Looking back on Censtor and PMRC

Prehistory of Censtor Corporation was described. A ten-year study of Nature’s neurological ways and means of processing information led the author to the research of computer-based technology, in particular, perpendicular magnetic storage technology. With funding Censtor Corporation was founded with Dr. Robert Noyce as Chairman of the board. Censtor Corporation created novel technology on perpendicular magnetic recording with the unique head in contact with the medium.     

Perpendicular magnetic recording—Its development and realization—

The principle of conventional magnetic recording is that magnetic fields are applied parallel to the plane of the magnetic medium. As described in this paper, the invention and development of a new method of placing the magnetized information perpendicular to the plane of the magnetic recording medium is presented. The yield in the mass production of high-density hard disk drives (HDDs) for perpendicular recording is much higher than that of HDDs for conventional recording. Consequently, it is estimated that as many as 75% of the 500 million HDDs to be shipped this year will use this technology.     

Read and write processes, and head technology for perpendicular recording

In this paper, we present a review of the write and read processes in perpendicular magnetic recording. We also discuss their impact, based on recording physics aspects, on design considerations for writers and readers. For the write process, we discuss fundamental write-ability limitations as well as possible paths to ultra-high areal density perpendicular recording. The impacts of different medium designs, geometrical scaling, and the breakdown of scaling, both in terms of write-ability and transition curvature, are shown based on different modeling techniques, including analytical formulas, finite element modeling (FEM), and micromagnetic simulations. Basic design rules as well as alternative designs that enable high areal density are briefly explained. For the read-back process, the relation between reader signal to noise ratio (SNR) and resistance, as well as reader resistance shunt and spacing loss, are discussed. Finally, we use a simple example to illustrate, both from a write as well as a read-back perspective, the complicated nature of perpendicular recording systems, and how different medium designs impact recording head technology for ultra high density perpendicular recording.     

Narrow-track perpendicular write heads

Narrow-track perpendicular write heads are reviewed. Because of the strong magnetic interaction between the write head and double-layered medium in perpendicular recording, various types of media are also considered. Current technology is discussed to illustrate design issues; then, for areal densities beyond 1 Tb/in², future technological requirements, including single-pole-type (SPT) heads for discrete track and bit-patterned media, are examined based on numerical simulations.     

A novel analysis method for noise of perpendicular recording media

An in-field magnetic force microscopy (MFM) observation technology was introduced to study media noise in perpendicular recording medium. Magnetization reversal field (H_R) distribution of the recording medium could be mapped by this technology. We have extracted media noise signal image from recording pattern. The noise signal image was converted to a contour image, in which peak areas represented high-noise areas. To clarify the relationship between H_R distribution and the media noise, we novelly combined the contour with the H_R-map. The results show that the media noise mostly gets generated in the lower-H_R areas, which indicates the lower-H_R area is one of the important origins of the media noise. For comparison, a simulation work was also performed by introducing a model based on experimental parameters. The simulation agreed with experimental results very well.     

Bidirectional recording performance of a perpendicular evaporated Co–CoO tape

It is the first report on the recording performance of a perpendicular metal evaporated (ME) tape measured with a giant magnetoresistive head. To solve the application difficulty of oblique evaporated tape media to linear scan tape systems, a perpendicular evaporated Co–CoO tape was proposed instead. The prepared sample showed perpendicular anisotropy with coercivity of 107.3 kA/m, M_rt of 3.9 mA and squareness of 0.25. Identical recording characteristics were obtained for both head-media moving directions, which enables the application of perpendicular evaporated Co–CoO tape to linear scan tape systems. The better carrier-to-noise ratio was also confirmed by comparison with a current advanced product of metal particulate tape, which can realize the higher recording density of linear scan tape systems using ME tape.     

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.     

Drive-based recording analyses at >800 Gfc/in using shingled recording

Since the introduction of perpendicular recording, conventional perpendicular scaling has enabled the hard disk drive industry to deliver products ranging from ∼130 to well over 500 Gb/in² in a little over 4 years. The incredible areal density growth spurt enabled by perpendicular recording is now endangered by an inability to effectively balance writeability with erasure effects at the system level. Shingled magnetic recording (SMR) offers an effective means to continue perpendicular areal density growth using conventional heads and tuned media designs. The use of specially designed edge-write head structures (also known as ‘corner writers’) should further increase the AD gain potential for shingled recording. In this paper, we will demonstrate the drive-based recording performance characteristics of a shingled recording system at areal densities in excess of 800 Gb/in² using a conventional head.Using a production drive base, developmental heads/media and a number of sophisticated analytical routines, we have studied the recording performance of a shingled magnetic recording subsystem. Our observations confirm excellent writeability in excess of 400 ktpi and a perpendicular system with acceptable noise balance, especially at extreme ID and OD skews where the benefits of SMR are quite pronounced. We believe that this demonstration illustrates that SMR is not only capable of productization, but is likely the path of least resistance toward production drive areal density closer to 1 Tb/in² and beyond.     

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.     

