Flying Height Modulation Research Articles

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%.

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.

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.

Static and Dynamic Slider Air-Bearing Behavior in Heat-Assisted Magnetic Recording Under Thermal Flying Height Control and Laser System-Induced Protrusion

The air bearing’s response to regions of elevated temperature on its bounding surfaces (the slider and disk) may be an important consideration in the head–disk interface design of heat-assisted magnetic recording (HAMR) systems. We implement the general non-isothermal molecular gas lubrication equation into an iterative static solver and dynamic air-bearing solver to evaluate the effect of localized heating of the air-bearing surface (ABS) due to the near-field transducer (NFT). The heat-dissipating components in our simplified HAMR design are the NFT, laser diode, and thermal flying height control (TFC) heater. We investigate the effect of each HAMR slider component on ABS temperature and thermal deformation and the slider’s flying height. The NFT induces a localized thermal spot and protrusion on the larger TFC bulge, and it is the location of maximum temperature. This ABS temperature profile alters the air-bearing pressure distribution, increasing the pressure at the hot NFT location compared with predictions of an isothermal air-bearing solver, so that the center of the pressure acting on the ABS is slightly closer to the trailing edge, thereby decreasing the pitch angle and increasing the minimum flying height. Other researchers have shown that the NFT’s thermal response time may be much faster than its protrusion response time (Xu et al. in IEEE Trans Magn 48:3280–3283, 2012). The slider’s dynamic response to a time-varying NFT thermal spot on the ABS while the combined TFC and NFT induced thermal protrusion remains constant is investigated with our dynamic air-bearing solver. We simulate the slider’s step response to a suddenly applied ABS temperature profile and a pulsed temperature profile that represents laser-on over data zones and laser-off over servo zones. The sudden (step) or rapid (pulse) increase in ABS temperature induces a sudden or rapid increase in pressure at the NFT location, thereby exciting the air bearing’s first pitch mode. For the slider design and simulation conditions used here, the result of the pitch mode excitation is to alter the position of the center of pressure in the slider’s length direction, thereby changing the pitch moment. In response, the pitch angle and minimum flying height change. The step response decays after approximately 0.15 ms. Because the laser duty cycle is much shorter than this response time, a periodic disturbance is predicted for the center of pressure coordinate, pitch angle, and minimum flying height. The peak-to-peak minimum flying height modulations are relatively small (only up to 0.126 nm); more significantly, the time-averaged minimum flying height increases 0.5 nm for the NFT that reached 208 °C compared to simulations of the isothermal ABS at ambient temperature.

Heater AC Voltage Induced Flying Height Modulations

For a thermal flying-height control (TFC) slider, its heater is usually provided with DC voltage. However, recently, both DC and AC voltages may be supplied to the heater. Unlike supplying AC voltage to the slider and disk in the past, the AC voltage to the heater will not only produce a thermal protrusion on the slider, but also leaves a part of the AC voltage on the slider/disk interface. The voltage acts as the electrostatic force and can be used for further control of the slider, even in the drive level. Simulations show that the flying height modulation is highly related to the AC frequency. By sweeping the AC frequencies while monitoring the flying height and pitch angle modulations, the first and second pitch modes of air bearing frequencies can be experimentally obtained without slider/disk contact. The roll mode frequency is also obtainable when the skew angle is not zero. The simulation results agree well with the experimental results obtained by a laser Doppler vibrometer (LDV). Therefore, the sweeping AC frequency method provides a practical scheme to obtain the air bearing frequencies without any slider/disk contact, even in the drive level.

Flying Height Modulation for a Dual Thermal Protrusion Slider in Heat Assisted Magnetic Recording (HAMR)

Thermal flying-height control (TFC) technology has been used in the last few years in hard disk drives (HDDs) to achieve a lower flying height for higher storage areal density. In this paper a slider with dual thermal protrusions, one produced by the laser used to heat the disk in HAMR and the other one used for TFC, is studied. The flying height modulations (FHM) due to disk waviness and external disturbances are compared for the dual protrusions, single protrusion and no protrusion sliders. The protrusions provide additional force and torque excitation to the slider. The protrusions can be viewed as a passive feedback loop to compensate the disturbance. The gain of the feedback loop depends on the stiffness of the protrusions, which is determined by the protrusions' heights and positions. Thus the complicated air bearing slider system with protrusions is simplified by casting the protrusion perturbation as a feedback loop and this simplified model helps to speed up the optimization of the TFC protrusion height and position. The numerical simulation results show that FHM of the dual protrusion slider is reduced significantly when the TFC protrusion height and location are optimized. External disturbance induced FHM can be suppressed as well. This scheme for dual protrusion optimization and analysis described in the work can benefit HAMR head design because the optimized dual protrusion provides a more consistent flying height and a greater ability to compensate external disturbances.

