Heat-assisted Magnetic Recording Head Research Articles

Study on the Thermal Decomposition of D-4OH PFPE Lubricant by Reactive Molecular Dynamic Simulation for HAMR

Heat-assisted magnetic recording (HAMR) is one of the ways of ultrahigh density storage technology. The pulsed laser in the HAMR head heats the nano-region of the disk to Curie temperature. The laser heating causes the decomposition of the PFPE lubricant, which indirectly leads to the failure of head-disk interface (HDI). The reactive molecular dynamics (MDs) simulations are performed to research the thermal decomposition of single and multiple D-4OH lubricants. The effect of three different heating rates on the thermal decomposition process of a single D-4OH lubricant is investigated. The decomposition processes of single and multi-D-4OH molecular chains are the same: the functional end groups degrade first and then the main chain. These results are useful for the optimization of PFPE lubricants for HAMR disks.

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.

Simultaneous multimethod scanning probe microscopy of complex nano-systems

In the twenty-first century, scanning probe microscopy characterization techniques have seen significant progress and are capable of probing complex structures and devices for a variety of near-surface features and phenomena with nanometer scale resolution. With modest customization, we can deploy these techniques for industrial metrology purposes in a simultaneous and multimethod system capable of shedding light on device function and failure modes, as well as determining the most efficient methods for data collection. To demonstrate this concept with a current, complex industrial device under development, several scanning probe microscopy techniques advantageous to the progress of heat-assisted magnetic recording heads were selected. This work describes simultaneous and multimethod approaches for performing heat-assisted magnetic recording head characterization using atomic force microscopy with scattering scanning near-field optical microscopy simultaneously performed with magnetic force microscopy or photo-induced force microscopy that could be extended to applications of other complex nanoscale devices. We demonstrate that the optical and magnetic fields are overlapping for fabricated heads, which is necessary for performing heat-assisted magnetic recording. We also observed that the multimethod atomic force microscopy methods show strong agreement between the measured optical and magnetic fields and the locale of their associated parts on the head.

Experimental study of media heat sink thickness impact on signal-to-noise ratio in heat-assisted magnetic recording

Recently, heat-assisted magnetic recording (HAMR) has been demonstrated to extend the areal density growth over the superparamagnetic limited. One key component of this technology is a heat sink layer in HAMR media, which benefits the thermal gradient and transition noise. However, the disadvantage of the heat sink layer (HS) has not been fully explored. In this paper, we investigate the background interference (BGI) impact as a result of the heat sink layer via the spin-stand tester. HAMR heads included a light delivery system that have measured a signal-to-noise ratio and down-track thermal gradient on a variety of HS thickness. Subsequently, we found that a thicker HS is a trade-off between the BGI and thermal gradient. Thus, it remains challenging to achieve an ultra-high areal density using the thermal media design.

Write head design for curvature reduction in heat-assisted magnetic recording by topology optimization

The reduction of the transition curvature of written bits in heat-assisted magnetic recording is expected to play an important role for the future areal density increase of hard disk drives. Recently, a write head design with flipped write and return poles was proposed. In this design, a large spatial field gradient of the write head was the key to significantly reduce the transition curvature. In this work, we optimized the write pole of a heat-assisted magnetic recording head in order to produce large field gradients as well as large fields in the region of the heat pulse. This is done by topology optimization. The simulations are performed with dolfin-adjoint. For the maximum field gradients of 8.1 mT/nm, 8.6 mT/nm, and 11.8 mT/nm, locally resolved footprints of an FePt-like hard magnetic recording medium are computed with a coarse-grained Landau-Lifshitz-Bloch model, and the resulting transition curvature is analyzed. Additional simulations with a bilayer structure with a 50% hard and 50% soft magnetic material are computed. The results show, that for both recording media, the optimized head design does not lead to any significant improvements in the written track. Thus, we analyze the transition curvature for the optimized write heads theoretically with an effective recording time window model. Moreover, we check how higher field gradients influence the curvature reduction. The results show that a simple optimization of the conventional head design is not sufficient for effective curvature reduction. Instead, new head concepts will be needed to reduce the transition curvature.

