Perpendicular magnetic recording — progress and prospects

The feasibility of high track density recording in a perpendicular magnetic recording system has been investigated. The properties of the magnetic field of a single pole type (SPT) head having a narrow track width were studied by using a large scale model of a head and a medium. The y-field gradient of the SPT head near the track edge is steeper than the x-field gradient of a ring head, and it was confirmed by Bitter technique that the recorded track width acculately coincides with the width of the main pole of the SPT head. Perpendicular recording using the SPT head also has superior off-track characteristics and the crosstalk signal is hardly detected even at very narrow track pitches. From a magnetics point of view, no problems exist in narrowing the track width of the SPT head in perpendicular recording. Narrow track recording and reproducing with SPT head of only 6.5 μm in track width could be achieved with sufficient signal to noise ratio in perpendicular flexible disk systems.

The soft magnetic underlayer (SUL) with high saturation magnetization in perpendicular magnetic recording media have been required essentially to achieve high recording density with high bit stability. However, stripe domain configuration caused by the induced perpendicular magnetic anisotropy and deterioration of surface smoothness in thick SULs degrades the recording performance. In this study, multilayers, with several kinds of interlayers for the SUL, were prepared in order to improve the soft magnetic properties and surface smoothness as well as reduce the media noise originated from the domain configuration.

The perpendicularly oriented crystal structure conditions of electroless plated film for high density perpendicular recording media were investigated mainly by (002) plane rocking curves and cross sectional SEM observation. A highly oriented narrow dispersion of c‐axis orientation normal to the surface plane was consequently obtained, and the columnar structure of hcp crystal with ca. 0.05 μm diam was confirmed. As a result of preparing disks A and B, which were chemically plated from an optimum bath and a more improved bath, respectively, the recording characteristics with a combination of ring head were obtained as recording densities of for disk A and 25,000 FRPI for disk B, in spite of a relatively thick magnetic medium. The di‐pulse waveforms, which are characterized with perpendicular recording, were observed at low bit recording density. It was indicated by the above results that the chemically plated Co alloy films are applicable as a perpendicular recording medium.

In order to enhance the perpendicular magnetic recording characteristics of an Fe-Co-O medium with a large perpendicular anisotropy, an Fe-Co underlayer with a coercive force of about 100 Oe was investigated. The voltage reproduced from the Fe-Co-O/Fe-Co double-layered medium with the most suitable film thickness is five times as large as that for a single-layered medium. However, an Fe-Co-O/Ni-Fe double-layered medium, the underlayer of which has a higher permeability, did not show such great improvement in reproduced voltage. These results suggest that the effect of the Fe-Co underlayer is different from the decrease of demagnetization field caused by the Ni-Fe layer in a Co-Cr/Ni-Fe double-layered medium. It was also verified that Fe-Co-O/Fe-Co double-layered flexible disks have a high wear resistance, with a lifetime of more than 10 million passes without a protection layer, because of the higher stiffness and stronger adhesive force between the Fe-Co layer and the polyimide substrate.

Effect of soft underlayer magnetic anisotropy on perpendicular recording process

Recording characteristics of low flying single pole head

The long-term bit error rate (BER) performance of partial response maximum likelihood (PRML) system in a perpendicular magnetic recording (PMR) channel with thermal decay is studied. A thermal decay model based on the experimental data for three CoPtCr-SiO2 PMR media with different thermal stability is obtained. Then, the performance of PR1ML channel for the 16/17(0,6/6) and 128/130(0,16/8) codes as a run-length-limited (RLL) code is evaluated by computer simulation using the model. Moreover, the relationships between the elapsed time and the BER are obtained for the combinations of two RLL codes and three PMR media and their performances are compared. The results show that for the combination of the 128/130(0,16/8) code and the medium with the poorest thermal stability, the degradation with the elapsed time is noticeable compared with the other combinations

Actual examples are given to show that low head impedance is extremely important in narrow track-width heads for high-density perpendicular magnetic video recording and reproduction, and that it strongly affects the wideband frequency characteristics and the noise level of the head. A main pole excitation probe-type head (MPD), with 7 μm track width, was fabricated to compare the dependence of frequency band and noise characteristics on head impedance. With high sensitivity and low impedance, the MPD head had a very favorable CN ratio of 54.5 dB, suggesting that in the future it will provide significant advantages for widebanddigital recording.

Poisson’s equation is solved exactly to obtain the medium demagnetization field for both a single pole head and a single pole with a trailing shield (shielded single pole) head designs in perpendicular recording. To write an isolated transition, for the semi-infinite single pole head, head imaging can be neglected when calculating the demagnetization field. To the semi-infinite shielded single pole head, when the pole separation is reduced, perfect imaging from both the head and the soft magnetic underlayer is a good approximation.

The dependence of recording performance on the medium parameters in a perpendicular recording system, composed of an auxiliary pole driven head and a double layer medium, is discussed. The soft magnetic back layer behaves as part of the head to enhance the magnetic interaction between the auxiliary pole and the main pole. For excessively thick back layer and Co-Cr layer, the reproduced output decreases due to shielding effect and weakened interaction. Since the main pole head parameters also closely relate to the recording performance, it was concluded that the medium and head design should be concurrently carried out.

