Media requirements and recording physics for high density magneticrecording

Relevant aspects concerning the ultimate achievable recording densities for particulate as well as for thin-film media are discussed. This review covers the entire range starting from micromagnetics of individual single domain particles, moving on to their magnetic behavior in a particle assembly under particular consideration of the structure being actually obtained in the process of manufacturing recording media, and finally embarking on an outline of recording physics. These considerations are not only carried out for longitudinally and perpendicularly oriented recording media but also for media having an arbitrary orientation of the easy axis of magnetization. All aspects are discussed and illustrated for the first commercially available thin-film medium on a flexible substrate, which is the metal evaporated tape, i.e., the obliquely deposited Co-Ni-O layer for the Hi8 video system     

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     

Considerations for applying solid state sensors to high density magnetic disc recording

To compete with optical recording densities of 10⁷or greater bits per square centimeter, equal magnetic disc areal densities must be achieved. Based on signal to noise considerations, inductive sensing of magnetic transitions written on magnetic media must give way to solid state magnetic sensors. There are fundamental considerations involved with implementing solid state sensors as read heads. These considerations embrace the basic sensor characteristics of size, resolution, sensitivity, signal to noise ratio and frequency response. Other equally important considerations involve packaging and head-wear resulting from head-media proximity demanded by the nature of the field to be sensed from the media. Finally, the solutions to these considerations must be simple and inexpensive.     

Shielded magnetoresistive head for high density recording

The fabrication process and the head material properties for shielded magnetoresistive heads with planarized lower shields using a tri-layered MR element are described in detail. Applying the etch-back process with low molecular weight polystyrene and CF₄/O₂ reactive ion etching, the residual step height for a lower shield is dramatically decreased to less than 5% of the initial step height. The tri-layered MR element consists of an MR layer, a magnetic separation layer (MSL), and a soft adjacent layer (SAL). 40-nm thick Ni ₈₁Fe₁₉ (wt.%) films were deposited by evaporation for use as an MR layer. Evaporated Ti MSL thickness was experimentally determined to be 20 nm. Amorphous Co₈₂Zr₆Mo₁₂ SAL films exhibited preferable magnetic properties as an SAL material. The fabricated shielded MR heads, using the tri-layered MR element with these NiFe, Ti, and CoZrMo films, provide superior capability to realize high recording density     

Position sensing for high track density recording

Progress towardhigher digital areal densities is related to the data track position sensing accuracy obtainable in future disk drives. In most systems available, position sensing is done remotely from the data track and subject to different dimensional stability conditions. To obtain higher track densities, position sensing reference information must be moved near the data track. Several techniques are available to embed reference information in the data track. All the techniques described have one of two basic principles of operation. One method uses comparison of reference signal amplitudes and the second method uses arrival time differences of reference signals. The time method is relatively unknown but offers the designer some new alternatives. This paper describes an overview of several methods. Relationships are derived to relate the influence of noise on both basic methods. Linearity, relative hardware simplicity, and track capture range are also discussed. It is our conclusion that the Amplitude or Tri-Bit method offers the greatest hardware simplicity, but is limited by noise and non-linearity at high track densities. The time dependent or Chevron method requires two data channels, but offers a relatively higher noise immunity, better linearity, and higher data capacity.     

超高密度磁存储的展望

磁性存储是最常用的海量存储技术,其记录密度越来越高,发展也越来越快。本文通过对信息记录、读出和存储三个过程的分析,对比了硬磁盘记录、垂直磁记录和磁光记录的优缺点,指出了采用垂直记录模式、非晶结构合金薄膜或铁氧体薄膜介质是实现超高密记录的方向,光辅助磁记录是很有希望的记录技术。同时,还指出量子磁盘技术是未来高密记录的方向。     

The physical limits of high density recording in metallic magnetic thin film media

The fundamental physical limits of high density, saturation recording in high hysteresis loop squareness magnetic isotropic metallic media have been investigated based on observations of micromagnetic structure of recorded Co-Re thin films. Study based on the configuration of the "feather-like" ripple structures of the Lorentz TEM image at the transition region and electron deflection shadow image of the surface field configuration of the recorded media indicate that the physical limit of high density recording in the metallic media is the size of magnetic clusters existing in the film. For a Co-10 at % Re film with Hcbetween 200 to 500 oe., thickness of about 50 nm and S* > 90%, the maximum linear packing density the film can provide, as determined from the cluster size, would be less than 25,000 FRPI.     

Corrosion-resisting Co-Pt thin film medium for high density recording

This paper describes a new material medium for high density longitudinal recording. Sputtered Co-Pt thin films will be shown to have excellent corrosion resistance and magnetic properties. Co-Pt thin films do not need a thick overcoat like plated Co-Ni-P films do, and have higher remanent flux density than ferrite thin films. Co1-xPtx(X=0-0.60) thin films prepared by r.f. diode sputtering have a maximum Hc value near X=20. The Hc, Bs and squareness, for 20 at.% Pt film are 1,100 Oe, 12,000 G and 0.80-0.90, respectively, at 0.1 μm film thickness. These values are not changed over 1-15 Watt/cm²power densities, corresponding to 6-85nm/min deposition rates. Films with more than 28 at.% Pt have no Bs change after immersion in water for over one month, indicating that the films are passive by this test, at least. Ni additions improve magnetic and corrosion properties. There is no Bs change for Co0.070Ni0.010Pt0.020films after immersion in water for over one month. Finally, 51 KFRPI linear recording density was obtained, at D50, using a Co0.70Ni0.10Pt0.20thin film disc with a 0.46 μm gap length head and a 0.12 μm head-medium spacing.     

