Unshielded magnetoresistive heads in very high-density recording

Unshielded magnetoresistive (UMR) heads provide very high signal levels and low noise but, because of their relatively large element height and an insensitive "dead zone" at the sensor edges, they have poor resolution. As a consequence, the signal diminishes dramatically as the recorded density increases and may be as much as 30 dB or a factor of 30 down at very high density. Various techniques have been used to increase the resolution and reduce the "peak-to-bandedge" ratio but they all reduce the bandedge signal as well and hence tend to lower the signal-to-noise ratio. We have found that a peak-to-bandedge ratio of more than 30 dB can be equalized and hence the standard UMR described by Hunt can be used to advantage in very high-density recording. This report describes results obtained with a UMR head reproducing 80 kFCI (3150 FC/mm) signals recorded on Kodak Isomax tape. Bandedge signal and low-density distortion were plotted versus bias field. Surprisingly, maximum high-density signal and minimum distortion occur at about the same bias field. Electronic, thermal, and magnetic noise were measured and tape-noise-limited performance was obtained. Equalized signals from a pseudo-random data sequence were examined with a transition interval analyzer as well as by eye pattern photograph. The transitions were well separated, and the eye pattern was well defined in both phase and amplitude.     

A new W-shaped single-pole head and a high density flexible disk perpendicular magnetic recording system

A new single-pole head with no auxiliary pole was developed for perpendicular magnetic recording. The head is called WSP head (W-shaped Single Pole head) because the head has a W-shaped side core which contributes to increase the recording and reproducing sensitivitiy. The head field of the new head has the same distribusion as that of an auxiliary pole head[1]. The recording and reproducing sensitivity of the head is equal to or higher than that of a ring-type video head. The head eliminates mechanical problems which prevent its application in perpendicular magnetic recording because we can locate the head on one side of the recording medium. As a possible application of the WSP head, a 3 1/2-inch flexible disk recording system was constructed. A linear recording density of the flexible disk system was 65.5 kbits/inch. This density is equivalent to 8 times that of the existing high-density 3 1/2-inch micro-floppy and 11 times that of a 5 1/4-inch floppy disk. In termes of information storage, this density gives a 4 megabyte unformated capacity on one side of a 3 1/2-inch flexible disk. The overwrite signal-to-noise ratio was greater than 30 dB and the peakshift displacement was less than 10 % at the linear dinsity of 65.5 kbits/inch.     

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...).     

Influences of Thermal Fluctuations on Recorded Magnetizations During Recording Process

The recording process is examined by computer simulation to clarify the reason why magnetic recording at a linear recording density of over 2000 kFCI is difficult in perpendicular magnetic recording. The recording medium is found to suffer from strong thermal fluctuations, even during the recording process. The recorded magnetization of the preceding recorded bit is decreased considerably by the reversed head field to write the succeeding bit. Numerical results show that this decrease is not due to normal recording loss, but rather to the thermal fluctuations that is enhanced by the reversed trailing field of the SPT head. Based on the results obtained herein, recording at over 2000 kFCI is believed to be possible by reducing the trailing field strength of the SPT head.     

High density magneto-optical recording using 0.67 μm band highpower laser diode

High-density optical recording on a magnetooptical disk has been achieved by a compact optical head that uses a 0.67-μm-band high-power visible-light laser diode. The recording density has been improved to 1.5 times that for conventional optical recording, using a 0.83-μm laser diode, as a result of the increase in linear density and track density. The C/N ratio for the readout signal is greater than 50 dB     

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.     

Overwrite as a function of record gap length

In disk recorders, a common head is used for both recording and reproducing. Old data are not erased but are simply recorded over. The partially erased old data signal appears in the new data as error-producing interference. The amount of erasure of the old data signal is called "overwrite", and 30 dB is typically required. To avoid the reproduce gap null, the head gap is usually chosen to be less than half of the 2F (or bandedge) flux change length. To achieve adequate overwrite using a gap this small, it is necessary to use a very thin recording layer. A simple explanation for this, involving greater depth of penetration for low density signals, is at best incomplete. The data below show that the overwrite phenomenon is complex. Overwrite spectra measured using a thick medium show large resonance peaks whose amplitude and position depend on the record gap, record current, and the densities of the overwriting signal and the signal being overwritten.     

