the laser absorption and the heat radiation of the locally heated recording magnetic disk are studied in Ref. [9], where the influence of the laser absorption and the heatradiation is considered combined. A new model, together with a finite element (FE) model for an integrated HAMR slider and an optical absorption model for a temperature profile of media hot spot, is then used to solve for the temperature rise and thermal protrusion on the slider body in Ref. [10]. However, because of the limitations of the experimental technology, the accuracy of the theoretical calculation is difficult to verify. As the development of the high-precision micro-electromechanical system (MEMS), tip-enhanced Raman spectroscopy (TERS) temperature measurement and other experimental technology progressed in recent years, the theoretical calculation of the near-field radiative heat transfer is gradually revised and improved so that the particular research on the influence of the near-field radiative heat transfer on the slider is badly needed to be supplemented. The flying height, i.e., the distance between the head and disk, has been reduced to be around 2 nm, which is much smaller than the characteristic wavelength of the electromagnetic waves. Due to the scale effect, such as the surface wave, evanescent wave, radiation tunneling, and so on, traditional radiation theories, such as [PERSON]'s law and [PERSON]'s law, cannot be used to predict the radiative heat transfer. Research on the near-field heat transfer is concentrated on the theory of heat transport by phonon tunneling, see Refs. [11; 12; 13]. In recent years [14], the theoretical and experimental studies on the near-field radiative heat transfer have obviously increased.