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Abstract

Heat-assisted magnetic recording (HAMR) is a fast evolving technology, and has been established as the next enabler of higher areal density in magnetic recording. After achieving high areal density capability, HAMR drive integration was recently demonstrated. In this paper, we discuss some of the recent learning from component performance and drive integration. We identify a key challenge in HAMR integration: erasure due to thermal background heating. The background heating was introduced to improve near-field transducer reliability and reduce laser power requirement. We present experimental and modeling data on the impact of thermal background, both in the cross-track (adjacent-track erasure) and down-track (self-erasure) dimensions.

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... With recent demonstrations of high areal density [1,2] and successful drive integration [3,4], heat-assisted magnetic recording (HAMR) [5] has proved to be a viable and promising technology for future magnetic data storage products. The commercialization of HAMR presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. ...
... Both of these are dimensional ranges that appear well beyond the capability of the PMR head used in the earlier publication [17], which was closer to 400 KTPI. This general trend is also followed by the path of areal density demonstrations previously published [1,2,4]. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. ...
... Under practical usage conditions we do need to expand this analysis and include the influence of multiple rewrites upon adjacent tracks. It has been illustrated before [4,23] that HAMR has the potential for no Wide Area or Side Track Erasure beyond 2 tracks from the test track, and also low Adjacent Track Interference (ATI) loss. When we previously investigated write clearance loss mechanisms [6] using the same technique for trimmed track analysis described above, we showed that curvature of the written track, and broadening did not appear to significantly change. ...
Article
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 transition width is 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 to those written with modern PMR writers. Both broadening and transition curvature are present cross track, that can be described reasonably well using LLG modeling and indicates the effective gradient profile dominates. However as we get to the track edge we see disagreement between the model and experiment, which is not currently reconciled. The curvature and broadening effect appears 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 vs. 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 to PMR. We revise NFT HMS requirements when we attempt to account for interference effects.
... With recent demonstrations of high areal density [1,2] and successful drive integration [3,4], heat-assisted magnetic recording (HAMR) [5] has proved to be a viable and promising technology for future magnetic data storage products. The commercialization of HAMR presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. ...
... Thus, the optimal bit aspect ratio (BAR) tends to be lower. This general trend is also followed by the path of areal density demonstrations previously published [1,2,4]. ...
... With a PMR writer, the written track width and ATI is mostly determined by the physical dimensions of the magnetic write pole and its shields, although a limited amount of tuning can be done by changing the writer current shape and amplitude. However, HAMR heads have the additional freedom to control the applied laser power, which can dramatically adjust the written track width and ATI, as well as the downtrack writing performance [3,4,16]. It was earlier shown that HAMR writers have more transition curvature than their PMR counterparts. ...
Article
Differences in the areal-density capability limits for heat-assisted magnetic recording (HAMR) and conventional perpendicular magnetic recording (PMR) are explored using spinstand measurements, drive footprinting, and micromagnetic modeling. The written track curvature is measured with a special technique that mitigates the cross-track averaging effects due to a finite read sensor width. Tracks written with HAMR heads are shown to have more curvatures compared with those written with modern PMR writers. Mitigation of written track curvature is demonstrated with two different HAMR writer designs. The curvature effect appears to challenge not only the downtrack bit resolution during readback, but also the cross-track written width with increased linear density (LD). Experimental measurements of a constant bit error rate for different LDs and track densities (TDs) indicate a significant opportunity for high TD 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.
... With the recent 1.0 [1] and 1.4 [2] Tb/inch2 basic technology demonstration, and drive level demonstration [3,4] heat-assisted magnetic recording (HAMR) [5] has proven to be a viable and promising technology for future magnetic data-storage products. The commercialization of HAMR presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. ...
... Error Bars are 95% confidence on the mean slope for each study. In order to better understand the source of these differences in sensitivity we explored micromagnetic readback modeling as outlined in [4] where the input variables were the pulse width (Pw50) and the media SNR mix at a fixed linear density, the electronic SNR was kept fixed. Figure 8 shows the contour map of BER sensitivity per 1 nm change in reader HMS as a function of the input Pw50, and the media SNR at a Fixed linear density of 1280 KFCI. ...
... Recording model simulations, using an LLG micromagnetic model as outlined in Ref [4], 128 bit Pesudo Random Bit Sequence written pattern repeated 50 times for each data point. The results of varying the writer clearance of HAMR under these conditions can be seen for 3 candidate NFT designs in Figure 12. ...
