Head-media Interface Research Articles

Numerical and experimental investigation of heat transfer across a nanoscale gap between a magnetic recording head and various media

With the emergence of Heat-Assisted Magnetic Recording and Microwave-Assisted Magnetic Recording, understanding nanoscale heat transfer at the head-media interface is crucial for developing reliable hard disk drives. There is a need to develop a methodology that uses a spacing-dependent nanoscale heat transfer coefficient, determined by using wave-based radiation and van der Waals force driven phonon conduction theories to predict head temperatures in hard disk drives. We present a numerical model to simulate the head temperature due to heat transfer across a closing nanoscale gap between the head and the media (nonrotating) and compare our results with static touchdown experiments performed with a head resting on three different media (Si, magnetic disks with AlMg, and glass substrates). The Thermal Fly-Height Control (TFC) heater in the head is powered to create a local protrusion, leading to contact of a resistive Embedded Contact Sensor (ECS) that is used to measure the temperature change. As the ECS approaches the media, enhanced phonon conduction heat transfer causes a drop in the ECS temperature vs TFC power curve. Our model shows that the introduction of van der Waals forces between the head and the media during computation of the head's thermal protrusion causes a steeper drop in the simulated ECS temperature curve, ensuring a good quantitative match with experiments for all of the media materials tested and different initial ECS-media spacings. We isolate the effect of air conduction on ECS cooling by comparing our simulations with experiments performed in air vs vacuum.

Pump-probe thermoreflectance measurements of critical interfaces for thermal management of HAMR heads

ABSTRACT For heat-assisted magnetic recording (HAMR) heads, a major reliability limiter is the peak near-field transducer (NFT) temperature. Since the NFT is nanoscale, heat sinking is controlled by materials and interfaces within a few 100 nm of the NFT. Heat sinks can be metallic to take advantage of the 10x-100x higher thermal boundary conductance (TBC) of metal/metal interfaces, versus nonmetal interfaces. Oxide formation at these interfaces can greatly decrease the TBC and contribute to NFT failure. Likewise, the thermal resistance of material between the NFT and media recording layer greatly influences the NFT operating temperature. Here we use pump-probe thermoreflectance techniques (FDTR, TDTR) to study metal-metal interfaces and detect partial oxidation of a buried metallic thin film, as well as evaluate the interface thermal conductance of amorphous-amorphous interfaces in a film stack representative of a HAMR head-media interface.

HAMR Thermal Modeling Including Media Hot Spot

All previous thermal modeling on heat exchange between hard disk drive (HDD) head and media treated media as a heat sink with uniform ambient temperature. However in heat-assisted magnetic recording (HAMR) system, the media temperature is no longer uniform and the temperature at the hot spot center can reach as high as 800 to 1000 K depending on recording layer's Curie temperature. In this paper, both media hot spot and the air bearing flow are included in formulating heat transfer across the head-media interface. Numerical results for a waveguide only slider and a slider with a metallic near-field transducer (NFT) are presented to illustrate the effect of HAMR media temperature.

(Invited) Linear Magnetic Tape Heads and Contact Recording

Linear tape recording heads, introduced in 1984, originally were cylindrical ferrite structures having shunt-biased magnetoresistive sensors. In 2000, heads transitioned to flat-lapped, ceramic wafers and anisotropic magnetoresistive (AMR) sensors fabricated in hard disk drive (HDD) facilities. In 2008, heads transitioned to giant magnetoresistive (GMR) sensors. Engineering tape heads has meant dealing with problems related to the head-media interface. Examples include sub-nanosecond pulses in sensors having metal shields (ca. 1994), signal losses and telegraph noise caused by parasitic electrical connections and spacing due to deposits that form at low relative humidity. Wear management, especially with 'green' media, and stick-slip effects are on-going challenges. A strategy was created for protecting GMR tape heads, leading to the discovery of polycrystalline Al2O3 films.

