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We present an area efficient test structure that allows measurement of the statistical distribution of SRAM cell read currents and write trip voltages for 1 million SRAM core cells. The data taken from measurements of wafers fabricated with a 90-nm and 65-nm CMOS process flow show that the device variations are Gaussian distributed for more than 1 million devices, covering more than 5 sigma of variation. The analysis of the measured SRAM performances validate Monte Carlo simulations. Index Terms--65 and 90 nm, low voltage, measurement structure, read current, SRAM, variation, write trip voltage.

The effect of thermomechanically actuated pole tip protrusion on adhesive forces is characterized through model and experiment. The roughness of a thermomechanically actuated region is characterized by atomic force microscopy. Using the extracted roughness parameters and estimated apparent area associated with thermal actuation, the intermolecular forces at the head-disk interface (HDI) are calculated using the ISBL (improved sub-boundary lubrication) code. Both roughness and nominal area of contact are found to be significant factors determining adhesive forces. The adhesive forces for various HDI designs--including thermal actuation--are also characterized experimentally in situ using commercial hard disk drive samples. The experimental results are found to be consistent with the model calculations and imply certain advantages for thermally actuated HDI designs. However, the experiments also raise concerns regarding the field application of the technology. Keywords: thermal actuation, adhesion, head-disk interface (HDI), head-media spacing (HMS), pole tip protrusion

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. Index Terms--Flying height, flying height modulation, head-disk interface, thermal flying height control, waviness.

This paper focuses on air bearing surface (ABS) design optimizations in order to reduce the lift-off force during the unloading process while satisfying the desired static flying performances. Since it takes a huge amount of computational time to solve time-dependent dynamic load/unload (L/UL) equations, an approximate lift-off force is created as a function of the air bearing suction force and flying attitude parameters by the kriging method. The design framework is employed in wrapping effectively and connecting the kriging model and the static analyzer to the optimizer. An optimization problem is formulated to minimize the amplitude of the lift-off force during the unloading process while keeping the flying height, pitch, and roll angles within suitable ranges over the entire recording band as well as reducing the possibility of slider-disk contact in steady state. Then, two different sizes of slider models are optimally designed for L/UL applications with 1-in disk drive. The L/UL simulation results show that the optimized ABS designs have reduced the lift-off force in the loading process by approximately 62% and 11% for pico and femto design respectively, while satisfying desired static flying performance. In addition, results demonstrated that the optimum slider incorporated with the suspension were not only properly unloaded onto the ramp but also smoothly loaded onto the rotating disk. Therefore, it is believed that the proposed design approach works efficiently in ABS designs for L/UL applications. Index Terms--Air bearing surface (ABS), kriging model, lift-off force, load/unload (L/UL).

Scaling the areal density, while maintaining a proper balance between media signal-to-noise ratio, thermal stability, and writability, will soon require an alternative recording technology. Heat assisted magnetic recording (HAMR) can achieve this balance by allowing high anisotropy media to be written by heating the media during the writing process (e.g., by laser light) to temporarily lower the anisotropy. Three major challenges of designing a HAMR head that tightly focuses light and collocates it with the magnetic field are discussed: 1) magnetic field delivery; 2) optical delivery; and 3) magnetic and optical field delivery integration. Thousands of these HAMR heads were built into sliders and head-gimbal assemblies, and optical and scanning electron micrograph images are shown. Scanning near-field optical microscopy (SNOM) characterization of the HAMR head shows that the predicted ~ [LAMBDA] A/4 full-width half-maximum (FWHM) spot size can be achieved using 488 nm light (124 nm was achieved). SNOM images also show that wafer level fabricated apertures were able to effectively eliminate sidelobes from the focused spot intensity profile. A magnetic force microscopy image of HAMR media shows that non-HAMR (laser power off) was not able to write transitions in the HAMR specific media even at very high write currents, hut transitions could be written using HAMR (laser power on), even at lower write currents. A cross-track profile is shown for a fully integrated HAMR head where the magnetic pole physical width is, ~350 nm, but the written track is ~200 nm, which demonstrates HAMR. A HAMR optimization contour shows that there is an optimum write current and laser power and that simply going to the highest write current and laser power does not lead to the best recording. Lastly, some prospects for advancing HAMR are given and a few key problems to be solved are mentioned. Index Terms--Heat assisted magnetic recording (HAMR), magnetic recording, recording head.

In order to achieve an areal density of 1 Tb/in (2) and beyond, not only the mechanical spacing between the slider and the disk but also the track misregistration (TMR) and the fly height modulation (FHM) should be reduced below current levels. But at reduced mechanical spacing there will be elevated excitation due to slider-disk contacts resulting in increased slider vibrations and head-disk interface (HDI) failures. Thus, there is a need to study the effect of slider-disk topography on the slider dynamics and stability at a higher level of complexity. In this paper, we do so by dividing the slider-disk surface features into three regimes based on the amplitude and the wavelength range of the features. Further, we have also proposed several ways of achieving an optimal slider-disk topography that can help reduce the slider vibrations and increase the stability of the HDI. Index Terms--Roughness, slider, stability, vibrations, waviness.