超高密度电学信息存储研究进展

21世纪是经济信息化、信息数字化的高科技时代，信息的爆炸式增长及电子器件持续微型化的要求需要小断研究和开发更高仔储密度、更快响应速度、更长存储寿命及可反复读、写的材料和器件。在纳米／分子尺度上实现存储功能的超高密度信息存储已成为当前信息领域一个倍受关注的研究热点。本文从存储材料和技术角度介绍了基于电学双稳态的超高密度信息存储最新研究进展。     

Thermal stability and the magnetization process in CoCrPt–SiO2 perpendicular recording media

The correspondence between the crystallographic texture and intergranular exchange coupling interactions, with the switching mechanism and the thermal response of the magnetization in CoCrPt–SiO₂ perpendicular recording media was investigated. Virgin hysteresis and isothermal remanence magnetization measurements both showed a three-stage process, which was interpreted to indicate that the Stoner–Wohlfarth coherent reversal mode is the dominant switching mechanism irrespective of the texture. For media samples with similar degree of texture, improvement in exchange decoupling of the media grains caused an increase in the onset field for the virgin magnetization process. The thermal decay of the magnetization, evaluated via the field-dependent viscosity coefficient peaked near the nucleation field, and the peak value showed a strong dependence on the strength of the exchange coupling interactions. A model establishing the role of the texture and exchange interactions in perpendicular recording media is put forth.     

Magnetic properties of soft layer/FePt--MgO exchange coupled composite perpendicular recording media

The magnetic properties of exchange coupled composite （ECC） media that are composed of perpendicular magnetic recording media FePt MgO and two kinds of soft layers have been studied by using an x-ray diffractometer, a polar Kerr magneto-optical system （PMOKE） and a vibrating sample magnetometer （VSM）. The results show that ECC media can reduce the coercivities of perpendicular magnetic recording media FePt-MgO. The ECC media with granular-type soft layers have weaker exchange couplings between magnetic grains and the magnetization process, for ECC media of this kind mainly follow the Stoner Wohlfarth model.     

Designing magnetics of capped perpendicular media with minor-loop analysis

A novel method of analyzing the switching field distribution (SFD) and magnetic correlation length of perpendicular magnetic recording media that uses major and minor magnetization loops is proposed. By applying the analysis to a series of capped perpendicular media, we found that a thick capping layer with a low saturation magnetization effectively reduced SFD without rapidly increasing the magnetic correlation length. Transmission electron microscope observation suggests that the SFD is narrowed by the increased uniformity of intergranular exchange coupling via the thick capping layer. Evaluations of recording characteristics demonstrated a close correlation between narrower SFDs and improved recording performance. Reducing exchange coupling dispersion is a clear solution for improving the performance of recording media.     

The role of nucleation field of media in heat-assisted magnetic probe recording

We characterize a method of heat-assisted magnetic probe recording on perpendicular media. Heating source is field emission current from a scanning tunneling microscope (STM) tip. Recording media are three kinds of magnetic films, Co/Pt, CoNi/Pt, and Co/Pd multilayers with different nucleation fields. Pulses with amplitude of 5 V were applied between the STM tip and the recording medium. Experiments show that magnetic marks with an average size of 180 nm were formed on both Co/Pt and CoNi/Pt films whose nucleation fields are greater than their saturation magnetization. No marks were observed on the Co/Pd film whose nucleation field is smaller than its saturation magnetization. A model is built to simulate the dynamic process of domain formation in probe-based magnetic recording system. Simulation results agree with experiments and it explains the effect of the nucleation field of medium in perpendicular recording.     

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.     

An analysis of medium noise of Co-Cr double-layer films in perpendicular magnetic recording

The medium noise in perpendicular magnetic recording is discussed. The perpendicular recording mode was expected to prevent zig-zag walls around transitions, and the experimental result of low transition noise in Co-Cr media confirms this. Also, the main origin of the noise is concluded to be irregular clusters of grains in the film.     

Single-layer and multilayered films for perpendicular recording based on a Co-Cr alloy

Results of materials research into the dependence of the magnetic properties of Co-Cr recording media on the structural properties are discussed in relation with results from recording experiments. The magnetic properties depend strongly on the preparation conditions. An increase in substrate temperature results in an increase of the coercivity, but also gives rise to inhomogeneities of the Co-Cr film in the growth direction and to lateral inhomogeneities due to Cr segregation. The inhomogeneities in the growth direction are suppressed by the application of a non-magnetic, amorphous Ge layer between the substrate and the Co-Cr layer, resulting in a magnetic layer with a large perpendicular anisotropy. From the slope of the hysteresis loops we obtain, using a model proposed by Kooy and Enz, an estimate for the mean size of the stripe domains, which is found to increase with increasing layer thickness and decrease with increasing substrate temperature. The change in the size of the stripe domains is consistent with the trends calculated from noise spectra obtained in recording experiments. Multilayers of thin Co-Cr layers alternated with non-magnetic Ge layers are also discussed.     

Playback performance of perpendicular magnetic recording tape media for over-50-TB cartridge by facing targets sputtering method

Perpendicular magnetic recording tape medium with laminated soft magnetic underlayers was successively formed on a 4.5 μm thin plastic film by the facing targets sputtering method at room temperature. The tape medium showed extremely high SNR values with sharp transitions in playback tests by a drum tester, compared to a linear format commercial tape medium. An areal density of 45.0 Gb/in.² of perpendicular tape medium was confirmed by a high-resolution contact tester. These results suggest that the perpendicular recording tape medium by the facing targets sputtering method is a promising candidate for a tape cartridge of over 50-TB.     

The current status and future of granular media

The current status of CoPtCr-SiO₂ granular media is briefly reviewed, and challenges relative to media technology exceeding 1 Tb/in² are discussed. It is effective to enhance grain isolation using oxide materials that easily precipitate at the grain boundary, and this technique is adopted in commercially available perpendicular recording media. Although some difficulties such as reduction of the grain size retaining the switching field remain, 1 Tb/in² could be achieved by using granular exchange-coupled composite-type media or related technologies. Discrete track media could be used for more than 1 Tb/in² recording. It could also take several years to further develop nano-processing technology and establish a cost-effective infrastructure. BPM offers great potential in achieving high recording density, although some necessary technologies are still too primitive to consider commercial production.