Wei Hua

Lube-Surfing Recording and Its Feasibility Exploration

Reducing the spacing between the magnetic head and the magnetic disk media is of crucial importance in enabling further increased recording density of the magnetic disk drives. The head-disk interface technology explored up to now can be classified into the following two categories: in-fly recording and in-contact recording. Pushing the areal density towards 10 Tb/in2 requires the magnetic spacing down to 3 nm level, which it is difficult for the conventional in-fly or in-contact schemes to achieve so small a magnetic spacing. This paper reports the authors exploration of lube-surfing recording mechanism-majority of slider surface flying over the disk surface and only the tiny protruded read/write head area surfing on the lubricant surface of the disk. The key factors for achieving such a mechanism include technology to sense slider-lubricant contact, nano-actuator and active contact depth control, advanced slider air-bearing design to make the slider well follow up the disk surface waviness and secure the contact depth, super-smooth slider and disk surfaces, and so on. Results presented in this work suggest that the read head can be a promising transducer for slider-lubricant contact detection. Results also suggest that contact depth control is important for this scheme and it is possible to have stable slider surfing on the lubricant, if the contact depth is within a certain range.

Effects of intermolecular forces on deep sub-10 nm spaced sliders

Summary form only given. The deduction of slider/disk spacing into sub-10nm range reveals some physical phenomena and interactions that are trivial and rare to be noticed for a slider flying far above 10nm spacing. The intermolecular forces, such as van der Waals and Casimir forces, become so important in the range that any negligence is expected to be impractical. The effects of intermolecular forces on the ABS design have been reported, and it is also found that the Casimir effect is serious in the head disk interface. It is an obvious challenge to introduce the intermolecular forces to the current HDI modeling so as to simulate a femto slider with a nanometer spaced flying height.

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.

A generalized heat transfer model for thin film bearings at head-disk interface

The physics of the heat transfer process from the slider of a hard disk drive to its disk is discussed. A generalized heat transfer model, which incorporates various molecular dynamics models, is proposed to solve the heat transfer problem in thin film bearings at head-disk interface. The proposed model considers the impact of molecular collisions between film molecules and solid surfaces, which plays an important role in the heat transfer of thin film bearings and is expected to improve the accuracy of predicting the thermal protrusion caused by the heating of the slider at the head-disk interface of hard disk drives.

Nanoscale roughness contact in a slider–disk interface

The nanoscale roughness contact between molecularly smooth surfaces of a slider-disk interface in a hard disk drive is analyzed, and the lubricant behavior at very high shear rate is presented. A new contact model is developed to study the nanoscale roughness contact behavior by classifying various forms of contact into slider-lubricant contact, slider-disk elastic contact and plastic contact. The contact pressure and the contact probabilities of the three types of contact are investigated. The new contact model is employed to explain and provide insight to an interesting experimental result found in a thermal protrusion slider. The protrusion budget for head surfing in the lubricant, which is the ideal state for contact recording, is also discussed.

Contact-induced off-track vibrations of air bearing-slider-suspension system in hard disk drives

Contact-induced vibration of air bearing-slider-suspension system is a crucial issue for slider flying stability and head positioning precision of 1xa0Tbit/in2 hard disk drives. In this paper, the contact-induced off-track vibrations of air bearing-slider-suspension system are investigated by simulation. A dynamic simulator is developed to calculate the interactions between the air bearing dynamics and vibrations of slider-suspension assembly. The simulation model consists of a finite element model of suspension assembly, an air bearing model based on the generalized lubrication equation, and a slider–disk contact model based on the probability distributions of surface roughness. A sequential method is used to couple all these models and analyses. The time history of the slider and suspension motions, together with the time-varying forces including air bearing force, air shear forces, contact force and friction force can be obtained. The effects of different contact conditions, such as the contact intensity, friction coefficient, and disk surface waviness on off-track vibrations are investigated numerically in details. The results reveal some mechanisms on how these factors contribute to the off-track vibrations of suspension assembly.

Dynamic Stability Analysis for Surfing Head-Disk Interface

Reducing the head-media spacing is of crucial importance in enabling future high density magnetic recording. Fly and lubricant contact interface, or surfing recording, is one of the possible interface solutions for 5 ~ 10 Tb/in2 of magnetic recording. One major challenge for surfing interface schemes is how to sustain the dynamic stability of the head disk interface (HDI). This paper explores the mechanism of sliders dynamic stability at near contact regimes, by means of a simulation study considering air bearings, surface adhesion and slider-lubricant and slider-disk contacts. The simulation results reveal that there are four HDI states, i.e. flying, bouncing, surfing and sliding states, when the slider approaches the disk. One major mechanism of HDI instability is due to the multiple stable equilibrium positions of HDI system. Minimizing the critical contact force by proper HDI design is the key to realize the lube-surfing scheme from the viewpoint of HDI dynamics and stability.

A nonlinear dynamics theory for modeling slider air bearing in hard disk drives

A nonlinear dynamics theory is presented for modeling air bearing slider systems. The nonlinear dynamic coefficients are acquired by fitting the database of the discrete linear dynamic coefficients. The linear dynamic coefficients for an air bearing slider around steady flying state are calculated by using a Reynolds-equation-based perturbation method. A nonlinear analysis shows that the natural frequencies of the system decrease with the increase of slider vibration amplitude, and that the softening feature of the system bends the resonance to lower frequency and complicates the system vibrations. The analysis correlates with experimental results.

Slider Design Optimization for Lube-Surfing Head-Disk Interface Scheme

The stringent magnetic spacing requirement of future high-density magnetic disk drives requires new head disk interface scheme. One possible approach is the lube-surfing scheme-where majority of the slider body is flying while the reader/writer (R/W) element is thermally protruded to contact and penetrate the disk lubricant. This scheme, however, has many challenges to overcome to be feasible. One of these challenges is a slider with improved disk waviness-following capability especially for waviness with wavelength less than half the slider length. This paper explores air bearing design strategies to improve the waviness-following performance of a Femto slider. We propose a ¿W-shaped¿ trailing pad slider and investigated the impact of pad layout, etching depth and trailing pad size to the disk waviness-following performance. We have also incorporated a trenched trailing pad structure to reduce air bearing pressure over the R/W element so as to minimize the loss in air bearing pressure during lube contact and to improve the actuation efficiency.

Intermolecular force, surface roughness, and stability of head-disk interface

To achieve 1-Tb∕in.2 magnetic recording areal density, the head/disk spacing, or the flying height (FH) of the slider has to be reduced to 2–3nm and the disk surface roughness and the slider air bearing surface (ABS) roughness need to be well considered when the flying height is in such a regime. This paper presents authors’ efforts in further study of the intermolecular force effects with the surface roughness in consideration and the investigation of the contact force caused by the surface roughness effects. A probability model is built to simulate the intermolecular force and the contact force, and these two forces are introduced into the modified compressible Reynolds equation governing the air bearing pressure of the slider. The simulation results show that the effects of slider/disk roughness on the intermolecular force are negligible. On the other hand, surface roughness plays important roles in the contact force and the contact vibration. Reducing the roughness of both the slider and the disk surf...