Past and present of perpendicular magnetic recording

Past and present of perpendicular magnetic recording (PMR) is described. PMR was born at a university institute 30 year ago and hard disk drive (HDD) industry is now converting the products from longitudinal magnetic recording (LMR) to PMR. The way to the present, however, was not flat. The research of PMR experienced the death valley in the 1990s, but was overcome by its unique research system. The commercialization of PMR was much later than it was expected in the beginning, which might have affected the position of HDDs in storage systems. The market of HDDs is expected to steadily grow towards the future being supported by PMR.     

Constructive ITI-coded PRML system based on a two-track model for perpendicular magnetic recording

In this paper, we study not only the new constructive inter-track interference (CITI) code based on the equalized level but also Viterbi detection algorithm taking into account the ITI from adjacent tracks for the perpendicular magnetic recording channel without a differentiator of two-track model. Although the investigation of this paper was not practical but theoretical, the result shows that the permissible percentage of ITI for conventional Viterbi detection to attain better performance compared with the case of single track is 26%, while the percentage for the proposed one is improved up to 50%. Further investigation has to be required under a more realistic system and moreover may be expanded into patterned media perpendicular magnetic recording.     

Preparation and recording characteristics of granular-type perpendicular magnetic recording media with thin intermediate layer

Granular-type media with thin Ru intermediate layer were prepared on a highly oriented high-B_s FeCo soft underlayer (SUL). A CoPt–TiO₂ recording layer on a Ru intermediate layer of only 2 nm had high-crystal orientation, high H_c of 6.5 kOe, and a high squareness ratio (SQ) of 0.99. The magnetic property of the SUL was also good. The recording performance was measured for the media with different Ru intermediate thicknesses by using a single-pole-type (SPT) head. The media had large reproduced output even for the Ru intermediate layer thickness of 2 nm.     

Adjacent-track interference in ultrahigh-density perpendicular recording system

In perpendicular recording system, the increase of track density is crucial to achieve ultrahigh areal density. At higher track densities, the adjacent-track interference (ATI) arises. In this work, ATI is studied by micromagnetic simulation. Two adjacent tracks are written successively. The track–track distance (TTD) and head–medium spacing are varied to analyze the write and read performance of these two tracks and to investigate the influence of ATI on recording performance. Simulation results indicate that when a track is written first, it is less vulnerable to ATI. ATI is stronger in a track with higher linear recording density. The head–medium spacing plays a significant role in the achievement of low ATI in perpendicular recording system. If the head–medium spacing is reduced to 5 nm, areal recording density above 540 Gb/in² could be realized.     

Read-write issues in ultra-high density perpendicular recording

Issues pertaining to the realisation of areal recording densities in excess of are discussed. Using a composite media structure with well defined magnetic and microstructural properties the potential for achieving using existing methodologies is clear. Higher densities will require the use of additional technologies.     

Analysis of waveform from perpendicular magnetic printed media

Edge printing is one of the perpendicular magnetic printing methods for writing servo signals with high speed, high accuracy and low cost. Sub-peaks of waveform from edge printed media are concerned as the sub-peaks can cause errors during the read-back process. In this study, in order to reduce sub-peaks, the influence of printing field, bit length and patterned magnetic layer thickness of master medium on sub-peaks is investigated by using a metal-evaporated (ME) tape as a slave medium. The results show that sub-peak to main peak ratio decreases with stronger printing field, smaller bit length and thicker patterned magnetic layer of master media.     

Micromagnetic modeling for heat-assisted magnetic recording

Heat-assisted magnetic recording (HAMR) is one of the candidate systems beyond the perpendicular recording technology. Here, a micromagnetic model and a heat transfer model are introduced to study the heating and cooling processes in the HAMR media; then, by integration of the SPT head and the laser heating source, the recording performance is simulated and investigated on a single track at an area density of 1 Tb/in². In the HAMR system, the temperature in the medium under the laser wave guide is increased by heating, and decreased by air bearing and heat conduction when the write process really occurred. The target of this study is to find the proper design of the head-laser assembly for optimum recording. It is found that the proper distance between the laser wave guide and the head's main pole rear/front edge is only 41.4/1.4 nm for optimum recording performance.     

Design and recording simulation of 1 Tbit/in patterned media

Design of patterned media for an areal density of 1 Tbit/in² with thermal stability is presented based on perpendicular M–H loops of the media. Required perpendicular magnetic anisotropy was estimated to be achieved with known materials. However, it is indicated that magnetostatic interaction between the dots becomes a limiting factor for achieving higher densities. Recording simulation using a Karlqvist pole head on the designed media exhibited possibility of the recording of 1 Tbit/in². Shift margin of the write head in the cross-track direction was found to be increased with elongated dots in the down-track direction. Recording simulation with an FEM-analyzed field of a side-shielded multi-surface pole head exhibited successful recording with increased cross-track shift margins as well as the effect of the elongated dot shape.     

