Head Disk Research Articles

Suppression of resonance in hard disk drive servo system with feedback resonant controller: a benchmark model study

In order to achieve the precision desired, the bandwidth of the read-write head positioning servomechanism of hard disk drive (HDD) is required to be very high to minimise the effects of external disturbances. However, any attempt to increase the bandwidth is hindered by increased gain in frequencies of mechanical resonances leading to oscillatory response. If not compensated properly, excessive or unbounded oscillation may even lead to instability. Cascade compensation is the popular method for compensating multiple resonant modes. However, this method requires a very precisely identified plant model. On the contrary, feedback controller works better in such situation which does not require a very accurate plant model. This paper presents a feedback resonant controller designed in a simple way to suppress the resonant modes of voice coil motor (VCM) actuator used in HDD servo system. Simulation results substantiate the effectiveness of the proposed design methodology.

Investigation of Near-Field Radiative Heat Transfer of Multilayered Structures in Heat-Assisted Magnetic Recording (HAMR)

Heat-assisted magnetic recording (HAMR) is a promising method to increase the area density to 1 Tb/in2 in hard disk drives. In HAMR, a pulsed laser is used to heat the magnetic medium above its Curie temperature. High temperatures can cause lubricant depletion, degradation, and carbon overcoat damage at the head disk interface. Near-field radiation effects in HAMR are important to contribute the heat transfer and hence the flying ability at the head disk interface. Therefore investigation of the near-field heat transfer is essential to optimize the head disk interface. An equivalent simulation model of the head disk interface for studying the radiative heat transfer is provided in this paper. The head can be simplified as a nanosphere, while the disk can be simplified to a semi-infinite multilayer structure, the thickness of every layer in the multilayer structure is nano-scale. And the distance between head and disk is smaller than 10nm. The near field heat transfer problem can be solved by combining the dyadic Green's functions with the fluctuations electrodynamics, and the scattering matrix approach is used in the multilayer structure. Considering both the nonmagnetic media and the magnetic media, several numerical computations are conducted.

Surface Texture Parameters of Perpendicular Magnetic Recording Disks

This paper presents the measured values of the surface texture parameters of commercially available perpendicular magnetic disks produced by five different vendors. In addition to the ordinary roughness parameter values, the asperity parameter values necessary for evaluating the mechanical characteristics of a head–disk interface with a clearance of less than 1 nm are evaluated. The mean asperity height is found to be 0.49 nm and the asperity density is 5044 $\mu \text{m}^{-2}$ . The asperity radius of curvature has an anisotropic nature, but its average value is approximately 20 nm. However, the peak radius of the highest asperity group associated with the asperity contact is close to 10 nm.

Analysis of the influence of the ramp and disk dynamics on the HDI response of 2.5-inch hard disk drive to a shock

Operational shock (op-shock) analysis has been developed to consider the dynamics of components of a hard disk drive (HDD). Op-shock simulation uses simplified models. However, the dynamics of the components affect the head–disk interface (HDI) response, especially the disk curvature and ramp–disk contact. This paper investigated the effect of the dynamics of HDD components on the HDI response. As the HDD is more sensitive to external shock and vibration, the effects of the dynamics of HDD components need to be considered for accurate op-shock analysis. The ramp–disk contact is an important factor in an op-shock simulation, because ramp–disk contact and disk deformation occur during the shock. Therefore, this op-shock simulation modeled the ramp–disk contact and considered the disk curvature. To obtain an accurate result, a two-way coupled method was used to investigate the HDD dynamic response during op-shock. Using the op-shock model, the fluid-structure coupling effect was investigated and compared with a decoupled model that used linear springs to replace the air bearing. Then, we investigated the effect of the ramp–disk contact and disk curvature on the HDI response during a shock.

