Heat-assisted Magnetic Recording System Research Articles

Effect of water on mechano-chemical reactions of perfluoropolyether lubricant films in heat-assisted magnetic recording: A reactive molecular dynamics study

A comprehensive understanding of the decomposition of perfluoropolyether (PFPE) films is crucial for establishing effective lubrication of heat-assisted magnetic recording (HAMR) systems. In this study, to examine the combined influence of heat, mechanical stress, and water, we conducted nonequilibrium reactive molecular dynamics simulations using our ReaxFF force field for the PFPE D-4OH films mixed with water. We thoroughly analyzed the reaction rates, types, pathways, and products, and identified vulnerable bonds. The results demonstrate that water significantly accelerates the reactions of D-4OH, particularly at lower normal pressures. We found that water molecules form clusters and are surrounded by the D-4OH end groups. Consequently, water induces hydrolysis of D-4OH or catalyzes reactions between D-4OH by dissociating C–O bonds in the end groups.

The Scaling Capability of Heat Assisted Magnetic Recording

In this study, we analyze the scaling potential of heat-assisted magnetic recording (HAMR) and the factors ostensibly preventing scaling. In principle, as each length in the problem scales by a factor of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1/x$ </tex-math></inline-formula> , we would expect the areal density (AD) of the system to scale as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${x}^{2}$ </tex-math></inline-formula> . To underpin the 20% CAGR expectation in the ASRC roadmap, we present a numerical investigation of HAMR where we consider the scaling of grain density, thermal spot size, reader dimensions, media velocity, and other factors. We also evaluate the relative importance of these scaling factors and assess the effect of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${K}_{u}{V}{/}{kT}$ </tex-math></inline-formula> on writing. The results show that reader, grain density, and thermal spot scaling are more critical in realistic HAMR systems. Finally, we highlight the importance of improving the reader by varying the width and the head electronic signal-to-noise ratio (HESNR).

Spin-Stand Measurements to Extract the Switching Distributions of Heat-Assisted Magnetic-Recording Media

A new measurement method to characterize the switching temperature distributions of heat-assisted magnetic recording (HAMR) media at recording time scales is presented. The method uses HAMR components in conventional recording systems, such as spin-stand testers and hard disk drives. Using dc noise measurements, a 2-D map of the probability of grains switching is extracted as a function of the medium temperature and of the applied field, which are controlled by the HAMR head laser power and writer current, respectively. These measurements give access to details of the HAMR media temperature and field distributions, as well as provide useful information related to the operation of the HAMR system.

Track width measurement in heat-assisted magnetic recording systems using the cross-track DC noise method

This paper uses a cross-track DC noise measurement technique to determine the written track width in heat-assisted magnetic recording (HAMR). The method is approached as a simple, no-extra-hardware-required technique on a spin-stand to determine the written track width as well as the DC noise amplitude at the center track in a HAMR system. The track width data show the expected trends with changing laser operating current () and write current (), and they are strongly correlated with track density capabilities and physical near-field transducer (NFT) peg width. The written track width is essentially defined by the which increases as increases. The method is useful for comparing the magnetic responses of different writer pole or NFT peg geometries, as well as media design comparison to provide insight into the head/component necessary to improve HAMR system performance, which is useful HAMR recording parameters for head/media designers and test engineers.

Numerical investigation of a light delivery device using metal/insulator/metal with a 3D linear taper waveguide and an input grating for heat-assisted magnetic recording.

Heat-assisted magnetic recording (HAMR), a new technique to overcome the data-density limitation, uses a laser to temporarily heat a nanovolume of a recording medium. Thus, this paper proposes a light delivery device that uses the metal/insulator/metal waveguide with a three-dimensional linear taper, and a grating added for an input, for HAMR. Our structure was calculated by finite-element method simulation. By design, 830 nm of light was delivered into a 50nm2 spot area with 63% coupling efficiency, and power intensity was enhanced 930 times. This achievement potential could be applied to the HAMR system in the future.