Current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) heads with a current screen layer were fabricated, and the recording performance was measured. An output voltage of 1.9 mV and head-amp signal-to-noise ratio (SNR) of about 30 dB were obtained from a 50-nm-wide head with an operating voltage of 120 mV. The MR ratio was 4%-5%, shield gap was 36 nm and resistance was 72 [ohm] With using the thermal fly-height control (TFC), the fabricated head showed a potential to yield a 382 Gb/[in.sup.2] recording (1252 kBPI x 305 kTPI). The current screen structure reduced the spin torque noise since just a low sensing current of 1-2 mA was required for obtaining a high output. Newly developed CPP-GMR films with a current screen layer showed the MR ratio of 18%-19% with the RA product of 0.2-0.3 [ohm] x [m.sup.2][micro]. Calculation showed that this film allows us to achieve 30 dB or more in the head-amp SNR when the sensor width was 40 nm or larger. The current screen CPP-GMR head is thus an attractive candidate that has a high potential suitable for an areal density of 500 Gb/[in.sup.2] or more. Below 40 nm, an all metal CPP-GMR head with the MR ratio of 10% or more would be the best candidate. Index Terms--CPP-GMR, magnetic head, magnetic recording, noise, spin torque.

Read heads using current-perpendicular-to-plane (CPP) giant magnetoresistance sensors have been fabricated and tested under highdensity recording conditions. A magnetoresistance of 5.5% and shield-to-shield spacing of 45 nm have been achieved by using an all-metal single-spin-valve with Heusler-alloy-based free and reference magnetic layers. Read heads with magnetic read widths ~45 nm were tested on perpendicular media, resulting in signals above 1 mV and signal-to-noise ratio ~30 dB. Linear densities in excess of 1050 kbpi were achieved with thermal fly-height control, compatible with recording areal densities of ~ 400 Gb/in (2). Current-induced spin-torque effects in the recording head were observed to result in rapid performance degradation above a threshold bias voltage of about 75 mV, corresponding to current densities > 10 (8) A/cm (2). Index Terms--CPP-GMR, magnetic recording, read-head, spin-valve.

We created a finite-element model of the head-disk assembly (HDA) of a small-form factor drive. We conducted operational shock simulation and modal analysis with this finite-element model, and investigated the pulsewidth effect of a half-sine acceleration pulse on the shock responses. We found that the umbrella mode of the disk is the dominant mode for operational shock. Numerical results of the pulsewidth effect on the maximum shock responses further confirm the conclusions in a previous work by the authors. Index Terms--Drive, hard disks, head, shock, simulation.

This paper presents a simulation study on the coupling effects of air bearing dynamics and suspension vibrations due to slider-disk contact or disk waviness. A simulator is developed to include the interactions between the air bearing, the slider-suspension assembly and slider-disk contact by considering the suspension, slider and air bearing as a whole system. From the simulation, the dynamic motion of read/write head in both flying-height (FH) and off-track directions, even on-track direction, can be obtained. It is helpful to characterize dynamics of head positioning and understand the interactions between the head disk interface (HDI) components. In addition, the effects of contact intensity, surface roughness, skew, and disk waviness on the read/write head positioning in both FH and off-track directions are investigated. The mechanisms of these factors' effects on the dynamic stability of read/write head positioning in both FH and off-track directions are also discussed in the paper. Index Terms--Aerodynamics, contacts, dynamics, hard disks, head-disk interface, magnetic disk recording, mechanical systems.

1/f-type of low frequency noise is widely observed in today's state-of-the-art tunneling-magneto-resistive (TMR) and current-perpendicular-to-plane (CPP), and giant-magneto-resistive (GMR) heads. In this paper, the 1/f-type magnetic noise arising from the thermally excited low frequency edge magnetization switch of the free layer is studied by micromagnetic simulation. It is found that although the free layer dimension is in the single domain regime, the edge magnetization random switching related to the thermal excitation and insufficient hard bias field on sensor edge can be a significant low frequency noise contributor in TMR/CPP GMR heads. A strong local hard bias (HB) field at sensor edge that pins the free layer edge magnetization is found to be more critical than a strong averaged HB field across the sensor to avoid magnetic 1/f-type noise. The simulation study shows that a high HB field gradient from sensor edge to center is preferred to maintain a low noise sensor while still achieving high sensitivity. This paper will also discuss the roles of free layer thickness, sensor geometry, current field, temperature, and air-bearing surface physical defect in the behavior of this edge flipping noise. Index Terms--1/f noise, current-perpendicular-to-plane (CPP), hard bias (HB), thermal excitation, tunneling-magneto-resistive (TMR).