Trapping Electron Assisted Magnetic Recording

Moving towards 10 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> areal density, finding a proper recording scheme with enough write-ability is the most challenging task of a magnetic recording system. Some recording schemes with enhanced write-ability, such as HAMR, MAMR, graded media, etc., have been proposed to achieve higher recording density. Here we propose a new alternative approach for enhanced writing-trapping electron assisted magnetic recording (TEAMR). In the TEAMR configuration, an electrical bias is applied to the main pole of the write head with the disk media and the other parts of head slider grounded. As the main pole area is very small, the electrostatic force produced by electrical potential is a few orders smaller than the air bearing force at the rear pad. Therefore, it will not affect the flying performance of the head slider. At the nanometer head media spacing, a very strong electrical field is produced in the head media interface. This strong electrical field will cause free electrons to accumulate (be trapped) at the interfacial surfaces of metallic magnetic grains. These trapped electrons are localized in the surface atoms of magnetic grains and will alter the valance-electron band filling of those surface atoms. For many magnetic materials, the extra band-filling electrons reduce the magnetic anisotropy energy and make it easier to be magnetically switched. In this work, the TEAMR effect was proved by the experiment study on Co alloy based commercial disk media. The first principle calculation on L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ordered FePt crystal shows that the magnetic anisotropy can be reduced to zero with around 0.38 electrons trapped into 1 unit cell of FePt. Further increase in trapped electrons will change the magnetic easy axis from out-of-plane to in-plane, which is considered as a negative magnetic anisotropy. With the magnetic anisotropy reduction at the surface atoms of each grain, micromagnetic simulation result shows that the effective switching field can be reduced to around 11% of anisotropy field for a 1.6 ? 1.6 ? 3.2 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> FePt grain. Thus TEAMR can be another good candidate for energy assisted recording requiring very little modification to the current perpendicular magnetic recording system.

Pressure Sensor Implementation for Head Media Spacing Reduction

Altitude performance is one of the key considerations in the head media interface design for hard disk drives (HDD). With conventional head media interface without thermal fly-height control (TFC), it is necessary to budget in the air bearing fly-height loss due to altitude effect. In order to reduce the sensitivity to altitude, some other performance need to be sacrificed. With TFC technology, there is an alternative approach to achieve superior altitude performance without trading off other performance. A pressure sensor is installed to the HDD to monitor the ambient pressure. When the pressure changes, the pressure sensor reports the pressure to the channel, and the TFC heater power is adjusted accordingly to maintain a constant fly-height at all altitudes. The pressure sensor has been implemented in the HDD successfully. The objective of this paper is to characterize the pressure sensor performance in the HDD application. The head-media spacing (HMS) was monitored with pressure sensor compensation turned off and on when exposed to different pressure to simulate altitude effects. The HMS was measured with harmonic sensor based on Wallace's spacing equation. It was demonstrated that the pressure sensor is effective in maintaining constant spacing at wide range of altitudes.

Self-heating in thin-film magnetic recording heads due to write currents

Self-heating due to write currents is a critical concern in thin-film magnetic recording heads; the reliability of the head-media interface and magnetoresistive sensors can be degraded by excessive temperature rise during writing operations. A combined experimental and theoretical study shows that the magnetic saturation of yoke structures and the heat transfer across the air bearing strongly influence the magnitude and spatial distribution of the temperature rise. This paper describes three-dimensional numerical heat transfer simulations that can accurately model self-heating as a function of the amplitude and frequency of write current pulses.

Head-media interface instability under hostile operating conditions

In the office environment, computers reliably transfer data to and from the hard disk drive and data transfer errors are extremely rare. However, when even ruggedized computers are operated in hostile conditions, valuable data can be lost as a result of read-write error due to either shock or vibration, seriously impairing drive performance. For this work, drives have been vibrated over large frequency ranges in order to characterize their data-transfer performance. In order to observe the cause of the problems, a drive has been modified with integral sensors to identify internal system failures and the results of these investigations are presented. These sensors have enabled the first unobtrusive tests of the drives' mechanical frequency response while the drive is in operation with sensors that can be easily and inexpensively embedded within a hard disk drive for use in a generic, smart, vibration resilient drive.

Near-Field Optical Recording without Low-Flying Heads: Integral Near-Field Optical (INFO) Media

The promise of near-field (NF) optics to increase the density in optical data storage, whether solid immersion lenses (SILs), apertures, or their variants, has not yet been fulfilled, largely because of the difficulties with the head-media interface at near-field dimensions, especially if removable media is a design goal. We introduce a new approach to NF optical storage, in which integral near-field optics (INFO), in the form of nano-cylinders, are formed in the media upon manufacture. Combining the advantages of near- and far-field optical data storage, we obtain double the track density of digital versatile discs (DVD) in removable rewritable (RW) phase change media. Potential for in-track density increase is also demonstrated. We briefly report on the modeling, manufacture, and performance of INFO media.

The Effect of Porosity on the Head-Media Interface

A lubrication model for the head-media interface is presented which includes the effect of porosity in the media coating. Experimental data is shown which illustrates the reduction in head-media spacing as porosity is increased. A modified Reynolds equation is derived to account for the effects of coating porosity. Other authors have considered a very thin porous layer to simulate a liquid lubricant or surface microstructure on a nonporous substrate. This study considers a porous layer that can be much larger than the bearing clearance. Darcy’s law is used in the porous layer. Velocity-slip effects, resulting both from rarefaction and the porous boundary, are considered. The modified Reynolds equation is applied to a simple capillary model of a porous layer as an illustrative example. The modified Reynolds equation was incorporated into a finite-element model for the head-media interface. Computations show reduced head-media clearance as porosity and permeability are increased in agreement with experimental data.

