Head-disk Interface Research Articles

Effects of SiO2 content on the nanomechanical properties of CoCrPt-SiO2 granular films

CoCrPt material is used for perpendicular magnetic recording media due to its high magneto-crystalline anisotropy that brings good thermal stability on the media. The addition of SiO2 between the CoCrPt grains offers benefits including lower noise and better thermal stability. It has been reported that the SiO2 content has strong effects on the media’s recording performance such as coercivity, anisotropy and noise. In this work, we focus on studying the effects of the SiO2 content on the nanomechanical properties of the media which are critical for the head-disk interface reliability. Variations of these properties with SiO2 content provide guidelines for optimum designs considering both recording and mechanical interface performance.

Thermal Diffusion Phenomena of the Functional Polymer Lubricants for the Heat-Assisted Magnetic Recording

Materials in a newly designed head–disk interface in the heat-assisted magnetic recording (HAMR) have to satisfy harsh operating conditions including cyclic thermal stresses generating large thermal gradient where the heated lubricants evaporate as well as thermally flow-out. Therefore, the stumbling block in the HAMR technology is to fully understand the system behavior under large thermal effects on the lubricant diffusion related to the depletion in the heated zone. The previous studies have investigated local thermal diffusion, depletion, and recovery phenomena of nonfunctional perfluoropolyether lubricant by studying the lubricant behavior at the center and the formation of a rim at the edge of heated zone, while the diffusion mechanism in the heated area has not been fully understood. In this paper, we examined the thermal effects on the lubricants for the application of HAMR in two aspects: 1) thermal diffusion for the replenishment and 2) lubricant evaporation for the loss of the lubricant by using the molecular dynamics.

Time-frequency signature investigation of operational shock-induced slider transient dynamics and damage by enhanced ensemble noise-reconstructed EMD

Abstract A continuous increase of the storage density of a hard disk drive (HDD), e.g., currently 1 Tb/in2, requires the physical clearance between magnetic head and disk to be reduced to angstrom level. At such ultra-thin clearance, a sudden shock occurred in HDD may destroy the head-disk interface caused by the unexpected impacts. The damage mechanisms involving in the operational shock testing and its dynamic analysis are crucial, which are few studied previously. Moreover, the meaningful interpretation to the slider/disk interactions is challenging to be addressed due to the weak local nonstationary dynamic characteristics contaminated by heavy noise. To reveal the damage mechanisms, this paper designed and conducted the experimental studies of shock-induced slider-disk interactions, and proposed enhanced ensemble noise-reconstructed empirical mode decomposition (EMD) based time-frequency method to identify the shock-induced slider transient dynamics. Hereinto, the neighboring coefficient denoising and minimax threshold are developed to enhance the weak shock-related signatures. Meanwhile, two kinds of time-frequency signature indicators are introduced to respectively describe the damage of one sudden shock and that of the whole shock process. The results show that the method can successfully identify the main modes of slider-disk interactions during shocking process and evaluate the damage degree, well agreed with the slider damage inspected by scanning electron microscope (SEM).

Sensing and Modeling Heat Transfer Between a Recording Head and Medium for Clearance Setting in HAMR

A thermistor-based proximity sensor enables identification of head-disk contact in a hard disk drive, allowing the setting of head-disk clearance. This sensor also provides thermal information, permitting validation of computational models used to design recording heads. We describe here a set of models, and a related set of experiments to probe thermal and mechanical characteristics at the head-disk interface. Example results are shown, leading to the conclusion that models with diffusive-ballistic equations are capable of simulating heat transfer through the gas bearing.

Recent Trends in Nano-Tribology of Head-Disk Interface in Hard Disk Drives

Recent Trends in Nano-Tribology of Head-Disk Interface in Hard Disk Drives

The effect of a thermal contact sensor on the temperature distribution and heat flux at the disk surface

Abstract The thermal response of the head-disk interface due to the presence of a thermal contact sensor is investigated. The temperature distribution near the contact sensor is calculated considering changes in the temperature of disk surface. The heat transfer in the air bearing near the contact sensor is determined. The results show that the contact sensor causes a high temperature of the disk surface, which is mainly generated by the conductive heat transfer. The increased disk temperature reduces the heat flux leaving the slider, which, in turn, causes an increased temperature of the contact sensor.

Numerical Study of Particle Rebound in the Head–Disk Interface

Particle intrusion into the head–disk interface (HDI) is becoming a concern in hard disk drives. Numerical studies of particle movement in HDI are of great importance for reducing particle-induced damage. In this investigation, we study the particle rebound mechanism in HDI. Not only the particle–slider collision but also particle–particle collision models are developed, which are divided into three types, including the collisions of particle and surface (P–S), particle and edge (P–E), and particle and particle (P–P). The subsequent response of particles after the collision, which includes rebound and adhesion, is calculated considering the collisional energy loss. The proposed adhesion/rebound model can more accurately present the particle trajectories in HDI and accumulation on the air bearing surface (ABS). We find that particles are likely to accumulate on the leading rail surfaces of the slider due to the air flow drag and Saffman lift. Considering the adhesion and rebound, particles with a larger diameter are more likely to fly away from the slider after the collision. The existence of particle rebound, to a certain degree, reduces contamination on ABS.