We refined a previously developed finite-element (FE) model of a Seagate Bali II hard-disk drive (HDD) to include compliance contributed by roller bearings at the spindle and pivot motors. We then performed drop-test simulations with the nonlinear structural dynamic codes LS-DYNA and ABAQUS/Explicit. In parallel, we subjected a nonoperating Bali II HDD to physical tilt-drop tests from drop angles of 10[degrees] and 45[degrees], representing a mild and a substantial shock input, respectively. We measured velocity responses with a laser Doppler vibrometer (LDV) at two points on the HDD. The first point was on the topmost read/write head at the center of its trailing edge, and the second point was on the top surface of the baseplate directly above the line of contact with the impact bar. In the FE studies, the baseplate velocity histories measured during the tests were used to prescribe input motions at the corresponding baseplate nodes. We compared computed and measured head velocity histories in order to evaluate the effectiveness of present state-of-the-art simulation tools for modeling head-slap events. Index Terms--Finite element, hard disk drive, shock, tilt drop test.

A pseudoray method is suggested for the initial design for the composition of combined optical CD-DVD read-write heads. The effectiveness of the method is shown in the design of the optical system for the CD-DVD read-write heads.

To aid understanding the physical mechanisms responsible for the perpendicular write head remanent field and thereby improve the write head design, we studied the remanence of single-pole perpendicular recording heads with trailing shield by using a contact scanning recording tester. We used two different pole-tip size heads in the study. We found that the remanence areas of the heads tested usually locate at the trailing edge corners of the pole tip. We also found that the remanence areas of both types of heads are also the head field easy-rising areas when the heads are driven by low write currents. Additionally, the current level that leads to remanence is the same for both types of heads. However, the remanence of the small pole-tip heads can be demagnetized by a much lower amplitude reverse current than that of the large pole-tip heads. Index Terms--Head remanence, perpendicular recording, pole-tip size, shielded head.

We have developed a lumped-element model for a thin-film giant magnetoresistive head with an inductive writer and used it to study the effect on the data rate capability. Our work shows that the reader can be approximated well by a resistor ([R.sub. eff]) and a capacitor ([C.sub.eff]) in parallel. The paper discusses the relative importance of different contributors to [C.sub.eff] and addresses the effect of [C.sub.eff] on the spectrum of the read-back signal and the electrical reader pulsewidth (PW50) in detail. The model predictions and spin-stand data agree reasonably well. The paper also reports on our study of the amount of writer-to-reader coupling in the transducer. Our work points out the importance of minimizing the parasitic capacitance between elements in the writer and reader and avoiding direct coupling between the writer and reader leads. Index Terms--High data rate, magnetic disk storage, magnetic heads, writer-to-reader coupling.

Ultrahigh storage densities of up to 1 Tb/[in.sup.2] or more can be achieved by using local-probe techniques to write, read back, and erase data in very thin polymer films. The thermomechanical scanning-probe-based data-storage concept called Millipede combines ultrahigh density, small form factor, and high data rate. After illustrating the principles of operation of the Millipede, a channel model for the analysis of the readback process is introduced, and analytical results are compared with experimental data. Furthermore, the arrangement of data-storage fields as well as dedicated fields for servo and timing control is discussed, and system aspects related to the readback process, multiplexing, synchronization, and position-error-signal generation for tracking are introduced. Finally, the application of (d, k) modulation coding as a means to further increase areal density is presented, and the effect on the user data rates discussed. Index Terms--Atomic force microscope, high-density data-storage system, MEMS, modulation coding, probe storage, servo control, thermomechanicai write/read/erase, timing recovery.

Structural resonance modes of the head stack and suspension assembly are one of the major limiting factors for achieving higher head-positioning servo performance in hard disk drives. This paper discusses the vibration suppression control of a PZT actuated suspension dual-stage servo system. A PZT strip on a PZT actuated suspension is utilized as a vibration sensor to control both the voice coil motor (VCM) actuator butterfly mode and the suspension sway mode. The vibration suppression controller is designed using the [H.sub.[infinity]] control design method. Experimental results show that an attenuation of 15 dB can be achieved for both the suspension sway mode and the VCM butterfly mode, and the rms of the head off-track motion due to airflow excited vibration can be reduced by approximately 30% with the proposed vibration suppression control scheme. Index Terms--Dual-stage servo, hard disk drive, vibration damping control, windage.