Slider Dynamics and Wear Behaviors at Subnano-Clearance Head–Disk Interface

Recently, to achieve a stable slider dynamics and a high recording density, a contact (lubricant surfing) recording scheme has been proposed in the literature. However, under contact/surfing condition, the interaction between the head and the disk may bring new tribological issues, which has not been fully understood so far. To amend that, this paper experimentally investigates the relation between the interfacial dynamics and the wear behaviors of head/disk at different contact conditions. To investigate the interfacial dynamics, slider was then flown at different contact stages, and slider vibrations in both the vertical and the downtrack directions were monitored using two laser Doppler vibrometers. The wear depth of slider was inspected after the experiment to correlate interfacial dynamics at different contact stages. According to our results, a severe contact is not necessary to generate serious wear, which has strong correlation with the interfacial dynamics. Compared with the structure modes, the air-bearing modes were found to have a dominant impact on slider wear. Moreover, nonlinear vibration has a much more detrimental effect on the slider wear, although this type of vibration accompanies small amplitude at high-frequency band. This paper helps understand the wear mechanisms of slider under different slider/disk contact conditions, and thus expands new designs for surfing/contact recording technologies.

Rotated Logging Storage Architectures for Data Centers: Models and Optimizations

We propose Ro tated Lo gging (RoLo), a new logging architecture for parallel disk-based mirrored storage systems for enhanced energy efficiency, which is one of the key concerns in modern data centers. By spreading destaging I/O activities among short idle time slots and proactively reclaiming the stale logging space, RoLo rotates loggers among a logical logging space pool formed collectively from the free storage space available among mirrored disks. We develop three flavors of RoLo, that is, RoLo-P/R/E, to emphasize performance, reliability, and energy efficiency respectively. Without the extra dedicated log disks and the corresponding centralized destaging, RoLo eliminates the additional hardware and energy costs, potential single point of failure and performance bottleneck. Furthermore, RoLo-P/R/E, applied to specific scenes correctly, can prolong the lifecycle of the disks and improve the system's energy efficiency by reducing the disk spin up/down frequency. We propose RoLo-S to further alleviate the performance bottleneck and energy consumption caused by frequent disk head seeks in on-duty logger disks. We have implemented RoLo and RoLo-S on real disk systems. Extensive trace-driven evaluations demonstrate the advantages of the three RoLo schemes over both a RAID10 system with centralized logging architecture and a typical RAID10 system, and the advantages of RoLo-S over RoLo.

Splitting of standing spin-wave modes in circular submicron ferromagnetic dot under axial symmetry violation.

The spin wave dynamics in patterned magnetic nanostructures is under intensive study during the last two decades. On the one hand, this interest is generated by new physics that can be explored in such structures. On the other hand, with the development of nanolithography, patterned nanoelements and their arrays can be used in many practical applications (magnetic recording systems both as media and read-write heads, magnetic random access memory, and spin-torque oscillators just to name a few). In the present work the evolution of spin wave spectra of an array of non-interacting Permalloy submicron circular dots for the case of magnetic field deviation from the normal to the array plane have been studied by ferromagnetic resonance technique. It is shown that such symmetry violation leads to a splitting of spin-wave modes, and that the number of the split peaks depends on the mode number. A quantitative description of the observed spectra is given using a perturbation theory for small angles of field inclination from the symmetry direction. The obtained results give possibility to predict transformation of spin wave spectra depending on direction of the external magnetic field that can be important for spintronic and nanomagnetic applications.

Optic nerve morphology in normal children

To expand the normative database of pediatric eyes analyzed with the Optovue optical coherence tomography (OCT). In a community-based, cross-sectional analysis, 77 healthy 5-year-old children were recruited from pediatric practices. No subject had any known ocular disorder. Their optic nerves were assessed using Optovue optical coherence tomography (OCT). Data were compared to the normative database obtained by Optovue for adults, ages 18-25, 40-45, and 55-60. Comparisons included thickness of retinal nerve fiber layer (RNFL) and ganglion cell complex (GCC), and optic nerve head disk area, vertical (VCD) and horizontal (HCD) cup:disk ratios, and cup area. In comparison to all adult age groups, the children's optic nerve heads were statistically less cupped (cup area, VCD, HCD), all with P values of <0.0003. No statistical difference was identified between the children and adults aged 18-25 and 40-45 with respect to RNFL or GCC. Both RNFL and GCC were thicker in the children compared to adults aged 55-60 (P < 0.003). Children's disk area's were marginally smaller than those of adults, the difference not reaching statistical significance. Our OCT results demonstrate that young children's optic nerves are statistically less cupped than those of all normal adults. The children's RNFL and GCC are statistically thicker than those of adults aged 55-60.

