Narrow track readout with magnetic garnet film

A novel magneto-optic readout head, which utilizes the modulation of width of straight domains in a garnet film in response to the fringing field from the recording medium, was evaluated from the viewpoint of narrow track readout. In a static transfer experiment with a garnet film having a domain width of 2.25 μm, a definite transfer to the garnet film with track widths of 3 μm or less was confirmed. An expression for the modulation degree of the domain width was derived, and its predictions agreed well with the experimental results. In the dynamic readout experiment, a carrier-to-noise ratio of 50 dB was obtained for a trackwidth of 4 μm and a wavelength of 10 μm. The cross-talk was -30 dB when the track-to-track spacing and track width were 10 μm and 5 μm, respectively. From these results the feasibility of narrow track readout based on this head was demonstrated. The readout performance, including the decrease of readable track width, the wavelength and crosstalk, can be greatly improved if a garnet film, having a domain width narrower than that used in this experiment is used together with a high-density recording medium.     

Analytical expressions for side fringing response and crosstalk with finite head and track widths

An analytical expression is derived that describes the response of a magnetic read head of finite width to a magnetized track of finite width shifted in the lateral direction. With this expression the side fringing response and crosstalk from adjacent tracks are calculated and a number of results is presented.     

Recording performance of discrete track patterned media fabricated by focused ion beam

We report on the recording performance of discrete track patterned media fabricated by focused ion beam (FIB). We investigated performance over a small area by spinstand read/write testing. Discrete track patterned regions show smaller magnetic track width and better signal separation between adjacent tracks and therefore higher track density than that of nonpatterned continuous media as a result of reduced side fringe effect and edge noise. We found that, at a designed groove depth of 4-8 nm, the shallow FIB etched grooves already provide good isolation between adjacent tracks, indicating the superiority of ion beam induced modification of magnetic properties in film media over physical modification of disk surface topography. This has implications for discrete track recording and media fabrication.     

Demagnetization-free longitudinal recording on flexible thin film metal media

The variation of demagnetization effects with media parameters for longitudinal contact recording has been investigated. Co-Re thin film metal media were sputtered onto flexible polyimide substrates. It was found that when the film thickness δ and demagnetizaton parameterB_{r}delta/H_{c}were less than 2 μ inch and 15 μinch, respectively, demagnetization-free longitudinal recording was obtained up to the recording density of 75 KFRPI at the head-to-medium spacing of 3 μ inch. As a result, recording densities of D50over 50 KFRPI were achieved with a 20 μinch gap head. For thicker films with larger demagnetization parameters, i.e.,delta geq 4 muinch andB_{r}delta/H_{c} geq 40 muinch, the longitudinal recording process approached the demagnetization limit. The results show that (with existing head field gradients) improvement in linear density of thin metal media can be obtained by an approximate factor of two before the demagnetization limit is reached.     

Spacing losses in finite track width reproducing systems

The Fourier transforms of the three field components of a finite width reproducing head are derived and given in analytic forms. These formulas represent three-dimensional analogs of the familiar two-dimensional exponential spacing loss factor. The on- and off-track head response to a longitudinally magnetized track is computed from these results and compared with experimental measurements on registration loss and read crosstalk. As long as the head-to-medium separation and medium thickness are small compared to the head width, the results are in relatively simple analytic forms.     

Micromagnetic Modeling of Track Squeeze and Erasure Versus Skew Angle

Micromagnetic modeling of adjacent track squeeze in perpendicular magnetic recording, and its effect on track density was studied as a function of skew angle. Based on amplitude 747 curve of track squeeze and reader off track capability of 10%, a loss of 6.5% of track density was predicted by micromagnetic model for head of 150 kTPI track density. The analysis also shows that erase width is smaller than magnetic write width and erase width formula based on pole geometry overestimates the effect of track density loss due to skew angle.     

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     

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     

The effect of the Bsof recording head cores on the magnetization of high coercivity media

Two types of recording heads, the cores of which had different values of saturation flux density Bsand almost the same values of effective permeability, were prepared and the recording characteristics with Ni-Co plated discs of varying coercivity Hc(410-900 Oe) were examined. The materials of the head cores were a sputtered alloy (Fe-Si-Al) film (B_{s} = 11 000G) and a single crystalline Mn-Zn ferrite (B_{s} = 3800G). Results showed that the Bsof the head core had to be about five times the Hcof the media to produce the beneficial effects of high Hcon short wavelength recording. From this point of view, the alloy film head has the advantage in high linear density recording because of its large Bs. By using an experimental laminated alloy film head of narrow-track width (60 μm), the core of which had a large effective permeability (such as 115 at 40 MHz), sine-wave signals of short wavelength (smaller than 1.5 μm) and of high frequency (such as 37 MHz) were recorded on a high Hc(900 Oe) plated disc and reproduced with the same head successfully.     