Self-assembled nano-size FePt islands for ultra-high density magnetic recording media

To find a method to form nano-size FePt alloy for ultra-high density magnetic recording media, this work concentrated on the formation mechanisms of nano-island FePt films on amorphous glass substrates. FePt films of different thicknesses (1-10 nm) were deposited on amorphous glass substrates and post-annealed at 700 °C for 10 and 30 min. The configuration of the film changed during the annealing process due to the surface energy difference between the glass substrate and FePt alloy. Investigation of the microstructures and magnetic properties of the ordered L1₀ FePt films revealed that the 1 nm FePt film annealed at 700 °C for 10 min had perpendicular magnetic anisotropy and good reproducibility of forming well-separated FePt nano-size islands for ultra-high density magnetic recording media.     

The promise of new particulate media for high density recording

The superiority of perpendicular recording derives from the very low demagnetization at high bit densities, and from the nearly perfect writing process when a single pole head is used in combination with a double layer medium. Recent experiments have shown that it is possible to record very high densities in the longitudinal recording mode by scaling down all the critical parameters to extremely small values. However, such extreme scaling will very likely be accompanied by some very difficult problems from the point of view of media imperfections, defects, yields and costs. The power of perpendicular recording derives in part from the ability to attain these very high bit densities without resort to extreme scaling of the critical system parameters. There is little doubt that in the long run perpendicular recording will predominate because of its superior performance derived from the advantages stated above. For the next several years, however, we have to look to new and improved particulate media (to satisfy the majority of the demands) which can be fabricated by using existing large capacity continuous web coating facilities. The best choice for satisfying the requirements of these tape-related large volume applications is to utilize the new particulate media which support a large degree of perpendicular magnetization (isotropic-high squareness, and perpendicular anisotropy particulate dispersions) rather than employing very high coercivity longitudianally optimized particulate media.     

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.     

Media for erasable magnetooptic recording

Amorphous rare-earth-transition-metal alloys are considered as materials for magnetooptic information storage. They can be prepared by evaporation or sputtering on glass or polymer substrates. The alloys are ferrimagnets and exhibit a uniaxial magnetic anisotropy. The magnetic and magnetooptic properties can be well tailored by the composition as well as the deposition conditions. The information is stored by memory magnetic domains which can be written by a thermomagnetic switching process. The reading process utilizes the magnetooptic Kerr effect. In both cases the temperature profile of the saturation magnetization, the uniaxial anisotropy, and in particular the coercivity are of primary importance. At present, the most prominent candidates for device applications are GdTb-FeCo and Tb-FeCo alloys. Carrier-to-noise values up to 61 dB (30 kHz) have been achieved using magnetooptic disks     

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.     

High density magnetic recording on a mini flexible disk drive

Small flexible disk drives have developed in a few years from 48 TPI (tracks per inch) single density, 125 kbyte products to 96 TPI double density, two-sided drives with 1 megabyte of capacity. The trend is toward more tracks per inch, with the industry goal currently at 144 TPI. The paper describes the technology that has been developed to make 144 TPI possible. This paper will discuss the evolution of drive technology, and point out some of the areas that still need work in order for 144 TPI to become a reality.     

Experimental studies of longitudinal and perpendicular high density recording

Single layer and double layer Co-Cr disks of various coercivities were sputter-deposited on rigid substrates and magnetic parameters measured. Record and playback properties were studied using both ferrite and thin film heads under identical system environments. A well optimized Ni-Co plated longitudinal disk was used as a benchmark throughout this investigation for direct comparison. With the objective of using "off the shelf" ring heads to bring up the perpendicular recording technology on rigid substrates, it was found that the performance of both our preliminary single and double layer Co-Cr perpendicular disks were at least as good as the well optimized longitudinal disk. The double layer disks have an added advantage of lower write current. Signal processing via Hilbert transform using both rectangular and Hamming windows was also studied and applied to the output waveforms.     

Q.P., an improved code for high density digital recording

This paper describes a new digital recording code format, Quadra-Phase (QP), designed for high density applications. QP is well suited for use on bandpass channels, such as a tape recorder channel, because its frequency response spectra matches the recording channel. This code is described and compared with other common PCM codes. The encoding and decoding processes are presented, and eye-patterns and spectra of QP and other codes are shown. QP achieves the same packing performance as NRZ, without the need for very low frequency channel response.     

Electrographic magnetic stylus recording; A high speed non-impact magnetic printing process

An electrographic magnetic printing process is described which uses magnetically attractable and electronically conductive dry toner material deposited directly onto a dielectric layer in response to electronic current flow from one member of an array of magnetically permeable styli. The imaging process is extremely flexible in imaging speed, dot/mm resolution, continuous grey scale, and materials.     

Noise in high performance thin-film longitudinal magnetic recording media

The problem of noise in thin-film longitudinal media is analyzed experimentally and theoretically. The physical mechanism for the noise is shown to be fluctuations in the geometry of the zig-zag transitions separating bit cells. The shifted-transition noise model is introduced as a means of quantifying the noise processes. Spatial, spectral, and autocorrelation properties are introduced. A calculation of the RMS noise voltage yields the characteristic noise versus density curves found experimentally, and clarifies their interpretation with respect to the signal-to-noise ratio. The corresponding experimental data for several plated and sputtered media are presented and analyzed in the light of the model predictions.