硬盘磁记录介质的现状与发展

讨论了现有介质、新介质的材料与结构,以及其工艺技术的现状。并展望了高密度硬盘磁记录介质的未来。指出高密度、高信噪比Ｓ／Ｎ的硬盘介质要求有高的矫顽力和小的磁记录畴。     

Tunneling magnetoresistive heads for magnetic data storage

Spintronics is emerging to be a new form of nanotechnologies, which utilizes not only the charge but also spin degree of freedom of electrons. Spin-dependent tunneling transport is one of the many kinds of physical phenomena involving spintronics, which has already found industrial applications. In this paper, we first provide a brief review on the basic physics and materials for magnetic tunnel junctions, followed more importantly by a detailed coverage on the application of magnetic tunneling devices in magnetic data storage. The use of tunneling magnetoresistive reading heads has helped to maintain a fast growth of areal density, which is one of the key advantages of hard disk drives as compared to solid-state memories. This review is focused on the first commercial tunneling magnetoresistive heads in the industry at an areal density of 80 approximately 100 Gbit/in2 for both laptop and desktop Seagate hard disk drive products using longitudinal media. The first generation tunneling magnetoresistive products utilized a bottom stack of tunnel junctions and an abutted hard bias design. The output signal amplitude of these heads was 3 times larger than that of comparable giant magnetoresistive devices, resulting in a 0.6 decade bit error rate gain over the latter. This has enabled high component and drive yields. Due to the improved thermal dissipation of vertical geometry, the tunneling magnetoresistive head runs cooler with a better lifetime performance, and has demonstrated similar electrical-static-discharge robustness as the giant magnetoresistive devices. It has also demonstrated equivalent or better process and wafer yields compared to the latter. The tunneling magnetoresistive heads are proven to be a mature and capable reader technology. Using the same head design in conjunction with perpendicular recording media, an areal density of 274 Gbit/in2 has been demonstrated, and advanced tunneling magnetoresistive heads can reach 311 Gbit/in2. Today, the tunneling magnetoresistive heads have become a mainstream technology for the hard disk industry and will still be a technology of choice for future hard disk products.     

Growth of Magnetic Disk Drive Industry and Future

1. IntroductionThe growth of personal computers (PCs) and theexpansion of information networks, featured by theInternet, lead to information storage demand surprisingly. No data storage equipment other than magnetichard disk drives (known as HDD) is able to offer a costperformance that matches the requirements of growing digital network era.Areal recording density of the HDD has been improved more than 3 million times, since RAMAC firstshipped in 1957. During the periods, there were many…     

Exchange coupling and its applications in magnetic data storage

The continuing scaling of magnetic recording is facing more and more scientific and technological challenges because both the read sensor and recording bit are approaching sub-50 nm regime with the ever increasing areal density in hard disk drives. One of the key and indispensable elements for both high-sensitivity sensors and high-density media is the exchange bias between a ferromagnetic and an antiferromagnetic layer or the exchange coupling between two ferromagnets via a non-magnetic spacer. In the nanometer regime, the exchange coupling between ferromagnet and antiferromagnet or two ferromagnets through a conductive spacer is governed by the intergrain exchange interaction which has its origin in electron spins. Interlayer exchange coupling in multilayer or trilayer essentially originates from the quantum confinement effect. In this paper, we first review the physical origin and various theoretical models of the two types of exchange couplings, followed by a review of the applications of the exchange bias and interlayer exchange coupling in data storage with emphasis on the advanced read sensor and advanced media including perpendicular media and patterned media.     

Self-assembled monolayers as lubricants for magnetic disk drives

Self-assembled monolayers (SAMs) are an attractive alternative to perfluoropolyether (PFPE) lubricants because they have a strong adsorption to a solid surface and have the potential to overcome the limitations of PFPE, such as thermodynamic instability, nonuniformity, and spin off. In this paper, we describe the possibility of SAMs as lubricants (or as a component of a lubricant system) for magnetic disk drives. We also discuss a strongly bonded hydrocarbon film deposited on an amorphous carbon film using a vacuum vapor deposition technique.     

