Areal Density Of Hard Disk Research Articles

Double-layer magnetic recording with multilayer perceptron decoder for single-reader/two-track reading in BPMR systems

To expand an areal density of hard disk drives, bit-patterned magnetic recording (BPMR) using a patterned medium instead of a granular medium as employed in perpendicular magnetic recording is attracting much attention as the next-generation recording technology. To further increase the storage capacity of BPMR, multi-layer magnetic recording can be combined with BPMR. Therefore, this paper considers double-layer magnetic recording with a single-reader/two-track reading technique for the staggered BPMR system, which is performed together with a rate-3/5 modulation code. This paper proposes to utilize a multilayer perceptron decoder to decode and estimate the log-likelihood ratio value of the recorded bit sequence that is obtained from the equalized channel. Simulation results show that at a bit-error rate of 10−4, the proposed system with a double-layer recording medium can achieve an improvement gain of about 1.2 decibel if compared to that with a regular single-layer recording medium, even in the presence of media noise.

Micromagnetic Simulation of L10-FePt-Based Transition Jitter of Heat-Assisted Magnetic Recording at Ultrahigh Areal Density.

The areal density of hard disk drives increases every year. Increasing the areal density has limitations. Therefore, heat-assisted magnetic recording (HAMR) technology has been the candidate for increasing the areal density. At ultrahigh areal density, the main problem of the magnetic recording process is noise. Transition jitter is noise that affects the read-back signal. Hence, the performance of the magnetic recording process depends on the transition jitter. In this paper, the transition jitter of L10-FePt-based HAMR technology was simulated at the ultrahigh areal density. The micromagnetic simulation was used in the magnetic recording process. The average grain size was 5.1 nm, and the standard deviation was 0.08 nm. The recording simulation format was five tracks in a medium. It was found that a bit length of 9 nm with a track width of 16.5 nm at the areal density of 4.1 Tb/in2 had the lowest transition jitter average of 1.547 nm. In addition, the transition jitter average decreased when increasing the areal density from 4.1 to 8.9 Tb/in2. It was found that the lowest transition jitter average was 1.270 nm at an 8 nm track width and a 9 nm bit length, which achieved an ultrahigh areal density of 8.9 Tb/in2.

Transfer Behaviors of PFPE Lubricant from Disk Surface to Slider

The rapid growth in storage areal density of hard disk drive (HDD) forces the flying height of slider has been currently reduced to less than 2 nm. In such a narrow spacing, perfluoropolyether (PFPE) lubricant is easier to transfer from disk surface to slider under fluid force field and heat transfer field, which is found to have a great effect on date storage reliability. Firstly, the lubricant transfer volume expression with respect to lubricant molecular weight is established. Secondly, the finite element models (FEM) of both Tri-pad positive pressure slider and flat slider are developed, and based on this, the PFPE lubricant transfer behaviors in HDD are investigated. Finally, four type unite cells composed of lubricant molecules and air molecules are established by using Materials Studio software, then both dynamic temperature and diffusion coefficients are researched based on molecular dynamics theory. Results show that the PFPE lubricant amount decreases with increasing pitch angles, rotation angles, slider flying height and disk rotation velocity, while it increases when heat source power increases. The diffusion coefficients of ZTMD lubricant is the maximum in the four lubricants we researched, while the temperature change ratio of Z-15 lubricant is the maximum.

Characterization of Laser-Induced Protrusion in HAMR by the Burnish Method

Heat-assisted magnetic recording (HAMR) has become one of the leading approaches to increase the areal density in hard disk drives to several Tb/in2. Laser-induced protrusion significantly impacts the HAMR heads’ performance and lifetime. Nonetheless, the pointy local protrusion amount is hard to accurately quantify by touchdown sensors. Based on the previous indirect thermal-mechanical studies with different sensors and techniques, a new methodology is developed to measure the laser-induced protrusion by burnishing the head during writing with laser powered ON and by utilizing atomic force microscopy scans. This burnish method requires the air-bearing surface (ABS) to physically contact the rotating disk without an excessive overdrive, which relies on the precise settings of the touchdown sensors and the detection methods. The time dependence of the burnish behavior is studied. The results show that its time constant is much longer than the pure thermal and mechanical time constants. The repeatability of the experiment is validated. The local protrusion induced by the laser is in the range of 2–5 nm based on current head design. Accordingly, the detailed studies about heater power effects and protrusion amount as a function of laser power have been conducted. In addition, the impact of varying degrees of stiffness of the ABS is discussed.