In this paper we introduce barium ferrite tape samples with a high recording characteristic in the conventional longitudinal magnetization mode, and explain the origin of their high output voltage at high recording densities, with reference to the effect of the magnetic characteristic and the behavior of remanent magnetization. Longitudinally and non-oriented barium ferrite recording media were prepared using the same magnetic paint based on a single barium ferrite magnetic particle. The barium ferrite tapes thus prepared were inferior to ultra-high density metal tapes with regard to surface roughness and magnetic energy, but exhibit better recording characteristics at high recording densities above 80 kFRPI or so. In particular, the origin of the high recording characteristic of longitudinally oriented barium-ferrite tape samples may be explained by the effect of the sample remanence curves, rather than by perpendicular magnetization.

The possibility of 300-500 Gbit/in/sup 2/ perpendicular recording using granular recording media has been investigated through micromagnetic simulation based on the Langevin equation. Writability and thermal stability in 10 years were obtained changing media parameters such as the grain size D, the grain separation d, and the thickness of the recording layer t/sub mag/ for proper combination of the grain saturation magnetization M/sub s-grain/ and the grain perpendicular anisotropy energy K/sub u-grain/. It was found that high-density recording is realized under the large grain separation, the large grain saturation magnetization, and the large grain anisotropy energy. The read/write calculation using ordered medium with D of 4.2 nm, d of 2.3 nm, t/sub mag/ of 12.0 nm, M/sub s-grain/ of 1313 emu/cm/sup 3/, and K/sub u-grain/ of 7.0 Merg/cm/sup 3/ confirmed the possibility of 1303 kFCI and 1954 kFCI perpendicular recording, leading to 325 and 488 Gbit/in/sup 2/ with 250 kTPI (track pitch of 102 nm).

Pd cluster seeds for a double-layered perpendicular magnetic recording medium composed of a Co/Pd multilayered film and a CoZrNb soft magnetic underlayer (SUL) were fabricated with an electrochemical treatment. By immersing the SUL into an aqueous PdCl/sub 2/ solution, the substitution of Pd for Co at the surface of the SUL took place, which was caused by the difference in electrochemical redox potential between Pd and Co. Uniformly distributed Pd clusters with 8 nm in diameter were successfully formed with the treatment for the optimized duration. Use of the Pd clusters as the seeds for deposition of the Co/Pd multilayered film increased the perpendicular coercivity and reduced the size of magnetic cluster of the film. These results suggested that the Pd cluster seeds led to the formation of magnetically isolated Co/Pd multilayered grains, improving the magnetic properties of Co/Pd multilayered film.

Although there is a necessity to achieve excellent overwrite (O/W) in magnetic disk write‐read equipment, the O/W mechanism has not been clarified sufficiently. Accordingly, the investigation of the O/W mechanism by experiment and by a finite element method simulation incorporating a magnetization model has clarified that during O/W, not only the field of the head but also the field generated by the magnetization distribution of the medium (inside field) and rewrite action exert influence on O/W characteristics. From the fact that the inside field changes with the recording density of the signal, modeling was done on the write density dependence of O/W characteristics and verification was made by simulation and experiment.Moreover, from the viewpoint of O/W, a comparison was made of perpendicular magnetic recording and longitudinal magnetic recording. As a result, for the combination of ring head and longitudinal film medium, although the O/W characteristics are degraded with the increase of the O/W signal recording density, in perpendicular magnetic recording the experimental result that the O/W erased value takes a maximum value when the recording density of the overwrite signal is half that of the prerecorded signal in perpendicular magnetic recording is obtained. Thus, it became possible to clarify the mechanism by using a model taking into consideration the inside field.

The recording density that has already been achieved with particulate media undoubtedly exceeds even the most optimistic expectations of most researchers in the field, who, long before the present state of development, had anticipated their replacement by thin metallic coatings or by some new and exotic formulation of novel particles. The old work horse not only survives but continues to lead the pack and even to demonstrate new spritness and vigor everytime it comes around the track! And if one judges by the recent IBM announcements, the old horse will continue to dominate the field well into the next decade. To be sure of course, he is not running unassisted, since the rules of the race permit a formidable array of electronic, electro-mechanical, and other props to be used, and they have been applied profusely and to great advantage. In this paper we do some crystal-ball gazing in trying to look at the future and to assess the attainable extensions in recording density using particulate media. Our analysis is primarily aimed in delineating the role of the magnetic parameters and the thickness of the coating and thus determining the way they would have to evolve in order to accomodate higher recording densities. The fundamental magnetic limits are several orders of magnitude beyond the present state of the art (4700 bits/cm, 235 tracks/cm). Therefore, it appears most likely that future advancements will be paced by other considerations-smaller particles with narrower range of shape and size distributions, uniform and well-aligned dispersions, thinner and smoother coatings, new types of transducers with better fringing field gradients and improved short wavelength and high frequency sensitivity, better track-following servos, more efficient codes, and a host of electronic signal processing, filtering, waveform shaping and compensation techniques. We conclude that these non-magnetic considerations will ultimately impose the upper bounds in recording density, which are estimated to be less than an order of magnitude beyond the present achievements (to about 10,000 bits/cm and 500 tracks/cm). The realization of this potential, however, is going to be increasingly more difficult, and it is not unlikely that we may have to switch horses along the way and to use thin metallic media, perpendicular recording, or some other not yet invented innovation.