Dynamic Performance of Head–Disk Interface in HAMR Using Molecular Dynamics Simulation Method

Heat-assisted magnetic recording (HAMR) technology is a great way to achieve higher storage density. In HAMR, dynamic performance of the head–disk interface (HDI) is critically important for the read/write stability. Lubricant bridge formation and lubricant transfer can significantly deteriorate HDI performance. In this paper, a 3-D full-atom model of a slider-lubricant-disk substrate system is established to study the dynamic performance of HDI in HAMR. The overlayers of the slider and disk substrate are made of diamondlike carbon, and the main lubricating ingredient is perfluoropolyether (PFPE). The formation and breaking of the lubricant bridge are investigated with the slider moving up and down. It is found that during the moving down process, PFPE molecules escape from the lubricant layer, and a lubricant bridge between the slider and disk is formed. During the moving up process, the lubricant bridge becomes thin and eventually breaks. The stable value of PFPE weight percentage on the slider during the moving up process is always larger than that during the moving down process. This is because of easier transfer of lubricant molecules to the slider in the low flying height region. The formation height is smaller than the breaking height, but these values are closer when the operational temperature is higher.

Coupled Effects of Surface Roughness and Accommodation Coefficient for Ultra-Thin Gas Film Lubrication

For modern magnetic disk drives, as the minimum flying height in the head-disk interface (HDI) is continuously reduced to about 1–2 nm, the surface roughness and the accommodation coefficient (AC) effects should be considered in addition to the gaseous rarefaction effect. However, only a few published papers had studied both the effects of these two parameters simultaneously. Based on the modified molecular gas film lubrication equation proposed for ultra-thin gas film lubrication, the coupled effects of the surface roughness, the AC, and the gaseous rarefaction in the HDIs are investigated for various groups of the surface roughness and the AC. Numerical results are presented and discussed to show the coupled effects on the pressure and the load carrying capacity of the film. Both the symmetrical and asymmetrical molecular interactions are studied and rational interpretations are given for the phenomena observed.

Investigation of Slider Dynamics Considering Bias Voltage and Lubricant Charge in the Unsteady Proximity Condition

The relationship between slider and lubricant becomes increasingly important as the mechanical spacing between slider and disk is reduced to satisfy the demand for higher areal density. At a reduced flying height, the slider easily contacts the lubricant, which can cause slider instability. This study analyzed slider dynamics to improve the head–disk reliability in the unsteady proximity condition, considering bias voltages between the slider, disk, and lubricant. Force–distance curves were measured using atomic force microscopy to investigate changes in lubricant performance induced by an applied voltage. Additionally, the touch-down power and take-off power were measured under various applied voltage conditions. Experiments were carried out to estimate slider instability as a function of charged disk and slider conditions, to improve the slider dynamics in the unsteady proximity condition. The effect of the bias voltage induced by a voltage applied to the lubricant was carefully examined to accurately understand slider dynamics. The relationship between the lubricant behavior and the applied voltage was investigated; the voltage applied to the disk was more influential in improving slider dynamics. Consequently, the effects of bias voltage and lubricant, as induced by a charged disk, should be considered when analyzing slider dynamics to improve head–disk interface reliability in an unsteady proximity condition.

Simulation of the head-disk interface gap using a hybrid multi-scale method

We present a hybrid multi-scale method that provides a capability to capture the disparate scales associated with modelling flow in micro- and nano-devices. Our model extends the applicability of an internal-flow multi-scale method by providing a framework to couple the internal (small scale) flow regions to the external (large scale) flow regions. We demonstrate the application of both the original methodology and the new hybrid approach to model the flow field in the vicinity of the head-disk interface gap of a hard disk drive enclosure. The internal flow regions within the gap are modelled by an extended internal-flow multi-scale method that utilises a finite-difference scheme for non-uniform grids. Our proposed hybrid multi-scale method is then employed to couple the internal micro-flow region to the flow external to the gap, to capture entrance/exit effects. We also demonstrate the successful application of the method in capturing other localised phenomena (e.g. those due to localised wall heating).

Real-time Visualization of Lubricant Films on Head Slider Using an Ellipsometric Microscope

As the recording density increased in hard disk drives (HDDs), the head disk interface (HDI) spacing has been decreased to 2-3 nm during its operation. The head slider may pick up the lubricant at this contact or near-contact HDI. Therefore, a real-time visualization of lubricant thickness distribution on a slider surface is essential to study the physical mechanisms for reducing the head disk interface spacing. A vertical-objective-based ellipsometric microscope (VEM) was presented as the effective method for real-time visualization of nm- thick lubricant films on disks with high lateral resolution. However, extinction coefficient of head slider is low, which means the intensity of reflected light may be reduced. In this report, the feasibility of VEM used for head observation is discussed, and the images of polar perfluoropolyether (PFPE) lubricant Zdol4000 applied on head slider are obtained by the current setup. It indicates that this method can be used in real-time visualization of nm-thick lubricant films on head slider.