磁性载体记录的稳定性探索

本文总结了有关贮存在各种磁性记录载体中信息的保存性的一些基本科学技术知识，包括：录音带，录像带，计算机磁带，软磁盘，硬磁盘，光磁盘等，全文分两期登出，本期登其第一部分，内容主要为：磁性记录材料的特性和稳定性，磁性粒子，下期内容为：粘合剂，聚酯带基，环境因素影响等。     

Water adsorption on lubricated fullerene-like CNx films

Humidity influences the tribological performance of the head-disk interface in magnetic data storage devices. In this work we compare the uptake of water of amorphous hydrogenated carbon (a-CH_y) and carbon nitride (a-CN_x) films, widely used as protective overcoats in computer disk drive systems, with two types of amorphous non-hydrogenated carbon (a-C and a-C_sp²) films, and fullerene-like carbon nitride (FL-CN_x) films. Carbon films were deposited on quartz crystal substrates by reactive dc magnetron sputtering in Ar/N₂ discharges. After deposition, some of the films were coated with a 2-nm-thick layer of Z-tetraol, a lubricant used in hard disk devices. A quartz crystal microbalance placed in a vacuum chamber was used to measure the adsorption of water at room temperature and at pressures of water corresponding to relative humidities in the range RH = 0 to 90%. Water adsorption and desorption is fast, indicating that equilibrium with ambient humidity is reached on time scales of minutes, much faster than the time scales for fluctuations in ambient humidity. The amount of water adsorbed on the non-lubricated amorphous carbon films is significantly higher than that on the fullerene-like films. The presence of the lubricant influences water adsorption but its impact differs on different carbon films.     

Fabrication of Flyable Perpendicular Discrete Track Media

Discrete track media offers many potential recording advantages over conventional continuous media in hard disk drives. In this study, we present a novel fabrication process for discrete track perpendicular magnetic media via electron beam lithography, ion milling, and the use of a protective Al sacrificial layer. Physical characterization of the media confirms the process is able to produce patterned tracks with no damage to the media. Spin stand analysis verifies the disks are flyable and capable of recording sharp transitions without any degradation in the magnetic signal     

Diamond-like film as a corrosion protective layer on the hard disk

A magnetic layer based on Co-alloys does not possess the necessary mechanical durability and corrosion resistance and must be covered with a protective layer. An accelerated electrochemical test has been conducted on a hard disk magnetic layer covered with amorphous hydrogenated carbon doped with nitrogen. From the potentiodynamic and DC polarization measurements of the corrosion current in a 0.15 M NaCl solution adjusted to different pH, it was concluded that an acid environment can largely decrease the reliability and lifetime of a hard disk. Lubricant can contribute to corrosion protection by covering voids and other overcoat imperfections and prevent water vapor penetration into the magnetic film. Atomic force microscope images show localized distribution of corrosion sites with a density of 1.2×10⁶ defects/cm². The overcoat also prevents a lateral spread of the corrosion products across a disk.     

Magnetic switching fields in square monolayer and bilayer nanodots

The increasing density in recording media has augmented the interest in magnetic structures with dimensions in the range of nanometers. One of the interesting nanoscale devices for these kind of applications are the square magnetic nanodots. However a very large integration of this structures in a 2D array leads to interactions between different dots due to the existence of strong magnetic poles at the dot borders. For certain dimensions they may also become unstable due to the high magnetostatic energy and may fall into circular magnetisation distributions, usually referred to as vortex. Magnetostatically coupled bilayers exhibit a higher stability in parallel magnetisation distributions due to a good magnetic flux closure between magnetic layers. Parallel magnetisation structures appear to be stable in bilayer nanodots for a wider range of dimensions rather than in monolayers. Therefore, the higher stability of the magnetisation in this structures as well as the reduction of the magnetic poles at the dot borders make this kind of structures interesting from the high density recording point of view. In this paper it is studied the different stable configurations for square magnetic dots in materials with two magnetic layers with dimensions suitable for a good magnetostatic coupling between them. Results are compared with dots in materials with one single magnetic layer with similar dimensions. Magnetic fields necessary to switch between the different stable configurations as well as the switching mechanisms are also studied.     