Article
Full-text available
We collate multiple experimental measurements of heat-assisted magnetic recording (HAMR) near-field transducer (NFT) and reader sensitivity measurements on spinstand to compare and contrast with the conventional perpendicular magnetic recording (PMR). The readback process shares many similarities, but differences appear due to the increased measured curvature of the prewritten track, which increases the observed pulsewidth (Pw50), lower readback amplitude due to reduced flux from the media transition (MrT), and increased coating thicknesses. We find that the reader head-to-media spacing (HMS) sensitivities and requirements converge toward the conventional scaling requirements. The HAMR write process is more complex due to the uncertainties associated with the optical properties and protrusion position of the NFT. However, accumulating multiple studies varying write HMS with coatings, NFT changes, and media changes, we consistently observe lower sensitivities compared with the conventional writer HMS, in line with modeled comparisons, leading to more relaxed requirements on the NFT clearance than the PMR write HMS. A 1.5 Tbpsi basic technology demonstration demo is shared using the HMS numbers in the bounds of the claims.
... In order to demonstrate the radial dependence described above, fully integrated heads and typical HAMR media were assembled into drives after first being screened on a spinstand. This is completed in a manner very similar to that use by C. Rea et al. [2] to characterize down track erasure in a HAMR drive. Drives were tested under fixed environmental conditions and temperature is the same for all drives tested and held constant for all measurements. ...
... Subtracting the power required to turn on the laser from the total laser power gives the amount of "effective" laser power that potentially could get transferred from the laser diode to the media, albeit subject to coupling efficiencies, etc. Additionally, because HAMR media includes a heat sink layer below the recording layer [2], if we were to stop the disk from spinning by setting ω equal to 0 in (3) and apply a constant laser power P such that P > 0, the temperature profile in the media would eventually reach a steady state due to thermal diffusion into the heat sink layer [7]. Therefore P in (1) and (3) is actually referring to only the laser power required to compensate for change in linear velocity. ...
... Optimizing on-track performance while not ceding the performance of neighboring tracks is another one of the key challenges in HDD integration [2]. ...
Article
Over the past year, heat-assisted magnetic recording (HAMR) has continued to make significant progress toward production and remains the most promising technology to enable areal density growth beyond 1 Tb/in<sup>2</sup>. In this paper, we present an experimental study on the effects of disk radius and head skew angle in a HAMR hard disk drive. We demonstrate the dependence of laser power on disk radius and the sensitivities to several additional factors that can potentially change that characteristic. We also contrast adjacent track interference and areal density capability performance in drive to conventional perpendicular recording and their respective sensitivities to radius and skew angle.
... Heat Assisted Magnetic Recording (HAMR) is the next generation hard disk drive technology that enables continued and significant areal density growth [2]. With recent high areal density capacity (ADC) demonstrations [3][4][5][6] and successful drive integration [7][8][9], HAMR has shown to be the most promising and viable technology for future magnetic data storage systems. HAMR introduces heat energy to the media which is delivered by a near-field plasmonic transducer during the write process. ...
... Assuming 50% workload load between HIMR bottom and top tracks, on average HIMR requires 8.6% higher total laser current than HAMR CMR. For HAMR SMR and HIMR, an increase in laser current requirement may reduce write reliability of the HAMR head [9]. The linear density in kilobits per inch (KBPI) for HAMR CMR, HAMR SMR and HIMR is shown in Figure 6. ...
... R ECENT high areal density demonstrations of 1 Tb/in 2 [1]- [4], [11] and higher, 3000 kFCI linear density [5], 1 million tracks per inch demonstrations [6], and successful drive integration [7]- [9] all have catapulted the heat-assisted magnetic recording (HAMR) technology into magnetic data storage device productization. The current stage of the HAMR integration into drives signifies the resolution of significant technical challenges [5], [10], [13]. ...
Article
Full-text available
Some aspects of the challenges to high track density heat-assisted magnetic recording (HAMR) are addressed with a focus on track interferences (adjacent as well as non-nearest neighbor erasure). Comparisons with corresponding conventional perpendicular magnetic recording (PMR) parameters are also made. Adjacent and non-nearest neighbor track erasure data from functional HAMR and PMR drives and simulation data of signal-to-noise ratio loss in HAMR are also presented.
... Assuming 50% workload between HIMR bottom and top tracks, on average HIMR requires 8.6% higher the total laser current than HAMR CMR. For HAMR SMR and HIMR, an increase in laser current requirement may reduce the write reliability of the HAMR head [10]. ...