Inverse Design of Near-Field Transducer for Heat-Assisted Magnetic Recording Using Topology Optimization

Heat-assisted magnetic recording (HAMR) is one of the next generation technologies in data storage that can increase the areal density to beyond 1.5 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Near-field transducer (NFT) is a key component of the HAMR system that locally heats the recording medium by concentrating light below the diffraction limit using surface plasmons. In this work, we use density-based topology optimization for inverse design of NFT for a desired temperature profile in the recording medium. We first perform an inverse thermal calculation to obtain the required volumetric heat generation (electric field) for a desired temperature profile. Then an inverse electromagnetic design of NFT is performed for achieving the desired electric field. NFT designs for both generating a small heated spot size and a heated spot with desired aspect ratio in recording medium are demonstrated.

Dependence of HAMR Transition Curvature on Bit Length

Transition curvature is a major contributor to transition broadening in current heat-assisted magnetic recording (HAMR) devices. In this article, in order to clarify HAMR performance versus bit length (BL), a detailed study of the dependence of HAMR transition curvature on BL is presented. With the help of micromagnetic simulations, HAMR bits with three different BLs (10.5, 15, and 21 nm) are written and transition curvatures are calculated. Both thermal exchange coupling composite (ECC) media and pure FePt media are tested. By tracking the change of transition curvature during the HAMR cooling process, an erase-after-write effect is observed. DC-erased media, introduction of intergranular exchange, and removal of dipole interactions are also examined for potential optimization of the HAMR system.

Dependence of optical laser power on disk radius, head-disk spacing and media properties in heat-assisted magnetic recording

In heat-assisted magnetic recording, minimization of optical laser power is important for the reliability of the head-disk interface. In this paper, a prototype heat-assisted magnetic recording system is used to investigate the relationship between the needed optical laser power and disk drive design parameters. In particular, the change of optical laser power, which is a function of the laser current, is investigated for different disk radii and write head flying heights while keeping the write width of the recorded signal constant. In addition, the dependence of laser power during writing is studied as a function of the thickness and material properties of a very thin “capping layer” on the top of the recording magnetic layer. The results show that laser power and media design parameters play a crucial role in heat-assisted magnetic recording devices.

Thermal Lagging of Multilayered Structure in Heat-Assisted Magnetic Recording Systems

Abstract In the present study, we numerically investigate the thermal lagging behavior on the hard disk drives in heat-assisted magnetic recording systems via the optical absorption model. The influences of overcoats, laser radius, relative scanning speed, interfacial thermal resistance, and the heat sink layer on the thermal lagging behavior are studied in detail. It is found that the thermal lagging distance, i.e., the horizontal distance between the location of the maximum temperature and the laser center, increases with an increment of speed and/or radius of the laser spot. The overcoats, the interfacial thermal resistance, and the heat sink layer have negligible effects on the lagging distance. Thus, the multilayered disk can be simplified as a single-layer disk for investigating thermal lagging distance. Meanwhile, the horizontal temperature gradient varies with these factors. Different overcoats result in different horizontal temperature gradient owing to the difference of in-plane thermal diffusivity. A laser with a smaller radius or a slower speed leads to a higher horizontal temperature gradient. The thermal resistance influences the horizontal temperature gradient insignificantly. This study may provide useful information for the design of hard disk drives for heat-assisted magnetic recording technologies.

Magnetic write field characterization in heat-assisted magnetic recording systems

Magnetic write field is a critical factor in determining the quality of the magnetic write transitions, which determine the linear density (LD) limit in heat-assisted magnetic recording (HAMR). We introduce a reversed DC band approach as a simple, no extra-hardware-Required technique in a spin-stand to assess the magnetic write field () contributions, independently in an HAMR system. Heads with different distances between near-field transducer and writer pole, and media with and without soft magnetic under layer were characterized. The estimated result strongly correlates with LD and signal-to-noise ratio, which agrees with previous modeling studies. These results give insight into the head/component required to optimize the HAMR’s system performance. The technique provides qualitative ranking of magnetic write fields and recording temperature, as well as laser operating current at Curie temperature for head/component designs.

