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.     

Parametric simulation of piezoelectric flying height control slider using shear-mode deformation

The piezoelectric flying height control slider has recently been implemented in magnetic recording disk drives to reduce the flying height. This paper performs the electromechanical simulation and air-bearing simulation to investigate the effects of the shear-model deformation on the static flying attitude of PZT slider. The location of PZT sheet and air bearing surface of slider are investigated to achieve a low flying height and robust head-disk interface. The results show that a short distance of the PZT sheet to the trailing edge of the slider can help to achieve a low flying height. A small center-trailing pad of the slider can also help to achieve a low flying height, but cannot prevent the reduction in pitch angle. The depth of the center-trailing pad does not change the reduction ratio of the pitch angle when increasing the drive voltage. A big pitch angle value is needed to avoid the pitch angle falling below zero at a high drive voltage.     

Simulation of a pad slider flying on a wavy surface

This paper describes following-up characteristic of a pad slider to a wavy surface. The pad slider comprises a leading pad and a trailing pad. We use a sine wave to replace an actual wavy surface. The response of the pad slider due to the sine wave is analyzed by computer simulation. The response of the pad slider is evaluated from variation in slider flying height (FH). A variation gain that is a ratio of FH fluctuation and wave amplitude is defined to explain the following-up characteristic of slider. To analyze the following-up characteristic, different wavelengths are used as parameter to calculate the variation gain. Because pads on the air-bearing surface of the slider are able to occur at two pressure peaks on the leading edge and trailing edge that enable a pitching motion of the slider, the slider can follow the disk waviness with a wavelength that is shorter than the slider length. The variation gain is 0.33 for the wavelength that is equal to the slider length. However, the slider cannot follow the disk waviness with a wavelength that is shorter than the pad length. The variation gain is greater than 1 for the wavelength 0.2 mm. To study the relationship between the variation gain and the dynamics of slider, a transient response simulation is carried out in order to investigate a natural frequency of slider. We set a projection on a plane surface. When the slider is flying over the projection, we can obtain a flying height response curve and a pitch response curve. The natural frequency of flying height and pitch angle can be known by FFT. The transient response of slider in pitch mode is compared with the variation gain. The simulation results make clear the fact that the following-up characteristic has correlation with the dynamics of the pitch model and the phase difference between a locus of head and a wave of disk surface.     

Visualization of lubricant pickup phenomena by lubricant thickness mapping on slider surface

A slider surface analyzing tester was developed to observe the lubricant thickness distribution on a slider surface by interferometry. We observed three phenomena related to lubricant pickup by the slider. The picked-up lubricant gathered around the boundary of the pad and recess area during the unloading interval, and the gathered lubricant flowed to the trailing edge of the pad after loading on the disk surface. There were two lubricant flows on the pad surface. The first was a circulation flow from the dynamic flying height protrusion area to the leading edge of the pad. The second was a circulation flow from the lubricant pool to the leading edge of the pad. Lubricant dewetting occurred on the slider pad surface when a thick layer of lubricant was adhered to the pad surface.     

Experimental study of slider–lubricant interaction with different slider designs

In this study, a new type of femto slider (Panda IV) with improved force arrangement and higher air pressure/higher stiffness at trailing edge of the slider was evaluated and compared experimentally with our previous type of femto sliders (Panda II and III). The evaluations were conducted on the Candela 5100 Optical Surface Analyzer (OSA) with the VENA CSS/load–unload system attached. Disks coated with a layer of 1.2 nm lubricant were specially prepared with a lubricant step for the comparison tests. Flying height (FH) of each slider was calibrated by bump disk with 3.5 nm bump height for maintaining the consistent FH during the tests. The relative amount of lubricant redistribution was analyzed with an OSA immediately after each slider is flown on-track for a period of time. It is found that the amount of lubricant transferred or the slider–lubricant interaction are minimized by reducing the size of slider’s central trailing pad. Finally, possible explanations on the lubricant transfer are discussed.     