The Instability of Angstrom-Scale Head-Disk Interface Induced by Electrostatic Force

When the storage density is coming into several Tb/in2, the head-disk spacing will drop into angstrom-scale regime [1]. At such small clearance, the electrostatic and intermolecular forces have become increasingly significant. It was reported that the electrostatic force produced by electronic potential difference between head and disk can be larger than the van der Waals force even if the slider and disk are both grounded[2],[3]. However, the electrostatic induced head disk interface instability and the relevant reliability were not well addressed in past. Therefore, this work investigates the instability of angstrom-scale head-disk interface induced by electrostatic force. First, the hysteresis effect of the touchdown-takeoff was investigated with/without eliminating the electronic potential difference. The electrical powers of thermal flying-height control (TFC) [4] head was supplied and then released to capture the hysteresis effect. It is found that a significant improvement of the hysteresis effect has been achieved when the electrostatic potential was eliminated. Second, a wear test over 60 hours when the head-disk clearance was set at an angstrom scale was performed. It is also found that the wear robustness has been greatly improved by eliminating the electrostatic potential. This study will help the design of the head-disk system for better reliability, especially for wear and flying stability.

Study of Head-Disk Interface Characterization Using Touchdown Sensor and Electromagnetic Signal in Hard Disk Drives

With the introduction of thermal fly-height control sliders, the local head-disk clearance can be reduced to a range from $\sim10$ nm to contact. This actively controlled touchdown (TD) has been used as a way to study the head–disk interface (HDI), the characteristics of which greatly affect the areal density and reliability of drives. In this paper, a high spatial resolution TD study is performed at the drive level using TD sensor signals and head-media spacing signals. Different flying stages, including passive flying, TD transition, and over push modulation, are identified from the experimental results. A correlation between the aforementioned signals at the passive flying stage is found to represent the disk surface property. The passive flying stage and over push modulation stage of TD are further analyzed in both the time and frequency domains. By establishing a correlation between the fly height and the temperature response, we can provide insight into the heat transfer model for the HDI.

Head–Disk Lubricant Transfer and Deposition During Heat-Assisted Magnetic Recording Write Operations

Lubricant accumulation was found on the media surface the instant when the laser is turned OFF during heat-assisted magnetic recording write operations. By changing the write cycles, laser ON/OFF duration, media, and head temperatures, we find that this lubricant accumulation is related to the change in head-media temperatures. The observed lubricant deposition process is restricted to a short time window (1-2 μs) after the laser is turned OFF. An equilibrium model of thermal displacement due to evaporation and condensation processes is presented and used to discuss the effect of the head-media temperature changes on the lubricant accumulation. Possible solutions to minimize the lubricant transfer and deposition are discussed.

Repeatable and Non-repeatable FH Modulation Study Near Head Contact

The lower head–media spacing (HMS) is always expected for achieving better recoding performance at high linear density. Besides the efforts to reduce the overcoat thickness of head and media, the low flying height (FH) and smaller FH modulation (FHM) are necessary to ensure the low HMS and good interface reliability. In the current HDD, the FH change is measured from the readback signal by the Wallace spacing loss equation. The real-time dynamic FH signals consist of repeatable and non-repeatable FH changes, or FHM. The repeatable FHM (RFHM) correlates with the disk morphology variations. It usually has the variation of transition parameter $a$ inside. Because the change of $a$ is the noise of the RFHM, it has to be very careful for the compensation of the RFHM by thermal FH control. Otherwise, it may cause unexpected head–disk contact. The non-RFHM (NRFHM) measurement is able to remove the media-related noise, which has a good testing accuracy of around 1 A. This paper studies both RFHM and NRFHM characteristics of four types of sliders with different air-bearing surface designs.

Investigation of Temperature Dependence of Raman Shift of Diamond-Like Carbon Coatings Used in Heat-Assisted Magnetic Recording