Comparison of media properties between hard/soft stacked composite and capping layer perpendicular recording media

The effect of soft layer thickness (t_Soft) of CoTaZr–SiO₂ and low Pt-containing CoCrPtO layers on media properties in hard/soft (H/S) stacked media is compared to media properties in conventional capping layer (CL) media. Coercivity and coercivity squareness in H/S stacked media continuously decrease with increasing t_Soft, while they increase in CL media. H/S stacked media with CoTaZr–SiO₂ layers having higher saturation magnetization and in-plane magnetic anisotropy constant exhibit stronger demagnetization effect. Compared to CL media, H/S stacked media with CoCrPtO soft layers improve signal-to-noise ratio and magnetic write width. However, the use of a relatively soft layer deteriorates adjacent track erasure and does not improve media writeability due to compensation effect between softer and harder layers to be used. These phenomena can be understood as undesirable side effects of a soft layer: higher demagnetization field and larger lattice mismatch.     

Read channel simulation based on microtrack and micromagnetic modeling for 1 Tb/in magnetic recording

Channel design for ultra-high-density perpendicular magnetic recording requires fast and precise modeling to generate readback signals corrupted by media noise. In this paper, we present a geometric-dependent approach to model random readback pulses, where a three-dimensional head and media combination for 1 Tb/in² density design is considered in the micromagnetic simulation. A systematic approach is developed to extract media noise statistics from micromagnetic modeling and generate readback pulses based on a fast microtrack model incorporating nonlinear effects. The feasibility of the proposed model is demonstrated through bit-error-rate (BER) simulation of a turbo equalization scheme over a low-density parity-check (LDPC) coded, general-partial-response (GPR) equalized perpendicular recording channel.     

3-D FEM analysis of thermal degradation in writing and reading characteristics of a perpendicular magnetic head

In order to achieve high-density recording, the detailed behavior of thermal degradation should be investigated. In this paper, the degradation of magnetization of high-density recording medium is examined using the 3-D finite element method (FEM) combined with the modeling of Stoner–Wohlfarth (SW) particles and Neel–Arrhenius switching probability. It is shown that the anisotropy field H_k suppressed the thermal degradation and the saturation magnetization M_s enhances it. The thermal degradation is also changed by the amplitude of magnetization.     

Anisotropy dispersion in (CoCrPt)1−x(SiO2)x perpendicular recording media

The distribution of easy axis orientation in perpendicular media is of technological importance because it affects the value of S^* (see Fig. 1), which quantifies the switching field distribution (SFD) and hence partially determines the data density achievable on a given medium. The distribution is controlled by the crystallographic orientation of grains and factors such as intergranular exchange and dipolar coupling. Due to strong demagnetising fields in the perpendicular orientation, traditional measurements of remanence as a function of angle are difficult to interpret and have required the use of large-scale computational models. In this work we have utilised the variation of coercivity H_C with angle, which has the advantage that at H_C the global demagnetising field is zero. Additionally, since such materials follow essentially the Stoner–Wohlfarth mode of reversal, the variation of H_C with angle, H_C(θ), is much greater than that for the remanence. We find that for (CoCrPt)_1−x(SiO₂)_x, where the level of exchange coupling is controlled, the distribution of magnetic easy axes is narrower when the exchange coupling is reduced, but dipolar coupling between the grains is strong and affects the magnetisation reversal significantly.     

Recording simulation beyond 2 Tbit/in on bit-patterned media with shielded planar head

A simple technique for bit-patterned media was proposed to increase achievable areal recording densities beyond 2 Tbit/in². Introduction of longitudinal magnetic anisotropy to the media indicated reduced effect of magnetostatic interaction between the dots. Recording simulation with a shielded planar pole head exhibited increased write shift margins in both down and cross track directions compared with that of the perpendicular anisotropy media. It was suggested that recording of an areal density of 2.5 Tbit/in² would be realized with a down and cross track margins of 3.5 and 4.0 nm, respectively. Better recording performance at high areal densities is expected if suitable head could be designed.     

The role of short exchange length in the magnetization processes of L10-ordered FePt perpendicular media

A three-dimensional micromagnetic model with non-uniform grain size distribution has been built up to study the magnetization process in FePt L1₀ perpendicular media. A 3D model of a single FePt magnetic grain is also set up for comparison. The high magneto-crystalline anisotropy K_u results in a short exchange length l_ex in FePt nanograins. Therefore a magnetic grain is divided into smaller grids on the order of l_ex. The simulated perpendicular and longitudinal loops are consistent with experiments, and it is explained why the measured perpendicular H_c is relatively smaller compared with the saturation field of the longitudinal loop in the FePt perpendicular medium.     