Dynamic instability of thermal-flying-height-control sliders at touchdown

With the wide application of thermal flying-height control (TFC) technology in the hard disk drive industry, the head-disk clearance can be controlled to as low as ~1?nm. At this ultra-low clearance, the air bearing slider is subject to relatively large interfacial forces, and it experiences more complicated dynamics, compared with the flying case. In this study we conduct a numerical analysis to investigate the dynamics of TFC sliders during touchdown. The general trend of the slider’s motion predicted by the numerical simulation qualitatively agrees with experimental findings. The touchdown process begins with a slight intermittent contact between the slider’s trailing edge and the disk, followed by a partial slider-disk contact at the trailing edge accompanied by a large pitch motion at the 1st air bearing mode; this pitch motion gets suppressed and the slider comes into stable sliding on the disk as the protrusion is further increased.     

Numerical investigation of slider dynamics induced by contacts with disk asperities

A time dependent Reynolds equation simulator combined with a finite element-based transient contact model between a slider and a disk asperity is used to study slider dynamics induced by contacts with disk asperities. The flying height change at the trailing edge of the slider is investigated as a function of asperity height, asperity diameter, and the spacing between the thermal protrusion of a thermal-flying control slider and a disk asperity. The effect of material properties of the disk asperities is studied. Slider vibrations corresponding to the first and the second pitch modes are excited by disk asperities.     

Plane strain sliding contact of multilayer magnetic storage thin-films using the finite element method

The sliding contact or scratch behavior of multi-layer thin-films such as those found in magnetic storage disks has been studied using the finite element method. A rigid cylinder sliding over a multilayered thin-film half-space was implemented to simulate the contact between a feature of the recording slider (such as the protrusion on the trailing edge of the slider, which is part of the thermal flying-height control, TFC) and the magnetic storage multilayer disk. The effects of different parameters such as normal load, friction coefficient and TFC radius on the von Mises, shear and principal stresses in the multilayer system were analyzed. Results showed that under sliding conditions, for a given normal load, the friction coefficient influences the location and magnitude of the plastic strain in the multilayer system. Repeated sliding contact was also performed to characterize its effect on the stress and strain behavior under various loading conditions and investigate shakedown behavior.     

Six-DOF vibrations of partial contact sliders in hard disk drives

A six-degree-of-freedom slider dynamic simulator is developed to analyze the slider’s motion in the vertical, pitch, roll, yaw, length and width directions. The modified time-dependent Reynolds equation is used to model the air bearing and a new second order slip model is used for a bounded contact air bearing pressure. The simulator considers the air bearing shear acting on the air bearing surface and the slider–disk contact and adhesion. Simulation results are analyzed for the effects of the disk surface micro-waviness and roughness, skew angle, slider–disk friction and micro-trailing pad width on the vertical bouncing, down-track and off-track vibrations of a micro-trailing pad partial contact slider.     

Mechanical stability of the interface between a padded slider and magnetic recording disk

Contact state, friction, and meniscus load between a padded slider and a smooth disk are discrete; friction is determined by the number of pads in contact and the meniscus load acting at each pad. By inducing frictional changes through varying sliding direction, we force the slider from one stable state to another and infer which pads are in contact from the corresponding changes in friction. We find the stability of a given contact state is influenced by friction coefficient, pad location, meniscus load, applied mechanical loads and applied moments. A model is proposed to account for these effects.     

Mechanical characteristics of a contact optical head slider operating on a metal-layered medium

 Advances in a digital network society require both higher density and higher transfer rates in all sorts of storage systems. Even in optical recording, the trend toward higher density and larger capacity requires novel surface-recording technologies that can drastically diminish head-to-medium spacing, resulting in an improvement in spatial resolution and, finally, a higher recording density. In this paper, we propose a novel contact optical head slider that is able to almost cancel the suspension load by generating hydrodynamic pressure, thus realizing a lower net contact force. A trial-manufactured contact slider being processed four sliding pads on air-bearing surfaces has indicated a gentle variation of both the acoustic emission signal intensity and the friction force as the circumferential velocity changes. Furthermore, a time-domain simulation was performed to investigate the effects of the damping of a medium surface (lubricant) both on slider bouncing and on contact force. Received: 5 July 2001/Accepted: 1 November 2001     

Readout characteristics of flexible monolithic optical head slider combined with visible laser light guide

Optical near-field recording is a candidate technology for overcoming the diffraction limit of conventional optical recording. In our previous work, we proposed a novel optical head slider for near-field recording that we call a flexible optical head slider. An air-bearing pad pattern is formed on the apex of a cantilever-like polymeric waveguide so that, by using the cantilever itself as the slider suspension, a single body structure incorporates the functions of the flying slider, suspension, and waveguide. This structure can be expected to provide several important advantages by miniaturizing head assemblies; simplifying the assembly and optical trimming processes; and producing a lighter head, thus allowing a wider tracking bandwidth. In this paper, we report the read-out signal evaluation of the flexible optical head slider. Using a slider with a sub-micron sized aperture, read-out of a test metal-patterned ROM (Read Only Memory) disk was successfully demonstrated down to a 0.2 μm linewidth line and space pattern.     