This paper describes a micro-electro-mechanical system (MEMS)-based active slider with microactuators. The proposal active slider uses PZT thin films as a microactuator and to control the slider flying height of less than 10 nm. The design procedure for the active slider is discussed. In addition, novel double-layered composite PZT thin film microactuators, which are very important functional microdevices, are studied. It is shown that the recently developed PZT thin films have better piezoelectric characteristics than conventional sol-gel derived PZT thin films and sputtered PZT thin films. The micromachining process for the active slider is also developed and the pico-size active slider is fabricated. The technical issues related to the fabrication of a MEMS-based active slider are discussed. Index Terms--Active slider, head disk interface, MEMS, microactuator, micromachining, PZT thin film.

A 2.5-rad/[s.sup.2] resolution micro-electro-mechanical system-based rotational accelerometer targeted for hard disk drive (HDD) application is presented. The device is composed by a 0.6-[micro]m BICMOS electronic interface that implements together with the MEMS sensor one single bit electromechanical [SIGMA][DELTA] conversion loop able to detect capacitive unbalancing as low as 50 aF (50 * [10.sup.-18] F). The produced bitstream is then processed by a digital chain and made available through a standard three-wire digital interface. The device is intended for feedforward compensation of external disturbances acting on computer HDDs to steadily keep the read-write heads on track and to allow greater track densities. Index Terms--Accelerometer, capacitive interface, MEMS, rotational, vibration compensation.

As technology moves toward 600 ~ 1000 Gb/[in.sup.2], the mechanical spacing between head-slider and disk media must be further reduced to a level of 3 ~ 4 nm. Such a reduction of the head-disk spacing will be accompanied with a lot new challenges. One severe challenge is how to reduce the flying height modulation caused by the surface waviness with an amplitude even as small as 0.4 ~ 2 nm. Shifting slider size from its current pico form-factor to femto form-factor seems to be a promising approach for the reduction of such flying height modulation. However, discussions on the femto slider are still limited to theoretical analysis and computer modeling/simulation until now. This paper reports authors' efforts in the design and fabrication of the femto sliders plus the comparative study of the dynamic performance between the femto and the pico sliders. Results indicate that the femto sliders are of smaller flying height modulation, compared with the pico sliders, if both the femto and the pico sliders are of similar air-bearing surface layout. Investigations also extend to the study of flying performance sensitivity of the femto slider to its possible parameter offsets caused by the manufacturing and integration processes. Results indicate that, in general, the flying performance of the femto slider is more sensitive to such offsets, compared with the pico sliders. For example, the flying height of the femto sliders is of greatly increased sensitivity to the gram load change, compared with the pico sliders. Such a phenomenon becomes more significant when the gram load is further reduced for the femto slider case. Index Terms--Air-bearing surface, etching and disk drive, femto slider, flying height.

Hydrogen-terminated Si(111) surfaces have been studied for tribological performance in the sliding of a diamond slider to obtain fundamental information on contact problems between an Si memory disk and the slider. The authors focus on the tribological role of the atoms on the contact surfaces and observe an extraordinarily low coefficient of friction:0.003 of the relevant sliding surfaces, a coefficient as low as the sliding system of diamond/Ag monolayer/Si(111). The sliding tests were carried out under ultrahigh vacuum lower than 1.5 x [10.sup.-7] Pa, with a load of 250 mN and 0.1 mm/s of speed. The Si(111) surface was H-terminated by hydrofluoric acid solution and also by hydrogen gas adsorption, before the sliding tests. By sliding both of the H-terminated Si surfaces, a very low and stable friction coefficient was observed, while the desorbed surface by heating at 600 [degrees]C showed a very high friction as 0.4 or higher. The observed friction reduction of two orders was attributed to a monolayer of hydrogen atoms terminating the Si (111) dangling bonds. The results are discussed in terms of the sliding mechanism, surface source of friction, and application to reduce memory disk surface damage as the result of probable contact between slider and disk, reduction of the friction of micro-machine sliding system, and design of a sliding system in a vacuum. Index Terms--Friction, lubrication, space technology, thin-film device, vacuum technology.

The role of particulate contamination on friction and wear at the head-disk interface (HDI) for load/unload (L/UL) picosliders and contact-start-stop padded picosliders is studied. L/UL picosliders performed better than padded picosliders in the presence of particulate contamination and in a clean environment. Disk wear was higher for harder and smaller sized particles. Disk wear decreases with an increase in disk speed in the presence of smaller particles as these can escape more easily through the HDI at higher speeds. Track following causes more and earlier disk damage as compared to sweeping. In the case of padded picosliders, HDI contamination causes wear of pads eventually leading to partial or complete removal of pads. Index Terms--Contact-start-stop, load/unload, pad wear, padded picosliders, particulate contamination.