Heat assisted magnetic recording (HAMR) can overcome the super-paramagnetic limit and increase areal density beyond 1 Tb/in2 in hard disk drives [1]. In HAMR, a pulsed laser beam is first used to heat the magnetic medium beyond its Curie temperature and then data is magnetically recorded during the cool-down process while the coercivity of the medium is still low. High temperatures at the head disk interface are caused by the laser heating process which leads to lubricant depletion, magnetic degradation, and carbon overcoat damage of the head/disk interface [2]. The carbon overcoat can also be graphitized or oxidized leading to poor corrosion protection and inferior tribological performance [2]. The heated spot size achieved by the pulsed laser is very small, approximately 25nm in diameter. In addition, the duration of a typical laser heating cycle, including the cooling process, is very short, on the order of one ns. The measurement of the temperatures in the heated laser spot is of great interest for HAMR technology since reliability is strongly affected by the temperature rise in the heated laser spot. It is very difficult, if not impossible, to measure the temperature rise in the laser heated spot using conventional methods such as thermocouple measurements. To simulate and investigate the near field effects in heat assisted magnetic recording, tip enhanced Raman spectroscopy can be used (Fig. 1). Tip Enhanced Raman Spectroscopy (TERS) combines an atomic force microscope (AFM) and a Raman spectrometer to investigate the near field effects between the tip of an AFM and the adjacent surface. In TERS, a laser is focused on the tip of a metallized AFM tip, mounted in a scanning probing microscope. The laser illumination causes localized heating of the probe tip and the disk surface. Plasmon interactions at the interface between the probe tip and the surface enhance Raman scattering from the laser illumination. The scattered light is measured with a spectrometer and then analyzed in order to quantify the temperature rise in the disk due to the near-field heating by the AFM tip.

Simulation of Particle Trajectory in the Head–Disk Interface

This paper presents a numerical method to simulate the particle trajectory in the head–disk interface (HDI). The simulator solves the reduced Navier–Stokes equation with the second-slip boundary conditions to get the air velocity distribution, then solves the motion equations of particles and obtains the trajectories of the particles in the HDI using the binary Lagrange interpolation and the fourth-order Runge–Kutta method. The calculated trajectories of particles follow well with the air streamlines in the recessed regions. The particle density, diameter, and initial entering velocity have different effects on particle movement. Furthermore, particle velocity behaviors in the recessed regions are investigated for the first time. The particle is accelerated or decelerated by the action of the drag force and becomes close to the velocity of local air flow. Such effects are more obvious for small particles than large particles. The particle velocity becomes very small when it is close to the leading edge of the trailing pad. The numerical results also show that the particle goes up when it crosses the transition regions, and the particle moves upward more sharply with higher initial entrance height due to the larger vertical velocity of the local air flow.

Enhancement of current-perpendicular-to-plane giant magnetoresistance in Heusler-alloy based pseudo spin valves by using a CuZn spacer layer

Enhancement of magnetoresistance output was attained in current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices by using a bcc CuZn alloy for the spacer. Pseudo spin valves that consisted of the Co2Fe(Ga0.5Ge0.5) Heusler alloy for ferromagnetic layers and CuZn alloy with the composition of Cu52.4Zn47.6 for a spacer showed the large change of the resistance-area products, ΔRA, up to 8 mΩ·μm2 for a low annealing temperature of 350 °C. The ΔRA value is one of the highest reported so far for the CPP-GMR devices for the low annealing temperature, which is essential for processing read heads for hard disk drives. We consider that the enhancement of ΔRA is produced from the spin-dependent resistance at the Co2Fe(Ga0.5Ge0.5)/CuZn interfaces.

A STUDY OF HEAD–DISK INTERACTION DETECTION IN THE HARD-DISK DRIVES

The reliability and performance of precision mechanical components that experience sliding under contact depend heavily on the friction and wear characteristics at the sliding interface. In order to improve the reliability of the sliding interface, there is a need to predict, measure and monitor any physical interactions at the head–disk interface (HDI). In the present work, the basic tribological characteristics of HDI were analyzed. The HDI during start–stop and constant speed operation using acoustic emission (AE) were studied. The Fast Fourier Transform (FFT) analysis of the AE signal was used to understand the interaction between the AE signal and the state of contact. In addition, we developed laser textured (LT) disk and the contact start–stop (CSS) tests were performed to investigate the effect of dimples on the stiction performance of the HDI. Furthermore, numerical analysis between the slider and disk surface pressure were performed using the boundary coordinate system and divergence formulation for the nonlinear Reynold's equation solution.

Dual track Wallace fly height measurement method to monitor slider motion in 2 dimension at near contact regime

Head media spacing of hard disk drive (HDD) is expected to continue its reduction in order to support areal density growth. At sub-nm clearance between the head and the disk, the intermittent head disk contact and interactions may happen. This does not only cause the fly height modulation but also induces the off-track motions as well. It is desirable to understand the characteristics of the 2D motions in the near contact regime, so as to enable further reduction of the clearance and to improve the reliability of HDD. This paper presents a method to measure instantaneous fly height (FH) motion and cross track motion concurrently by using read back signal from the dual data tracks written at different frequencies. The method is able to separate the FH motion and cross track motion of the head.

