A Co/Pd multilayer film with perpendicular coercivity of 2.2 kOe and remanence ratio (SQ) of unity was prepared by electron beam evaporation in vacuum. In the MFM image of signal patterns of 4 kFRPI recorded using a ring-type MIG head, many reversed domains were observed. However, when the film was magnetized along the film normal direction using an electromagnet (H = -13 kOe), only few reversed magnetic domains were observed, which was consistent with SQ = 1. Therefore, the reversed domains in the signal patterns were induced in the recording process. dc erasing was also studied with the magnetic field inclined to the film normal. The domain structures were almost the same when the perpendicular component of the field was kept constant while the in-plane component was varied, implying that the in-plane field component did not contribute to the formation of the reversed domains. It was found that reversed magnetic domains were easily induced even by a weak reversing magnetic field applied along the film normal. Hence, although the possibility of an insufficient recording head field was not excluded, it seemed more likely that the reversed magnetic domains in the signal patterns were caused by some erasing effect of the ring-type MIG head. For a Co/Pd multilayer medium with a negative nucleation field in the perpendicular M-H loop, a stronger reversing field was needed to induce the reversed magnetic domains. No reversed magnetic domains were observed in the MFM image for signal patterns of 4 kFRPI in this medium, indicating that a negative nucleation field was effective to suppress the formation of reversed magnetic domains.

The origins of baseline shift were discussed considering the measured off-track properties using a wide write head with track widths of 97 µm and a narrow read head with track widths of 2.7 µm. The baseline shift increased when the read head was moved close to the track edge. Beyond the track edge, baseline shift decreased to negative values. The impulse response curve of the MR head to the perpendicular magnetization was estimated from the readback waves of the MIG head and the MR head. The response curve depended on the recorded track width. When the recorded track was narrow, the undershoot of the response curve was smaller than that of the head field based on the 2D double-gap ring head model with infinite track width. This small undershoot induces sensitivity of the DC-component of the recorded magnetization and causes the baseline shift. To calculate the readback waves of the MR head for single-layer perpendicular recording media with narrow-track recording, the effect from stray field at the recorded track edge must be included in the impulse response curve of read head.

The medium noise of single-layer perpendicular recording media is known to be suppressed by reducing the magnetic domain size and achieving a higher squareness ratio (Mr/Ms = SQ) in the perpendicular M-H loop. The media with smaller domain sizes exhibit a small slope at Hc in the M-H loop due to exchange de-coupling between adjacent grains, which requires a sharp head field to acquire high recording performances. Reduction of the medium thickness would be effective for recording as only a sharp head field near the head surface could be used. Thus, the effects of reduced recording layer thickness in single-layer perpendicular recording media on read/write performances were investigated using Co/Pd multilayer media with a small loop slope having thickness, δ, of 46, 22 and 10 nm, and with a steeper loop slope having δ of 40 and 10 nm. It was found that the recording performance on small loop slope media could be improved in terms of signal level by reducing the recording layer thickness, which indicated that the recording on the media was sensitive to the recording head field. The results in the simulation analysis were similar to those obtained experimentally, indicating that the change in recording layer thickness could be mainly regarded as that in the head-medium spacing. Thinner media with steeper loop slopes could acquire a narrower dipulse width. The recording resolution of the present media, however, was determined under the influence of the domain structure and the size. Finally, for media with small loop slopes, the same SNR of 38 dB at 100 kFRPI was obtained for thicknesses of 22 and 10 nm, which was larger than that for a thick medium of 46 nm thickness by 8 dB. For both the steep loop slope media, the obtained SNR was 35 dB at 100 kFRPI.

Effect of dispersions of medium parameters and structure on the recording performance was systematically investigated. Moderately increased M-H loop slope is effective for obtaining higher thermal stability, smaller saturation fields, and higher resolution. It was found that the most influential factor is the dispersion in anisotropy field, Hk. Small Hk dispersion reduced the noise when exchange coupled media were used. Reduced grain size and a stacked structure of the media were expected to give a restricted gain in the signal to noise ratio.

Shift margins in down and cross track directions and skew angle were investigated using micromagnetic simulation with a shielded planar head for patterned media with an areal density of 1 Tbit/in2. The shift margins were quantitatively estimated using parameters of the head field and the magnetic properties of media. It is essential to use a head with a higher field gradient and a medium with a small field width between saturation and nucleation fields, to obtain a larger down track shift margin, and a head with a narrower cross track field distribution to obtain a larger cross track shift margin and skew angle margin.