Experiments on slider lubricant interactions and lubricant transfer using TFC sliders

Future magnetic storage density targets (>4 Tb/in. ²) require subnanometer physical clearances that pose a tremendous challenge to the head disk interface (HDI) design. A detailed understanding of slider-lubricant interactions at small clearances and contact is important to not only address magnetic spacing calibration and long term HDI reliability but also to meet additional challenges imposed by future recording architectures such as heat assisted magnetic recording (HAMR). In this work, the behavior of the disk lubricant is investigated through controlled tests using TFC sliders which are actuated to proximity (i.e. backoff) and into contact (i.e. overpush) on one specific half of the disk per rotation by synchronization with the spindle index. Observations for lubricant distribution in contact tests (i.e. overpush) reveal an accumulation of lubricant on the disk near the onset of contact suggesting a migration of lubricant from the slider to the disk as the slider approaches the disk. Experiments also reveal that there is a similar deposition of lubricant even in the absence of contact for backoff tests. Furthermore, light contact tests result in significant lubricant rippling and depletion with associated slider dynamics. The lubricant rippling frequencies correlate well with the slider’s vibration frequencies. Interestingly, strong overpush may lead to stable slider dynamics (for certain air bearing designs) that is also associated with noticeably lower lubricant distribution (compared to the light contact case), and the greatest lubricant changes are observed only at the onset and the end of contact. This paper reveals the complex nature of slider-lubricant interactions under near-contact and contact conditions, and it highlights the need for further studies on the topic to help design a HDI for recording architectures of the future.     

Effects of environmental temperature and humidity on thermal flying height adjustment

Thermal actuated sliders are being widely used in today’s hard disk drive industry for its advantages of easier control of flying height (FH) and less risk of contacts with the disk. This article uses a coupled-field analysis method, which includes an air bearing model, a heat transfer model and a thermal-structural finite element model to investigate the FH changes of thermal actuated sliders at various environmental conditions. The mechanism of water vapour’s contribution to air bearing pressure loss is explained and a new humidity model is proposed to calculate this pressure loss. The temperature effects are also considered in the simulation models. It is observed that the environmental temperature and humidity have significant effects on slider’s FH changes, but their effects on the thermal protrusion height are limited. A humidity sensitivity study is also made and the results are discussed. It is found that the slider with thermal protrusion on its trailing pad will be more sensitive to the humidity. Besides air bearing stiffness, some other factors such as peak pressure, protrusion shape and air bearing surface (ABS) design will also contribute to the slider’s humidity sensitivity.     

Characterization of slider-lubricant interaction with tribo-current

Lube-surfing recording combined with thermal fly-height control (TFC) technology is considered as a promising head-disk interface (HDI) scheme for further increasing magnetic areal density to 5?C10?Tbits/in². To realize this alternative technology, however, a lot of tricky issues are required to be solved. Among them, how to characterize the flying of slider in the lubricant or light lube-contact by the slider is probably one of the tough but inevitable challenges. In this study, the slider/lubricant/disk contact induced tribo-current is investigated with a modified media-tester in which the TFC slider is electrically isolated with the rest of the tester. The measured tribo-currents versus the heater voltages or the powers to the slider??s heater clearly indicate three different intensity regions of tribo-current, by which the three different contact types, namely, non-contact, lube-contact and solid-contact can be differentiated clearly. This method provides a promising way for accurately studying of lube-surfing recording.     

