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.     

Active-head slider with piezoelectric actuator using shear-mode deformation

The active-head slider technology for hard disk drive is one of the most promising means to decrease flying height. This paper describes a piezoelectric flying height control slider which has a faster dynamic response compared with conventional active-head sliders. This slider can be also adapted to the conventional slider-fabrication process. PZT layer located near the magnetic head has shear mode deformation by applying electric voltage between the upper and lower electrodes when the flying height of magnetic head needs to be decreased or increased. We fabricated a prototype with single crystal Si substrate for feasibility study. Our evaluation of the prototype revealed that the piezoelectric constant of the shear mode deformation was 0.88 nm/V, and the dynamic response was 50 kHz. The shape of the air bearing surface was optimized by simulations using a robust design method. We found that the stroke was 8 nm for an applied voltage of 11 V if the flying height was 11 nm with no deformation in PZT.     

Novel piezoelectric actuator control slider using transverse-mode deformation

This work proposes a novel structure design of active slider with a piezoelectric actuator at the top of the slider body and a soft layer between the substrate and head elements, to efficiently control the flying height of slider. A parametric simulation was performed to obtain an optimize dimension of PZT actuator and the most efficient soft material. Then three types of slider structures were designed and investigated with considering air bearing effects to achieve large actuation efficiency.     

Intermolecular force and surface roughness models for air bearing simulations for sub-5 nm flying height sliders

When the spacing between the slider and the disk is less than 5 nm, the intermolecular forces between the two solid surfaces can no longer be assumed to be zero. The model proposed by Wu and Bogy (ASME J Trib 124:562–567, 2002) can be view as a flat slider–disk intermolecular force model. The contact distance between the slider and disk needs to be considered in this model when the slider-disk spacing is in the contact regime. To get more accurate intermolecular force effects on the head disk interface, the slider and disk surface roughness need to be considered, when the flying height is comparable to the surface RMS roughness value or when contact occurs. With the intermolecular force model and asperity model implemented in the CML air bearing program, the effect of intermolecular adhesion stress on the slider at low flying height is analyzed in the static flying simulation. It is found that the intermolecular adhesion stress between the slider and the disk has slight effect on the slider-disk interface for a flying slider.     

Simulation of the head disk interface for discrete track media

This paper investigates the effect of discrete tracks on the steady state flying behavior of sub ambient proximity sliders. A finite element based air bearing simulator is used to simulate the flying characteristics of sliders over a grooved disk surface. Sliders flying over discrete track disks “see” a disk surface that consists of ridges and grooves. The air bearing pressure build-up for sliders flying over discrete track disks is different from that for sliders flying over plane disks. Low air bearing pressure can be expected for those regions of the slider that are positioned over grooves, while high air bearing pressure exists over ridges. The air bearing characteristics are determined for several pico and femto-type air bearing sliders flying over discrete track disks. An empirical equation is obtained describing the loss of flying height of a slider flying over discrete track disks.     

Active flying-height control slider using MEMS thermal actuator

Today’s head/disk interface design has a wide flying height distribution due to manufacturing tolerances, environmental variations, and write-induced thermal protrusion. To reduce the magnetic spacing loss caused by these effects, we developed an active head slider with a nano-thermal actuator. The magnetic spacing of these sliders can be controlled in situ during drive operations. After simulating the heat transfer in the slider to obtain the thermal deformation of the air-bearing surface, we fabricated a thermal actuator using thin-film processing. An evaluation done using a read/write tester showed a linear reduction in the magnetic height as electric power was applied to the actuator. The actuator’s stroke was 2.5 nm per 50 mW with a time constant of 1 ms. There was no significant impact on the reliability of the read element.     

Air-bearing surface chemical modification for low-friction head–disk interface

As a chemical modification of slider air bearing surface (ABS), fluorine ion implantation (FII) was conducted. FII treated ABS was fabricated to reduce slider surface energy, which is expected to reduce adhesion and friction force between the slider and disk. Tribological performance, such as slider flyability, was experimentally investigated. Touchdown speed, corresponds to a flying height of about 0.4 nm, was reduced by FII treatment. Slider touch-down and take-off hysteresis was also improved by reducing the adhesive force of the lubricant by fluorine-ion implantation. AE output and friction force were also reduced in the drag test. FII treated slider showed a 45% reduction of AE amplitude and 14% reduction of friction force. FII treatment was confirmed to be effective in improving slider flyability.     