Analysis of dropout peakshift in magnetic tape recording

Dropout is the term used to describe sporadic and long-duration signal loss in a tape recording system due to media defects or debris generated by the head-media interface. The main mechanism believed to cause dropouts is spacing loss during writing and/or reading. Dropouts are a major limiting factor in the performance of the magnetic tape channel, which makes understanding them a critical issue for future density improvements. In this paper, we analyze the peakshift behavior of signals during a dropout, which constitutes a significant but unexpected side effect. This peakshift was found to be caused during the recording process. Both experimental analysis and modeling have been used in this study.

Numerical and experimental study of the particle contamination in a head/media interface

Trajectories of particles moving in a head media interface are simulated by a numerical approach. The simulation result shows that there are several areas on the ABS of a selected slider where particles easily accumulate. This numerically predicted result is consistent with the results obtained experimentally using a system designed for testing particle contamination. Therefore, the method can be used as a tool in slider designs for reducing particle contamination.

Micro-Tribological Studies on Fluorinated Carbon Films

Micro-tribology is a key technology in micro-mechanics and the magnetic recording head-media interface. The atomic-scale surface state is very important in micro-tribology. However, the concepts of micro-tribological material design are not clear. The top-surface must be modified to improve micro-tribological properties. To improve adhesion and strength, hard carbon films containing silicon deposited by an electron cyclotron resonance (ECR) plasma depositor are examined for friction and wear using a reciprocating tribometer. Fluorinated silicon-containing films are deposited to reduce atomic-scale wear. Micro-tribological properties of this film are investigated by ultra-low load wear-testing, leading to the following conclusions: (1) The micro-tribological characteristics of silicon-containing carbon film are improved by fluorination. Fluorination decreases the surface energy, evaluated by contact angle to the water, and reduces the micro-wear on an atomic scale. (2) The adhesion to the substrate and the strength of the carbon film are greatly improved by adding small quantities of silicon. These films also have significantly longer lubricating lives.

HEAD-MEDIA INTERFACE OF STRETCHED RECORDING DISK

Head-media interface of CoCr and metal particle stretched recording disks is investigated using a slotted spherical slider. Acoustic emission output of the stretched disk scarcely depends on the flying height. Frictional coefficient in high speed region of the stretched disks is higher than that of a rigid disk which is almost zero. In contrast the coefficient in low speed region are lower than that of the rigid disk. These peculiar running behavior must be based on elasticity of the stretched medium. The change of spacing is less than 0.1μm below 6m/sec of running speed. Using the slotted spherical slider with the optimal radius of curvature even the CoCr stretched disk shows the durability over 100 million passes. These results suggest the possibility of a high density recording disk system based on such "quasi-contact head-media interface".

ADVANCEMENTS IN RESEARCH OF MAGNETIC RECORDING

This paper reviews some of the advancements in the research and development of magnetic recording media and heads for applications in high-density information storage systems. In particular, advancements in the understanding of the fundamental mechanisms in perpendicular and longitudinal recording processes, media, and heads are discussed. Advancements in the designs and materials of media and heads, calculations of recording performance, and new measurement techniques are reviewed. It is concluded that the development of magnetic recording technology is accelerating and needs rapid advancements in the research of media, heads, and the head-media interface. Interactions between university and industry can assist in advancing magnetic recording research to achieve the ultra-high areal densities and reliability required for future information storage systems.

Development of a Three-Dimensional Noncontact Digital Optical Profiler

A noncontact three-dimensional optical profiler for measuring surface roughness is described. The system consists of a reflection microscope, Mirau interferometer with a reference surface mounted on a piezoelectric transducer, CID detector array, frame grabber, and micro-computer. Interferometric phase-shifting techniques are used to obtain surface height information. The height measurements are processed by a computer to obtain topographical statistical parameters, which are useful in predicting tribological and magnetic performances of the head-media interface in magnetic storage systems. Sample data are presented for magnetic media (tape, floppy disk, and rigid disk), a magnetic head, a silicon wafer, and a glass slide.

An Optical Profilometer for Surface Characterization of Magnetic Media

Conventional surface-characterization techniques either are not sophisticated enough to provide complete surface-topographical data or cannot be employed because of the relatively low hardness of magnetic media. An optical profilometer has been developed which provides a noncontact method of obtaining surface characteristics from a magnetic medium. The system consists of a standard Leitz reflection microscope, a Mirau interferometer controlled by a piezoelectric transducer, a linear array of photodiode detectors, and a microcomputer. The combination yields a system that measures the optical-height variations of surfaces to a high degree of precision. This height variation is processed by a computer to provide surface-topographical statistical parameters, which are useful to predict tribological and magnetic performances of the head-media interface. Sample data of magnetic media (tape, floppy disk, and rigid disk) are presented.