Showing papers by "David B. Bogy published in 2020"

Measurement of angstrom-level laser induced protrusion using touchdown in heat-assisted magnetic recording

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.

Numerical and Experimental Investigation of Nanoscale Heat Transfer Between a Flying Head Over a Rotating Disk

Abstract: With the minimum fly height less than 10 nm in contemporary hard-disk drives, understanding nanoscale heat transfer at the head-disk interface (HDI) is crucial for developing reliable head and media designs. While flying at near-contact, the fly height and spacing dependent nanoscale heat transfer are significantly affected by interfacial forces in the HDI (such as adhesion force, contact force etc.). Moreover, with the emergence of technologies such as Heat-Assisted Magnetic Recording and Microwave-Assisted Magnetic Recording, head failure due to overheating has become an increasing concern. In this study, we present a numerical model to simulate the head temperature profile and the head-disk spacing for a flying head over a spinning disk and compare our results with touchdown experiments performed with a magnetic recording head flying over a rotating Al-Mg disk. In order to accurately predict the fly height and heat transfer at near-contact, we incorporate asperity based adhesion and contact models, air & phonon conduction heat transfer, friction heating and the effect of disk temperature rise in our model. Our results show that the incorporation of adhesion force between the head and the disk causes a reduction in the fly height, leading to a smaller touchdown power than the simulation without adhesion force.

On the Wave Speed of Thermal Radiation Inside and Near the Boundary of an Absorbing Material

Abstract: Planck's law describes thermal radiation into vacuum from a black body in thermal equilibrium. This law can be easily adapted to describe radiation into a transparent medium with a constant refractive index, and it admits a less trivial extension to radiation into a transparent medium with a nonconstant refractive index. However, this law cannot be straightforwardly generalized to describe thermal radiation into absorbing media and, in particular, to describe thermally exited electromagnetic fields inside the radiating body itself. We first analyze Planck's law and show why it cannot be straightforwardly extended to radiation into an absorbing medium. The derivation of this law relies on the assumption that a radiated field admits decomposition into normal modes, which cannot exist in absorbing media that are characterized by a complex-valued refractive index n=n′+in″, whose imaginary part describes the rate of energy dissipation. Correspondingly, the speed of electromagnetic waves in absorbing media c=c0/n, where c0 is the speed of light in vacuum, is also complex-valued, which suggests that the conventional concept of a complex valued wave speed is not suitable for modeling thermal radiation. We demonstrate that complex-valued wave speeds adequately describe waves that carry signals, such as radio waves and laser beams. Such waves decay because they pass some of their energy to the medium. The energy absorbed by the medium is eventually reradiated, but in studies focused on the transmission of signals, the reradiated fields are ignored as noise. In order to study thermal radiation in an absorbing material, one must treat the material and the radiation together as a closed system. The energy in such a system is conserved, and its distribution between the material and radiation does not change in time. This radiation admits decomposition into normal modes, which makes it possible to extend Planck's law to radiation into absorbing materials. This paper proposes a model of thermal radiation in an absorbing medium as a closed, energy conserving system. The radiation field has normal modes that correspond to an effective speed of wave propagation. Assuming that an absorbing material and the radiation in it are in thermal equilibrium, we show that deep inside the material, the average speed of photons is given by a frequency and temperature-dependent expression c∗=c0/(1+e−ℏω/κT). While this result is independent of the material, we further show that close to the boundary of the medium, the speed of thermal radiation depends in a complex way on the refractive index and the extinction coefficient of the material, as well as the direction of propagation and the distance from the material's surface.

A critical review of published approaches to nanoscale thermal radiation

Abstract: Radiative heat transfer across nanoscale-thick layers attracts considerable attention because of its importance for modern technology, and also because of the evidence that conventional methods of radiative heat transfer fail at such small scales. This paper analyzes several approaches to this problem that have been proposed since the late 1960s when the first adaptations of the classical Stefan–Boltzmann law of radiative heat transfer to smaller scales were presented. It is shown that while all authors agree that thermal radiation is described by Maxwell’s equations for electrodynamics, the methods of these studies drifted from deductive reasoning based on these equations toward heuristic guesses. This paper identifies critical points of several of these previous studies that are responsible for the lack of a suitable theory of radiative heat transfer across nanoscale layers despite almost 50 years of effort. Among these points are: (1) attempts to describe heat transfer using statistical distributions that are limited to equilibrium systems that cannot produce any heat flux; (2) application of the so-called fluctuation–dissipation theorem when its conditions are not satisfied; (3) the failure to distinguish between different kinds of evanescent fields; (4) an unjustified assumption that resonant surface waves can transfer heat by tunneling.

Computation of intermolecular forces that carry sound across vacuum gaps

Abstract: A recently understood ability of sound to cross narrow vacuum gaps between material bodies has numerous implications in modern technology, where it opens additional channels for the transfer of heat and acoustic signals between narrowly separated objects. Acoustic vibrations in a body are characterized by perturbations of its surface and of its material density, which affect molecules of another body via intermolecular forces. Methods of continuum mechanics allow the description of the acoustic waves initiated by external body forces, including van der Waals forces. The expressions for such forces have been known for many years, but their applications to configurations relevant to sound transmission are often based on restrictive approximations and asymptotic expansions, which are rarely satisfied in practical situations, where amplitudes of molecular displacements may be in a picometer range. This paper develops a method of accurate computations of van der Waals forces caused by such perturbations with scales comparable with intermolecular distances in a material. The accuracy of the developed approach is limited only by calculation errors, and the scope of its applications is not only limited to acoustic perturbations but also includes the description of van der Waals forces caused by arbitrary molecular density perturbations, by corrugated surfaces, contaminated materials, etc.