This paper provides a systematic micromagnetic analysis on the magnetization processes in various miniaturized single-turn head designs. The calculation results show that a thick yoke yields the formation of a magnetization vortex through the yoke thickness, resulting in efficient flux conduction at small yoke dimensions. A vortex structure is also important for the pole tips to maintain zero remanence after writing. Following this understanding, a hollow yoke structure has been studied to promote the efficient flux conduction in the yoke and flux closure mode in the pole tips at the remanent state. The simulation results are elucidating for efficient single-turn head designs at small yoke dimensions. Index Terms--Narrow tracks, single-turn write heads, thin film heads, write heads.

Multi-track magnetic tape heads for use in high performance, high density storage subsystems are going through a rapid period of change in response to the continuing demand for increased capacity and data transfer rate. The luxuries of wide read and write tracks and low coercivity media have disappeared, increasing the problem set posed for tape head design and manufacturing. The multi-track nature of the head is becoming more complex with the move to full thin film write head structures akin to the thin film disk head. The dramatic increases in track density while maintaining interchange requirements are resulting in significant dimensional reductions in the magnetoresistive (MR) read devices. The increasingly complex new heads dictate stringent process control and correspondingly more lenient head designs in order to achieve an acceptable yield. This paper summarizes the technology transition and the factors that drive these head changes from the media type and track density issues to the recording density and recording code advances. Index Terms - Recording heads, magnetoresistive (MR) heads, thin film heads, tape recording.

In a magnetic rigid-disk drive, information to and from the data surfaces is transferred by a system that consists of a read/write preamplifier, conductors and the read and write transducers. Printed-circuit conductors are now replacing the connection between the head and the read/write preamplifier that has conventionally been designed using twisted-pair wires. This paper provides a practical method of simulating the electrical behavior of these interconnects, and through comparisons with measurements, demonstrates the validity of the design procedure.

GMR heads with NiO/NiFe/Co/Cu/NiFe films have been designed, fabricated, and their read performance was tested. Calculations showed that output was reduced significantly when the exchange coupling field on the pinned layer was smaller than 200 Oe due to tilting of the magnetization of the pinned layer. When the pinned layer of NiFe(1 nm)/Co(1 nm) was used, the coupling field from NiO was 360 Oe which would be large enough to ensure the full output. The readback waveform of the fabricated head was noise-free and well biased. The obtained output was about six times as large as that of a conventional AMR head. When the sensing current was increased, the output increased linearly up to the current density of 20 MA/[cm.sup.2], while the peak asymmetry was not affected much by the current. The output Increased linearly with the read track width, except for small dead zones at both ends of the track. The microtrack profile was symmetric, reflecting the symmetric magnetization configuration of the free layer without the medium field.

Successful use of a NiFe/Co90Fe10 bilayer as a soft magnetic free layer in spin-valve films with a GMR enhanced structure comprised of NiFe/Co90Fe10/Cu/Co90Fe10/FeMn is outlined. The GMR ratio of the spin-valve film with Co90Fe10 is over 7% and the coercivity of the free NiFe/Co90Fe10 bilayers is less than 5 Oe. A merged inductive/spin-valve head was fabricated with a read track-width of 1.3 [[micro]meter] and a read gap-length of 0.26 [[micro]meter] using spin-valve film with a Ta(50 [Angstrom])/NiFe(45 [Angstrom])/Co90Fe10(30 [Angstrom])/Cu(32 [Angstrom])/Co90Fe10(22 [Angstrom])/FeMn(100 [Angstrom])/Ta(100 [Angstrom]) structure and 260 [Angstrom] thick domain control Co78Cr10Pt12 magnet layers. Its read/write performance was tested on a low noise CoCr17Pt5Ta4 thin film disk with an Mr[center dot]t of 0.41 memu/[cm.sup.2] and a coercivity of 2500 Oe. There is no Barkhausen noise in the readback waveform. The result of the microtrack sensitivity profiles reveals an effective read track-width of 0.8 [[micro]meter]. A normalized output for a track-width of 880 [[micro]volt]pp/[[micro]meter] and D50 of 166 kFCI was obtained. Using a PRML channel, a bit error rate of less than [10.sup.-8] was obtained without error correction at a data rate of 3.3 MB/s and at a linear density of 217 kBPI on a thin film disk with an Mr[center dot]t of 0.72 memu/[cm.sup.2]. Thus, 5 Gbit/[in.sup.2] density recording with a linear density of 217 kBPI and a track density of 23 kTPI is possible.

We have succeeded in designing, fabricating, and testing GMR/Inductive heads. 1 [[micro]meter] trackwidth CoFe spin-valve read elements showed high sensitivity even after head fabrication due to its good thermal stability. About 1.5 [[micro]meter] trackwidth a-CoZrNb write elements with Bs (Saturation flux density) of 1.35T fabricated by trench filling process showed high potential for Gbit, density recording. As a result of the feasibility study, 18kTPI and 140kBPI are thought to be within the scope by combining the dual element heads with low-noise media.

