TMR and squeeze at gigabit areal densities

The track misregistration (TMR) required to achieve gigabit areal densities is calculated for a set of components based on the average bit error rate for a file. The average error rate is a statistical average, based on the TMR of the file, of the on-track and off-track error rate. The on-track and off-track error rates are dependent on squeeze from adjacent tracks. In this analysis, the average error rate is calculated using measured error rate profiles of the center track. For a file average soft error rate of 1×10^-6, the results show, for a particular head/disk combination designed for gigabit recording and operating at a linear density of 158 kbpi, that a track pitch of 4.0 μm (3.35 ktpi) can be achieved with a TMR of 0.635 μm. Obtaining this TMR value in a file is one of the challenges to recording of 1 Gb/in² in magnetic storage products     

Patterned media write designs

Recording design alternatives are analyzed for 100 1 Gbit/in² vertical and horizontal patterned media, written at 1 Gbit/s with one and two pole heads. Bit write fields, disturb fields, and switching speeds are calculated as part of a system analysis of the theoretical potential of patterned media to address possible physics limits for conventional “featureless” continuous film media (these include thermal decay, nonlinear transition shift and transition noise, and high writer pole M_s requirements near the limit of known materials). The patterned media “bits” are assumed oriented vertically or horizontally by crystalline anisotropy, and smaller than 100 nm to allow single-domain, 1/2 ns switching. M_sH_c V/kT>500, implying thermal stability including media demagnetization and write head remanent fields, are nearly an order of magnitude areal density extensibility beyond 100 Gbit/in² with present-day head/media magnetic requirements     

The "millipede" - nanotechnology entering data storage

Present a new scanning-probe-based data-storage concept called the "millipede" that combines ultrahigh density, terabit capacity, small form factor, and high data rate. Ultrahigh storage density has been demonstrated by a new thermomechanical local-probe technique to store, read back, and erase data in very thin polymer films. With this new technique, nanometer-sized bit indentations and pitch sizes have been made by a single cantilever/tip into thin polymer layers, resulting in a data storage densities of up to 1 Tb/in². High data rates are achieved by parallel operation of large two-dimensional (2-D) atomic force microscope (AFM) arrays that have been batch-fabricated by silicon surface-micromachining techniques. The very large-scale integration (VLSI) of micro/nanomechanical devices (cantilevers/tips) on a single chip leads to the largest and densest 2-D array of 32×32 (1024) AFM cantilevers with integrated write/read/erase storage functionality ever built. Time-multiplexed electronics control the functional storage cycles for parallel operation of the millipede array chip. Initial areal densities of 100-200 Gb/in² have been achieved with the 32×32 array chip     

Side fringing fields and write and read crosstalk of narrow magnetic recording heads

In magnetic recording systems the side fringing fields of magnetic recording heads are responsible for crosstalk from adjacent tracks and eventually for partial erasure of adjacent tracks, thereby limiting the attainable track density. In this paper we derive analytical expressions for the magnetic field near the side of a recording head and calculate the cosine transform of the longitudinal field component, with the head side angle and gap length as parameters. The field of a head of zero width is also considered. Due to the side fringing field the written track is somewhat wider than the geometrical head width; the increase in width being approximately proportional to the maximum field strength in the recording medium and the head-to-medium distance. The amplitudeuof the read crosstalk signal from an adjacent, infinitesimally narrow track is calculated and it appears that it can be approximated byu/u_{0} = 0.5 exp (-2pi x/lambda), where u0is the on-track signal (with zero head-to-medium spacing),xis the distance between track and head side, and λ is the wavelength. Maximum track densities are calculated for a specified crosstalk-to-signal ratio and a given head width and wavelength. For a wavelength of 10 μm, a head width of 5 μm, and a crosstalk of -20 dB, the track density is limited to about 130 tracks/mm, assuming a track width equal to the head width. When the track is taken to be 5 μm wider than the head to account for the effects of the write process, no guardband at all is needed for -20 dB crosstalk and the limit to the track density is 100 tracks/mn.     