The relationship between disk surface acceleration and head-to-disk interaction

One dominant trend in magnetic disk recording has been rapidly increasing areal bit density. This has been accompanied by lower head flying heights, which has caused the disk surface smoothness and flatness to be of greater concern. One technique for evaluating the surface is to fly a 3330 type slider equipped with a piezoelectric crystal instead of a read/write head. The output of the piezo-electric crystal is monitored as the full recording band of the disk surface is traversed. Isolated bursts of activity are interpreted as head contacts with flaws on the disk surface. However, even in the absence of such head hits there may be a high output from the burnish head, which we refer to as "burnish noise". Heads flying on such high burnish noise disks generate substantial audible noise accompanied by high head wear rates. In this investigation we have shown the relationship between burnish noise and the high frequency axial acceleration of the disk surface.     

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     

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     

Extremely high bit density recording with single-pole perpendicular head

Since perpendicular magnetic recording is free from recording demagnetization, high-density recording up to the intrinsic limit of a recording medium is possible. This prediction was verified experimentally in a flexible disk system using a single-pole head and a Co-Cr/Ni-Fe double-layer medium. We could record and reproduce signals up to 680KFRPI. The recording bit length at the highest density was of the order of the Co-Cr columnar diameter.     

Pulse-read on erasable thermal phase-change superresolution disks

New erasable thermal phase-change superresolution (EPSR) disks composed of mask and recording layers can increase recording density by the detection of the below-diffraction-limited marks within the readout spot. The formation of the aperture and the readout signal on the EPSR disk were analyzed. The feasibility of optically designed EPSR disks was evaluated by thermal simulation. A carrier-to-noise ratio of 32 dB at a mark size of 0.4 mum, 8 dB higher than that of a conventional disk, was obtained by application of a pulse-read method to the EPSR disks at a wavelength of 780 nm and a numerical aperture of 0.55.     

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.     

Slider-disk clearance measurements in magnetic disk drives usingthe readback transducer

A new method is described for noninvasively measuring the mechanical clearance between a recording head slider and the magnetic medium in hard-disk drives. The method is based on the detection of the pulse width of the read transducer output. A variation in clearance produces a proportional variation in pulse width. The proportionality factor can be determined by simulating the spacing loss using a digitized isolated impulse, typical for the respective head-disk combination. Instrumentation is presented that provides an output voltage proportional to the ratio (PW_x/T) of the pulse width at x% of the peak amplitude PW_x and the period T of the readback signal. This instrumentation measures the absolute slider-disk clearance by evacuating the air from the disk drive. The bandwidth is such that clearance dynamics can also be measured. This allows clearance measurements as well as the detection of undesirable slider-disk interactions, in situ, in fully operational disk drives     

High coercivity sputter-deposited maghemite thin-film disk

The fabrication and read-write characteristics of a high-coercivity sputter-deposited maghemite (γ-Fe2O3) thin-film disk medium is discussed. By employing a low sputtering gas pressure, severe internal strain is introduced into the films. This internal strain markedly increases coercivity. Furthermore, the films are composed of fine crystallites (300-400 Å in diameter), which result in extremely low media noise. A high coercivity (1060 Oe) γ- (Fe0.954Co0.02Ti0.02Cu0.015)2O3thin-film disk medium with a thickness of 0.095 μm exhibited superior read-write characteristics (e.g., a 2493 FRPM recording density D50and a 42 dB signal-to-noise ratio). These results show that sputter-deposited maghemite films have a promising potential for use as high-density disk media.     

An experimental investigation of the effect of medium thickness and transducer spacing on the read-back signal in magnetic recording systems

An experimental investigation has been made of the dependence of the read-back signal on medium thickness and read transducer spacing for magnetic recording particulate disk file systems using saturation recording. The novel technique employed permits the study of the effects of transducer spacing and medium thickness without the need for changing either the medium specimen or the read transducer. For the ranges of transducer spacing (1.25-6.0 μm) and medium thickness (0.0- 1.75 μm) investigated, excellent agreement with theoretical predictions was found for peak signal amplitudes and half-pulsewidths of isolated pulses. It was also found that the -6 dB pulse packing density, calculated from half-pulsewidth measurements of isolated pulses, agrees well with experimental results.     