Physical limitation of the conventional longitudinal media has been actualized recent days according to the dramatic increase in storage area density with a growth ratio of more than 100%/year so that perpendicular recording is being watched with keen interest. Development in perpendicular recording media from the proposal to actual usage for HDD system are reviewed in terms of historical back ground, recent media design and preparation, new media proposal and recording performances, mainly basing on the author's results. In case of perpendicular media, physical limit of thermal stability could not be so serious but media noise reduction with sophisticated preparation method has been research topics as the breakthrough for the commercialization. Improvements on Co-Cr system alloy media were made by introducing proper additives and under layers so that a high storage density over 60 Gb/inch2 has been achieved. Far higher density up to 1 Tera bits/ inch2 is expected to be possible by using newly proposed Co/Pd multilayer or Fe-Pt metal compound films. It is prospected that breakthrough for the future progresses would be mechanical issues such as head medium spacing and tribology problems rather than magnetic properties of media.

A modified frequency domain method for analyzing nonlinear waveform distortion in a magnetic recording process is presented. The measurement technique combines a 5th harmonic measurement technique, which uses a specific 30-bit pattern including dibits, and a precompensation technique for the dibits. The 5th harmonic voltage ratio given by the former technique includes the amount of NLTS (Nonlinear transition shift) and PE (Partial erasure) in dibits. The latter precompensation technique is employed to evaluate the PE as the minimum in the 5th harmonic voltage ratio. The true NLTS can be estimated from the amount of distortion and the evaluated PE. The high accuracy of the technique was confirmed by an examination using a pulse pattern generator with varied phase and amplitude. Finally, the effects of medium properties such as coercivity and squareness on the nonlinear distortions have been investigated by applying the technique to particulate flexible media. The NLTS increased with squareness from 3.5% to 7% while PE was less than 6% for any squareness at a recording density of 76 kFRPI. When coercivity became large, NLTS and PE decreased. The direction of NLTS for Ba-ferrite media agreed with that for a perpendicular Co-Cr thin-film medium.

Time decay of magnetic properties in perpendicular magnetic recording media was studied. It was suggested that magnetization in media with a low energy ratio, KV/kT, of 50 is thermally stable in the absence of a demagnetizing field while coercivity exhibits a large time dependence. Magnetization in perpendicular recording media exhibited an appreciable time decay even for films with a large energy ratio of 300. The decay is attributed to the small perpendicular squareness due to a large perpendicular demagnetizing field acting in the media. The recording density dependence of the time decay in the output was explained in terms of the change in the demagnetizing field with the density. It is concluded that the use of media with large squareness as well as large energy ratio effectively reduces time decay in the output.

Pole-tip-driven structure, which is composed of a coil wounded at the main pole tip, is favorable for obtaining a sharp and strong head field as a single-pole-type head. Three kinds of pole-tip-driven-type heads with different yoke and coil structures are investigated in terms of magnetomotive force dependence of head field and effect of coil recession. Field calculation by finite-element method (FEM) showed that the three heads exhibited the same field sensitivity in spite of the difference in distribution of coil exciting field and magnetization of the main pole. In a lower range of magnetomotive force the heads showed different dependence of field sensitivity on the coil recession. However, there was not much difference in degradation of sensitivity in a region near the saturation of field. Thus, the importance of reducing coil recession was confirmed as reported earlier.

Ring head recording on single-layer perpendicular recording media was studied by a simple simulation analysis based on a loop tracing method considering only the perpendicular component. Although the assumed model was primitive, the simulation results qualitatively well explained the experimental results such as a decrease in output at high recording currents and its relaxation upon using a smaller gap-length head. The simulation results revealed that achievable recorded magnetization is, in general, much smaller than the saturation value due to a broad distribution of the ring head field, but a medium with a steeper slope in the perpendicular M-H loop could improve the recording performance. This was confirmed experimentally for the medium with a steeper loop slope, though the medium exhibited a larger medium noise at high densities. It was suggested that the development of perpendicular recording for higher output and lower noise could be performed for both media with a small and steep loop slope. The former should be improved by means of the recording head while the latter by the media. A large improvement is expected for both cases.

To develop ultra high density magnetic recording systems, a quasi-static system with a reciprocating medium motion against a stand still head was developed for a read/write tester. Double-layered perpendicular recording media and merged GMR heads assembled on a conventional head-gimbal assembly (HGA) were applied to evaluate the read/write performances. A smooth sliding contact motion was achieved, however, the head-to-medium spacing was varied depending on the head motion direction. The spacing in the reverse running direction of the conventional head slider seems to be smaller than that of the flying height in a high-speed spin stand. A merged ring head was suitable for perpendicular magnetic recording in the case of the reverse direction sliding.