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...).     

Analytical modelling of perpendicular recording

An analytical model for perpendicular magnetic recording is presented which is capable of calculating 'ab-initio' the magnetisation distributions written by an 'arbitrary' head field into both single and double layer recording media. Magnetisations are shown to follow the head field distribution existing at the medium's top surface. Application of the theory allows prediction of many of the fundamental features of the perpendicular write/read cycle, e.g. transition widths, replay pulse shapes, D50values etc.     

Possibilities of perpendicular magnetic recording

The potential of perpendicular magnetic recording using a single-pole head and a double-layered medium has been investigated theoretically by computer analysis and compared with that of longitudinal magnetic recording. In conventional longitudinal recording, a recording demagnetizing loss due to the change of magnetization mode from semicircular to circular shapes occurs with increasing recording level at high bit density. In perpendicular magnetic recording, the perpendicular magnetization mode is maintained regardless of recording level even at an extremely high bit density of 571 kFRPI. This indicates that the perpendicular recording medium has a very high recording resolution, where a single bit size approaches several diameters of the microcrystalline particles of the Co-Cr layer. An ultrahigh density at which the recording area for 1 bit will reach 1 μ² at present and 500 Å₂ in future should be possible     

Interpretation of spectral response in perpendicular recording

A previously published voltage spectrum in perpendicular recording is analyzed here utilizing the exact fields of a probe head and keepered medium. It is concluded that sharp transitions have been written at a negligible spacing. Previous analysis which utilized a duality between a ring head and a probe-keeper head yielded a non-zero spacing or recorded transition length. Measurements of D50for varying record geometries are analyzed in terms of scaling laws.     

On the transition length in perpendicular recording

Using the analytical model previously described by the authors [1],[2], magnetization transitions and transition lengths are calculated as a function of relevant physical parameters. When a probe head is used to record on a medium with a permeable under-layer, the readback voltage and the transition length are determined mostly by the head-medium separation. The calculated results agree with experimental observations of the variation of high frequency readback voltage with medium thickness.     

The switching criterion in perpendicular magnetic recording

This paper presents a theoretical analysis of the effect of the "switching criterion", the level at which self-consistency is assumed in calculations on the perpendicular magnetic recording process. It can be proven that in a perpendicular recording configuration with an ideal keeper layer and a recording layer with a rectangular hysteresis loop, the switching criterion in stand-still recording situations is immaterial, because self-consistency is reached at all depth levels simultaneously. If either the keeper layer is absent, or the recording layer's hysteresis loop is sheared, it is shown that the higher the level at which self-consistency is assumed, the sharper the stand-still recorded transitions will be.     

Writer pole tip remanence in perpendicular recording

We studied the remanent field from perpendicular writer pole tips by micromagnetic modeling. The pole tip remanence has two causes: 1) the residual flux from the yoke due to undesirable material properties and/or yoke design and 2) the geometry of the pole tip. An optimal yoke design for low remanence includes a wide and short yoke with relatively shallow yoke flare angle around 30/spl deg/ that favors transverse major domains. Horizontal anisotropy in the yoke and the pole tip also helps suppress the vertical magnetization, mainly in the yoke, which yields less residual flux into the pole tip, hence lower remanence. The remanent field is also a strong function of the throat height, i.e., the pole tip length, due to the shape anisotropy effect. Micromagnetic modeling shows that multilayer lamination, both in the yoke and in the pole tip, is an effective way to achieve low remanence. It also eliminates the sensitivity of remanence to the throat height. To improve the robustness of the remanence against the stray field, an antiferromagnetic coupling (AFC) between the lamination layers may also be necessary. Our results apply to both single pole heads and shielded pole heads.     

Magnetic alumite disc for perpendicular recording

Details of making are given of perpendicular magnetic alumite, discs obtained by an aid of anodization with the additional new process named "pore widening" which was effective to control the coercive force. The electrodeposited fine iron needles were in single crystalline state. The perpendicular orientation of magnetization was confirmed. The magnetic recording, characteristics of rigid discs indicated a hilgh potential for use as a perpendicular recording medium.     

Wavelength response of perpendicular magnetic recording

From the measurements of the surface field of a recording medium, it was obtained that an almost ideal step change distribution of magnetization is formed in perpendicular recording for digital signals. The fact represents that the upper limit of usable recording density is determined only by the resolution of a reproducing head. Therefore, the reproduction by a perpendicular head was successfully analyzed by using the reciprocity theorem. By introducing thickness loss of main pole of the head, the satisfactory coincidence was obtained between the calculated and the measured results in the wavelength response even at the densities of more than 150 kMRPI. In reproducing process, the magnetic interaction between the main pole and the medium acts so as to decrease the effective spacing between them.     

Exchange-biased soft underlayers for perpendicular recording

We inserted NiFe/CoFe/antiferromagnetic-MnIr/CoFe layers between two CoTaZr soft layers to enhance the exchange-bias field (H/sub eb/) and then evaluated the effect of this lamination on the spike noise and recording characteristics of CoCrPt-SiOx media with an exchange-biased soft magnetic underlayer (SUL). The two CoTaZr layers were exchange-biased radially throughout the disk, and a higher H/sub eb/ was obtained for the upper CoTaZr layer. By using the laminated SUL, spike noise was suppressed even when the total thickness of the CoTaZr layers was increased to 300 nm. Although the medium had a high H/sub c/ of 7.0 kOe, a fairly good overwrite and signal-to-noise ratio were obtained. As another application of exchange biasing, we also examined the possibility of combining exchange biasing and antiparallel-coupled (APC) soft layers; i.e., a pinned APC SUL. An exchange-bias field from the pinning layers to the lower CoTaZr layer and an exchange-coupled field between the two CoTaZr layers were successfully applied. The medium with the pinned APC SUL showed no spike noise throughout the disk, and wide-area adjacent track erasure was effectively suppressed.     