Article
Heat-assisted magnetic recording (HAMR) is the next-generation hard disk drive technology which enables continued and significant areal density growth. There are currently three write architectures for the layout of tracks in hard disk drives: conventional magnetic recording (CMR), shingled magnetic recording (SMR), and interlaced magnetic recording (IMR). The read-back architecture multiple-sensor magnetic recording (MSMR) can be combined with the three different write architectures to increase the areal density by using two or more readers to read-back the same track. In this paper, we compare the areal density capability of HAMR CMR, HAMR SMR, and HAMR IMR combined with read-back with one reader, MSMR with two readers, and MSMR with three readers.
... W ITH recent demonstrations of high areal density [1], [2], [26], [30] and successful drive integration [3], [4], the heat-assisted magnetic recording (HAMR) [5] has proved to be a viable and promising technology for future magnetic data storage products. The commercialization of HAMR presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. ...
... The main optical components include a laser diode which provides the optical energy, and a near-field transducer (NFT) which confines the light to a tiny spot on the media. Integrating those components into HAMR drives is a significant challenge, which requires extended understanding of near-field optics as well as nanoscale thermal, mechanical, and magnetic behavior and multiphysical coupling [1], [2]. Over the last decade, the industry has made significant progress in HAMR integration, such as novel and reliable NFT devices [3]- [5], more robust head disk interface (HDI) materials [6], [7], and thermal management of the HAMR head and media [8], [9]. ...
Article
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.
... Modern HAMR heads include a light delivery system consisting of a laser diode module integrated on the top of the slider of the recording head, a number of optical waveguides, and other optical elements to guide the light to the bottom of the slider in immediate proximity of the magnetic recording pole of the write head and a plasmonic near-field transducer (NFT) to focus the light to a sub-diffraction spot size of 50 nm or smaller. All the elements of the HAMR recording head were described extensively in [2], [3], [5], [15], and [21]. Here, we are using a state-of-the-art head providing a relatively strong, wide area magnetic field of about 10 kOe and an optical spot size of 50 nm, enabling TG in the media in the excess of 10 K/nm as measured by a sideband modulation technique described in [22]. ...
Article
Full-text available
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. IEEE
... The recording time window, which determines the writing quality, also changes as media speed changes [15]. This thermal process on the media also plays an important role in adjacent track interference and adjacent bit interference [16]. ...
Article
Heat assisted magnetic recording (HAMR) promises to deliver higher storage areal density than the current perpendicular magnetic recording (PMR) product. A laser is introduced to the HAMR system to heat magnetic media to reduce the media coercivity. The thermal response of the media becomes very critical for the success of the magnetic writing process. The study of thermal response time in HAMR relies on the setup configurations, such as laser spot sizes, the way that laser energy is delivered to media and the media structures. In this paper, the thermal response time of HAMR media under three different heating methods is systematically investigated through experiments and numerical analysis. A lumped model is built to simplify the heat conduction problem to understand the difference in thermal responses under various experimental conditions. Dominant layers are identified under those experimental conditions. The transient thermal response is mainly determined by the dominant layers. Engineering the dominant layers helps the most in optimizing the thermal performance of the media. Our study clearly suggests that, for HAMR systems, optimizing the thermal properties of the heat sink layer is the key to reducing variations in the transient thermal process resulting from changes in the linear speed.
... With recent demonstrations of high areal density [1,2] and successful drive integration [3,4], Heat-Assisted Magnetic Recording (HAMR) [5] has proved to be a viable and promising technology for future magnetic data storage products. The commercialization of HAMR presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. ...
Article
Full-text available
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 Multi Sensor Magnetic Recording (MSMR) are explored using spinstand measurements. Tracks written with HAMR heads are shown to have more curvature compared to 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 2 reader MSMR, than the PMR. This difference we assign to the lower electronic signal to noise 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 2 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) signal to noise ratio, 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.
... A lot of progress has been made to improve the reliability of the NFT, light delivery, and integrated HAMR heads [18]. High TGs (>10 K/nm) have been achieved with modern head and media combinations and contributed to the progress of areal density illustrated later in this paper. ...