An Experiment Study of Head-to-Media Spacing Sensitivity in Ultra-high Density Magnetic Recording Systems

The decrease in flying height due to the higher recording areal density of hard disk drives (HDDs) has had a huge impact on data storage technology. The head protrusion that induced by ambient temperature can further reduce flying clearance by a few nanometers. However, the head-disk interface reliability should be also emphasized; since when the head flies too close to the media, the recording head may be burnished. To investigate the head-to-media spacing sensitivity, we use the spin stand tester and analyze the sensitivity through the measurement of overwrite (OVW), micro-track profile (MTP), and signal-to-noise ratio (SNR). The experiment results imply the significant relationship between the OVW and the heater-induced writer protrusion (H-IWP). Furthermore, we verify that the properly reduction of head-to-media spacing can also enhance the both of writability and readability.

Structural Size Effect with Altered Temperature on MgO-based Magnetic Tunnel Junction Device During Current Flow

Recently, the increase in high areal density of hard disk drive with the small bit cells affects the structural variation in the magnetic recording head. Therefore, the structural size effect on the altered temperature in the tunneling magetoresistance (TMR) head with the MgO-based magnetic tunnel junction structure was investigated. Results indicate that the temperature increment in the MgO layer and the antiferromagetic layer decreases significantly with decreasing the thickness of the MgO barrier. This is because the generated heat during the current flow is proportional to the TMR resistance which can be estimated by the resistance of the MgO barrier layer. The efficiency of the write/read process with the thermal effect in the magnetic recording head is the important parameter realized for development of the storage technology.

The Importance of Depth-Varying Fields for MAMR Switching-Field Reduction

Microwave assisted magnetic recording (MAMR) was proposed to increase the data storage areal density in hard disk drives [1]. With a localized AC field generated by a spin torque oscillator (STO), MAMR significantly reduces the switching field when the AC field frequency matches the ferromagnetic resonance frequency of the media elements [2]. In this study we present micromagnetic modeling of media switching-field reduction due to AC fields at GHz frequencies. We employ a 4-layer media model based on current exchange-coupled media with the top layer describing a large lateral exchange “cap” with three low lateral exchange “oxide” layers below. We examine hysteresis loops in two scenarios: depth invariant applied fields (external and AC), and depth invariant external field with depth varying AC field. We then do a DC-band write study with realistic writer field and macrospin-based STO field to gauge switching-field reduction in a scenario more similar to actual recording.

A Study of SNR and BER in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) is a promising approach to increase the areal density of hard disk drives. The purpose of this paper is to provide a computational framework for optimizing the HAMR system. In order to do that, an integrated HAMR simulation system was built. We then studied several factors that could affect the playback signal-to-noise ratio (SNR), such as the switching time of the writing field, the peak temperature of the heat spot, the standard deviation of anisotropy field and exchange parameters, and the size of the reader element. We found that bit length (BL) becomes the key factor for the value of SNR for small, e.g., 10 nm, BLs. We established a relationship between the SNR and the bit error rate so that it will be easy to predict the storage capacity based on the playback SNR. This curve is used to predict the possible storage capacity for different BLs.