A noval tape head contour is proposed that allows very small head tape spacing at high velocities using three transverse bleed slots and a single radius cylindrical head. A finite element based numerical model is used to. determine the head-tape spacing as a function of physical design parameters including tape roughness, tape speed and tape tension. The Greenwood-Williamson theory of contact between rough surfaces is applied to account for asperity contacts as the head-tape spacing decreases below the peak-to-valley surface roughness. The sensitivity of the new head design on geometrical parameters is studied and the variation of spacing and contact pressure is determined as a function of physical and geometrical design parameters. The numerical results are compared with interferometric measurements of the head-tape spacing using head contours based on our numerical design. Excellent agreement is found between numerical predictions and experimental measurements and it is concluded that the spacing at the head-tape interface is limited by surface roughness.

This paper describes materials development, device modeling, and initial test results for a novel GMR reproduce head for ultrahigh density recoding ([approximately] 10 Gbit/[in.sup.2]). It consists simply of a 20-bilayer GMR multilayer, self-stabilized by intrinsic antiferromagnetic interlayer coupling, and self-biased by sense current into a bipolar, field-gradient-sensing bias state. Analogous to a dual magnetoresistive (DMR) such a GMR-DMR head is capable of achieving very high linear resolution without shields. Tape-test results using evaporated NiFe/Cu and sputtered NiFeCo/Cu agree reasonably well with theoretical expectations. 1/f noise in these GMR multilayers will also be discussed.

Three sub-25 nm fly height sliders are presented for near contact recording. The designs are geared towards the goal of achieving 10Gb/[in.sup.2] areal density. The optimization procedure presented shows promise for facilitating achievement of this goal. The dynamic simulations show the stability of these designs when disturbed from their steady state conditions.

This paper presents the discrete-time LQG/LTR design of a disk drive track following servo system. The servo compensator designed through linear-quadratic Gaussian control combined with loop transfer recovery (LQG/LTR) consists of a Kalman filter for state estimation and state feedback for control. The desired tracking servo performance is first formulated through a frequency shaped return ratio of the Kalman filter and subsequently recovered at the output of the plant/compensator loop through the automatic design of a discrete-time linear quadratic (LQ) regulator. Particular attention has been given to modeling the calculation time delay and bias force estimation. The excellent robustness and performance characteristics of a continuous time LQG/LTR design are theoretically unachievable due to the extremely low sampling rate and nonminimum phase plant characteristics. However, both time and frequency domain simulations show that reasonable stability margins and performance can still be recovered. This technique nearly eliminates all the trial and error typical of a conventional pole placement design of a similar system. The direct discrete-time design can handle extremely low sampling rates associated with embedded servo systems. The technique can also be used for designing multi-rate and multi-input servo systems.

A complete read channel device using PR-IV signalling and maximum likelihood detection is described. Descriptions, simulated performance and measured results of the digital adaptive feedback loops (AGC, FIR tap weights, DFE tap weights, frequency and phase) in the read channel are presented. Analog FIR filter and flash A/D converter performance is presented. The weighted averaging servo demodulation technique used in the device is described and compared to integration. This monolithic CMOS device consumes less than 0.75 W when reading at 85 Mbps, and occupies a step-and-repeat area of 24.6 [mm.sup.2].

A fully integrated digital read/write channel for magnetic recording applications is described. The device employs partial response maximum likelihood (PRML) sequence detection when reading data to enhance bit error rate vs. signal-to-noise ratio performance. The sequence detector is designed to accept input pulses exhibiting four non-zero samples, sampling occurring once per channel bit time. Channel optimization is facilitated through the inclusion of sixty-two 8-bit registers. The device is fabricated in a 0.8 micron double-metal CMOS process and supports data rates as high as 64 Mbits/sec.

This paper presents a number of considerations that have to be taken into account when designing read/write amplifiers for magnetoresistive-read/inductive-write recording heads. It discusses various concepts for implementation of magnetoresistive (MR) pre-amplifiers, the noise and bandwidth obtainable, different techniques for biasing the MR sensor and a number of alternative signal sensing schemes. Different forms of required sensor protection are dealt with as well. The flagging of a number of possible mis-operations is reviewed and, finally, the write amplifier design for these MR/inductive heads is briefly touched upon.

We present a novel method to determine the motion of the slider's flying height and off-track direction simultaneously in a hard disk drive. The results show the relative movement of the slider in two dimensions, both during thermal actuator controlled contact and full flying condition. The method separates the position error signal (PES) and the flying height signal from the readback signal. The method includes writing dual-frequency pattern tracks adjacently, with the readback signals from the tracks filtered for the in situ motion measurement. The harmonic ratio method is used to determine the flying height variation of the head, whereas the conventional servo burst detection method is used to determine the PES. Index Terms--Flying height, measurement, position error signal, two-dimensional.