Spacing Control in Heat-Assisted Magnetic Recording

The spacing between the flying head and the rapidly rotating disk in hard disk drives continues to decrease in order to grow the areal density. In heat-assisted magnetic recording (HAMR), the spacing control complexity is compounded by the additional protrusion resulting from the laser heating. Compared to the writer induced and thermal fly-height control protrusion, the near-field transducer (NFT) protrusion is a faster and more local protrusion that requires new HAMR-specific spacing control techniques. In this paper, we will review the simulation and experimental studies which shed light on both the steady-state and transient characteristics of the NFT protrusion. Simulation analysis reveals the scale of the protrusion and its effects on spacing control. Experimentally, we have demonstrated a novel technique to characterize the NFT protrusion and assist in setting the spacing for each HAMR head. Based upon the knowledge gleaned from characterization, compensation schemes have been developed. Some of the schemes have already been successfully demonstrated.

Measuring Thermal Gradient in HAMR Using Pseudorandom Bit Sequences

The downtrack thermal gradient (DTTG) is a key metric determining the performance of heat-assisted magnetic recording heads. It describes the change in temperature with respect to the position at the recording point, which determines the transition width and consequently the linear density capability. Here, we present a method for measuring the DTTG by stepping the laser power while writing a series of pseudorandom bit sequences (PRBSs). The sharp change in laser power changes the diameter of the thermal spot produced in the media by the laser and near-field transducer, which in turn produces a shift in the location of the written transitions. The magnitude of this shift is measured from the readback signal and may be used to calculate the thermal gradient. The PRBS-based technique described here has the additional advantage of enabling corrections for a number of errors in the calculation of the thermal gradient, including the nonlinear dependence of the transition shift on the magnitude of the laser power modulation and changes in the curvature due to the change in the size of the thermal spot.

Effect of material optical properties on thermo-plasmonics of heat-assisted magnetic recording devices

Plasmonics enables unparalleled focus of optical energy well beyond the diffraction limit of light. The coupling of light with the electrons of a metal produces a surface wave known as surface plasmon. Surface plasmons exhibit unique characteristics, including strongly enhanced local fields, high sensitivity to changes in the local environment, and energy localization to extremely small volumes not restricted by the wavelength of exciting light. However, a significant fraction of optical energy of surface plasmons is dissipated as heat. A novel application of loss-enabled plasmonics is heat-assisted magnetic recording (HAMR), in which a plasmonic near-field transducer (NFT) is used to heat a sub-diffraction-limit region of the magnetic medium of the hard disk to lower its coercivity and enable the write pole of the magnetic head to write data in closely packed magnetic bits. The NFT must deliver a sufficient fraction of the incident laser power to the magnetic medium to increase the temperature of the heated magnetic bit above its Curie temperature, without causing excessive self-heating that could limit the lifetime of the plasmonic transducer. This investigation was focused on the development of an electromagnetic-thermal model of a typical HAMR head stack that can yield simulation results illustrating the effect of material optical properties of various stack layers on thermo-plasmonic performance. The hybrid analytical/numerical approach derived in this study provides an effective means of tuning the optical properties of stacking layers in HAMR devices to optimize the thermal performance and forms a general computational framework for selecting optimum optical properties for other plasmonic devices.

Heat-Assisted Magnetic Recording’s Extensibility to High Linear and Areal Density

Heat-assisted magnetic recording (HAMR) is being developed as the next generation magnetic recording technology. Critical components of this technology, such as the plasmonic near-field transducer and high anisotropy granular FePt media, as well as recording demonstrations and fully integrated drives have been reported. One of the remaining ongoing challenges of magnetic recording in general and HAMR in particular has been the demonstration of high linear density recording, approaching the grain-size (GS) limit of the recording media, and a clear pathway to smaller GSs while maintaining good magnetic properties and distributions. This paper will demonstrate the extensibility of FePt-based media down to the 5 nm center-to-center range. A linear recording density of 3000 kilobits per inch (kbpi), or a bit length of 8.5 nm, approaching the GS limit of this media, has been demonstrated on recording media with a slightly larger GS of 7 nm center-to-center, and using an HAMR head with high thermal gradient >10 K/nm. Key parameters of the media include the microstructure, the thermal design and magnetic properties, most importantly the tradeoff between achievable GS, media moment–thickness product, Mrt, and the distributions of the magnetic switching field and the Curie temperature. Further optimizing the composition, growth, and architecture of the media stack to achieve all the prerequisite magnetic and thermal properties for high signal-to-noise ratios in the smallest demonstrated GS media allows linear recording densities of up to 4000 kbpi, and areal densities in the 3–4 tera-bits-per-square-inch range can be extrapolated based on geometrical scaling.