HAMR Versus PMR: A Comparative Study of Signal-Noise Performance

A micromagnetic simulation-based comparative study between perpendicular magnetic recording (PMR) and heat-assisted magnetic recording (HAMR) has been performed. Signal-to-noise ratio (SNR) is adopted as the metric of comparison, and various system parameters are considered, including fly-height (FH), write-track distance, and rotary motion of the head-stack-assembly (skew). It is found that even a HAMR system with a wide 60 nm full-width at half-maximum thermal profile demonstrates better SNR performance when compared to an optimized PMR system at a magnetic FH of 8 nm. The impact of skew on a single reader system is found to be minor for both PMR and HAMR, but the impact on a dual reader 2-D recording system differs substantially. In particular, the resolution for small bit detection depends greatly on reader shield orientation to the HAMR-written curved transition beneath the read free layer.

Novel Ionic-Liquid-Type Lubricant for Heat-Assisted Magnetic Recording System

In this paper, a new thermally stable ionic-liquid (IL)-type lubricant has been developed. Since heat is applied intensively on the surface of magnetic disks in heat-assisted magnetic recording (HAMR) systems, a thermally stable lubricant is expected to prolong the life of HAMR drives. To verify this concept experimentally, the depletion of ultrathin film lubricants was evaluated for IL- and perfluoropolyether (PFPE)-type lubricants by far-field laser irradiation. For PFPE, lubricant depletion decreased with increasing bond ratio, whereas lubricant depletion for IL did not depend on its bond ratio. The lubricant depletion quantities of ILs were small and comparable to that of PFPEs with the highest bond ratio.

Experimental study of lubricant depletion induced by pulsed laser irradiation heating in heat-assisted magnetic recording

In this study, we experimentally investigate the lubricant depletion as well as the temperature distribution caused by pulsed laser irradiation heating and discuss the fundamental characteristics. It was determined that the maximum temperature increased as the duty ratio increased. However, the rate of the maximum temperature change due to the duty ratio was comparatively small. Therefore, it was suggested that the maximum temperature was almost constant when the duty ratio was more than 50% under experimental conditions. The thermal distribution or the thermal gradient also depended on the duty ratio and increased as this ratio decreased. It was also determined that the depletion of the lubricant film due to pulsed laser heating was smaller than that due to continuous laser heating. Moreover, it depended on the duty ratio and increased as the duty ratio increased. In addition, it depended on the full width at half maximum (FWHM) of the temperature distribution due to pulsed laser irradiation heating and increased as the FWHM is increased. Therefore, it is also suggested that the lubricant depletion issue will not be as critical in actual heat-assisted magnetic recording system because the FWHM would be very small under practical circumstances.

Effect of Recording Conditions on the Downtrack Thermal Gradient in Heat-Assisted Magnetic Recording

Downtrack thermal gradient (DTG) is calculated for heat-assisted magnetic recording (HAMR) systems in both spin-stand experiment and micromagnetic model. DTG is an important parameter in HAMR. It describes the change in temperature in the vicinity of the write position. Recording conditions and media properties affect the switching process, which in turn affects the measured DTG. Trends in DTG versus different recording conditions such as linear velocity and applied field showed very good agreement between experiment and model. The model is also used to quantify the effect on DTG from parameters that are difficult to measure, such as Curie temperature distributions, anisotropy field distributions, and curvature. Finally, a breakdown of the contribution of the different conditions and properties considered on DTG is presented.

Effective heat dissipation in an adiabatic near-field transducer for HAMR.

To achieve a feasible heat-assisted magnetic recording (HAMR) system, a near-field transducer (NFT) is necessary to strongly focus the optical field to a lateral region measuring tens of nanometres in size. An NFT must deliver sufficient power to the recording medium as well as maintain its structural integrity. The self-heating problem in the NFT causes materials failure that leads to the degradation of the hard disk drive performance. The literature reports NFT structures with physical sizes well below 1 micron which were found to be thermo-mechanically unstable at an elevated temperature. In this paper, we demonstrate an adiabatic NFT to address the central challenge of thermal engineering for a HAMR system. The NFT is formed by an isosceles triangular gold taper plasmonic waveguide with a length of 6 µm and a height of 50 nm. Our study shows that in the full optically and thermally optimized system, the NFT efficiently extracts the incident light from the waveguide core and can improve the shape of the heating source profile for data recording. The most important insight of the thermal performance is that the recording medium can be heated up to 866 K with an input power of 8.5 mW which is above the Curie temperature of the FePt film while maintaining the temperature in the NFT at 390 K without a heat spreader. A very good thermal efficiency of 5.91 is achieved also. The proposed structure is easily fabricated and can potentially reduce the NFT deformation at a high recording temperature making it suitable for practical HAMR application.