Role of Intergrain and Intragrain Coupling on Switching Field of Granular Exchange Coupled Composite Media Using Microwave Assisted Magnetic Recording

Microwave assisted magnetic recording is one interesting magnetic recording technology to improve writability for high areal density of hard disk drive. In this work, the role of intergrain and intragrain coupling and microwave field is investigated by micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The results show that the switching field, Hsw, of media with microwave field, Hac, of 10 GHz is lower than that for media without Hac. The exchange coupling between nearest neighbor grains hardly affects the Hsw of media and slightly affects the slope of hysteresis loop. In addition, the slope of hysteresis loops and Hsw decrease with increasing exchange coupling in each grain. The results from this work can lead to obtain the improvement writability and increasing areal density of hard disk drive.

Free Layer Characterization of TMR Head with Multi-Stripe Height by Using Ferro Magnetic Resonance Analyzer (FMRA)

As continued reducing the track width (TW) of magnetic read sensor for gaining areal density in hard disk drive (HDD) industry, stripe height (SH) is needed to be reduced as keeping aspect ratio(TW/SH) for maintain resistance and signal constant. This paper is proposed to characterize the free layer behavior of tunneling magneto resistive (TMR) head with multiple SH by using ferro magnetic resonance analyzer (FMRA) that is able to analyze by layer-level state without applying external field. The results are found that, at shorter SH, effective stiffness field of free layer is increased which is based on decreasing FMR amplitude and increasing FMR frequency. As a result, magnetic read sensor is shown less sensitivity and signal to noise ratio (SNR) with shorter SH. The SH is a correlated function of the effective stiffness field in free layer that is referred to changed shape anisotropy field which has an effect to read ability.

Experimental and Simulation Study of Thermal Protrusion-Induced Head-Disk Contact Instabilities in Hard Disk Drives

Stability of the thermal fly height control slider has profound effects on the read-write performance and reliability of the head-disk interface. As magnetic storage technologies evolve aggressively toward higher areal density in hard disk drives, the fly height around the transducer area on the slider has achieved a read-write physical spacing of \({\sim }2\) nm, and is expected to be even lower to meet the magnetic storage road map in the near future. At such a low spacing, instabilities can occur in terms of slider vibration and prominent lubricant transfer from the disk to the slider, resulting in significant interface change that raises reliability concerns. In this paper, we experimentally investigate the thermal protrusion induced slider instability at contact proximities, and perform numerical simulations to explain the measured dynamics at head-disk contacts. The results suggest that as the slider's thermal protrusion area approaches the disk surface, instabilities mainly come from down-track force excitations. These excitations lead to more significant vibrations in the down-track direction and on the slider's leading edge rather than its trailing edge. The behavior of slider vibration at disk proximity is also dependent on the head-gimbal-assembly design.

Microwave Assisted Magnetization Reversal on Exchange Coupled Composite Media

To overcome superparamagnetic limit, microwave assisted magnetic recording (MAMR) is one interesting magnetic recording technology. Therefore, the effect of microwave on magnetization reversal in media should be analyzed. In this work, we propose the MAMR to decrease switching field (coercivity, Hsw) in exchange coupled composite (ECC) media by using the micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The Hsw of single layer and ECC media without microwave field is 110.90 and 7.7 kOe, respectively. When the oscillating microwave field is added, Hsw of single layer media with microwave frequency of 2.5 - 40 GHz is lower than 110.90 kOe. Likewise, Hsw of ECC media with microwave frequency of 5 - 16 GHz is lower than 7.7 kOe and has the lowest value of 4.9 kOe at frequency of 10 GHz. The results from this work lead to solve superparamagnetic limit and increase areal density in hard disk drive.

Lubricant transfer mechanism and process between slider and disk

The increasing of areal density of hard disk drives promotes the decreasing of the slider flying height. Lubricant transfer between slider and disk, caused by reducing slider flying height, plays an important role in affecting slider flying stability. In this study, the improved coarse-grained, bead-spring model is used to investigate the mechanism of lubricant transfer between slider and disk by molecular dynamics simulation. The effects of lubricant thickness on disk surface, lubricant type, and local temperature difference on the slider surface on lubricant transfer are studied. We observe that the amount of lubricant transferred to the slider sharply increases with the increase of lubricant thickness value on the disk surface. Increasing the number of hydroxyl groups in an individual lubricant molecule can greatly reduce the volume of the lubricant transferred to the slider. In addition, the local temperature difference on the slider surface can increase the volume of lubricant transferred to slider. What is more, the increasing of the number of hydroxyl groups contained in an individual molecule can considerably improve the influence of the local temperature difference on the lubricant transfer between slider and disk.

