Head-medium Spacing Research Articles

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

In heat-assisted magnetic recording (HAMR), a laser is employed above the read-write transducer to provide energy to the media, lowering its coercivity. However, the laser also brings thermal energy diffusion inside the slider and induces an extra angstrom-level protrusion, which we call laser-induced protrusion (LIP). The LIP needs to be taken into consideration in HAMR due to the significance of head-media spacing. This paper focuses on laser heating on the millisecond timescale during flying in the HAMR conditions. When the laser is turned ON for milliseconds, the LIP forms in the short term (∼μs) and fly height change (FHC) happens in the long term (∼ ms) due to the crown/camber change, resulting in a smaller touchdown power (TDP). Thus, the touchdown power change (ΔTDP) is measured and the LIP is isolated using the time constants. A component-level HAMR stage is used to study the effects of laser-on time, laser current, and linear velocity on the ΔTDP. The experimental results show that the FHC needs ∼ 28 ms to reach the steady state and that the protrusion size presents a two-stage linear relation with the laser current separated by a threshold. The LIP size is reduced by about half when operating from 12 m/s to 24 m/s.

Investigations of Adhesion under Different Slider-Lube/Disk Contact States at the Head–Disk Interface

Adhesion is the key factor influencing the failure of the hard disk drive operating under ultra-low flying height. In order to mitigate the negative effects of adhesion at the head–disk interface (HDI) and promote further development of the thermal flying height control (TFC) technology, an adhesive contact model based on the Lifshitz theory accounting for the thermal protrusion (TP) geometry of TFC slider, the layered structures of the head and disk, and the operation states of the slider was proposed to investigate the static contact characteristics at the HDI. The simulation results demonstrated the undesirable unstable regions during the transitions between different operation states and the necessity of applying TFC technology. The reduction in the head–media spacing (HMS) was found to be achieved by properly increasing the TP height, decreasing the thickness of the lubricant layer or the thickness of the diamond–like carbon (DLC) layer during the flying state or the TP–lube contact state. At the TP–DLC contact regime, the attractive interaction was stronger than other states, and the strong repulsive interaction made the HMS difficult to be further reduced through the increase in the TP height or the decrease in the lubricant thickness.

A method for monitoring head media spacing change in a hard disk drive using an embedded contact sensor

Growth in the demand for higher capacity hard disk drives has pushed the requirement for head-media spacing to sub-nanometer levels. The drop in operational clearance makes a head-disk interface more susceptible to potential head-wear and contamination related issues. Such degradation processes are often accompanied by a noticeable shift in the head-disk clearance. Hence monitoring an interface for a spacing change can be helpful in early detection of its imminent failure. In this paper, we present a method to detect the change in head-disk spacing using an embedded contact sensor (ECS). This technique involves the analysis of ECS dynamic response for an interface that is subjected to heater induced spacing modulations. As the head moves closer to the disk surface, the magnitude of the ECS frequency components can be used to determine the ‘characteristic spacing’ which can be used as a metric to detect any physical change for a given interface.

Materials for heat-assisted magnetic recording heads

Abstract

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

Abstract

Correlation of Disk Topography Waves with Nanometer-Scale Lubricant Moguls and Magnetic Head Media Spacing

Magnetic recording disk carbon overcoats are lubricated with nanometer-thick films of perfluoropolyether lubricant. It is well-known that lubricant thickness redistribution takes place due to air shear stress oscillation at air bearing resonant frequencies and also due to shear stress oscillation induced by disk topography waves on test tracks. We extended this work to demonstrate correlation between surface topography and lubricant redistribution on whole disk surfaces. Lubricant moguls are shown to form over regions of the disk surface which have topography waves that are half the slider length, and the lubricant thickness peak is out-of-phase down track from the topography peak height. There is a critical relative humidity above 20% beyond which moguls are readily formed by the slider flying at 10 nm without thermal fly-height control. The significance of the lubricant redistribution for drive magnetic performance has long been the subject of debate. The slider flying height modulation measured by magnetic head media spacing was in good agreement with the dynamic air bearing simulation based on the measured disk surface topography. Measured head media spacing image data on the same disk surface before and after lubricant redistribution had the same length scale as the correlation between topography and lubricant thickness variation. These results demonstrate that lubricant thickness redistribution on the order of atomic diameters can degrade magnetic performance, and that the surface topography waves alone can degrade areal density by as much as 2%.