A Comparative Study of Perpendicular Media

Three types of perpendicular recording media were compared using simulations. Exchange coupled composite (ECC) type media could support minimum track pitches up to 18 nm less than a single layer medium, allowing significantly higher areal recording densities. The recording performance of all media was highly susceptible to grain-to-grain variations of uniaxial anisotropy K_u and, in the case of ECC media, to variations in the interlayer coupling strength. Areal densities of around 500 Gb/in² should be achievable using current technologies, higher densities require improved manufacturing tolerances     

A measurement of signal-to-noise ratio versus trackwidth from 128 µm to 4 µm

Bulk-erased tape noise will vary as the square root of the reproduce head trackwidth if the noise signal is uncorrelated across the track. Recent models of erased noise involve clusters of interacting particles that could be as large as a few microns. As the trackwidth (TW) approaches the cluster size, the noise should become correlated across the track and the tape-noise-limited signal-to-noise ratio (SNR) should become constant as TW is further reduced. A check of this idea using inductive heads is impractical. Magnetoresistive (MR) heads have very high signal and low noise and so are well suited for this task, but precautions must be taken to minimize thermal and Barkhausen noise. A multichannel MR head having TW from 128 μM to 4 μM was built to explore the areal reproduce density limits of MR heads and measure the bulk-erased SNR versus TW for 3M 5198 tape. Tape-noise-limited performance was achieved with the narrowest 4μM TW channel at a wavelength as short as 1μM. A wideband-equalized SNR of 20 dB was demonstrated with this channel at a wavelength of 1.27 μm or 40 kilo flux changes per inch (40 kFCI), at a tape speed of 38.2 cm/sec. With a 100% guard band, or an 8-μm track-to-track pitch, this corresponds to the very high areal storage density of 127 MFC/in². The SNR was found to vary assqrt{TW}down to TW = 4 μm, which indicates that the expected particle cluster size must be smaller than 4 μm in the crosstrack direction.     

Comparative study of MR head designs with abutted vs. overlaidpermanent magnet bias

We have developed Soft-Adjacent-Layer (SAL)-biased MR head products with two types of permanent magnet (PM) domain stabilization schemes, namely, abutted and overlaid. Both types of MR heads are capable of achieving recording density of 500 Mb/in² with linear bit densities ranging from 100-118 kbpi. While transfer curves with a transverse sweeping field showed similar behavior for both PM schemes, the overlaid structure has a large hysteresis in its transfer curve under a longitudinal sweeping field while the abutted junction was hysteresis free. Micromagnetic modeling showed that the hysteresis for the overlaid scheme arises from the varying field response of the MR element in the active region, under the leads and under the PM. Magnetic test shows that the abutted MR head with a PM defined TW had a slower rise in sensitivity and had an effectively narrower magnetic track width. On the other hand, the overlaid MR head with an active region defined by the leads with MR under them has a steeper rise in sensitivity, a broadened magnetic read track and side readings     

Performance study and analysis of dual-element head on thin-filmdisk for gigabit-density recording

Inductive-write and magnetoresistive-(MR)-read dual-element heads with very narrow tracks and gaps have been designed, fabricated, and tested on thin-film media of high coercivity and squareness. The results not only show excellent writeability at modest write currents but also the existence of a narrow region of optimum write current, limited by the onset of self-erasure by the write head at high write currents. This leads to significant degradations of overwrite, signal amplitude, trackwidth, linear resolution and disk-noise-induced peak-jitters. A peak-jitter approach is shown to be useful in characterizing many aspects of recording performance. A peak-jitter evaluation of signal-to-noise behavior reveals not only satisfactory overall performance but also the dominance of disk noise as well as a concentration of the disk noise at the track edges. Peak-jitter evaluations of offtrack and squeeze behavior clearly demonstrate the narrow-track capabilities of these recording heads for high areal density operation     

Comparison of readout characteristics between magnetooptic transferheads and magnetic heads