Experimental and direct numerical analysis of hard-disk drive

The purpose of this paper is to study methods for enhancing the reliability and performance of hard-disk drives (HDD) because it is essential for improving recording density, speed of data access, and output signal. This study also investigates various techniques that can be used for head/disk contact detection. The acoustic emission (AE) and friction signal characteristics were observed with respect to the durability of the head/disk interface (HDI) under various operating conditions using a contact start-stop (CSS) test. In addition, to study the influence of surface topography on the stiction performance of the HDI, a modified and polished laser pump was proposed and CSS investigations were accomplished. Moreover, the static and dynamic properties of an HDD air slider were studied using a finite element method (FEM).

Effect of Air and Helium on the Head–Disk Interface During Load–Unload

The tribological characteristics of the head–disk interface are investigated during load–unload for air and helium-filled drives as a function of the pitch static angle and the roll static angle between slider and disk. A custom-made experimental tester inside a sealed environmental chamber was used to determine the regions of “safe” pitch static angle and “safe” roll static angle in air and helium environment during the load–unload process. The presence of head–disk contacts during load–unload were evaluated by measuring the acoustic emission signal and the decrease in rotational speed of the spindle. Scanning electron microscopy and optical surface analysis were used to investigate wear of the slider and the redistribution of lubricant on the disk surface after 10,000 load–unload cycles. The results indicate that the tribological performance of the head–disk interface is improved in helium environment compared to air environment.

Materials challenges for the heat-assisted magnetic recording head–disk interface

Abstract

Head flying characteristics in heat assisted magnetic recording considering various nanoscale heat transfer models

The thermal issues in heat-assisted magnetic recording (HAMR) technology have drawn much attention in the recent literature. In this paper, the head flying characteristics and thermal performance of a HAMR system during the touch-down process considering different nanoscale heat transfer models across the head-disk interface are numerically studied. An optical-thermal-mechanical coupled model is first described. The coupling efficiency of the near field transducer is found to be dependent on the head disk clearance. The shortcomings of a constant disk-temperature model are investigated, which reveals the importance of considering the disk temperature as a variable. A study of the head flying on the disk is carried out using an air conduction model and additional near-field heat transfer models. It is shown that when the head disk interface is filled with a solid material caused by the laser-induced accumulation, the heat transfer coefficient can become unexpectedly large and the head's temperature can rise beyond desirable levels. Finally, the additional head protrusion due to the laser heating is investigated.

Electrostatically Tunable Adhesion in a High Speed Sliding Interface.

Contact hysteresis between sliding interfaces is a widely observed phenomenon from macro- to nanoscale sliding interfaces. Most such studies are done using an atomic force microscope (AFM) where the sliding speed is a few μm/s. Here, we present a unique study on stiction between the head-disk interface of commercially available hard disk drives, wherein the vertical clearance between the head and the disk is of the same order as in various AFM-based fundamental studies but with a sliding speed that is nearly 6 orders of magnitude higher. We demonstrate that, although the electrostatic force (dc or ac voltage) is an attractive force, the ac-voltage-induced out-of-plane oscillation of the head with respect to the disk is able to completely suppress the contact hysteresis.

Molecular Dynamics Simulation of Lubricant Transfer at Head–Disk Interface

Molecular Dynamics Simulation of Lubricant Transfer at Head–Disk Interface

Rigorous derivation of diffusion equation for submonolayer lubricant film

In magnetic hard disk drives, it is important to secure the stability and durability of head–disk interface in relation to a submonolayer lubricant film because the spacing between a head and lubricant surface has to be reduced to ~0.5 nm to achieve a high density recording far beyond 2 Tb/in2. As a succeeding paper of the newly proposed diffusion equation for a submonolayer liquid film, this paper presents a rigorous derivation of the disjoining pressure (DP) from the Lennard–Jones potential (LJP) and formulated the rigorous diffusion equation incorporating the DP. The flow equation for a submonolayer film and viscosity model are the same as in the previous paper. The difference of the rigorous DP and diffusion equation from the previous ones are not significant except in a small thickness regime less than the van der Walls (vdW) distance. Theoretical relationship between the vdW distance in DP and the molecular force equilibrium distance in LJP is elucidated. Rigorous expressions of the conventional DP and diffusion equation for multilayer film are shown. Superiority of the submonolayer diffusion theories to the conventional theory are demonstrated by comparing their theoretical diffusion coefficients with Waltman’s experimental data.

Nanotribology of Head-Disk Interface in Heat-Assisted Magnetic Recording

Nanotribology of Head-Disk Interface in Heat-Assisted Magnetic Recording