Tunneling magnetoresistive heads for magnetic data storage

Spintronics is emerging to be a new form of nanotechnologies, which utilizes not only the charge but also spin degree of freedom of electrons. Spin-dependent tunneling transport is one of the many kinds of physical phenomena involving spintronics, which has already found industrial applications. In this paper, we first provide a brief review on the basic physics and materials for magnetic tunnel junctions, followed more importantly by a detailed coverage on the application of magnetic tunneling devices in magnetic data storage. The use of tunneling magnetoresistive reading heads has helped to maintain a fast growth of areal density, which is one of the key advantages of hard disk drives as compared to solid-state memories. This review is focused on the first commercial tunneling magnetoresistive heads in the industry at an areal density of 80 approximately 100 Gbit/in2 for both laptop and desktop Seagate hard disk drive products using longitudinal media. The first generation tunneling magnetoresistive products utilized a bottom stack of tunnel junctions and an abutted hard bias design. The output signal amplitude of these heads was 3 times larger than that of comparable giant magnetoresistive devices, resulting in a 0.6 decade bit error rate gain over the latter. This has enabled high component and drive yields. Due to the improved thermal dissipation of vertical geometry, the tunneling magnetoresistive head runs cooler with a better lifetime performance, and has demonstrated similar electrical-static-discharge robustness as the giant magnetoresistive devices. It has also demonstrated equivalent or better process and wafer yields compared to the latter. The tunneling magnetoresistive heads are proven to be a mature and capable reader technology. Using the same head design in conjunction with perpendicular recording media, an areal density of 274 Gbit/in2 has been demonstrated, and advanced tunneling magnetoresistive heads can reach 311 Gbit/in2. Today, the tunneling magnetoresistive heads have become a mainstream technology for the hard disk industry and will still be a technology of choice for future hard disk products.     

Performance estimation for maximum-likelihood detection forchannels with transition noise

Transition noise is known to be a major cause of errors for high density magnetic recording. This noise is signal dependent and can be modeled as multiplicative noise in a linear channel model. A maximum-likelihood algorithm was considered for detection of signals in such noise. In this work, the performance of the detector, based on this algorithm, is compared to the traditional Viterbi algorithm (VA) and a modified Viterbi algorithm (MVA) by computer simulations. Results show an improvement of up to 5 dB In signal-to-noise-ratio (SNR) under typical conditions with a reasonable complexity     

Fabrication and MFM study of 60 nm track-pitch discrete track media

Discrete track magnetic recording media with a 60 nm track pitch and prewritten servo patterns were fabricated and tested for read/write performance, and a feasibility analysis of the embedded servo was performed. The fabrication process consisted of ultraviolet nanoimprint lithography (UV-NIL) and sequential ion beam etching on a conventional perpendicular magnetic recording medium. Magnetic patterns were written to the fabricated tracks at 700 kilo flux changes per inch (kFCI) using a spin stand and were read using magnetic force microscopy (MFM), with a resulting signal-to-noise ratio (SNR) of 12.15 dB. The servo pattern was also visualized with MFM. These results demonstrated the feasibility of writing to a 30 nm wide discrete data track and the workability of the embedded servo pattern.     

A technique for measuring pole tip saturation of low inductance heads

Increasing the coercive force of a magnetic recording medium normally improves high density digital performance. However, in rigid disk systems, the head is not in intimate contact with the disk. In addition, the ferrites employed as head materials have much lower saturation magnetization than the metals normally used in other types of heads. Under these conditions, the head field may be inadequate to fully saturate recording media of higher than normal coercive force. In the development of the latest disk products, increasing the coercive force has not improved performance but has increased overwrite modulation. This situation has not been improved by increasing write current amplitude. Pole tip and core saturation of the record head has been suspected as the cause of these observations. This paper describes a method of characterizing saturation effects in low inductance heads such as those used with rigid disks. Evidence of the deterioration of performance due to pole tip and core saturation is shown from isolated pulse measurements on a rigid disk with NiZn and MnZn ferrite heads.