A New AFM-Based Technique to Detect the NFT Protrusion on HAMR Head

Heat assisted magnetic recording (HAMR) is anticipated to increase the areal density in hard disk drives to multiple Tb/in2. However, the newly introduced protrusions caused by additional heat sources must be taken into account in head-disk spacing control. The light absorption by the near field transducer (NFT) and the heat dissipation into the slider through the laser delivery system can result in the temperature rise well over a hundred degrees Celsius. In response to such a big temperature change, an instantaneous protrusion at the NFT region is expected. This protrusion has a direct impact on the head-magnetic spacing (HMS) and therefore affects writing performance. Due to its small lateral size below 1 micrometer, the NFT protrusion is difficult to probe. In this paper, we introduce a new technique, which is based on atomic force microscopy (AFM) and allows for a simultaneous topography imaging and NFT protrusion mapping on the air bearing surface (ABS) with a nano-scale resolution. The measured NFT protrusion profile of a nonflying head indicates the local min-fly point at the location predicted by our simulation results. A further quantitative analysis proves the dependence of NFT protrusion on the laser power, which agrees very well with modeling and can offer a good guidance to the head-disk interface management during recording.

Simulation studies of contact sensor for disk defect mapping

The keep increasing areal density in hard disk drive demands reducing the clearance between the read/write element and the disk surface. A clearance of 2 nm could be achieved using thermal-flying-height technology (Kurita et al. IEEE Trans Magn 41(5):3007–3009, 2005). In such a low-clearance flying regime, it is important to ensure the reliability of the head-disk interface. Head carbon overcoat wear should be taken into account and it requires ultra-smooth disk surface. Conventional method for disk inspection is using glide tester (Nayak et al. IEEE Trans Magn 39(2):729–734, 2003). However, with lower and lower clearance, some small defects on the disk surface can no longer be neglected, and these small defects cannot be detected by glide test. Thus a thermal-contact sensor integrated in a magnetic-head slider was proposed by Shimizu et.al to detect defects on disk surface (Shimizu et al. IEEE Trans Magn 47(10):3426–3432, 2011). Their experimental results confirmed that the contact sensor has equivalent sensitivity for detecting head-disk contact to that of a conventional acoustic emission sensor. And the defect mapping with this method was more sensitive than measurements with an optical surface analyzer. All these results were obtained using experimental method. Thus, numerical simulation studies are necessary in order to investigate the contact mechanics between asperities and the contact sensor. In this paper, a simplified contact model was developed to study contact mechanics between contact sensor and the disk asperity using finite element method. Different parameters’ effect on the signal of a contact sensor was investigated. These results are of significance for the development of a thermal contact sensor in the hard disk drive for higher recording density.

Experimental Study of Dual Heater Slider Touchdown Detectability

With the increase of areal density in hard disk drive (HDD), the mechanical spacing between magnetic head and disk has decreased to sub~1nm. Under such ultra-low spacing, accurate spacing control becomes more important for lowering down the risk of head-disk interference and getting good bit per inch (BPI) performance. Dual-heater slider technology has been considered as a most useful solution. This paper introduces the advantages of dual heater in reading/writing spacing control. Furthermore, as a key issue in precise spacing control, the touchdown detectability of dual-heater slider was also investigated using three TD detection methods under writing and reading mode. Result shows that a single method is difficult to get the best TD detectability under both writing and reading mode. Instead, hybrid method may be a feasible solution.