To demonstrate the highest areal recording density of tape media, we have developed a metal evaporated tape with higher magnetic anisotropy of 2.5 x [10.sup.5] J/[m.sup.3] and finer magnetic activation volume of 2.7 x [10.sup.-24][m.sup.3] of a Co-CoO recordin la er de osited on a g yp smoother base film. A capability of an areal recording density of 23.0 Gb/[in.sup.2] was confirmed using a giant magnetoresistive head as a read head. The smoother surface roughness improved signal-to-noise ratio more than 2 dB. Index Terms--Areal recording density, giant magnetoresistive head, magnetic tape recording, metal evaporated.

The spin transfer modified Landau--Lifshitz--Gilbert equation is used to model a permalloy bar with configurations for current, transverse field, and magnetizations resembling those of a read head. The trends of noise power and noise peak frequency against sensing currents and transverse fields with different polarities are calculated and analyzed by symmetry and asymmetry properties, by magnetization dynamics, and by extended linear stability analysis that is applicable to shape anisotropy. With the linear stability analysis, it is found that smaller track width (with aspect ratio fixed) has larger field stability and larger current-density stability. Index Terms--Spin transfer noise, stability analysis, thermal noise.

Exchange-coupled composite (ECC) media consisting of hard and soft coupled regions has been predicted to exhibit attempt frequencies that are much larger than conventional perpendicular media due to the presence of two degrees of freedom. As a result, adjacent track erasure (ATE) and archivability are adversely affected, whereas writability is significantly enhanced due to thermal fluctuation assisted recording. Recording heads for ECC and perpendicular media, designed dimensionally for recording at 250 Gbits/[In.sup.2], are evaluated within this context. In comparison, recording on ECC media is shown to yield about a factor 2 advantage in areal density. Index Terms--Attempt frequency, exchange-coupled composite (ECC) media, perpendicular recording, thermal fluctuations.

A new method to detect magnetic recording head/media contact point is introduced in this paper. By comparing it with a common method using an acoustic emission sensor, the new method is simple, but yet still effective enough to control fly high or clearance between head and disk so as to improve the performance of perpendicular magnetic recording (PMR) at ultra-high density. The method can be easily implemented on head/disk spin-stand testers or in hard disk drives. Index Terms--Disk drives, magnetic head, magnetic recording, perpendicular magnetic recording.

The lattice Boltzmann method (LBM) is utilized to simulate the nanoscale flow physics of air bearings in the head-disk interface. In the high Knudsen number flow analysis of air bearings, the slip boundary model is very important to guarantee the accuracy of solution. In this paper, the Laugmuir slip model for the rarefied gas flow was incorporated and its feasibility and accuracy was examined in nanoscale flow simulations. It was shown that our LBM can solve the fluid flow of air bearing with high efficiency due to its complex geometry handling capability and high accuracy comparable to the Boltzmann transport equation in the slip flow regime. The LBM model developed in this paper could serve as a powerful tool for the design of advanced air-bearing systems. Index Terms--Air bearing, head-disk interface, Langmuir slip model, lattice Boltzmann method.

Reactive ion etching processes for texture fabrication on air bearing surface is investigated to improve the etching selectivity between [Al.sub.2][O.sub.3] and TiC. Etching parameters, such as gas flow rate, pressure, ICP power, and RF power, are optimized. The high selectivity etching between [Al.sub.2][O.sub.3] and TiC is achievable by the ICP reactive ion etching with C[F.sub.4]. The selectivity between [Al.sub.2][O.sub.3] and TiC can be larger than 6, it means that the [Al.sub.2][O.sub.3] is only etched 1 nm to fabricate texture with more than 5-nm height. The experimental results show that the textured sliders fabricated with the developed process can prevent sharp increase of friction force and reduce contact vibration. Index Terms--Air bearing surface, head disk interface, reactive ion etching, textured slider.

The intensity of slider-lubricant interaction is characterized by the amount of lubricant transferred from free lubricant region to bonded lubricant region in this study. The effect of electrostatic force on the slider-lube interaction is investigated by comparing the intensity of lubricant transferring under the same flying height but different electrostatic force conditions. Bump disks are used to calibrate slider and ensure the consistency of flying height under different experimental conditions. Results indicate that the slider-lubricant interaction can be increased significantly when sufficient interface voltage is present at the head-disk interface. The amount of lubricant transferred across the head-disk interface is found to increase with the increase in the applied interface voltage. Index Terms--Electrostatic force, head-disk interface, lubricant transfer and magnetic disk drive.

