R&D: Measurement of Angstrom-Level Laser Induced Protrusion Using Touchdown in HAMR

Applied Physics Letters has published an article written by Qilong Cheng, University of California at Berkeley, Berkeley, California 94720, USA, Haoyu Wang, Shanghai Jiao Tong University, Shanghai 200240, China, Siddhesh V. Sakhalkar, and David B. Bogy, University of California at Berkeley, Berkeley, California 94720, USA.

Abstract: “In heat-assisted magnetic recording (HAMR), a laser is employed above the read-write transducer to provide energy to the media, lowering its coercivity. However, the laser also brings thermal energy diffusion inside the slider and induces an extra angstrom-level protrusion, which we call laser-induced protrusion (LIP). The LIP needs to be taken into consideration in HAMR due to the significance of head-media spacing. This paper focuses on laser heating on the millisecond timescale during flying in the HAMR conditions. When the laser is turned ON for milliseconds, the LIP forms in the short term (∼μs) and fly height change (FHC) happens in the long term (∼ ms) due to the crown/camber change, resulting in a smaller touchdown power (TDP). Thus, the touchdown power change (ΔTDP) is measured and the LIP is isolated using the time constants. A component-level HAMR stage is used to study the effects of laser-on time, laser current, and linear velocity on the ΔTDP. The experimental results show that the FHC needs ∼ 28 ms to reach the steady state and that the protrusion size presents a two-stage linear relation with the laser current separated by a threshold. The LIP size is reduced by about half when operating from 12 m/s to 24 m/s.

As data generated worldwide are growing explosively, it is crucial to increase the areal density of traditional storage devices to satisfy the requirements. The industry has reached a consensus that heat-assisted magnetic recording (HAMR) is probably the most promising technology for hard disk drives (HDDs) to improve the areal density over 1 Tb/in².^1,2 To realize this higher storage density, the head-disk interface (HDI) spacing should be less than 2–3 nm. Thus, a thermal fly height control (TFC) element is utilized to induce a microscale thermal protrusion to decrease the HDI spacing at the head-write transducer. It is also required that magnetic materials with a smaller grain size are used for the media, but data writing is challenging because of their higher coercivity.^3,4 To assist the writing process, a laser is introduced in HAMR as shown in Fig. 1. The laser travels through a waveguide element and a near field transducer (NFT) element to locally produce a nanoscale hot spot on the rotating disk, lowering the coercivity of the magnetic layer.^2,5 The laser diode has an efficiency of around 50% and half of the input energy heats the whole slider body, leading to the crown/camber change.⁶ The rest of the energy that passes through the waveguide is partially absorbed by the head carbon overcoat, forming an extra localized angstrom-level protrusion, which we call laser-induced protrusion (LIP). The LIP needs to be considered in the HDI spacing control and compensated for during flying in the HAMR conditions.⁷ The LIP may also cause material failure, leading to the degradation of the HDD performance⁸“