Adaptive flying height modulation control of hybrid active slider with thermal and piezoelectric actuators

Hybrid active slider is an effective means to increase the storage density of hard disk, but its effectiveness is compromised by the flying height modulation (FHM), the bounding vibrations associated with the slider. There is a need to reduce the FHM through real time control. The hybrid active slider exhibits a very complex dynamic behavior which causes a big challenge for the traditional controller relying on an exact dynamic model. Without the requirement of an exact knowledge of the dynamics of the slider, this paper proposes an adaptive control scheme to control the flying height modulation. It is designed from the model with uncertain parameters and can guarantee the convergence of FHM. The details of the controller design and the proof of its performance are presented, and simulation results are provided to verify the effectiveness of the controller.

Effect of reduction of slider pitch-mode vibration on magnetic-recording performance at low-clearance head–disk interface

Aiming at improvements of both stability of slider flying and magnetic-recording performance under a low-clearance condition, a “narrow-grooved slider” was constructed and demonstrated at drive level. The proposed slider has narrow grooves on its center pad of an air-bearing surface for attaining a high-damping effect on pitch-mode resonant vibration of the air bearing. The relationship between the high-damping effect and the pitch-mode resonant vibration was studied, and the magnetic-recording performance at the lube/slider interaction regime was improved. In this study, first, flying-height modulation (FHM) of the slider was analyzed in the frequency domain by using a fast Fourier transform. Compared with the narrow-grooved slider, a non-grooved slider showed a larger increase in high-frequency modulation of gap flying height when the clearance was reduced to near “zero” at which the slider is starting to interact with lube. Furthermore, by means of drive-level experiments, sector error rate (SER) as a function of flying clearance was investigated. Under a low gap flying height condition, HDDs with the non-grooved slider showed slight SER degradation during slider/lube interaction. However, the narrow-grooved slider did not show any SER degradation at the same gap flying height; the damping effect of the narrow-grooved slider suppressed high-frequency FHM, thereby preventing SER degradation in the slider/lube interaction region.

Investigation of the Dynamic Characteristics of Light Delivery for Thermal Assisted Magnetic Recording

Several new technologies have recently been proposed for high areal density greater than 5 Tb/in2 as next generation storage devices. Among these technologies is thermal assisted magnetic recording (TAMR). However, there are several dynamic and realistic problems associated with the current method of TAMR light delivery that must be resolved. In this paper, we investigate and discuss the dynamic characteristics for two kinds of light delivery systems using experimental and simulated results. We concluded that the optical fiber TAMR light delivery system had higher vertical and pitch stiffness. This could result in higher flying height modulation (FHM) on the disk surface and reduced TD-TO performance. Furthermore, since the optical fiber TAMR light delivery system has a lower positive pitch angle during unloading process, as soon as the air bearing disappears after the limiter is engaged, the contact possibility between the slider and the disk increases in unloading process. In addition, for the mounted laser diode (LD) TAMR light delivery system, we constructed a thermal-structural coupling finite element (FE) model of the entire TAMR system and simulated the thermal distribution and structural expansion according to temperature distribution. The results showed that the thermal problem was caused by heat produced by the LD, which can affect the head disk interface (HDI) problem in high areal density.

Dynamic Head-Disk Interface Instabilities With Friction for Light Contact (Surfing) Recording

Recent advances in hard-disk drive technology involve the use of a thermal fly-height control (TFC) pole tip protrusion to bring the read/write recording elements of the slider closer to the disk surface and thus achieve terabit per square inch recording densities. A dynamic, contact mechanics-based friction model of the head-disk interface (HDI) that includes roughness and accounts for the TFC geometry and its influence on the HDI dynamics is presented. The model is based on physical parameters and does not include any empirical coefficients. Experimental flyability/touchdown measurements were performed and used to examine in detail the HDI contact criterion in the presence of surface roughness and dynamic microwaviness. Using the model, a procedure is outlined that identifies the optimal clearance and light contact conditions, i.e., the amount of thermal actuation that minimizes, both, the clearance, as well as the flying height modulation. Through calculation of the time varying interfacial forces, mean pressure and shear stress at the HDI can be predicted and used to characterize the contact regime. Based on our results, a light contact regime with reduced bouncing vibrations and low stresses (thus, low wear) that would enable surfing recording is identified.