Simplified molecular gas film lubrication equation and accommodation coefficient effects

As the spacing between the flying head/slider and the rotating disk in hard disk drives (HDDs) continues to decrease, the interaction between the molecular gas and the surfaces of the disk and the head/slider becomes significant. The influence of surface accommodation coefficient (AC) is an important factor to govern the static characteristics of the head/slider. Starting from the polynomial logarithm fitting equations of Poisueille flow rate and Couette flow rate, a new simplified molecular gas film lubrication (MGL) equation is proposed to simulate the ultra-thin air bearing film in HDDs. The new MGL equation is simpler than that of the polynomial logarithm form of MGL equation. The new approach produces very good approximations for both Poisueille flow rate and Couette flow rate with very little differences to those based on the original MGL equation. The new simplified MGL equation is solved by using a meshless method, called least square finite difference (LSFD) method. Effects of ACs on the static characteristics of air bearing films in HDDs with ultra-low flying heights are investigated. Numerical results show that effects of ACs on the static characteristics are significant for the case of symmetric molecular interaction. On the other hand, effects of ACs at the disk surface on the static characteristics are significant for the case of non-symmetric molecular interaction, while effects of ACs at the slider surface on the static characteristics are weak.     

An efficient thermal actuator design for the thermal flying height control slider

In this work, we propose a novel thermal actuator by designing a thermal insulator, a thermal conductor, and their combination to the traditional thermal heater. The thermal-structure simulation coupled with air bearing simulation is used to simulate the actuation by the thermal actuator, as well as the effects on flying performance of slider being actuated. The simulation results show that an additional 0.8–1.1 nm flying height reduction can be obtained by applying the proposed thermal actuator when the flying height of TFC slider is about several nanometers.     

Nanoscale dynamics of MEMS TFC active slider

With enabling of a thermal actuator, an air bearing slider can fly at sub-nanometer level spacing on a magnetic disk while the recording elements are functioning. At such spacing, the slider stability and head-disk interface reliability remains to be understood. In this study, a novel understanding on dynamics of the MEMS thermal flying-height control (TFC) slider in touchdown process is developed. By using average and variational-iteration methods, closed-form spectrum estimations of slider vibration are derived. The derived formulation offers an insight of the relationship between spectrum and interface parameters. Physics-based simulation is also conducted to quantify the spectrum of slider vibrations as a function of varied interfacial parameters. To further extend the analytical and numerical analysis, the experimental study of a TFC slider while flying on a rotating disk at sub-nanometer spacing are performed. The analysis reveals the dominance of the air bearing force among other interfacial forces at sub-nanometer spacing.     

Influence of disk curvature on slider attitude under operational shock conditions for a 2.5-in. hard disk drive

Recently, the number of disks in hard disk drives has increased, and the gap between the slider and disk has decreased. These changes make the contact between the ramp and disk easily. External shock and ramp–disk contact can cause change in disk curvature. Such a change in disk curvature affects the air bearing pressure between the slider and disk. However, disk curvature has not been considered in the previous research. Thus, in this study, we investigated the influence of disk curvature on slider dynamics. Disk curvature was calculated from a transient shock analysis, and was then applied to slider dynamic analysis. As a result, disk curvature reduced the shock performance, by decreasing the minimum flying height and increasing the pitch and roll angle of the slider.     

Effects of disk surface roughness on static flying characteristics of air bearing slider by using a combined method of Reynolds equation and rough disk surface

Reynolds equation was modified with adding the surface roughness parameters to analyze the effects of disk surface roughness on the static flying characteristic of an air bearing slider. However, the modification demands the complicated mathematical expressions and related knowledge of physics and mathematics. In this paper, a combined method of Reynolds equation without introducing the roughness parameters and rough disk surface is proposed to investigate the effects of disk surface roughness on the static flying characteristics of an air bearing slider, it is different from those models of modified Reynolds equation introducing the disk surface roughness used by many researchers. More importantly, this method avoids the complicated numerical calculation resulted from the mathematical expressions including the Peklenik parameter \(\gamma\) and roughness R_a. By using an Ω air bearing slider, we investigated the effects of disk surface roughness on the static flying characteristics of this slider, the results show that the Peklenik parameter \(\gamma\) and roughness R_a have a significant influence on the pressure distribution, the load carrying capacity and the location of the pressure centre.     

Direct Monte Carlo simulation of air bearing effects in heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is a promising high density recording technology in current hard disk industry. It is proposed to use a heat source from the slider system for heating up the recording media in order to increase its storage density. The heat generated from a heat spot on the disk and/or the higher slider body temperature in HAMR system could affect the slider air bearing and flying height. This paper studies the heat effects on slider air bearing characteristics by using the direct Monte Carlo simulation (DSMC) method. The simulation results show that the heat spot less than 50?nm in diameter could not affect much to the air bearing; however, its location should be away from the bearing pressure peak to minimize the heat spot effect. Furthermore, high temperature slider could increase the bearing pressure and force and the trend of force increment is independent of the flow channel length.     