Hard-particle-induced physical damage and demagnetization in the head-disk interface

Test stand experiments were performed in which 44 μm stainless steel particles and 0.3 μm alumina particles were introduced at the head-disk interface (HDI) of hard disk drives (HDD), and their damage to the HDI was observed and evaluated. An Olympus HDI reliability tester was used to fly the sliders at different radii. Optical microscopy, atomic force microscopy (AFM) and magnetic force microscopy (MFM) were used to examine the physical and magnetic conditions of the sliders and the disks after the particle experiments. The results showed that both particle types were able to produce scratches and demagnetization but in very different ways. The alumina particles could become entrapped in the HDI to cause physical damage to both the sliders and disks, whereas the larger stainless steel particles could not get into the HDI. For the alumina particles plastic deformation and high temperatures occur simultaneously to cause demagnetization when a particle-induced scratch is formed. Stainless steel particles can induce high-speed slider/disk contact and physical damage to the slider’s trailing edge through their contributions to flying instability.     

Ramp unloading “footprints”

 When ramp unloading, the radial distance a slider moves between first touching the ramp and lifting from the disk is called its “footprint”. Footprints can detract from available recording area. Footprint values were measured for different sub-ambient style air-bearing designs and unloading parameters on a common suspension. Specifically, data was taken over a wide ranging values of PSA/RSA (slider pitch/roll static attitude respectively). Sliders with nominal PSA/RSA regularly lifted gracefully and unaided by suspension limiters within a few mils of travel. As PSA approach zero, however, sub-ambient lifting forces persist, leading-edge limiters engage and footprints extend to 12 mil. Increasing unload velocity increases footprints and slider dynamic vertical motion for nominal PSA/RSA. Surprisingly, for low PSA, slider motion decreases for increasing unload velocity when the leading edge limiter engages. Received: 1 June 2001/Accepted: 21 September 2001     

Tri-state dynamic model and adhesive effects on flying-height modulation for ultra-low flying head disk interfaces

Physical spacing between a flying slider and a rotating disk is projected to be 3 nm in order to achieve extremely high areal recording densities of 1 Tb/in². In such ultra-low flying-height regimes, two imminent obstacles that need to be overcome are intermittent head/disk contacts and strong intermolecular adhesive forces at the head/disk interface (HDI). Head/disk contact can cause large vibrations of the recording slider and possibly damage the disk and slider due to large contact forces. Strong adhesive forces disturb the balance of forces in a flying slider by pulling it down onto the disk and increasing the possibility of catastrophic HDI failures by doing so. This paper describes a dynamic model that includes contact and adhesive forces. Specifically, a lumped parameter single degree-of-freedom, three state nonlinear dynamic model representing the normal dynamics of the HDI and asperity-based contact and adhesive models were developed and coupled together to predict the performance of ultra-low flying sliders. The validity of the proposed dynamic model was confirmed in terms of flying-height modulation (FHM) by experimental measurements using ultra-low flying HDIs. It was found that the amplitude and frequency components of the dynamic microwaviness play an important role in slider dynamics. Furthermore, the effect of adhesive force on FHM was investigated and design guidelines for reduced FHM were suggested.This research was supported by a grant from the Information Storage Industry Consortium (INSIC) and the National Science Foundation under grant number CAREER CMS-0227842. Gary Prescott and Thomas Pitchford of Seagate Technology provided the spindle motor and HGA samples. The authors gratefully acknowledge this support.     

Performance analysis of an integrated piezoelectric ZnO sensor for detection of head–disk contact

Integrated capability for detection of head–disk contact is desired for magnetic sliders with near-contact flying height. At the same time, fabrication of added features should be compatible with the existing slider micromachining process which is highly specialized and cost sensitive. Aimed at meeting the two requirements, a novel sensor configuration is explored in the present study. The new sensor configuration consists of a piezoelectric zinc oxide (ZnO) thin-film sensor sandwiched in the magnetic slider on its trailing side. Coupled structural and piezoelectric finite-element analysis for a sensor–slider–suspension assembly was performed to investigate the dynamic sensing performance. Output voltages on the millivolt level were obtained under typical head–disk interactions. The second in-plane bending mode of the slider was found to be the major contributor to the output voltage. Parametric study further showed that a thicker ZnO layer generally generated a larger output, while the thickness of the slider overcoat had only minimal effect. Simulation results from harmonic and transient analyses demonstrated that the piezoelectric thin-film ZnO sensor was sufficiently sensitive for detection of head–disk contact.