Approaches to reduce effect of short-range interactions in head disk interface

Different approaches to reduce the effect of short-range interactions on slider’s flying stability—reduction of pad width at trailing edge and increase of air pressure on trailing pad—are compared for the flying stability and flying height modulation. The static and dynamic simulations show both approaches can reduce the effect of intermolecular force and electrostatic force on the slider’s flying stability. On the other hand, the increase of air pressure on trailing pad can also reduce the flying height modulation caused by disk waviness, but reduction of pad width at trailing edge increases the flying height modulation. Hence, the increase of air pressure on trailing pad is more suitable for the application of the ultra low flying height in hard disk drives.

Effects of Radial and Circumferential Disk Features on Slider Dynamics

As the head-disk spacing is reduced in order to achieve higher areal density, dynamic stability of the slider is compromised by a variety of proximity interactions. One of the key factors is the disk topography, which provides dynamic excitation to the low-flying sliders and strongly influences the dynamics and stability of the slider. Several experimental and numerical studies have been done on the fly height modulation caused by the disk topography along the track (circumferential) direction. In this paper, we investigate the effect of the disk topography in the circumferential as well as the radial directions and use a new method to measure the 2-D (true) disk topography for disks with three different kinds of roughness and waviness features. We used a dynamic air-bearing simulator to simulate the slider dynamics over the measured topographies. Simulations used 1-D topographies along the track direction as well as the actual 2-D topographies. Comparison of the slider dynamics over the different 1-D disk topographies indicates a strong dependence on the circumferential roughness and waviness features. Further, the comparison of slider dynamics and frequency content over 1-D and the 2-D disk topographies indicates that the radial features, which have not been studied intensively until now, are also very important in determining the actual slider dynamics, as they dictate the variation of the adjacent tracks in the circumferential direction. The simulations with 2-D disk topography are more realistic than the 1-D simulations. Further, the effect of the radial features can be reduced through effective air-bearing system design.

Flying Characteristics on Discrete Track and Bit-Patterned Media With a Thermal Protrusion Slider

Future disk drives may have discretized tracks or bit patterns for increased thermal stability and higher areal density. This affects the surface topography, resulting in different flying characteristics of the currently used air bearing slider. The actual flying height drop versus the pattern density and pattern depth has not been measured yet. However, it has been recently simulated. We have made flyable patterned disks with angular sections having various topographies. This allowed the measurement of flying height modulation and clearance drop within one revolution in different sections of the disks. Slider-to-disk clearance changes were measured using pulsed thermal protrusion. Furthermore, we investigated the slider dynamics and disk-lubricant changes during the transition from a flat zone to a patterned zone. This is of relevance since the servo sectors have different patterns compared to data zones on the disk leading to dynamic excitations and flying height modulation. These effects may still be detectable even after disk planarization since they are periodic and may excite the air bearing resonance. Based on our experimental observation, we have established an empirical equation that predicts slider clearance loss versus pattern density. The clearance change was measured when the slider air bearing was transitioning from a large scale pattern (in micrometers) to a small scale pattern (in nanometers), indicating increased lift with an increased number of walls/holes-even at a similar nominal pattern density. Functional drives were built that contained discrete track media with imprinted servo sectors.

Towards fly- and lubricant-contact recording

Reducing the spacing between the magnetic head and the magnetic media is of crucial importance in enabling future high-density magnetic data storage. The magnetic spacing has been reduced to a level that we have to explore new schemes for the further reduction of the spacing. This paper reports authors’ efforts towards fly- and lubricant-contact recording scheme. The targeted objective is to minimize the mean-plane spacing between the slider and disk surfaces and to allow the further reduction of the overcoat thickness. The slider will still fly over the disk surface. The read/write head part of the slider will be protruded to contact lubricant—to minimize the mean-plane mechanical spacing. The key factors for achieving such a mechanism include super-smooth slider and disk surfaces, advanced slider air-bearing design to significantly reduce flying height modulation caused by disk waviness and flatness, nano-actuator design, position feedback control scheme to control the contact depth, and so on. The preliminary experimental data confirm the feasibility of such a lubricant-contact scheme. The slider air-bearing design presented in this work also indicates the feasibility of high-performance slider design, which can well follow the disk waviness. A parameter, anti-modulation factor, is introduced for the evaluation of slider's performance in reducing flying height modulation caused by disk waviness.