Composite media for perpendicular magnetic recording

A composite perpendicular recording media consisting of magnetically hard and soft regions within each grain is proposed. Application of applied field initially causes the magnetization of the soft region to rotate and, thus, change the angle of the effective field applied to the hard region. This important change in the effective field is enabled by an exchange layer that moderates the interaction between the two regions. Energy arguments show that the resulting performance (as measured by the ratio of energy barrier to switching field) is similar to the previously proposed tilted media, while avoiding some of the difficulties. In particular, fabrication of the proposed composite media appears to be significantly easier than that of tilted media.     

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     

Fundamental properties of perpendicular magnetic recording

A two dimensional computer simulation model of the magnetic recording process has been, developed to assess the fundamental properties of perpendicular magnetic recording. It was found that vertical anisotropy media are well suited to abrupt and closely spaced magnetic transitions. The demagnetizing fields support short transitions in the perpendicular mode as opposed to degrading transitions in the longitudinal mode. However, gapped heads may have difficulty in delivering high intensity vertical fields to saturate the medium. Novel write head designs might be required.     

Analytical solution for side-fringing fields in perpendicular recording

Through the use of conical coordinates and in the limit of small pole lenght, flying height, and medium thickness, the three-dimensional field from a beveled probe-type recording head with a permeable magnetic underlayer has been determined in a simple closed form. For a given bevel angle the spectral response to perpendicularly magnetized media off the side of the head depends on one dimensionless parameter. The field solution is used to find the quasi-exponential falloff of this response as a function of the bevel angle. For a large bevel angle this recording geometry is shown to be less sensitive to side pickup than a conventional gapped recording head.     

High-anisotropy nanocluster films for high-density perpendicular recording

This paper reports results on the synthesis and magnetic properties of L1/sub 0/:X nanocomposite films, where L1/sub 0/=FePt, CoPt, and X=C, Ag, etc. Two fabrication methods are discussed: nonepitaxial growth of oriented perpendicular media, and monodispersed nanoparticle-assembled films grown with a gas-aggregation source. The magnetic properties are controllable through variations in the nanocluster properties and nanostructure. The films show promise for development as recording media at extremely high areal densities.     

Wear properties of perpendicular magnetic recording media

Wear properties of sputtered Co-Cr/permalloy double layer on flexible substrate with and without an inorganic protective top layer were studied under simulated tribological conditions. A soft head slider (ex. glass) gave even severer damage to media than the hard one (ex. Al2O3-TiC), and a 200 Å thin inorganic protective top layer was effective enough to enhance resistance against wear. Read-write experiments were also done with use of the double layer flexible media with the protective top layer (200 Å in thickness) by running them on a specially designed driver unit with an Al2O3.TiC spherical SPT head. An out-put signal loss was no more than 3dB even after 10 million times of the head passes.     

Noise of low-coercivity single-layer perpendicular recording media

The noise of single layer CoCr media is investigated. A model is developed by treating the medium as a collection of columns. It appears that there is a strong correlation between magnetization direction of the columns. Neighbouring columns tend to have the same magnetization polarity, indicating exchange interaction. It is concluded that the media consist of domains. The domain width increases with the medium thickness. These domains can also result in a poor signal response at short wavelength.     

Computer modeling of the write process in perpendicular recording

A self-consistent computer simulation model utilizing a ring head and single layer recording medium has been developed to study perpendicular recording. In the model it is assumed that the medium switches uniformly following the volume averaged field. The optimum recording field obtained from the model calculation at various head-medium spacings permits an estimation of head saturation limitations. The roll-off curve, which is the linear superposition of the alternating voltage pulses, is in good agreement with experimental data. The write spacing loss is spacing dependent and is 120d/λ (dB) at contact for a typical recording case. For contact recording the imaging effect is important in the write process. Optimum recording at contact is shown to produce a high overshoot and sharp leading edge in the transition while leaving the magnetization level below the coercivity; this yields an apparent "negative" transition length in terms of the arctangent transition model.     

Co-Cr perpendicular magnetic recording tape by vacuum deposition

The relation between the incident angle and the crystallographic orientation of a vacuum deposited Co-Cr film is discussed. Also presented are the magnetic properties and the orientation of both a Ni-Fe underlayer and the Co-Cr film for the double layer medium, and the experimental results about the composition distribution in the co-cr film. The films were deposited on a transporting polymer substrate by continuous vacuum deposition. It is found that the orientation of the Co-Cr film is determined only by the incident angle at the initial point of the film formation, and that deposition efficiency more than 50% can be achieved easily. A double layer medium with Ti film under the Ni-Fe film (Co-Cr/Ni-Fe/Ti medium), which is suitable for perpendicular magnetic recording, is produced by vacuum deposition. Auger depth profile in radial direction of the column of the Co-Cr film shows directly that there is Cr segregation near the columnar grain boundaries.     

High-density perpendicular recording media with large grain separation

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).