Fabrication and experimental study of Al2O3-TiC sliders with piezoelectric nanoactuators for flying height control

A gap flying height (FH) of less than 5 nm between the read/write element and the surface of the disk is required for ultrahigh density recording. A stable and constant FH must also be sustained in the presence of altitude and temperature changes and manufacturing tolerance. A FH adjustment or controlled slider that is capable of adjusting its gap FH has been proposed previously. In this paper we demonstrate an inexpensive and low-temperature approach for integrating piezoelectric materials in the fabrication of current small form-factor Al₂O₃-TiC sliders. A bulk PZT sheet is bonded onto the back of row-bars and the sliders are separated by a standard dicing process. It requires no deep reactive-ion etching (DRIE) or high temperature processes and is suitable for mass production. A conventional design and a new special air bearing surface (ABS) design have been fabricated and tested by an optical profiler and a FH tester. A nonflying actuation stroke of 0.6–0.8 nm/V has been observed. The FH measurements showed that the ABS plays a key role in increasing the actuation efficiency, which also agrees well with the numerical analysis.     

Interactions between nano-spacing flying head sliders and ultra-thin liquid lubricant films with non-uniform distribution in hard disk drives

This paper describes the effect of ultra-thin liquid lubricant films on air bearing dynamics and flyability of less than 10 nm spacing flying head sliders in hard disk drives. In particular, the effect of non-uniform lubricant film distributions on head/disk interface dynamics are studied. The disks with lubricant on one half of disk surface thicker than the other half were used in this study. The dynamics of sliders is monitored using acoustic emission (AE) and the interactions between the slider and disk are investigated experimentally. The disks were also examined with a scanning micro-ellipsometer before and after each test. Complicated slider responses were observed and clarified. In addition, it was found that the periodic lubricant film thickness modulations or non-uniformity caused by the slider-disk contact interactions could be observed. It is suggested that this lubricant film thickness non-uniformity will be one of the technical issues in order to achieve ultra-low head/disk contact interface of less than 10 nm.     

Numerical simulation of molecularly thin lubricant film flow due to the air bearing slider in hard disk drives

In this paper we numerically study the evolution of depletion tracks on molecularly thin lubricant films due to a flying head slider in a hard disk drive. Here the lubricant thickness evolution model is based on continuum thin film lubrication theory with inter-molecular forces. Our numerical simulation involves air bearing pressure, air bearing shear stress, Laplace pressure, the dispersive component of surface free energy and disjoining pressure, a polynomial modeled polar component of surface free energy and disjoining pressure and shear stress caused by the surface free energy gradient. Using these models we perform the lubricant thickness evolution on the disk under a two-rail taper flat slider. The results illustrate the forming process of two depletion tracks of the thin lubricant film on the disk. We also quantify the relative contributions of the various components of the physical models. We find that the polar components of surface free energy and disjoining pressure and the shear stress due to the surface free energy gradient, as well as other physical models, play important rolls in thin lubricant film thickness change.     

Thermal protrusion induced air bearing frequency variations

The thermal flying height control sliders are widely used in hard disk drives. This paper studies the relationship between the air bearing frequencies and the thermal protrusion values of a thermal flying height control slider by simulation. Simulation results show that the second pitch mode of the slider increases significantly as the thermal protrusion increases. This feature could be used to in situ monitor the thermal protrusion values and the flying heights of the slider. Discussions on all the air bearing frequency modes are given, and the reasons of the frequency variations due to thermal protrusion are explained in details.     

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.     

Simulation of thermal flying height control slider with built-in contact sensor

This work investigates the piezoelectric contact sensor in the thermal flying height control (TFC) slider. A finite element model is built for the thermal flying height control slider with a piezoelectric contact sensor, which is used to detect the contact between the slider and disk. A constant force is applied at the maximum thermal protrusion point of air bearing surface. The simulation results show that the ZnO sensor with shear-mode is more sensitive to contact force than that with transverse-mode. The sensitivity of contact sensor can be increased by reducing the cross-sectional area of sensor, increasing the thickness of sensor, and choosing a short distance of sensor to air bearing surface. In addition, the thermal-stress effects from TFC heater on contact sensor are significantly large and the amplitude of thermal-stress inducing output voltage is orders larger than that induced by contact force. However, by optimizing the distance of sensor to ABS, it is possible to eliminate the thermal-stress effects. Finally, the response time of thermal-stress induced electrical voltage of contact sensor is about 0.3?ms.