Scanning transmission electron microscopy analysis of grain structure in perpendicular magnetic recording media

The key component of a hard disk medium is a Co-based magnetic layer (ML) grown on a Ru seed layer. The ML nanostructure, composed of less than 10 nm grains, is believed to be controlled by this seed layer. We successfully used scanning transmission electron microscopy energy dispersive spectrometry simultaneous composition-based imaging and Moire? pattern analysis for determining the mutual structural and orientation relationship between the two layers revealing a grain-to-grain agreement. The method presented here can be utilized for observing structural correlations between consecutive polycrystalline thin film layers in general.     

Optimizing tape‐burnishing/wiping process of magnetic recording media through Taguchi method

For the ever‐increasing recording density in hard disk drives, ultra‐low gliding height media are required. It is an immense challenge for the tape‐burnishing/wiping process of media to reduce the asperities effectively and efficiently while no scratches occur. The purpose of this paper is to characterize and then optimize the tape‐burnishing/wiping process as so to minimize the asperities without leaving any scratches on the media surface. A Taguchi experiment design method is adopted to analyze the data and acquire an optimal level combination of process parameters. The resulting optimal combination is practically implemented in tape‐burnishing/wiping the several magnetic recording media, which reveals that the average pass ratio of 5 nm glide avalanche testing increases nearly doubly to a level of 96%. Copyright © 2008 John Wiley & Sons, Ltd.     

Perspectives for 10 terabits/in2 magnetic recording

Magnetic recording technology has come a long way, since the introduction of the first hard disk drives (HDD) in 1956. The areal density has grown by a factor of 200 million times and the HDD has stayed as a main candidate for mass storage of information. In order to maintain its lead over other competing technologies, HDD industry continues to invent several technologies. Having introduced perpendicular recording technology in the last 5 years, the industry is looking at introducing bit-patterned media or heat-assisted magnetic recording in the next five years. The researchers--looking at a longer term--are investigating 10 Tbits/in2 as the next major milestone. The issues and probable candidates for 10 Tbits/in2 magnetic recording technology are described from a material perspective.     

Interlayer exchange coupling effect of L1(0) CoPt based exchange coupled composite media

In this work, effects of exchange coupling of soft magnetic layer on switching field and magnetization reversal behaviour of CoPt-SiO2(soft)/CoPt-SiO2(hard) exchange coupled media were investigated. With increasing the thickness of the soft layer, both the coercivity and magnetization squareness of composite media decreased. Soft layer thickness 4 nm and below was more effective to significantly reduce the switching field than that above 4 nm. More incoherent switching behavior was observed with increasing soft layer thickness.     

Voltage pulsing for localized clearance measurement

Mechanical clearance is a critical parameter of the head/disk interface (HDI) for sliders flying in close proximity in hard disk drives. It depends on a variety of HDI parameters, such as the air-bearing design, disk roughness, and the disk lubricant. An experimental setup was developed to determine the mechanical clearance locally at any location on the disk. Voltage pulsing is used to add an additional force component to the complex HDI equilibrium. The applied force is attractive in character and therefore reduces the mechanical spacing between the slider and the disk. If the magnitude of the applied voltage is changed, a slight contact between the slider and the disk can be realized. Simultaneous measurement of the induced slider displacement using a laser vibrometer allows the quantification of the mechanical clearance. The effect of disk roughness and microwaviness on the mechanical clearance is investigated.     

Comparison of Micromagnetic and Point Probe Model in Magnetic Force Microscope Simulation

We developed a micromagnetic model of magnetic force microscopy (MFM) tip to compare it with the simple point probe model. We simulated the MFM signal to provide an understanding of the measurement of the field generated by the write head in perpendicular recording hard disk drives. When the magnetic pole density at the air-bearing surface of the head's main pole is increased from 0.2 T to 1 T, the MFM tip with vertical anisotropy shows a flower-state magnetization, while the tip with horizontal anisotropy has more complicated switching modes. It is found that the signal ratio of the two MFM tips with vertical/horizontal anisotropy does have a one-to-one correspondence to the average magnetic field in the tip; however, the signal ratio may change sign because of the magnetic moments' switching in the tip with vertical anisotropy. The result of micromagnetic simulation is quite similar to that of the point probe model, and has good agreement with experiments.     