Lubricant Transfer Model at the Head-Disk Interface in Magnetic Storage Considering Lubricant–Lubricant Interaction

Interactions among molecularly thin lubricant–solid and lubricant–vapor interfaces become increasingly strong when the slider approaches closer to the disk (or media) surface, which is desirable for higher storage areal densities. Lubricant transfers from the disk to the slider as the head-media spacing decreases below a critical value, leading to loss of protection for the solid thin films. The present work proposes a new criterion for lubricant pickup to occur by improving the lubricant transfer model via considering interactions between the lubricant films, in addition to lubricant–solid interaction. The effect of lubricant roughness is also included in the proposed model, which is then used to predict critical solid distances as a function of lubricant roughness and average lubricant thickness. It is found that the interaction force between the slider solid and the disk lubricant plays a weaker role than that between the lubricant on the slider and the lubricant on the disk in determining the critical solid distance. The physics for the dependence of the critical clearance on the bonding ratio is explained, and a linear model is developed.

A single asperity sliding contact model for molecularly thin lubricant

Molecularly thin lubricants are important in protecting the recording head and the rotating disk in a magnetic storage hard disk drive from mechanical damage induced by contact. Direct contact is more likely to occur at lower head-media spacing, which is the distance between the rotating magnetic disk and the head that reads/writes the data, and is highly desirable to be extremely low (in the nanometer range) for Terabit/in2 areal densities. Solid contact mechanics of the head-disk interface has been well addressed through extensive experiments and modeling, by neglecting the effect of the molecularly thin lubricant. Considering the important role of the lubricant, it is necessary to investigate the contact mechanics that include the lubricant. The present study develops a mechanics-based model to account for the nanoscale phenomena including slippage, nonlinear viscosity and bonded lubricant fraction, thus providing a measure to bridge the hydrodynamic lubrication and solid contact regimes at the nanoscale. Results show that the friction coefficient increases with the bonded ratio, which correlates with experimental observations.

Repeatable and Non-repeatable FH Modulation Study Near Head Contact

The lower head–media spacing (HMS) is always expected for achieving better recoding performance at high linear density. Besides the efforts to reduce the overcoat thickness of head and media, the low flying height (FH) and smaller FH modulation (FHM) are necessary to ensure the low HMS and good interface reliability. In the current HDD, the FH change is measured from the readback signal by the Wallace spacing loss equation. The real-time dynamic FH signals consist of repeatable and non-repeatable FH changes, or FHM. The repeatable FHM (RFHM) correlates with the disk morphology variations. It usually has the variation of transition parameter $a$ inside. Because the change of $a$ is the noise of the RFHM, it has to be very careful for the compensation of the RFHM by thermal FH control. Otherwise, it may cause unexpected head–disk contact. The non-RFHM (NRFHM) measurement is able to remove the media-related noise, which has a good testing accuracy of around 1 A. This paper studies both RFHM and NRFHM characteristics of four types of sliders with different air-bearing surface designs.

Study of Head-Disk Interface Characterization Using Touchdown Sensor and Electromagnetic Signal in Hard Disk Drives

With the introduction of thermal fly-height control sliders, the local head-disk clearance can be reduced to a range from $\sim10$ nm to contact. This actively controlled touchdown (TD) has been used as a way to study the head–disk interface (HDI), the characteristics of which greatly affect the areal density and reliability of drives. In this paper, a high spatial resolution TD study is performed at the drive level using TD sensor signals and head-media spacing signals. Different flying stages, including passive flying, TD transition, and over push modulation, are identified from the experimental results. A correlation between the aforementioned signals at the passive flying stage is found to represent the disk surface property. The passive flying stage and over push modulation stage of TD are further analyzed in both the time and frequency domains. By establishing a correlation between the fly height and the temperature response, we can provide insight into the heat transfer model for the HDI.