Planarization of Discrete Track Recording Media to Improve Flyability of Magnetic Recording Sliders

Discrete track recording (DTR) media which consist of discrete tracks are presently considered a new paradigm to achieve ultra-high areal density in hard disk drives (HDDs). However, due to the presence of the grooves, stable flying of magnetic recording sliders over DTR media is a question of great concern. In this study, planarization of DTR media with hydrogen silsesquioxane (HSQ), a nonmagnetic material, is investigated. Scanning electron microscopy (SEM) of planarized disks showed very good ldquogap fillingrdquo properties of HSQ in the groove area during the planarization process. Atomic force microscopy (AFM) measurements indicated that the residual groove depth of planarized DTR media decreases with decreasing rotating speed of the disk during spin-coating. Improved flyability of magnetic recording sliders over DTR media was observed after planarization.

Magnetic Properties of Patterned CGC Perpendicular Films With Soft Magnetic Fillings

To increase the areal density of hard disk drives (HDDs), discrete track media (DTM) and bit-patterned media (BPM) are being extensively studied. In the DTM and BPM, it is essential to isolate data tracks or bits. Unlike isolating the data tracks or bits using the conventional procedure involving physical etching, we attempted to isolate the tracks or bits with a soft guard band using an area-selective ion irradiation method. We prepared hard and soft (H/S) patterned films by nanoimprinting followed by ion irradiation. We confirmed the magnetic isolation of tracks and bits using magnetic force microscopy (MFM) images and magnetooptical Kerr effect (MOKE) signals. The results of an analysis of the magnetic properties of patterned coupled granular continuous (CGC) films with weak exchange coupling confirmed the validity of this novel approach for the fabrication of DTM and BPM.

Development of high density magnetic recording media for hard disk drives: materials science issues and challenges

<title/>The tremendous increase in areal density of hard disk drives is mainly ascribed to harmonic development between magnetic recording media and heads in their scaling, especially allowing a commercial transition from the longitudinal to perpendicular recording system. This paper reviews the main features, recent breakthroughs and future potentials of both the longitudinal and the perpendicular media from a viewpoint of materials science. Special attention is firstly paid to the ‘trilemma’ problem for the media, i.e. the compromise among writability, thermal stability and signal to noise ratio (SNR). The evolution of media materials these years are then addressed with emphasis on the thermodynamic origin of magnetically induced phase separation of Co-Cr based alloys, which governs media noise and coercivity, and its applications to the current longitudinal media. The materials challenges for media to achieve 500 Gb in.−2 and above are further predicted from the viewpoints of thermal stability improvement and microstructure control of media materials, and their engineering issues have been discussed for the current Co-Cr based alloys, potential FePt and CoPt ordered, phase separated Co-W based alloys and magnetic rare earth compounds. Finally, the future media approaching 1 Tb in.−2 and beyond are addressed with respect to the principles, progress, engineering challenges and future directions.

Low damage smoothing of magnetic materials using off-normal gas cluster ion beam irradiation

A damage-free smoothing process for magnetic materials using a gas cluster ion beam (GCIB) has been studied. As the areal density of hard disk drives (HDDs) has increased, the flying height of the magnetic recording head above the disk has been decreasing. In order to further reduce the flying height and improve the sensitivity of sensors, a damage-free smoothing technology for magnetic materials is imperative. A GCIB process has been proposed as a novel smoothing technique for various materials. With this process it is expected that the damage will be low, since the gas clusters are aggregations of a few to thousands of atoms or molecules. In this paper, oblique GCIB irradiation has been studied in order to achieve low damage smoothing of PtMn and NiFe thin films. The surface morphology after GCIB irradiation was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A smooth surface with an average roughness of less than 1 nm was obtained by using oblique GCIB irradiation. Irradiation damage, such as fluctuations in the ratio of the material components and oxidation of the surface, were investigated by secondary ion mass spectrometry (SIMS). The fluctuations were suppressed by using oblique GCIB irradiation with incident angles larger than 80°. Consequently, irradiation from a GCIB at grazing incidence can be applied to achieve low-damage smoothing of magnetic materials.