The readout characteristics of a magnetooptic transfer (MOT) head were compared with those of a magnetic head. Magnetic recording/readout was done on a CrO₂ flexible disk by using a head with a track width of 5 μm. A Bi-substituted garnet film with a domain width of 1.2 μm and an He-Ne laser spot focused down to 3 μm were used as the MOT head. Readout waveforms from both heads were surprisingly similar. The maximum carrier-to-noise ratio obtained was 50 dB (bandwidth: 30 kHz) for both heads. Experimental data for off-track and crosstalk characteristics demonstrated that the MOT head was suitable for use as a high-track-density readout head. The potential advantages of multitrack readout using MOT heads are described     

A magnetic head for 150 MHz, high density recording

A multilayered magnetic head that can read and write at 150 MHz on metal particle tape with a coercivity of 120 kA/m (1500 Oe) has been developed. Ten 2-μm layers of Fe₆₈Ru₈Ga₇ Si₁₇ alloy, with 100 nm of SiO₂ used as spacer, form the magnetic-core thickness and the track width. The head was tested in a rotary recording system at a relative head-to-tape speed of 73 m/s. At a linear density of 4000 fc/mm (100 kfc) and 150 MHz, the measured single frequency signal to 300-kHz-slot noise was 33 dB (RMS-RMS). The measured frequency response curve agrees with theory and indicates a head-to-tape spacing of 70 nm at high speed. The read efficiency of the head decreases from 37% at low frequency to 15% at 150 MHz     

Thermal effect limits in ultrahigh-density magnetic recording

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in². This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at “stability ratios” (K_uV/K_BT)₀ ≃35 ± 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in ² conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits     

Thin-film inductive heads for perpendicular recording

A modified thin-film magnetic head for perpendicular recording in rigid disk drives with improved read/write characteristics, especially at high areal bit densities, is presented. The head on which the modified design is based is described. It combines the advantages of single-pole heads and thin-film heads, writing with the sharp field edge of the leading pole and reading like a thin-film head. To increase the writing efficiency and improve the yield, the sequence of magnetic layers in the head is changed; the second layer of the four-layer head is embedded in the substrate, where it can be placed much closer to the pole tip of the first layer. The improved write capability depends mainly on the position of the embedded layer. In addition, there results an improved magnetic flux guidance from the embedded layer into the pole tip layer, providing the potential for a significant improvement in fabrication yield. The embedded-layer approach also allows a further increase in areal density. The results of read/write tests and the write-wide and read-narrow characteristics are presented     

High bit density (100 Mb/cm2) with single-layer Co-Crmedia and ring heads in perpendicular magnetic recording

The signal and noise of single-layer Co₇₉Cr₂₁ media are measured with ring heads to estimate the area density that can be achieved. Densities as high as 100 Mb/cm² (1 μm²/bit) are expected when a signal-to-noise ratio sufficient for an error probability less than 10 ^-5 is required. As a comparison, densities estimated from data from metal-evaporated tape and CrO₂ tape are given. In the frequency response of the single-layer media, an additional minimum was observed for a wavelength slightly larger than the gap length. This is probably caused by the bipolar nature of the perpendicular recording field of a ring head     

Properties of side-shielded read heads in longitudinal and perpendicular recording

Side-shielded (SS) read heads were fabricated, and their magnetic track widths were calculated and measured. The measurements in longitudinal recording show that SS heads exhibit sharper profiles compared with side-unshielded heads. To examine the effect of side shielding, we studied the dependence of the magnetic read width on write density using calculations and experiments. The calculations indicate that the SS head can reduce the skirt of the microtrack profile even at low densities, while the side-unshielded head cannot. This result was qualitatively found in an experiment. We also studied the SS effect in perpendicular recording and found better performance. The calculations predict that SS can strongly reduce the skirt of the microtrack profile even in perpendicular recording. We observed a sharper profile in an SS head compared with a side-unshielded one.     