Dual track Wallace fly height measurement method to monitor slider motion in 2 dimension at near contact regime

Head media spacing of hard disk drive (HDD) is expected to continue its reduction in order to support areal density growth. At sub-nm clearance between the head and the disk, the intermittent head disk contact and interactions may happen. This does not only cause the fly height modulation but also induces the off-track motions as well. It is desirable to understand the characteristics of the 2D motions in the near contact regime, so as to enable further reduction of the clearance and to improve the reliability of HDD. This paper presents a method to measure instantaneous fly height (FH) motion and cross track motion concurrently by using read back signal from the dual data tracks written at different frequencies. The method is able to separate the FH motion and cross track motion of the head.

Media Roughness and Head-Media Spacing in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording involves the transfer of energy to the recording medium via optical means. To enable high areal density, the recorded track must be smaller than the diffraction limit of focused light, which is accomplished by using a near-field transducer (NFT) with a corner or peg with small dimension. Energy transfer using such a transducer is a near-field effect, and therefore is highly sensitive to the spacing between the NFT and the medium. Since the recording medium has some surface roughness, there will be a variation in the NFT-to-medium spacing and this will impact the amount of energy transferred from the NFT. We model the effect of Gaussian surface roughness on NFT energy transfer and predict surface temperature variations for a rough surface. In addition, we illustrate how changing the head-medium spacing changes the impact that roughness has on surface temperature variation. We combine these modeled predictions with spinstand measurements of recorded data and conclude that the effect of media roughness results in only limited temperature excursions above the nominal recording medium temperature.

A HAMR Media Technology Roadmap to an Areal Density of 4 Tb/in$^2$

This paper discusses heat-assisted magnetic recording (HAMR) media requirements and challenges for areal densities (AD) beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Based on recent roadmap discussions the focus is primarily on granular chemically ordered L10 FePtX-Y-perpendicular media with reduced average grain size down to 〈D〉 = 3-5 nm relative to current CoCrPt based perpendicular magnetic recording (PMR) media with average grain size 〈D〉 = 7-9 nm. In HAMR media the combination of thermal conductivity and Curie temperature TC determines the required laser power during recording. Key challenges are sigma variations of D and TC which need to be reduced to σD/D ~ 10-15% and σTC/TC ~ 2%. In addition AD is limited by switching field distribution (SFD) and thermal spot size. The key goal going forward is to optimize heads, media, head-media-spacing (HMS) and read-back channel technologies to extend AD to 4 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and beyond.

Damage layer study of the overcoat deposited on the top magnetic layer of hard disks

This paper presents the effect of nanoscale damage layer on magnetic results obtained from the optimization of media overcoat deposition parameters on the top magnetic layer on magnetic hard disks. We have investigated the effects of interface interaction between the overcoat deposition parameters on the top magnetic layer on the media by using a plasma enhanced chemical vapor deposition (PECVD) on next generation hard disks. The goal is to achieve a reduced damage layer, lower head media spacing (HMS) and a higher spectrum of signal to noise ratio (SpSNR) optimized by using Taguchi experimental design with a four-factor three-level (L9) orthogonal array. An analysis of variance (ANOVA) was carried out to interpret the measured coercivity (Hc), HMS and SpSNR. It was found that source gas type is the most significant factor with a percentage contribution effect of 59.8% on HMS and 51.7% on SpSNR. The bias voltage is the second most significant factor with its percentage contribution being 24.2% on HMS and 31.0% on SpSNR. Overall, the optimum SpSNR was obtained using a C2H2 source gas, −100V bias voltage, 50V anode voltage and 20sccm gas flow rate, respectively.

Transition Parameter “a” Variation of Individual Writing Process

The jitter noise is the dominant media noise in perpendicular recording. It is proportional to cross track correlation and the square of transition parameter “a” [1]. Besides the efforts of minimizing the value of “a,” it is also very important to control the variation of “a” in each individual writing process. In this work, from the time domain head media spacing measurement, there is a significant signal amplitude variation arising from individual writing process at the same writing conditions. After removing the change related to disk morphology, the amplitude difference of individual writing reflects the variation of “a.” Reducing such “a” variation is helpful to control the transition noise.

