Nanoscale thermal transport aspects of heat-assisted magnetic recording devices and materials
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at-assisted magnetic recording hot zone In a heat-assisted magnetic recording (HAMR) system, a metallic near-field optical transducer delivers highly localized optical energy to a magnetic medium, where it is absorbed as heat via ohmic dissipation, raising the temperature of the medium, typically 400–500°C above its Curie temperature. The near-field optical transducer also gets hot because it is lossy. Thus, we designate the near-field transducer (NFT), the adjacent medium, and air gap as the “hot zone” in the recording system. Figure 1 shows this region, drawn approximately to scale, and identifies some of the nanoscale heattransfer issues, which are addressed in this article, that must be considered to analyze and design heat-assisted magnetic recording systems.
Nanoscale heat-transfer issues relevant to HAMR Heat conduction is the primary heat-transfer process in HAMR. Macroscopically, heat conduction in diffusive regimes is described by the heat diffusion equation and Fourier’s Law, which relate the heat flux (q ) (W m–2) to the temperature gradient (∇T) (K m–1) by the thermal conductivity (k) (W m–1 K–1),
q = − k ∇T .
(1)
A wide range of thermal conductivities exists within a HAMR device at room temperature, ranging from ∼10–1 W m–1 K–1 for the organic lubricants, to ∼100 W m–1 K–1 for the amorphous SiO2 and Al2O3 head components, to ∼101 W m–1 K–1 for a metal alloy such as FePt, and to ∼102 W m–1 K–1 for metals such as Au. Heat conduction at elevated temperatures is critical to the operation of HAMR. The variation of k with T is shown in Figure 2a for several HAMR relevant materials.1–4 More generally, in an anisotropic medium, Fourier’s Law can be expressed based on a thermal conductivity tensor. While HAMR media are anisotropic, the full tensor description can be simplified because the thermal conductivities differ in the in-plane (r) and cross-plane (z) directions, which are also the two principal directions of heat propagation, qr = − kr
dT dT and q z = − k z . dr dz
(2)
The implications and utility of this anisotropy for the operation of HAMR will be discussed in the section on media.
James A. Bain, Department of Electrical and Computer Engineering, Carnegie Mellon University, USA; [email protected] Jonathan A. Malen, Department of Mechanical Engineering, Carnegie Mellon University, USA; [email protected] Minyoung Jeong, Department of Materials Science and Engineering, Carnegie Mellon University, USA; [email protected] Turga Ganapathy, Department of Mechanical Engineering, Carnegie Mellon University, USA; [email protected] doi:10.1557/mrs.2018.6
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• VOLUME 43 • FEBRUARY Mount 2018 • www.mrs.org/bulletin ©available 2018 Materials Downloaded MRS fromBULLETIN https://www.cambridge.org/core. Royal University, on 13 Feb 2018 at 11:11:27, subject to the Cambridge Core terms of use, at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2018.6
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Nanoscale thermal transport aspects of heat-assisted magnetic recording devices and materials
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