I. INTRODUCTION
It is well known that areal density of perpendicular magnetic recording (PMR) is limited to approximately 1 terabit per square inch (Tb/in2) due to its fundamental limit known as superparamagnetic limit [Wood 2009]. To achieve an ultrahigh storage density, a smaller grain size and a very high coercivity (Hc) medium is required in order to maintain the signal-to-noise ratio (SNR) and magnetic stability. However, current magnetic write fields ( Hw) cannot write on the higher coercivity medium (Hw ≪ Hc). Thus, the writing process must be assisted to reduce coercivity of the magnetic medium. In recent years, heat-assisted magnetic recording (HAMR) has been proposed as one of the leading candidates [Rausch 2013, Wu 2013, Rea 2014]. This technology utilizes a near-field transducer (NFT) to heat a tiny spot on a high magnetocrystalline anisotropy medium before recording data at room temperature [Kryder 2008].