The Head-Disk Interface Roadmap to an Areal Density of Tbit/in2

This paper reviews the state of the head-disk interface (HDI) technology, and more particularly the head-medium spacing (HMS), for today’s and future hard-disk drives. Current storage areal density on a disk surface is fast approaching the one terabit per square inch mark, although the compound annual growth rate has reduced considerably from ~100%/annum in the late 1990s to 20–30% today. This rate is now lower than the historical, Moore’s law equivalent of ~40%/annum. A necessary enabler to a high areal density is the HMS, or the distance from the bottom of the read sensor on the flying head to the top of the magnetic medium on the rotating disk. This paper describes the various components of the HMS and various scenarios and challenges on how to achieve a goal of 4.0–4.5 nm for the 4 Tbit/in2density point. Special considerations will also be given to the implication of disruptive technologies such as sealing the drive in an inert atmosphere and novel recording schemes such as bit patterned media and heat assisted magnetic recording.

Magnetic Recording Near the Grain Size Limit

The writing quality of magnetic recording improves with increasing sharpness of the written transitions. Ultimately, the transition sharpness is limited by the grains themselves, and no longer depends on the head field gradient. In this study, we varied the head field gradient by changing the head-media spacing and characterized the write quality by transition jitter and equalized signal-to-noise (eSNR) measurements. Consistent with a recording in the grain size limit, we discovered that the write quality did not improve with smaller head to medium spacings. We also found that the measured values for the jitter and the eSNR are close to what is estimated for grain size limited recording.

A method for determining ultrathin DLC film thickness by spectroscopic ellipsometry

The accurate measurement of ultrathin DLC overcoat thickness becomes a key factor in the research and fabrication of advanced magnetic recording heads and media with the ever-decreasing head-media spacing. Spectroscopic ellipsometry can meet this requirement when the suitable measurement method is adopted. For ultrathin DLC film, there is the strong correlation between its optical constants and thickness, which makes ellipsometry measurement difficult to yield a unique and accurate solution. In this study, we adopt a combined analytical method to determine ultrathin DLC films with a thickness range of 10---50 A. The uniqueness and accuracy of the measured thickness are verified by Uniqueness Test tool and X-ray reflectivity (XRR), respectively. The results show that the adopted method is able to uniquely and accurately measure the thickness of ultrathin DLC film. This method is expected to be of value for thickness measurement of other ultrathin dielectric or metallic films.

Head-disk interface design in magnetic data storage

Superior electrical conductivity and thermo-mechanical response of graphene can significantly improve areal density in magnetic data storage. The current head media spacing in hard disk drives (HDD) is 7.2 nm, and replacing conventional carbon overcoat (COC) with graphene will drastically reduce head media spacing, increasing areal density to eight times its present value. A paradigm shift in HDD systems can also be achieved via selection of a combination of new lubricants and unconventional architecture of COC systems. Here, we evaluate the feasibility of graphene overcoat (GOC), by understanding GOC-lubricant interactions. We further introduce new alternative head-disk interface (HDI) designs consisting of buffer/lubricant layers (i.e., graphene/carbon nanotube (CNT) or fullerene/perfluoropolyether (PFPE)). These hybrids could further enhance tribological performance including the reduction of wear and friction while drastically increasing areal density of data storage devices. Our study here will lead to vigorous investigation of HDI in magnetic data storage, including heat-assisted magnetic recording (HAMR), with tuned atomistic design criteria.

Relaxed Head Media Spacing by High Resolution DDSV Reader at Multi- $\hbox{Tb/in}^2$ Areal Density

At 10 Tb/in2 areal density and bit aspect ratio (BAR) of 4, the 1 T bit length is only 4 nm. By proportional scaling, it requires the head media spacing (HMS) to be around 2 nm. In such a small spacing, it is extremely hard to squeeze in the medium overcoat and lubricant for having a reliable head disk interface. This small HMS demand is mainly determined by the readback process for the reader to have large signal intensity and high signal resolution. The differential dual spin valve (DDSV) reader incorporates a metallic gap layer between the free layers of two spin valves. The opposite pinned reference layers generate the differential operation and reproduce the pulse signal from the transition of perpendicular magnetizations. With the readback resolution defined by the thicknesses of gap layer and two free layers, the DDSV reader gains more resolution than the conventional reader. Unlike conventional reader also using two shields to define signal resolution, the shields of DDSV reader is only for the elimination of inter-symbol interference. This allows the shield to shield spacing of DDSV reader to be much larger, which has smaller shunting effect and promotes the signal strength. With stronger signal intensity and higher readback resolution, the DDSV reader is able to reproduce good quality signal at much higher HMS than the conventional reader.