Effect of Chemically Adsorbed Molecules on the Viscous Friction of Nanometer-Thick Liquid Lubricant Films Coated on a Diamond-Like Carbon Surface

Nanometer-thick liquid lubricant films are used for lubrication of miniaturized mechanical systems such as hard disk drives and microelectromechanical systems. In particular at the head–disk interface of hard disk drives, the lubricant thin film is coated on a diamond-like carbon surface, and the films are usually composed of both chemically and physically adsorbed lubricant molecules. The combination of these two types of molecules leads to high lubrication performance. However, the detailed mechanism is not fully understood. The aim of this study was to clarify the effect of chemically adsorbed lubricant molecules on the viscous friction of lubricant films. Two different samples were tested: One was the lubricant film composed of both chemically and physically adsorbed molecules, and the other was the lubricant film with only physically adsorbed molecules. We used a fiber wobbling method (FWM), which we developed in our previous study, to measure the viscous friction. The FWM uses a spherical-ended glass fiber as a shearing probe and enables friction forces in the order of 0.1 nN to be measured at precisely controlled nanometer-scale gap widths. For the fair comparison of friction forces between the samples with different compositions, the contact area between the probe tip and the lubricant film must be the same. To determine the contact area in the friction measurement, we improved the FWM system to enable simultaneous measurement of friction force and contact area. Experimental results showed the 1-nm-thick layer of chemically adsorbed molecules in the lubricant film effectively reduced the viscous friction.

Electrostatic Force Manipulation Methodology: Principles, Mechanisms, and Setup for Head–Disk Interactions Monitoring

To achieve higher hard disk drive areal density, the head–disk clearance had been reduced to $F_{\mathrm {es}}$ ) across the head–disk interface were investigated. It was shown that the distribution of the applied $F_{\mathrm {es}}$ was different for the two setups. This led to further studies of the effects of the $F_{\mathrm {es}}$ distribution on interactions monitoring. The results showed that when an alternating $F_{\mathrm {es}}$ was distributed at the pole region, the magnitude of the electrodynamically excited first-harmonic vibration increased significantly as the gap fly height was reduced. It was very sensitive to spacing change and useful for the head–disk interactions monitoring at the near-contact regime. The second harmonic was found sensitive to head–disk contact. When the alternating $F_{\mathrm {es}}$ was distributed at the slider body, the magnitudes of the first and second harmonics were larger due to larger effective surface area. However, the fly height of the slider body was mainly affected by the protrusion push-up effect, and hence the first and second harmonics showed a reverse trend as compared with the case where $F_{\mathrm {es}}$ was distributed at the pole region. In addition, slider pitch motion was also significantly affected. The effective EFM setup was constructed to demonstrate its effectiveness in head–disk interactions monitoring and contact detection.

New classification of acute papilledema in children with severe malaria

Raised intracranial pressure is a feature of cerebral malaria in children living in Africa. We investigated specific clinical optic disc features of papilledema to establish their prognostic significance in this encephalopathy. We developed a classification of acute papilledema and tested it against disease outcome. Kenyan children admitted with severe falciparum malaria (cerebral or impaired consciousness) underwent dilated fundal examination using direct and indirect ophthalmoscopy. Clinical features of the optic disc were systematically recorded and compared to the child's outcome. Poor outcome defined as death or neurological impairment on discharge was used to construct and test a clinical classification of papilledema. Forty-five children were examined (26 cerebral malaria, 17 severe malaria with an impaired conscious level or prostration) of whom seven had a poor outcome (three died, four had residual neurological impairment). Loss of the optic disc cup and marked optic disc elevation were significantly correlated with a poor outcome (P < 0.05). Increasing severity in the proposed classification of acute papilledema was positively correlated with a poor outcome (P < 0.05, chi-square test for trend). Loss of the optic disc cup and marked elevation of the optic disc head appear to be correlated with poor outcome in children with severe malaria whereas the presence of dilated veins suggests a good outcome. The proposed classification of acute papilledema is useful as a prognostic indicator and may be applicable to other encephalopathies with raised intracranial pressure.