Narrow track magnetic head fabricated by ion-etching method

Manufacturing narrow track magnetic heads is difficult as track widths become very narrow. A new flying-head manufacturing method is established by applying ion-etching. Optimum conditions for ion-etching of the ferrite material were determined. More than 10 μm ion-etched depth and even less than 5 μm track width were obtained. Through experimental and theoretical evaluation, it is proven that ion-etched heads have equivalent or better read/write characteristics than mechanically manufactured heads.     

A design concept of array heads

This paper describes a novel design concept and experimental hardware data of array heads for close-packed track recording. The heads are batch fabricated on wafers in a linear fashion. These 60-turn thin-film inductive heads are designed with 6 μm pitch helical coils and planar side-by-side P1/G/P2 yokes structure. The linear head array is placed along the upstream-to-downstream direction of the track. By skewing the array slightly off the track direction, each head of the array aligns to an individual track. In this case, the track pitch is about 5 μm, which is the yoke height. With this head arrangement, even though a thermal expansion causes the head-to-head distance to increase along the upstream-downstream direction, it does not cause a thermal induced track misregistration problem. The increased head-to-head distance only affects the timing of signals between tracks, which can be compensated by the channel electronics. Thus, the thermal induced track misregistration problem is eliminated using this design. The guard bands between tracks are not necessary, and a close-packed track recording is possible. A state of the art head impedance of the 60-turn head is obtained: 11 Ω and 0.40 μH. The gap-to-gap pitch is 100 μm. The overall head-to-head isolation is greater than 50 dB at 10 MHz. Such a large isolation is realized by suppressing the capacitive coupling between lead wires using a ground plane and grounded wall structures. The tight winding of the helical coils reduces the magnetic coupling between the heads     

Reduction of the average access time with thin-film head arrays

The average access time, ta, of a moving head disc file can be drastically reduced by the application of multi-track Thin-Film Head Arrays. It is shown analytically that ta= ts+ 16/15(Sm/a)^1/2, where tsis the settling time, Sm, the maximum seek length and a, the constant acceleration and deceleration. When eight arrays each containing 25 thin-film heads are used to cover a 50 mm span of disc recording surface with a track density of about 40 track/mm, the average access time on the order of 2 to 6 m sec. can be achieved depending on the settling time, with present batch fabricated vertical inductive head design and constant acceleration and deceleration of 10,000 to 20,000 cm/sec².     

A fixed read/write head utilizing Winchester technology

The traditional goal when designing a fixed-head disc is to combine reliability, compactness and low cost. The usual technique of achieving these goals is to use a large slider containing many read/write heads. When recording density is increased, the heads must operate at a lower flying height, and a large slider interferes with the typical disc profile. Data General's 6063 fixed head disc drive solves this problem and achieves low cost and reliability by using a three-track ferrite slider of the proven Winchester technology. The disc employs a serpentine gimbal spring which results in a compact head assembly that allows each slider the necessary freedom of movement without affecting other sliders in the assembly.     

Recording characteristics of electroless-plated perpendicular media for flexible disks

The read/write characteristics of electroless-plated perpendicular magnetic recording media for flexible disks were studied using commercial VHS and 8-mm VTR ring heads. Excellent results were shown for a medium composed of Co-Ni-Re-P plated on a non-magnetic electroless-plated Ni-P layer. A Ni-P underlayer of only 500 Å greatly improved the read/write characteristics of the disk. Reproduced signals over 300 kFRPI were observed for a system using a VHS head, andD_{50} = 134kFRPI was obtained using an 8-mm head.     

The Feasibility of$+15$Gb/in$^2$High-Density Recording With Barium-Ferrite Particulate Media and a GMR Head

The feasibility of a high-density magnetic particulate recording medium with a thin magnetic layer of evenly dispersed high-coercivity barium-ferrite particles was investigated. Thermal stability, writability, and an areal recording density of 17.5 Gb/in$^2$were confirmed for a flexible disk system using a giant magnetoresistive head.