Materials for heat-assisted magnetic recording heads
- PDF / 1,620,536 Bytes
- 6 Pages / 585 x 783 pts Page_size
- 39 Downloads / 308 Views
he heat-assisted magnetic recording environment The geometry of a heat-assisted magnetic recording (HAMR) head is shown in Figure 1. The primary optical elements are the laser, focusing optics (“light path”), and near-field transducer (NFT). In a HAMR head, the magnetic localization and gradient problem of conventional perpendicular recording has been replaced with a thermal localization and gradient problem. The recording industry has favored plasmonic nanostructures as the primary technology to heat the recording layer. There are several categories of plasmonic NFTs; we focus here on the “lollipop” NFT, where the plasmon is generated on a circular disc and a rectangular peg is used for concentration.1 The NFT peg heats the media, which is moving in the down-track (DT) direction, as shown in Figure 1a. The recording process happens in between the peg and the pole where the media is cooling and the magnetic field is strong. At 10 K rpm (30 m/s), the NFT has ∼1 ns to provide enough power to the FePt recording layer in order to heat it past its Curie temperature of ∼400°C. The net power required to heat the track is approximately 0.15 mW. However, delivering the energy to the recording layer is only a few percent efficient,2 resulting in tens of mW of optical power being absorbed by nanostructures in the head. In terms of the power flux density,
the peg has ∼50-nm cross-track (CT) width and 25-nm DT height, leading to a power flux density of ∼150 TW/m2, some 25 million times higher than the surface of the sun. Figure 1c–d shows the impact of this power density on the NFT temperature, which has a peak value >300°C and thermal gradients that are locally >1 K/nm. The impact of this extreme thermal environment on head engineering is profound. High temperatures adversely affect the reliability of the recording head through activation of many Arrhenius-type processes, including migrative (diffusion, interdiffusion, void migration), reactive (oxidation, reduction, corrosion), interfacial (interface reactions, migration of materials and voids along interfaces) and phase-change processes (amorphous to crystalline transitions). All of these can render irreversible head damage over time scales comparable to the aggregate harddrive workload. The isothermal effects are further compounded by cyclic effects that are unavoidable since the laser must be temporarily reduced in power while the head is flying over the servo wedges that maintain the CT position to within 1–2 nm. Servo wedges are periodic magnetic patterns along the data tracks that are written into the media during drive manufacturing and serve to keep the head located and centered on a specified data track. Further stress originates from the high-T-specific head– disc interaction. Mismatches in the coefficient of thermal
Michael C. Kautzky, Seagate Technologies, USA; [email protected] Martin G. Blaber, Seagate Technologies, USA; [email protected] doi:10.1557/mrs.2018.1
100
• VOLUME 43 • FEBRUARY University 2018 • www.mrs.org/bulletin 2018 Materials
Data Loading...
