Thermal Behavior of Magnetoresistive Heads: An Analysis of the Thermal Asperity Problem

It is well known that the resistance of a magnetoresistive (MR) or giant magnetoresistive (GMR) head, and therefore its output, varies as its temperature changes. This causes uncertainty in the interpretation of magnetic output, and this uncertainty becomes more important when an asperity or particle passes by or comes into contact with the slider, causing a voltage transient during read back. The temperature variation during non-contact is caused by changes in the cooling of the air bearing surface as the flying height changes. When contact occurs an even more significant temperature spike, called a ‘thermal asperity’ (or TA), is caused by frictional heating at the contact interface. These temperature fluctuations are analyzed in this paper. Results show that the temperature of the MR read coil is influenced by bias current in read coil, slider materials and flying height (which is sensitive to surface topography). The temperature variation without contact causes MR output signal variations which can be used to characterize surface topography. The flash temperature rise that occurs with asperity contact can be as much as 150 degrees (C) or more at the contact interface, but it lasts less than a microsecond. The magnitude of the TA temperature spike is affected by contact force, sliding velocity, and geometry and properties of slider and disk materials, including surface films.

Polydispersity Effects in Evaporation of Perfluoropolyether Thin Films

The evaporation rates of bulk liquid and thin films of an alcohol-derivatized perfluoropolyether have been studied experimentally and computationally. We find that the time dependence of the evaporation rate in both cases is dominated by the polydispersity, and can be described very well by a model that incorporates the molecular weight distribution, molecular-weight-dependent Arrhenius parameters of evaporation, and Raoult’s law of vapor pressures. Minor corrections to the model that account for surface interactions are necessary in the case of thin film evaporation.

Comparison of Energetic Carbon Deposition Processes for Use As Ultra-Thin Disk Overcoats

Three different processes, Plasma Enhanced CVD (PECVD), Ion Beam (IB), and Cathodic Arc (CA) have been used to deposit highly energetic carbon films in the 2–10 nm thickness range in commercial, high throughput disk manufacturing tools. The deposition conditions used are typical of those required for disk manufacturing. Raman spectroscopy, I-V measurements, nanoindentation, and AFM based scratch testing have been used to characterize the structural, electrical, and mechanical properties of the films. The measured maximum hardness for the PECVD and IBD films are 28 and 25 GPa, respectively, and found to be influenced by the hardness of the softer substrates for the 70–120nm films available for measurement. The scratch resistance of the CAC films is ∼2× the scratch resistance of the IBD films and 25% greater than the PECVD films. Addition of nitrogen to the films produced by both the PECVD and IB techniques reduces the hardness of the films. Both the Raman and I-V data suggest increasing concentrations of sp2 bonding result from these nitrogen additions.

Ultrathin CNx Overcoats for 1 Tb/in2 Hard Disk Drive Systems

Carbon nitride films were grown on silicon and hard disk substrates using pulsed dc magnetron sputtering in a single cathode deposition system. Substrates were mounted on a specially designed rotating holder that allowed 45° tilt angle and substrate rotation about the surface normal up to 20 rpm. AFM scans over 10×10 μm2 showed that 50 nm thick CNx films prepared under optimum substrate bias conditions have r.m.s. surface roughness almost four times lower than those prepared without substrate tilt and rotation. We observed a two-fold reduction in corrosion damage for hard disk substrates with 1 nm CNx overcoats deposited with substrate tilt and rotation. This improved performance is likely a result of more efficient and uniform momentum transfer parallel to the surface during deposition in this configuration.

PFPE Lubricant Bonding to Carbon Overcoats

Continued reduction in the head-disk spacing of magnetic data storage systems and the resulting increase in the frequency of head-disk contacts will place increasing burdens on the perfluoropolyalkyl ether (PFPE) lubricant and amorphous carbon (a-C) overcoat used to protect the surfaces of magnetic media. In addition, environmental conditions such as temperature, humidity, and contamination that influence the lubricant-overcoat interactions become increasingly important to the tribological performance of the head-disk interface. It is of utmost importance to obtain a fundamental understanding of the molecular interactions at the lubricant-overcoat interface in order to maintain the reliability of future hard disk drives. Recent progress has generated insight into the heterogeneous nature of the a-C overcoat surface, the interaction mechanisms of PFPEs with a-C overcoats, the effects of humidity on lubricant-overcoat interactions, and the evaporation kinetics of PFPE lubricants.

