Mode Hopping Impact on NFT Protrusion Measurement in HAMR

In terms of head media spacing (HMS) control in heat-assisted magnetic recording (HAMR), it is important to measure laser power-induced protrusion in writing. A near-field transducer (NFT) in the head provides confined heat to the media. However, heat generated by the laser diode is partially absorbed in the NFT, which leads to localized protrusion (NFT protrusion). Since the NFT protrusion area is narrow, it is difficult to measure by conventional touch down-based methods. Burst writing scheme (BWS) is one of the proposed methods for NFT protrusion measurements. The BWS implements two writing modes, steady-state writing in which the NFT is fully protruded and burst writing in which NFT protrusion is quite small. In the BWS, HMS is swept by the thermal fly height control (TFC) heater, which is embedded in the head. The readback amplitude curve as a function of TFC power in each writing mode is used for the NFT protrusion calculation. Readback amplitude usually increases as the TFC power increase when spacing between head and media gets closer. However, for steady-state writing, unintentional readback amplitude drop has been observed in some heads and it affects NFT protrusion results directly. From the relation between readback amplitude and thermal sensor output, which reflects NFT temperature, we found that laser mode hopping causes amplitude drop. Since readback amplitude in one revolution of continuous writing varies by several mode hopping, average amplitude that contains several laser modes is effective for the plotting of stable steady-state writing curve. We successfully mitigated amplitude drop by using the amplitude average in continuous steady-state writing. As one example, NFT protrusion measured by this method was twice as large as those measured with conventional method by avoiding the small NFT protrusion estimation due to amplitude drop.