Shengkai Yu

Air-Bearing Design Towards Highly Stable Head–Disk Interface at Ultralow Flying Height

Pushing recording density towards tera-bit per square inch and beyond requires reducing flying height to 2.5-3 nm or below. One critical challenge at such ultralow flying height is the possible head-disk crash or unstable head-disk spacing due to short-range interaction forces, such as electrostatic force, intermolecular forces, lubricant interaction force, and so on. Slider design and design strategy are investigated in this work aiming at significantly increased stability of the head-disk spacing at ultralow flying height. Nonzero surface roughness leads to a roughness-limited possible minimum flying height. A stable head-disk interface requires a full air-bearing domination even at a roughness-imposed minimum flying height. Here, the air-bearing domination means that both air-bearing force and air-bearing stiffness are larger than the combination of various short-range forces and the corresponding stiffness. Investigations presented in this paper indicate that high pressure and high-pressure concentration technology are effective approaches to extending the domination of air-bearing force towards such a roughness-limited possible minimum flying height. Slider designs, proposed by authors, exhibit satisfying flying height stability even at the roughness-limited minimum 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.

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.

Experimental Study of Lubricant Depletion in Heat Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) is a promising choice to overcome the superparamagnetic limit in magnetic recording and further increase the areal recoding density of a hard disk drive. However, HAMR brings about serious lubricant depletion problems on the disk surface due to the high temperature in the heat assisted writing process. Experimental studies of the lubricant depletion under HAMR conditions are still very limited so far. It is essential to do experimental studies under real HAMR conditions or under equivalent conditions if a stand-alone laser is used to emulate the HAMR system. In this work, a self-developed HAMR tester is introduced. A method to control the repeatability of laser heating temperature is explained. Lubricant depletion, accumulation, loss, the percentage of accumulation to depletion, and the depth of depletion are determined quantitatively. The effects of laser power, total laser-on-time and laser-off-time in one laser heating and cooling cycle on lubricant depletion are studied experimentally with the HAMR tester. From the experimental studies, it is found that lubricant accumulation at the edge of the lubricant depletion track takes a considerable percentage of the lubricant depletion and the lower the laser heating temperature, the higher the percentage is. Furthermore, media cooling time plays a significant role in lubricant depletion for the media without a heat sink layer and on glass substrate.

Evaporation of Polydisperse Perfluoropolyether Lubricants in Heat-Assisted Magnetic Recording

An evaporation model based on kinetic theory is developed to analyze the depletion of polydisperse perfluoropolyether lubricants caused by the rapid laser heating in a heat-assisted magnetic recording (HAMR) system. The simulation studies show that the distributions of molecular weights in a polymer, which are usually characterized by the polydispersity index (PDI), have significant effects on the lubricant depletion. As PDI increases, the lubricant loss at the beginning of laser heating becomes more severe due to preferential evaporation of lighter molecular weight components in the mixture. Therefore, lubricants with high molecular weights and low PDIs should be more favorable in HAMR application.

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.

Light Delivery System for Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording (HAMR) technology needs a light delivery system with an incorporated near-field transducer (NFT) as a minute heat generator. A novel system for light delivery with a triangular spot size converter (SSC) and an optical fiber is proposed. The advantages of using a triangular SSC are optical efficiency and integration tolerance between the optical fiber and the NFT. It also has good affinity to conventional head fabrication process. The optical fiber enables flexible laser location. It was confirmed that fabricated triangular SSC can propagate light and reduce beam spot. Computer simulations and experiments demonstrated possibility of integration of light delivery system between NFT, SSC, and optical fiber. Flyability of a pico slider with an optical fiber was also demonstrated. It shows that the optical fiber does not have a big impact on flyability below 10 nm flying height and off-track vibration.

Flying height adjustment by slider's air bearing surface profile control

A new active slider structure is proposed for the flying height adjustment. A bulk PZT ceramic is bonded onto the backside of slider directly. The PZT actuator adjusts the flying height by control of air bearing surface profile. The total thickness of the active slider is same as a standard pico slider. This structure is easy to fabricate and compatible with standard product, it also keeps the air bearing surface intact. The thicknesses of the PZT and the glue are optimized with numerical simulations. This active slider has been demonstrated and the observed flying height adjustment is 2.3nm.