The dispersive and polar components of surface energy, which are typically obtained from the two-liquid geometric method through contact angle measurement, have been extensively used to study the interfacial properties of perfluoropolyether (PFPE) lubricated media used in hard disk drive industry. In this paper, an alternative approach, Lifshitz-van der Waals and Lewis acid-base (LW/AB) theory, which provides additional information on the nature of lubricant/overcoat interaction, has been applied to the study of the surface energy of PFPE-coated media. Here, the surface energy is decomposed into Lifshitz-van der Waals (LW), Lewis acid, and Lewis base components. The results of Lewis acid and Lewis base components of surface energy suggest that the bonding of functional PFPEs with a carbon overcoat is a Lewis acid-base interaction; the overcoat is an electron donor and functional PFPEs are electron acceptors. It was also found that the LW surface energy has a sequence of Ztetraol < Zdol < AM2001 < A20H < Z03 for films with the same thickness, which is in the reverse sequence as found in the bonded thickness of these PFPEs. A molecular-level bonding mechanism is proposed to explain the experimental results. Index Terms--Head-disk interface, Lifshitz-van der Waals and Lewis acid-base theory, Perfluoropolyether, surface energy.

We developed a thermal flying-height control (TFC) slider to control the flying height of magnetic recording heads. The slider basically consists of a small heater fabricated near the read/write element. This study discusses the effect of heater size and heater location on the change in the flying height at the read/write element. We also discuss the resulting temperature rise due to the additional heat applied by the heater. Specifically, we have found that small heaters generally resulted in lower heater power per unit change in the flying height and lower head temperature rise per unit change in the flying height. In terms of heater location, we have found that a heater closer to the air-bearing surface (ABS) also tends to result in a larger change in the flying height because of the larger protrusion shape. However, the head temperature rose significantly. Therefore, shorter ABS/heater distance was a trade off lower power against higher rise in head temperature. We concluded that smaller heaters and the shorter ABS/heater distance are better as long as head reliability is ensured. Index Terms--Flying height, magnetic head slider, microactuator, thermal protrusion.

In an earlier report, flyability failures due to siloxanes were investigated comparing disks with either Z-Dol or X-1P lubricant. On the Z-Dol disk, the head-disk interface failed at 47 h (h) and the slider accumulated silicon oxide smears, whereas the X-1P disk did not fail (truncated at 300 h) and the slider remained clean. The contrasting performance was rationalized by an assumed mechanism in which degraded lubricant was essential. In this paper, we demonstrate that siloxanes can be tribochemically converted into silicon oxides without any lubricant on the disk or slider. Lubricant degradation is, therefore, not needed for the formation of silicon oxides from siloxanes. We find that siloxane oligomers on Z-Dol disks form micrometer-sized droplets 100 nm in height, whereas on X-1P disks, siloxanes spread out as a very thin film. The siloxane droplets on Z-Dol disks may lead to flyability failure because they exceed the fly height. The presence or absence of siloxane droplets on disk surfaces may be explained in terms of surface wetting properties characterized by contact angle measurements. Index Terms--Hard disk drive, head-disk interface, head smear, lubricant, silicon oxide, siloxane.

This paper presents finite-element analysis of the stress-induced magnetic anisotropy in perpendicular magnetic recording (PMR) heads developed during fabrication. The intrinsic stresses of various thin films used in the heads were measured using wafer curvature, while the stress in the yoke region was measured using X-ray diffraction. The measured stresses are provided. The initial strains due to the intrinsic stresses in the films were modeled using equivalent thermal strains and the calculations were verified by correlations with the stress measurements. Finite-element simulations were performed for calculating the stresses in full PMR heads. Detailed simulation procedures are described. Computed results are presented on how sensitive the magnetic anisotropy in the write pole are to the pole tip length and the slider lapping and the compressive stress in overcoat as well as the ambient temperature. Index Terms--Finite-element method (FEM), intrinsic stress, perpendicular magnetic recording (PMR), stress-induced magnetic anisotropy.

This paper presents a finite element analysis of alternating write-current-induced pole tip protrusion (WPTP) generated in longitudinal magnetic recording (LMR) heads. An integrated magneto-thermal-mechanical model is developed for the simulations of the electromagnetic Joule heating generated in the LMR writer, the slider temperature distribution, and the slider deformation during writing. The computed results are compared with the experimental measurements in terms of writer resistance and inductance, the WPTP, and temperature rise at reader location. It is found that eddy currents are strongly induced near the surfaces of write poles, while the eddy currents induced in write coils are negligibly weak. The results show that the eddy current loss in the write poles dominates over the Joule heat generated in the write coils. It is also found that the maximum protrusion appears near the write pole in the overcoat and the protrusion profile is sharper than that induced by equivalent direct write current and coil resistance. Index Terms--Eddy current, finite element method (FEM), longitudinal magnetic recording (LMR), write-current-induced pole tip protrusion (WPTP).