Article
Full-text available
Heat-assisted magnetic recording (HAMR) is being developed as the next generation magnetic recording technology. Critical components of this technology, such as plasmonic near-field transducer (NFT) and high anisotropy granular FePt media, as well as the performance and reliability of fully integrated drives have been reported. This paper will focus on the progress and challenges of HAMR media, including microstructure and thermal design as well as the testing and characterization at high field and high temperature. Due to the importance of the Curie temperature distribution, σ TC, for HAMR, we present a newly developed temperature-dependent complex ac susceptibility method to extract σ TC for HAMR media. Such novel magnetic characterization methods have been used in combination with other high field magnetic metrology and spin-stand recording to provide feedback for continuous improvements of HAMR media. Together with NFT and write head design, the thermal design, σ TC, and microstructure of the media are key factors to reduce the transition jitter below 2 nm as demonstrated in a previously reported 1 Tb/in2 HAMR demonstration. Here, we report the further improvements by significantly enabling higher linear density (>2500 kfci) HAMR and steady progress in areal density to 1.402 Tb/in2.
... A lot of progress has been made to improve the reliability of the NFT, light delivery, and integrated HAMR heads [18]. High TGs (>10 K/nm) have been achieved with modern head and media combinations and contributed to the progress of areal density illustrated later in this paper. ...
Article
Full-text available
Heat assisted magnetic recording (HAMR) is being developed as the next generation magnetic recording technology. Critical components of this technology, such as plasmonic near field transducer and high anisotropy granular FePt media, as well as performance and reliability of fully integrated drives have been reported. This paper will focus on the progress and challenges of HAMR media, including microstructure and thermal design as well as the testing and characterization at high field and high temperature. Due to the importance of the Curie temperature distribution, ---TC, for HAMR, we present a newly developed temperature dependent complex AC susceptibility method to extract ---TC for HAMR media. Such novel magnetic characterization methods have been used in combination with other high field magnetic metrology and spin-stand recording to provide feedback for continuous improvements of HAMR media. Together with near field transducer and write head design, the thermal design, ---TC and microstructure of the media are key factors to reduce transition jitter below 2nm as demonstrated in a previously reported 1 Tb/in2 HAMR demonstration. Here we report further improvements enabling significantly higher linear density (>2500 kfci) HAMR recording and steady progress in areal density to 1.402 Tb/in2.
Article
We discuss background interference (BGI) arising from the read-back of pre-existing data and adjacent tracks on a recording medium. A signal-to-noise ratio analysis is used to measure the impact of BGI in heat-assisted magnetic recording in a spin-stand tester. The signal-to-noise ratio for a data track with an alternating-field background was compared to a data track written over a pair of background tracks. We considered the effect of background-track offsets, the distance between the background and data tracks. BGI, which depends on the background-track offset, can degrade read-back performance. The dibit extraction technique is able to separate the linear dibit response and the BGI in the read-back waveform.
Article
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 severe curvature issue than PMR. HAMR curvature could cause poor bit error rate (BER) and limits the maximum areal density capacity (ADC). 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 has 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 (HS) and internal layer (IL) media material or geometry, it could also achieve curvature cancellation of ~ 40% with increase thermal gradient (TG). 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.
Article
Two-Dimensional Magnetic Recording (TDMR) offers the opportunity to provide areal density gain, but questions were unanswered as to how the gain is achieved and what can be done to maximize the gain from this new technology. In this paper we offer some reasons why different investigators might report different Areal Density gain opportunities. We present data collected on a spin stand with two reader heads and processed with a commercially available Field Programmable Gate Array (FPGA) TDMR channel. The implications of this work should provide guidelines on reader geometries, placement, and performance.
Article
The areal density capability of a magnetic recording disk drive is highly dependent on the application or market segment. In this paper we define a new areal density metric which represents what areal density is possible under ideal recording conditions. This proposed areal density metric enables the industry to standardize and compare the areal density capability of magnetic recording disk drives across various recording technologies independent of market segment. We demonstrate the performance of experimental TDMR and HAMR technology components measured using the proposed areal density metric.
Article
Full-text available
Heat-assisted magnetic recording (HAMR) is being developed as the next-generation magnetic recording technology. Critical aspects of this technology, such as plasmonic near-field transducer (NFT) and high anisotropy granular FePt media, have been demonstrated and reported. However, progress with areal density was limited until recently. In this paper, we report a basic technology demonstration (BTD) of HAMR, at 1.007 Tbpsi with a linear density of 1975 kBPI and track density of 510 kTPI, resulting from advances in magnetic recording heads with NFT and FePtX media. This demonstration not only shows significant areal density improvement over previously reported HAMR demos, more significantly, it shows HAMR recording at a much higher linear density compared to previous reports. It is an important milestone for the development of such a new technology. Many challenges still remain to bring this technology to market, such as system reliability and further advancement of areal density.