Sub-mm disk waviness characteristics and slider flying dynamics under thermal FH control

The flying dynamics and flying stability of a slider are the key issues at sub-5 nm flying height (FH) under thermal FH control. The resonant frequencies of current sliders are at 100 kHz level. At present linear disk velocities, the disk waviness at sub-mm level and 10 micron level can excite the resonant modes of the slider and induce FH modulation. This work uses the triple-harmonic method to monitor the dynamic FH signal during the process of thermal FH control. As the FH reduces, the same disk waviness characteristics excite larger dynamic FH modulation.

A double common-path heterodyne interferometer for the measurement of flying height modulation

A double common-path differential heterodyne interferometer is proposed to measure the flying height modulation (FHM) of a slider in a hard disk drive. A transverse Zeeman He–Ne laser is employed as the light source and high-speed phase measurement is applied for signal processing. The resolution and the sampling frequency of FH measurement are 0.1 nm and 500 kHz, respectively. According to the symmetrical optical path design, the environmental noise and Abbe error from the disk can be adaptively compensated. The experimental results show that the stability of the system is sub-nanometer in a normal laboratory without air conditioning, and the FHM of ±3 nm is achieved when the pitch of the rotating disk is around 1.2 µm.

Study of a Spherical-Pad Head Slider for Stable Low-Clearance Recording in Near-Contact Regime

A slider with an air-bearing surface that has a small spherical pad around the read/write elements on the trailing center pad was developed to reduce the meniscus and friction forces and slider clearance loss. While the slider which has a spherical pad of a radius less than 10 mm, the meniscus force is reduced to less than 10% of the air-bearing lift force, thus, flying height modulation is decreased and instability are suppressed. Evaluation using an air-bearing surface model with a spherical pad showed that slider clearance was increased by 1 nm at a 5-nm nominal flying height. Evaluation using touchdown-takeoff testing of a spherical pad slider fabricated by depositing carbon on the center pad air-bearing surface by means of the lift-off resist technique showed that the spherical pad slider had a very small friction force and acoustic emission output up to a 5-nm interference height. It thus provides instability-free sliding in the near-contact regime.

1105 ニアコンタクト浮上に対する素子部球状スライダの適用検討(要旨講演,一般セッション:情報機器コンピュータメカニクス)

A slider with an air-bearing surface that has a small spherical pad around the read/write elements on the trailing center pad was developed to reduce the meniscus and friction forces and slider clearance loss. Flying height modulation and slider instability are suppressed by the reduction of the meniscus force to less than 10% of the air-bearing lift force and the use of a spherical pad with a radius of less than 10 mm. Evaluation using an air-bearing surface model with a spherical pad showed that slider clearance was increased by 1 nm at a 5-nm nominal flying height. Evaluation using touchdown-takeoff testing of a spherical pad slider fabricated by depositing carbon on the center pad air-bearing surface by means of the lift-off resist technique showed that the spherical pad slider had a very small friction force and acoustic emission output up to a 5-nm interference height. It thus provides instability-free sliding in the near-contact regime.

Optimal Slider-Disk Surface Topography for Head-Disk Interface Stability in Hard Disk Drives

In order to achieve an areal density of 1 Tb/in2 and beyond, not only the mechanical spacing between the slider and the disk but also the track misregistration (TMR) and the fly height modulation (FHM) should be reduced below current levels. But at reduced mechanical spacing there will be elevated excitation due to slider-disk contacts resulting in increased slider vibrations and head-disk interface (HDI) failures. Thus, there is a need to study the effect of slider-disk topography on the slider dynamics and stability at a higher level of complexity. In this paper, we do so by dividing the slider-disk surface features into three regimes based on the amplitude and the wavelength range of the features. Further, we have also proposed several ways of achieving an optimal slider-disk topography that can help reduce the slider vibrations and increase the stability of the HDI.

Low Flying-Height Slider With High Thermal Actuation Efficiency and Small Flying-Height Modulation Caused by Disk Waviness

To sustain an ultra-low and stable flying height (FH) is crucial for achieving high areal densities in magnetic recording. Recently, a new method called thermal flying height control (TFC) has been introduced to the latest generations of disk drives for precise control of slider-disk spacing. It is noted that the TFC technique is able to eliminate almost all static FH loss, but is unable to circumvent dynamic FH loss such as flying height modulation (FHM) due to disk waviness. It is therefore advantageous to have both high thermal actuation efficiency and low FHM due to disk waviness characteristics in a TFC slider design. This paper investigates the effects of air bearings on the thermal actuation efficiency and the capability in following disk waviness of the TFC sliders. Air bearing surface (ABS) design strategies for TFC slider are proposed and investigated with simulations. The results show that both excellent thermal actuation efficiency and strong capability in following disk waviness can be achieved through proper arrangements of air bearing pressure distribution on the ABS of TFC sliders.