Electromagnetic and Thermomechanical Analysis of Near-Field Heat Transfer in Heat-Assisted Magnetic Recording Heads

Heat-assisted magnetic recording (HAMR) is a relatively new data storage technology that promises to satisfy the increasing demands for higher storage densities. Heat transfer across the nanometer-size head-disk gap does not follow classical heat transfer principles during the operation of an HAMR hard-disk drive. The HAMR technology relies on laser heating of a sub-diffraction-limit region of the magnetic medium to lower its coercivity and allow the magnetic head to write data in the form of tightly packed bits. Near-field heat transfer strongly affects the thermal protrusion produced by laser heating. A typical HAMR head model with all key components was used in a coupled electromagnetic-thermomechanical analysis to investigate the effect of near-field heat transfer on the thermal characteristics of the magnetic head. The effects of various heat sources and read/write conditions on the development of the thermal protrusion and the surface temperature distribution at the trailing edge of the slider body are examined in the context of simulation results.

Nanostructure, structural stability, and diffusion characteristics of layered coatings for heat-assisted magnetic recording head media

The intense laser heating in heat-assisted magnetic recording (HAMR) has been a major hindrance to HAMR technology from becoming commercially viable. Thermal damage of the near-field transducer (NFT) and write pole (WP) embedded in the trailing edge of the magnetic head due to failure of the protective carbon overcoat after prolonged heating at an elevated temperature are major obstacles. Therefore, the main objective of this study was to develop an effective coating method for HAMR heads. This was accomplished by introducing a new class of layered coatings consisting of ultrathin amorphous carbon (a-C) overcoat, adhesion (SiN) layer, and buffer (NiCr or TaOx) layer sequentially deposited onto Au and FeCo base layers to mimic the layer stacking of NFT and WP elements, respectively. The structural stability of the a-C overcoats and diffusion characteristics of each comprising layer under conditions of heating at 350 °C for 30 min in an Ar atmosphere were investigated by high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). For most stacking configurations the HRTEM/STEM and EELS results generally revealed some layer intermixing and minute carbon atom rehybridization in the heated a-C overcoats. The findings of this investigation suggest that further optimization of the developed layered coatings can provide a viable solution to thermal damage of HAMR heads.

Ultrathin amorphous carbon films synthesized by filtered cathodic vacuum arc used as protective overcoats of heat-assisted magnetic recording heads

Despite numerous investigations of amorphous carbon (a-C) films, a comprehensive study of the feasibility and optimization of sub-5-nm-thick a-C films deposited onto the write pole of heat-assisted magnetic recording (HAMR) heads is lacking. The main objective of this study was to identify the role of pulse substrate bias voltage and C+ ion incidence angle on the structure and thickness of 1–4-nm-thick a-C films deposited by a rather new thin-film deposition method, known as filtered cathodic vacuum arc (FCVA). The cross-sectional structure of a-C films synthesized under various FCVA conditions was examined by high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). It was found that film growth under process conditions of low-to-intermediate substrate bias voltage (in the range of −25 to −100 V), low ion incidence angle (10°), very short deposition time (6 s), and fixed other deposition parameters (65% duty cycle of substrate pulse biasing and 1.48 × 1019 ions/m2·s ion flux) yields a-C films of thickness ≤4 nm characterized by a significant content (~50–60 at%) of tetrahedral (sp3) carbon atom hybridization. A threshold where sp3 hybridization is greatly reduced due to limited film growth was determined from the HRTEM/STEM and EELS measurements. The results of this study demonstrate the viability of FCVA to produce extremely thin and uniform protective a-C films with relatively high sp3 contents for HAMR heads.

Visualizing the bidirectional optical transfer function for near-field enhancement in waveguide coupled plasmonic transducers

We report visualizations of the bidirectional near-field optical transfer function for a waveguide-coupled plasmonic transducer as a metrology technique essential for successful development for mass-fabricated near-field devices. Plasmonic devices have revolutionized the observation of nanoscale phenomena, enabling optical excitation and readout from nanoscale regions of fabricated devices instead of as limited by optical diffraction. Visualizations of the plasmonic transducer modes were acquired both by local near-field excitation of the antenna on the front facet of a waveguide using the focused electron beam of a scanning electron microscope as a probe of the near-field cathodoluminescence during far-field collection from the back facet of the waveguide, and by local mapping of the optical near-field for the same antenna design using scattering scanning near-field optical microscopy as a probe of the near-field optical mode density for far-field light focused into the back facet of the waveguide. Strong agreement between both measurement types and numerical modeling was observed, indicating that the method enables crucial metrological comparisons of as fabricated device performance to as-modeled device expectations for heat-assisted magnetic recording heads, which can be extended to successful development of future near-field-on-chip devices such as optical processor interconnects.