Article
Full-text available
In this paper, we report a micromagnetic modeling study on the recording processes in heat-assisted magnetic recording. By solving coupled Landau-Lifshitz-Bloch equations, recording simulations are performed for granular FePt-L10 thin film media. The calculated signal-to-noise ratio shows strong dependence on recording field amplitude, especially for media of small size grains. It is found that low field amplitude yields unsaturated recording whereas high field amplitude causes transitions to be broadened immediately after writing. High thermal gradient will alleviate the transition broadening, allowing high field amplitude to be employed so that completed magnetization in the recorded bits can be achieved in small grin size media. The study concludes that the ability in obtaining the expected signal-to-noise ratio performance at very small grain pitches would critically rely on whether sufficiently high thermal gradient can be achieved in the media.
Article
Full-text available
Heat-assisted magnetic recording is a promising approach for enabling large increases in the storage density of hard disk drives. A laser is used to momentarily heat the recording area of the medium to reduce its coercivity below that of the applied magnetic field from the recording head. In such a system, the recording materials have a very high magnetic anisotropy, which is essential for the thermal stability of the magnetization of the extremely small grains in the medium. This technology involves new recording physics, new approaches to near field optics, a recording head that integrates optics and magnetics, new recording materials, lubricants that can withstand extremely high temperatures, and new approaches to the recording channel design. This paper surveys the challenges for this technology and the progress that has been made in addressing them.
Article
The commercialization of heat-assisted magnetic recording (HAMR) presents some significant technical challenges that need to be resolved before the widespread adoption of the technology can begin. In this paper, we present some HAMR data from prototype drives and discuss some of the challenges related to protrusion management, recording performance optimization, and drive power requirements within the drive.
Article
Heat-assisted magnetic recording (HAMR) limitations and extendibility are studied in light of the recent 1.0 Tb/in2 technology demonstration. The paper examines HAMR specific technology challenges, including switching field distributions at elevated temperature, saturation noise, and near-field transducer (NFT) thermal spot-size limits. While current HAMR recording density ( ~ 1 Tb/in2) is limited by switching field distribution and thermal spot size, ultimate HAMR density (up to 5 Tb/in2) is determined by achievable recording-layer magnetic anisotropy and grain size.
Article
Heat assisted magnetic recording (HAMR) is one of the potential candidates for future hard disk drives. In order to identify critical factors in HAMR recording, a combined micromagnetic-thermal-FEM modeling is used to simulate the dynamic recording process. Based on simulation results, the recording field angle is one of the critical parameters directly affecting recording at high linear densities and high data rates. The higher field angle is necessary to ensure sufficient magnetization switching speed since the switching needs to occur before medium anisotropy field increases to beyond the recording field strength during cooling. The modeling study found the medium Gilbert damping constant is also a critical parameter for the same physical reason.
Article
An algorithm is developed for determining the signal to noise ( S / N ) ratio for an arbitrary data sequence by a time domain correlation analysis. By measuring the correlation coefficient of time domain readback voltages, r [ V ( t ), V ( t + nT )], the S / N for an arbitrary data sequence can be determined. The authors applied this method to obtain the S / N ratio for a magnetic recording thin film medium written by a 127-b pseudorandom sequence at different linear recording densities. The new algorithm may also provide a simple way to integrate the S / N measurement into a digital oscilloscope
Article
Adjacent-track interference (ATI) in a dual-layer perpendicular recording system arises during both writing and reading. During writing, the stray fields from the write head can cause side writing and side erasure. A dc-erase method using media with a low nucleation field is used to identify the extent and polarity of these fields. In media with a strong anisotropy in the soft underlayer (SUL), experimental results reveal that the fields causing ATI have a strong cross-track asymmetry due to interactions between the writing flux and the magnetization of the SUL. On readback, cross-track amplitude profiles taken at long wavelengths show a distinctive "side bump" predominantly on just one side of the track profile. This asymmetry is again found to be associated with the magnetization in the SUL. A theoretical study yields expressions for the lateral extent of the side reading and a critical bit length at which side reading becomes significant. Both of these are consistent with experimental observations. The onset of side erasure during writing is further equated with the time-dependent nucleation field of the medium. Theoretical arguments on thermal decay in the presence of external and demagnetizing field indicate that the threshold for ATI-associated side erasure is expected to follow a time dependence similar to Sharrock's formula for dynamic coercivity.
  • A Q Wu
  • Y Kubota
  • T Klemmer
  • T B Rausch
A. Q. Wu, Y. Kubota, T. Klemmer, T. B. Rausch, et al., " HAMR1.0 T/in2 demo, " IEEE Trans. Magn., vol. 49, no. 2, pp. 779–782, Aug. 2013.