Head-Disk Interface Issues for Near Contact Recording

As the magnetic recording density increases towards hundreds of Gb/in2, both the magnetic spacing and head-disk clearance decrease to < 10 nm. By one estimate, the magnetic spacing for 1 Tb/in2 is about 6 nm and the read width is ∼ 30 nm. There are at least two different approaches to achieving this. The first one is an extension of the traditional flying interface and the second is contact recording. In the former case one needs to be concerned about maintaining adequate clearance both at sea level and at higher elevation whereas in the latter case the wear and corrosion of the heads and disks may pose major challenges. In the flying regime, an accelerated test to assess the relative integrity of the head-disk interface is described here. This is accomplished by monitoring the acoustic emission, capacitance or friction between the head and the disk as the ambient pressure is reduced. The pressure at which an abrupt change in the above signals takes place is called take-off pressure (TOP). This is also known as altitude avalanche measurement. With this method it is possible to compare different disk and head designs at the full velocity of the slider. We present results correlating the TOP with disk roughness and the influence of disk lubricant. An example of how head-disk interference takes place in a disk drive will be given for an experimental 10 nm flying slider. The effects of radial flying height profile, take-off height of the disk, and the disk curvature on mechanical spacing are presented. The results of changes occurring on the air bearing surface and the disks after long term flyability test are discussed.

Tribology of High Density Tape Storage Systems

Decreasing track width and tape thickness to increase the volumetric recording density of tape drive systems will result in the failure of tracking. The decrease in the tape thickness causes the vertical displacement of the tape forwarding position, which is controlled by pressing the lower tape edge against the lead of a stationary drum. The position of the upper tape edge is controlled by the height of the flanges of the roller guides beside the drum. The displacement of tape forwarding position causes the failure of tracking, especially with narrow track. The reduction of the static friction coefficient between the tape and the roller guides in a drive was found to be effective to reduce the excessive force with which the tape edges are pressed to the flanges or the lead, which will cause the displacement of the tape forwarding position.

Self-Adjusting Flying Height Sliders for Operation in the Near-Contact Recording Regime

The viability of the concept of “self-adjusting flying height” for giant magneto-resistive (GMR) recording heads designed for operation in the near-contact recording regime (flying heights below 10 nm) is demonstrated experimentally. In the present context, the realization of the latter concept relies upon the micro-texturing — production of deterministic, spatially coherent surface topography patterns — of potential contact regions on the recording heads. Through the manipulation of micro-texture pattern feature geometry and spatial distribution, the apparent area of contact, or bearing area ratio, in such regions can be varied in a controllable manner. The latter affords a means to enable the transition of head-disc interfaces (HDI) initially operating under interference contact conditions, into the fly regime without impairing the mechanical and electrical integrity of the HDI. Experimental results attesting to the feasibility of the concept self-adjusting flying height for GMR heads are presented and discussed. Practical aspects associated with the implementation of the latter concept are also discussed.

The Future of Magnetic Recording

Magnetic information storage technology has made astounding progress since its invention over a hundred years ago. For the last several years, storage packing densities in hard disk drives have doubled every year! This frantic pace is expected to soon slow because of the some very fundamental limits that are becoming increasingly evident in the technology. Conventional magnetic recording technology is expected to ultimately reach densities of several hundred Gigabits per square inch and data-rates of a few Gigabits/s (current products are ∼25 Gbit/sq.in. and over 0.5 Gbit/s). We examine the key limiting factors and then try to develop a consistent geometry and set of material properties that could support a density close to one Terabit per square inch. Finally we speculate about the external characteristics of a small hard disk drive that would store one Terabyte of information [1].

Predicting Fly-Height Modulation and Contact Forces in Ultra-Low Flying Head-Disk Interfaces Using a Tri-State Switching Dynamic Contact Model

In order to achieve higher recording densities up to 1 Tbit/In2 using conventional recording technologies, the recording slider will need to “fly” within 5 nm or less from the rotating disk. In such ultra-low flying height regimes, intermittent head/disk contact is unavoidable. Head/disk contact can cause large vibrations of the recording slider in the normal and lateral (off-track) directions as well as damage the disk due to large dynamic contact forces. This paper describes a simple continuum mechanics-based model that includes the dynamics of a flying head/disk interface (HDI) as well as the contact dynamics. Specifically, a lumped parameter one degree-of-freedom, three state nonlinear dynamic model representing the normal dynamics of the HDI and an asperity-based contact model are developed. The effects of realistic (dynamic microwaviness) and harmonic input excitations, contact stiffness (surface roughness) and air-bearing force during contact on fly-height modulation (FHM) and contact force are investigated. Based on the tri-state model predictions, design guidelines for reduced FHM and dynamic contact force are suggested.