Determination of lubricant film thickness on a particulate disksurface by atomic force microscopy

The ability of the atomic force microscope (AFM) to measure the lubricant film thickness on the surface of particulate disks is demonstrated, and experimental results are presented. AFM measure the thickness of the lubricant film at a particular location on the disk surface with a lateral resolution of the order of the AFM tip radius, ~1000 Å. For an unused disk, 50-70 Å of lubricant thickness is found. After the disk has been in use for several years, the lubricant thickness decreases to 35-55 Å. In both cases, the lubricant is uniformly distributed on the disk surface, although somewhat more uniformly on the used disk. For all types of disks studied, most of the lubricant resides below the disk surface, presumably in the medium's porosity, with only a molecular thin film of lubricant on the surface     

USE OF THIN SOLID FILMS AT HEAD/MAGNETIC MEDIA INTERFACES

The lubrication of head / disk interlaces in magnetic hard disk drives by physically processed thin films was investigated. Sputtered gold films showed no useful endurance as a protective / lubricating film. Ion-plated carbon films and hydrogenerated carbon films (i-carbon) were effective for preventing tribological damages in magnetic recording media, while amorphous carbon films (a-carbon) were ineffective. The longest durability was found for i-carbons. Raman scattering showed that i-carbons contained much sp³ component, and microindentation test signified that i-carbon films were twice as hard as amorphous carbon films. As a protective film, i-carbon seems very promising, but hard materials should be selected as heads to avoid quick wear of the head. Sufficient performance of i-carbon films may lead to application of thin solid lubricants to hard disk drives.     

Multichannel molecular switch with a surface-confined electroactive radical exhibiting tunable wetting properties

A multichannel surface molecular switch based on self-assembled monolayers (SAMs) on gold of a novel polychlorotriphenylmethyl radical has been fabricated. These SAMs are electrically commutable between two stable redox states that reveal distinct magnetic, optical, and wetting properties that can be employed as read-out mechanisms. The high stability and reversibility of systems like the one reported here further supports the vision of using molecules in the electronic devices of the future.     

PVD-Abscheidung von CoSm-Schichten für magnetische Maßstäbe

PVD deposition of hard magnetic – CoSm coatings for magnetic scales Magnetic functional layers have been used for decades as reliable and highly accurate information carriers, for example in hard disk storage or on magnetic cards. In addition to data storage, magnetic layers also offer great potential in mechanical and electromechanical elements for measuring the position, speed or angle of moving components. In the context of electro-mobility, the demand for high-precision magnetic measurement systems is also increasing. Furthermore, magnetic layers can be used for encoding workpiece information or as an element of miniaturised drives. Various processes are used for the production of magnetic layers, which have their specific advantages and limitations. For thinner layers, galvanic processes are often used, while sintering processes, for example, are used for thicker layers. However, the economic production of high-performance coatings in the range of a few micrometers to a few tens of micrometers represents a technological challenge. At the Fraunhofer IST, soft and hard magnetic coatings for various applications have been under development for a long time. A special low-pressure coating process, hollow cathode gas flow sputtering (GFS), which has also been developed and industrialised at Fraunhofer IST over the last 20 years, is used for this purpose. In cooperation with ITK Dr. Kassen GmbH, hard magnetic coatings were developed with this process, which are used for measurement scales for high-precision positioning in compact microscope stages. In operational practice, these coatings have proven to be extremely reliable and robust against outside influences, so that a high reliability could be achieved even under difficult conditions. The developed layers allow the magnetic coding with reproducible marks, which in combination with a special evaluation algorithm provide very high positioning resolution. Due to the integrated measuring system, the microscope stages enable an absolute determination of the position, without referencing, with a position resolution in the nanometer range.