Nanoscale thermal transport aspects of heat-assisted magnetic recording devices and materials

Abstract

Head flying characteristics in heat assisted magnetic recording considering various nanoscale heat transfer models

The thermal issues in heat-assisted magnetic recording (HAMR) technology have drawn much attention in the recent literature. In this paper, the head flying characteristics and thermal performance of a HAMR system during the touch-down process considering different nanoscale heat transfer models across the head-disk interface are numerically studied. An optical-thermal-mechanical coupled model is first described. The coupling efficiency of the near field transducer is found to be dependent on the head disk clearance. The shortcomings of a constant disk-temperature model are investigated, which reveals the importance of considering the disk temperature as a variable. A study of the head flying on the disk is carried out using an air conduction model and additional near-field heat transfer models. It is shown that when the head disk interface is filled with a solid material caused by the laser-induced accumulation, the heat transfer coefficient can become unexpectedly large and the head's temperature can rise beyond desirable levels. Finally, the additional head protrusion due to the laser heating is investigated.

Infrared Near-Field Transducer for Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is one of the next generation data storage technologies that aims to increase the areal data density to beyond 1 TB/in2. Near-field transducer (NFT), which is a type of plasmonic nanoscale antenna, is a key component of the HAMR system. The laser in an HAMR system typically operates in the near infrared wavelength range between 700–900 nm. We explore the plasmonic behavior and performance of a number of NFTs at longer wavelength of 1550 nm. The different plasmonic modes in the NFTs at different wavelengths are analyzed, and the correlation of the NFT performance with variations in the geometrical design is investigated. Thermal performances of NFT are also studied and compared with the thermal performances when NFTs are operated near 800 nm. It is found that there is a significant improvement in the figures of merit when using longer wavelengths and this can provide an important strategy to realize higher performance data storage devices.

Heat-Assisted Recording: Advances in Recording Integration

Differences between heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) for signal processing of the written track by one and two reader multisensor magnetic recording (MSMR) are explored using spinstand measurements. Tracks written with HAMR heads are shown to have more curvature compared with those written with modern PMR writers. We introduce two signal processing techniques in order to determine the merits and explore the recording physics tradeoffs for this complex system. To first order, we see twice as much bit error rate (BER) gain for the HAMR written track—on readback—using two reader MSMR, than the PMR. This difference we assign to the lower electronic signal-to-noise (SNR) ratio for the HAMR written track. In order to replicate practical conditions, we stress the MSMR system by applying increased adjacent track squeeze and displacing the two readers from the center of the track. Under these conditions we can resolve differences between the PMR and HAMR systems. Increased adjacent track squeeze appears to trim the broad and curved track edges, until the on-track signal starts to degrade. Increased offset of the readers from the center of the track offers improved the media (spatial) SNR, but we fail to harvest all those gains into BER across the range. We hypothesize that this is due to increasing distortion, a combination of track edge noise and reading of the encroaching tracks.

Data Rate Effects on Transition and Remanence Noise in a Modeled Heat-Assisted Magnetic Recording System

The heat-assisted magnetic recording performance at varying data rate conditions is investigated using an ensemble waveform analysis method. This method decouples the transition signal-to-noise ratio (SNR) and the remanence SNR from the total spatial media SNR. For data rates below 2200 Mbpsi, transition and remanence noises are largely independent of writer field rise time (RT) values less than 150 ps. Above a data rate of 2200 Mbpsi, the transition SNR is significantly degraded for the slowest RTs considered. For all data rate values, the remanence SNR sees minimal degradation with increasing RT. At the highest data rate, at least a 1 dB loss is observed compared with the lowest data rate, across the RT range considered. For comparison, spin-stand data were also collected. The modeled results were contrasted to bit error rate (BER) trends obtained from the measurement. The spin-stand data show a significant BER degradation as a function of increasing RT at higher data rates. This trend is likely explained with the increase in transition noise observed in the model. These results suggest that moderate writer field RTs do not degrade either transition or remanence noise significantly. If RTs can be kept low, higher data rates may be allowable from a media noise perspective.