Two-track squeeze and adjacent track interference (ATI) are major barriers to increasing track density in hard disk drives (HDD). These depend on skew angles made by a magnetic head and circumferential direction on a magnetic disk. This paper describes relationships between the skew angle and the magnetic core width (MCW) which affects two-track squeeze and ATI performance. We propose a design concept of a track pitch profile at different skew angles considering MCW. Equivalent robustness of ATI performance on different skew angle conditions is obtained with the optimized track pitch.

In this paper, we propose a new method to estimate and effectively reduce the effect of thermal asperity (TA) in the perpendicular magnetic recording (PMR) channels with the state trellis. The TA is estimated from the state trellis, then its average is used to modify the equalized signal entering the Viterbi detector. For the partial response (PR) targets with DC component, the proposed method with a maximum-likelihood detector can improve the bit error rate performance by more than an order of magnitude when TA occurs and degrades when the giant magneto-resistive (GMR) nonlinearity and base line wander (BLW) effects are present. Unlike the previous studies, this method allows the use of PR targets with DC component under the presence of TA.

In this paper, we consider the behavior of an autoregressive (AR) detector for partial-response (PR) signaling against offtrack interference (OTI) environment in perpendicular magnetic recording. Based on the behavior, we derive the optimum branch metric to construct the detector by the Viterbi algorithm. We propose an optimum AR detector for OTI that considers an optimum branch metric calculation and an estimation of noise power due to OTI in order to calculate an accurate branch metric. To evaluate the reliability of soft-output likelihood values calculated by our proposed AR detector, we demonstrate a bit error rate performance (BER) of low-density parity-check (LDPC) codes under OTI existing channel by computer simulation. Our simulation results show the proposed AR detector can achieve a better LDPC-coded BER performance than the conventional AR detector. We also show the BER performance of our proposal can keep within 0.5 dB of the case that perfect channel state information regarding OTI is used in the detector. In addition, we show that the partial-response maximum-likelihood (PRML) detector is robust against OTI even if OTI is not handled by the detector.

The long-term bit error rate (BER) performance of partial response maximum likelihood (PRML) system using an adaptive equalizer in a perpendicular magnetic recording (PMR) channel with thermal decay is studied. A thermal decay model based on the experimental data giving the amplitude change of reproducing waveforms with the elapsed time for CoPtCr-SiO2 PMR medium is obtained. The BER performance of PR1ML channel for the 16/17(0,6/6) run-length-limited (RLL) code is evaluated by computer simulation using the model. The relationship between the ratio RJ of the jitter-like media noise power to the total noise power at the reading point and the required SNR to achieve a BER of 10-4 is also obtained and the performance is compared with that of the conventional equalization. The results show that the significant improvement in SNR by utilizing the adaptive equalization is recognized over all RJ compared with the conventional equalization.

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.

A reactive sputtering method using an Electron-Cyclotron-Resonance (ECR) microwave plasma was used to deposit Ni-Zn ferrite thin-films for a soft magnetic backlayer of Co-containing spinel ferrite thin-film perpendicular magnetic recording (PMR) media. The Ni-Zn spinel ferrite thin-films with a preferential orientation of (100) and a relatively low coercivity of 15 Oe were obtained at a high deposition rate of 14 nm/min and at a temperature below 200 degrees C. Although post-annealing treatment in air at 200 degrees C was effective to decrease the coercivity of the Ni-Zn ferrite thin-films, the saturation magnetization and initial permeability decreased and the surface smoothness was deteriorated simultaneously. The Ni-Zn ferrite thin-films prepared by ECR sputtering are promising as the backlayer of the perpendicular magnetic recording medium, but further improvement is required in terms of the soft magnetic properties, the grain size and the surface roughness.

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.

Co-containing ferrite thin-film/Mn-Zn ferrite thin-film double-layered perpendicular media were prepared using reactive ECR sputtering and magnetron sputtering methods, and their magnetic and structural properties and recording characteristics were studied. The Mn-Zn ferrite thin-film backlayer had saturation magnetization of 3.5 kG and coercivity of 60 Oe. Reproduced voltage for the Co-containing ferrite thin-film/Mn-Zn ferrite thin-film double-layered medium was about twice of that for the Co-containing ferrite single-layer medium.

Co-containing ferrite thin-film media deposited by a reactive-ECR-sputtering at a low substrate temperature of 150 degree Celsius were oxidized by ECR plasma. The magnetic properties and recording characteristics of the media were improved by the oxidation with maintaining a smooth surface. The media showed high D50 of 203 kFRPI in MIG head recording and reproduction. The Co-containing ferrite thin-film is feasible to be used as a protective overcoat layer.

In recent years, perpendicular magnetic recording have progressed rapidly. It will not be long before perpendicular magnetic recording is put into practical use. However there have been few tools contributing to the optimum design of perpendicular magnetic recording media and heads except computer simulations. The authors have introduced a simple method based on the concept of self-consistent magnetization to analytically predict a transition parameter in terms of parameters of recording media and writing heads. Moreover we have discussed the origin of media noise by using a time-domain analysis of readout voltage and Voronoi cell model analysis. In this paper, main parameters to realize high bit density recording over 100 Gbit/inch2 is discussed first through these methods, and then the current status, the future problems and the prospects in perpendicular magnetic recording technology are described.

Magnetoplumbite type Ba ferrite (BaM) disks were prepared on Pt and Pt-Ta underlayers using facing targets sputtering apparatus. Pt underlayers are more effective than ZnO underlayers to promote c-axis orientation of BaM layers, especially for extremely thin BaM films. Pt-Ta underlayer was used to decrease the grain size of BaM layers. BaM/Pt-Ta disks revealed larger S/N ratio than BaM/Pt disks because of their larger signal output and lower media noise level. BaM disks with 50 nm thick BaM layers revealed lower noise level and larger S/N ratio than that with 100 nm thick BaM layers due to smaller grain size.