Materials challenges for the heat-assisted magnetic recording head–disk interface

Abstract

Materials for heat-assisted magnetic recording heads

Abstract

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.

Head and Media Design for Curvature Reduction in Heat-Assisted Magnetic Recording

Recording curvature in magnetic data-storage technology has long been one of the significant challenges impacting on recording performance. Despite curvature occurrence in the conventional recording techniques such as perpendicular magnetic recording (PMR), heat-assisted magnetic recording (HAMR) is demonstrated to induce much more severe curvature than PMR. HAMR curvature could cause poor bit error rate and limits the maximum areal density capacity. Here we have theoretically predicted and demonstrated various approaches for curvature reduction from the aspect of either altering the near-field transducer head design or recording medium design. Optical and thermal modeling have indicated that by utilizing a crown-shape peg to change the thermal source profile and compensate for thermal expansion and rounding effect, it could potentially improve curvature figure of merit (FOM) and achieve curvature reduction by ~45%. In terms of the recording media design, by altering the heat sink and internal layer media material or geometry, it could also achieve curvature cancellation of ~40% with increased thermal gradient. The combined approach from both HAMR head and media perspectives with balanced recording FOMs, could potentially realize significant curvature reduction to be of similar or better recording curvature level to PMR.

Nanoscale temperature of plasmonic HAMR heads by polymer imprint thermal mapping

ABSTRACT Polymer imprint thermal mapping (PITM) is a high-resolution thermal mapping technique that is especially valuable for nanoscale plasmonic devices. PITM leverages a ∼50 nm polymer film coating that crosslinks irreversibly with temperature, which records the peak temperature rise of the surface in the local, linear reduction of polymer film thickness. Using AFM to measure topography before and after heating, but not during operation, PITM sidesteps plasmonic artifacts seen in other near-field thermometries, where the probe tip disturbs and is heated directly by the near- and far-field radiation around the plasmonic device. This is notably troublesome for characterizing heat-assisted magnetic recording (HAMR) heads for next-generation hard disk drives. HAMR heads use near-field transducers (NFTs) to focus light on a magnetic media, heating a nanoscale region to its Curie temperature to enable magnetic writing. The PITM proof-of-concept was introduced at The Magnetic Recording Conference (TMRC) in 2015: here, we present a mature technique capable of benchmarking finite-element thermal simulations of nanoscale devices.

Efficiently reducing transition curvature in heat-assisted magnetic recording with state-of-the-art write heads

Curvatures of bit transitions on granular media are a serious problem for the read-back process. We address this fundamental issue and propose a possibility to efficiently reduce transition curvatures with state-of-the-art heat-assisted magnetic recording heads. We compare footprints of conventional with those of the proposed head design on different media, consisting of exchange coupled and single phase grains. Additionally, we investigate the impact of various recording parameters, such as the full width at half maximum (FWHM) of the applied heat pulse and the coercivity gradient near the write temperature of the recording grains. The footprints are calculated with a coarse grained model, based on the Landau-Lifshitz-Bloch equation. The presented simulations show a transition curvature reduction of up to 40%, in the case of a medium with exchange coupled grains and a heat pulse with a FWHM of 40 nm. We further give the reason for the straightening of the bit transitions, by means of basic considerations with regard to the effective recording time window of the write process. Besides the transition curvature reduction, the proposed head design yields an improvement of the transition jitter in both down-track and off-track directions.

High Track Pitch Capability for HAMR Recording

Differences in the areal density capability limits for heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) are explored using spinstand measurements and micromagnetic modeling. The written track curvature and the transition width are measured with a special technique that mitigates cross-track averaging effects due to finite read sensor width. Tracks written with HAMR heads are shown to have more curvature compared with those written with modern PMR writers. Both broadening and transition curvatures are present cross track, that can be described reasonably well using Landau-Lifshitz-Gilbert modeling and indicates the effective gradient profile dominates. However, as we get to the track edge, we see disagreement between the model and the experiment, which is not currently reconciled. The curvature and broadening effects appear to challenge not only the downtrack bit resolution during readback, but also the cross-track written width with increased linear density. We discuss different near field transducer (NFT) designs, and comparing narrow versus wide thermal profile NFT designs, we can see that track pitch appears to weakly optimize to a maximal cross-track gradient point. Experimental measurements of constant bit error rate for different linear and track densities indicate a significant opportunity for high track density recording using HAMR. The difference appears to be related to the ability for HAMR to address high track pitches with a minimal increase in risk of adjacent track interference compared with PMR. We revise NFT head-to-media spacing requirements when we attempt to account for interference effects.