Crucial points in perpendicular recording

Since the introduction of perpendicular recording on a floppy disc by IWASAKI in 1977 and its equivalent design on a rigid disc (SPH-like sensor + double-layer medium) in 1981, many tests have been carried out on different R/W sensors. For each test the main goal was the fci record or the improvement of the magnetic layer. Seen from the recording system point of view, the head and the medium are looked at as a unit through a specification, unchanging with increasing area density. For example, a minimum of 26 dB and 70 % must be achieved for the S/N ratio and the resolution respectively. By considering the noise of the best electronic channel (with a thin film head), and ignoring mechanical and medium noises, the output signal must be at least 250 μv pop. For a 50 Kfci application, however, a sensor does not yet exist. Using a ferrite head with a 1.2 μm gap length to write on FeTbGd, the level of the signal will not be high enough to be used. It is improved with a 0.6 μm gap head but then, the field doesn't allow us to write ! Such problems exist also with thin film heads or SPH like sensors on rigid discs. To improve the R/W process, the trend is to use a double layer medium e.g. CrCo/FeNi. The results show that this direction is not necessarily the best. For example, when erasing or over-writing with the head, some domains appear in the FeNi film which create noise from the track or its edge. Another example is the fact that the optimum parameters for a medium such as CrCo are not always compatible with the characteristics of the head (i.e. Hc, the thickness, the crystallographic orientation, the bit stability compared to the write field, the signal, the noise...).     

Head-to-media spacing losses in perpendicular recording

The perpendicular recording process is essentially demagnetization-free at high bit densities, and the head-to-medium spacing losses become perhaps the most constraining factor in realizing the ultimate capabilities of this technology. In this study we investigated the recording losses resulting from head-to-medium spacing for a double layer madium using a single-pole head and a narrow gap ring head. The spacing was introduced by sputtering Ti overlayers onto the CoCr film in the range of 0.02 microns to 0.18 microns. The recording experiments were performed using a tape deck and a closed tape loop running at low speed. For a spacing d and a wavelength λ, the spacing loss when writing and reading with the single pole head was found to be -99d/λ (dB) for any bit density up to 4Kbpmm (kilo-bits per mm). The spacing losses for the ring head, however, depend on bit density, and are much larger at lower bit densities. In an attempt to separate the writing from the reading spacing losses, we recorded with the single pole head and read back with the ring head. The results of this experiment show that the large spacing losses observed with a ring head at the lower bit densities are primarily incurred during the writing process.     

Effects of the writing process and crosstalk on the timing accuracy of pulses in NRZ digital recording

The position at which a NRZ transition is recorded is dependent on the write current, the effect of increasing the current being to shift the recorded position beyond the trailing gap edge. The shift is more marked with a thick medium than a thin coating, and is worsened if the write head pole tips are approaching saturation. If the recording field changes magnitude when reversed, positive pulses become displaced relative to negative, and pulse pairing occurs. When recording different patterns on adjacent tracks, peak shifts can occur due to writing crosstalk aiding or opposing the head field and the shifts may well impose an upper limit to track density.     

High density recording with perpendicular Co-Cr-Nb media and ring heads

Effective double layer structure was investigated by adding Nb to the sputtering source of Co-Cr thin film perpendicular magnetic recording tapes. The output from the tapes was measured with a ring head through to the short wavelength, λ50=0.19 μm (D50=267KFRPI).     

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     

Recording by an ideal single-pole head onto an array of Stoner-Wohlfarth magnetic particles

This paper presents a theory of recording by a probe head onto an array of individual magnetic particles. The theory predicts the area packing density of particles and shows that head-to-medium spacing is the prime factor in determining its value. The paper investigates the trade between attainable packing densities and variations in medium properties.     

Design and characterization of ion-implanted tracks for 16 Mbit/cm2storage density bubble memory devices

The ion-implanted propagation tracks with contiguous disk patterns (CD tracks) have been confirmed to be better for high density propagation tracks (≥ 16 Mbit/cm²) than those with snake patterns (snake tracks), because of less interactions between bubbles on the other side in the same track. The CD tracks with 1.8 µm × 2.0 µm cell size for 0.5 µm bubbles have been evaluated. The large operating bias field margin of 12.4 percent is obtained at the quasi-static operation with rotating field HRof 60 Oe. The minimum rotating field is 40 Oe. Interdigital folded minor loops are proposed and operated. The proposed minor loops are composed of straight propagation tracks connected alternately to relax the in-side turns. The overall operating margin of 8.4 percent (46 Oe) is obtained at HR= 60 Oe. The feasibility of 16 Mbit/cm²storage density bubble memory devices is confirmed.     

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     

A crosstalk model considering leakage fluxes and methods to reducecrosstalk between read/write head and pre-erase head

A head combined with a read/write (R/W) and a pre-erase gap is adopted for 4 MB flexible disk drives. Reducing the crosstalk coupling between the erase head and the R/W head is an important factor in the design. We developed a simple design method to analyze the crosstalk using a three-dimensional finite-element method (3D-FEM). We found that there are two flux paths contributing to the crosstalk in the reproducing process: one is the path through the ferrite cores, and another is the path fringing and reentering into the R/W core. Balancing the fluxes through these two paths, it was possible to realize both high efficiency and low crosstalk simultaneously, Furthermore, the crosstalk during the recording process was analyzed using dc-bias recording theory     

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.     

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