High-frequency MFM characterization of magnetic recording writer poles

By means of high-frequency magnetic force microscopy (HF-MFM), different magnetic recording writer poles are characterized in the frequency range 200– 1500 MHz using the dual-vibrational HF-MFM technique. The stray fields emanating from hard disk writer poles stemming from different makers are compared to each other, and the field distributions obtained at different carrier frequencies are analysed. At low carrier frequencies, the maximum field is located directly at the gap, whereas at high frequencies two distinct maxima before and after the gap are observed.     

Dependence of error patterns and BER performance on write pre-compensation in GPRML channels

Nonlinear transition shift (NLTS) influences the performance of PRML channels in perpendicular magnetic recording. For NLTS, it is thought that write pre-compensation is useful to write the transition at the intended position. In this paper, we study the influence of NLTS on the BER performance of GPRML channels and show the appropriate amount of pre-compensation for GPRML channels both with and without a post-processor (PP) to achieve better BER performance. The results show that the widening the shortest bit length by NLTS brings the BER improvement to GPRML channel and the write pre-compensation is useful for a GPRML channel with PP.     

Perpendicular magnetic recording technology at 230 Gbit/in

A perpendicular recording system that allows areal densities beyond 200 Gbit/in² has been designed and tested to investigate the major challenges in perpendicular magnetic recording. The integrated write/read head has a trailing shield to improve the write head field gradient and a conventional CIP–GMR reader. The medium is a low-noise CoPtCr-based oxide medium with a CoTaZr soft underlayer. On track byte error rates at ∼ 50 Mb/s are better than 10⁻⁴ at ∼1000 kbpi. Using a 15% off-track criterion at 10⁻² byte error rate, track densities between 200–240 ktpi are realized, yielding areal densities of 210–233 Gbit/in². High-resolution magnetic force microscopy (hrMFM) has been employed to investigate the write characteristics of these heads with improved cross-track resolution. Using a quantitative analysis method, many parameters, such as transition curvature and transition width, are estimated from the hrMFM image. Significant transition curvature is found, which increases the width of the read head response to a transition, T₅₀, by 2–3 nm.These results give insights into the recording physics of perpendicular recording and in particular point out improvements required for achieving even higher areal densities.     

Thermal stability and switching field of hard/soft-stacked perpendicular media

The time dependence of remanence coercivity and thermal stability were investigated for hard/soft-stacked media consisting of a magnetically hard granular layer underneath a very thin soft layer with a large saturation magnetization, M_s. The values of remanence coercivity at measurement times t′=10³ and 10⁻⁵ s (pulse field) were measured, and defined as H_r and H_r^P. The remanence coercivity on the recording time scale, H_r (1 ns), and the energy barrier, ΔE/kT, were evaluated by fitting H_r and H_r^P to Sharrock's equation taking into account the power law variation of the energy barrier, n. The value of H_r (1 ns) for a (Co–Pt)–SiO₂ (9 nm)/Co–SiO₂ (2 nm) stacked medium with an interfacial coupling control layer was about 9 kOe, which was less than half of that of a (Co–Pt)–SiO₂ (9 nm) conventional medium (=21.3 kOe). The value of ΔE/kT for the stacked medium was about 111 (n=0.7), and was not significantly different from the conventional medium. Moreover, no significant difference in the rate of decrease of H_r with increasing temperature was observed between media with and without interlayers. These results indicate that the use of a thin soft layer with high M_s was effective at significantly reducing H_r with no notable change in thermal stability.     

Iterative decoding algorithm using attenuated information in PMR with bit-patterned medium

In this paper, a low-density parity check (LDPC) coding and iterative decoding system is studied in perpendicular magnetic recording (PMR) with a bit-patterned medium. We propose the efficient iterative decoding algorithm attenuating reliability information, and evaluate the sector error rate (SER) performance of the LDPC coding and iterative decoding system by R/W computer simulations. It is clarified that the proposed systems provide better SER performances than that of the conventional system at an areal density of 1 Tbit/in² and can also prevent the error floor which appears in the performance of conventional system.     

Simulations of continuous/cluster-pinned recording media

Continuous/cluster-pinned recording media, consisting of cuboid clusters of various sizes exchange-coupled to a continuous hard layer, were modelled to investigate their suitability for high density data storage. The pinning field due to the clusters was determined by modelling domain-wall motion in the continuous layer. Larger clusters, a thinner continuous layer and increased saturation magnetisation of the clusters all increased the pinning field. Simulations of recording demonstrated the feasibility of using domain-wall pinning to control the written bit size in continuous media.