Employing reactive sputtering using an electron-cyclotron-resonance microwave plasma without oxidation process, high coercivity ferrite thin-films with perpendicular magnetic anisotropy were successfully prepared without NiO underlayer at low substrate temperature. The ferrite thin-film deposited on glass substrate had smooth surface and were composed of small grains. Perpendicular recording was performed on the ferrite thin-film hard disk. The ferrite thin-films with high coercivity could be prepared on flexible film substrates (Polyimide and PET).

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.

Ferromagnetic Co77Cr20Ta3 layers were deposited on a Pt seed layer by a facing targets sputtering apparatus. The Co-Cr-Ta and Pt crystallites revealed better c-axis orientation at a substrate temperature Ts above 200C. Relatively high perpendicular coercivity Hc⊥ of 2.5 kOe was obtained for the bilayered film with a Co-Cr-Ta layer thickness, δCo, of 50 nm deposited at Ts of 250C. Although the Co-Cr-Ta/Pt medium with δCo of 100 nm exhibited lower recording density than a Co-Cr-Ta/Cr longitudinal one, its noise level became small at the high-density recording range. Measurement of the anomalous Hall voltage clarified that the bilayered film with δCo as small as 30 nm revealed larger perpendicular magnetization than the single layer. The Pt seed layer is effective for depositing thin ferromagnetic Co-Cr-Ta layers below 100 nm in thickness.

Nonlinear phenomena in perpendicular magnetic recording employing a single-pole head and a double-layered medium were investigated. First, measurement of linear superposition in the time domain indicated than the amount of nonlinear transition shift (NLTS) was less than 10 nm. It was concluded that the nonlinearity was caused by transition shift, not by waveform distortion. By interpreting the results, we proved that the NLTS was strongly related with head field gradient and interference field from recorded magnetization. Dependence on head parameter was examined by experiments. Based on the results, a single-pole head with which transition shift can be reduced was proposed. Pseudo-random sequence analysis revealed that NLTS was several percent even at 318 kFRPI, or at a bit interval of 80 nm, which agreed with the result of measurement of linear superposition in the time domain analysis. Experiments showed that NLTS increases the shortest bit length, in contrast with the case of longitudinal recording.

Extremely narrow track width of deep submicron range is examined in perpendicular magnetic recording. Head field distribution of a single-pole head analyzed by 3-dimensional computer simulation results in a sharp gradient, but relatively large cross-sectional area is required to maintain head field strength. Based on this design concept, a lateral single-pole head is described and proved to attain track width of 0.4 µm. In addition, multilevel partial response appropriate to the new multitrack recording system is proposed.

Tremendous progress has been made in magnetic data storage by applying theoretical considerations to technologies accumulated empirically through a great deal of research and development. In Japan, the recording demagnetization phenomenon was eagerly analyzed by many researchers because it was a serious problem in analogue signal recording such as video tape recording using a relatively thick magnetic recording medium. Consequently, perpendicular magnetic recording was proposed as a method for extremely high-bit-density recording. This paper describes the theoretical background which has resulted in the idea of perpendicular magnetic recording. Furthermore, the possibility of magnetic recording is discussed on the basis of the results obtained theoretically by magnetic recording simulators. Magnetic storage has the potential for extremely high-bit-density recording exceeding 1 Tb/cm2. We propose the idea of 'spinic data storage' in which binary digital data could be stored into each ferromagnetic single-domain columnar particle when the perpendicular magnetizing method is used.

Co-Zr and Co-Zr-Ta amorphous films were prepared by the Kr sputtering method for use as the backlayers of Co-Cr perpendicular magnetic recording tape media. The effect of the addition of Ta to Co-Zr thin films was also investigated. Lower substrate temperature was required to prepare amorphous Co-Zr films with excellent soft magnetic properties. The relationships among Ta content X, magnetostriction constant λ and magnetic characteristics such as coercivity Hc and relative permeability µr were clarified. A method of evaluating λ of soft magnetic thin films deposited on polymer sheet substrate has been presented. Films with composition of (Co95.7Zr4.3) 100-X TaX at X of 10 at.% possessed sufficiency soft magnetic properties such as low Hc below 80 A/m and high µr above 600. Addition of Ta was effective in changing change the sign of λ from positive to negative. It was found that the negative magnetoelastic energy and the smaller λ caused the soft magnetism.

The authors have studied the demagnetization phenomenon which is observed in a conventional CoCrTa/CoZrNb double-layered (DL) perpendicular recording medium. The authors have also investigated the effects of an in-plane hard magnetic layer in a triple-layered (TL) perpendicular recording medium. The in-plane hard magnetic underlayer is made of CoSm or CoCrTa/Cr and is laid under the CoZrNb soft magnetic layer. In the DL medium, a demagnetization phenomenon i.e. decrease of the readback signal, is observed when the CoCrTa layer has a strong perpendicular orientation and the CoZrNb underlayer has a low coercivity. The amount of the signal decrease depends strongly on the accumulated disk revolutions. This demagnetization is considered to be caused by fact that the recorded magnetization in the CoCrTa layer is reduced by the magnetic field generated from the domain walls in the CoZrNb layer, since the CoZrNb layer is very sensitive to a magnetic environment such as geo-magnetism and domain walls move as the disk rotates. On the other hand in the TL medium, the hard magnetic layer has an effect of pinning the magnetic domain in the CoZrNb layer, by which the demagnetization problem is successfully prevented. The hard magnetic layer remarkably reduces the domain walls in the CoZrNb layer and contributes to medium noise reduction. Thus the TL medium presents a higher SN ratio than DL medium.