Abstracts of Magazine FUJITSU 2007-1 (VOL.58, NO.1)

Special Issue: HDD Technologies

• New Trends of Enterprise Hard Disk Drives

Fujitsu is a major provider of high-performance, high-reliability hard disk drives (HDDs). Advanced storage systems require new HDDs with higher performance, lower power consumption, and a smaller footprint. These requirements have led to the development of the 2.5-inch small form factor (SFF) HDDs and the Serial Attached SCSI (SAS). This paper describes the benefits of the 2.5-inch SFF and SAS and explains that the trend towards these technologies is a technical necessity. We then introduce our latest AL-10 series of enterprise HDDs, which all use a common elemental technology. The HDDs introduced include a 3.5-inch 15 000 rpm HDD, 2.5-inch 10 000 rpm HDD, and the world's first 2.5-inch 15 000 rpm HDD.

• 2.5-inch Hard Disk Drives for Mobile Equipment

Fujitsu has been developing 2.5-inch hard disk drives (HDDs) to meet the needs of notebook personal computers and other compact consumer products. This paper introduces the MHW2160BH and MHX2250BT. These HDDs offer an areal recording density of 130 Gbit/in², which is currently the world's highest. The MHW2160BH is 9.5 mm thick, has two 2.5-inch disks rotating at 5400 rpm, and a storage capacity of 160 GB. The MHX2250BT is 12.5 mm thick, has three 2.5-inch disks rotating at 4200 rpm, and a storage capacity of 250 GB. The high areal recording density of these HDDs was achieved using perpendicular recording technologies and head flying height control technologies. Both of these HDDs have a Serial ATA interface for high-speed data transfer.

• HDD Interface Technology

To enable faster data transfer, the interface of hard disk drives (HDDs) has been changed from a parallel interface to a serial interface. Currently, the main interfaces being used are the Fibre Channel (FC), Serial ATA (SATA), and Serial Attached SCSI (SAS) interfaces. Until now, the HDD market segments corresponded to the serial interfaces that were available. However, because of the recent demands for cost reduction, interfaces are now being used for different segments. As a result, the HDD market segments have become fractionalized, and the correspondence between market segments and serial interfaces has been lost. For example, some enterprise HDDs now use a conventional low-end SATA. This paper describes the features of these serial interfaces and the interface technology of the HDD controller that enables serialization of the interfaces. It also discusses the technological problems of future HDD interfaces and their possible solutions.

• Servo Track Writing Technology

To increase the recording density of hard disk drives (HDDs), the technology for writing the servo tracks used to position the recording heads on the data tracks must be improved. Servo track writing (STW) is commonly performed using the HDD's own heads, with the positioning done using an external actuator. Recently, however, the demands for higher productivity and STW quality have been growing and a new method is needed. As a major manufacturer of HDDs, Fujitsu is developing various STW technologies to meet this need. This paper introduces the main STW methods and requirements and describes the factors that reduce STW quality. Then, it describes media STW and magnetic printing STW, which are typical external STW methods.

• Nano-Scale Simulation Technologies

Physical simulation technologies are employed in various areas of hard disk drive (HDD) development and design. To further increase HDD recording density, it is necessary to achieve nano-order precision in heat dissipation analysis and manufacturing process analysis. To meet this requirement, Fujitsu has developed and is actively using simulation techniques for analyzing head element protrusions and the ion milling and deposition processes. This paper focuses on the following analyses that are performed to increase HDD recording density: the analysis of head element protrusions to minimize the head flying height and the analyses of ion milling and deposition performed to reduce the size of head element structures.

• HDI Technology for Increasing Storage Density and Reliability of HDDs

Various technologies are being developed to simultaneously achieve high-density storage and high-reliability in hard disk drives (HDDs). Some examples are technologies for reducing the spacing between the head and disk magnetic layer while providing an accurate grasp of the conditions at the nano level and technologies for controlling the head flying height at the nano level. In this paper, we describe the following head-disk interface technologies that Fujitsu has developed: a co-axial impact collision ion scattering spectroscopy technique for evaluating the insulation properties of lubricants, a technique for evaluating the insulation properties of the overcoats used on magnetic storage devices based on the drop method, and a chemical modification technology for the flying surface of head sliders.

• Development of Novel Lubricants for Ultra Low Flying Height

The disk surfaces of hard disk drives (HDDs) are coated with a layer of lubricant that is only several nm thick to maintain the reliability of the head-disk interface (HDI). To increase the recording density, the HDD flying height has been reduced to as low as about 10 nm. Since lubricant technology is one of the most important technologies for lowering the flying height, Fujitsu is making continuous efforts to develop novel lubricants for ultra low flight applications. Through these efforts, we have developed a lubricant whose chemical structure has been adjusted to realize a molecular monolayer having no significant change in molecular weight. Because of this feature, this lubricant should provide a solution to the trade-off problem with conventional, low molecular weight lubricants; namely, the fact that they provide good flight characteristics but poor HDI reliability because of their high evaporation rate.

• Ultra High-Density Perpendicular Magnetic Recording Technologies for Hard Disk Drives

Perpendicular magnetic recording has been investigated for ultra high-density magnetic recording since 1977. Important problems with perpendicular recording were wide-area data erasure, a deterioration of the bit error rate (BER) due to noise from the soft-magnetic underlayer (SUL), and the high cost of producing a thick SUL to secure an adequate BER at high recording densities. To overcome these problems, we developed the following technologies: a granular recording layer, an anti-parallel structure (APS)-SUL, and a trailing shielded head. By applying these technologies, we improved the density of perpendicular magnetic recording to over 300 Gbit/in². In this paper, we describe our head/media investigations and a signal processing study that compared partial response targets containing a DC response.

• Advanced Technologies in Synthetic Ferrimagnetic Media

Recently, synthetic ferrimagnetic media (SFM) have been widely employed as high-density recording media for hard disk drives (HDDs). The signal-to-noise ratio (SNR) and thermal stability of magnetically written bits have been improved by three new SFM technologies. The first technology greatly improves the SNR in the lower areal magnetization region by increasing the magnetic anisotropy energy of the stabilizing layer without degrading the thermal stability. The second is an exchange enhancement technology that uses a Co-based layer adjacent to the Ru exchange coupling layer to improve the thermal stability without degrading the SNR. On SFM media that incorporate this second technology, stable, non-percolating high-density patterns of up to 900 kfci were clearly observed using high-resolution magnetic force microscopy (MFM). The third technology uses a separated recording layer with a Ru/CoCr/Ru exchange-coupling layer (separated SFM) to provide a more than 50% reduction of medium noise power with the same or better thermal stability than that of conventional SFM. These technologies enable the design of practical media for recording densities over 150 Gbit/in². This paper introduces these new SFM technologies.

• TMR Film and Head Technology

We have developed Al-O barrier magnetic tunnel junctions (MTJs) and achieved a magneto-resistance (MR) ratio of 27% with a resistance-area product (RA) of about 3Ω·µm². Moreover, we conducted research on the use of Ti-O and MgO barrier magnetic tunnel junctions as new low barrier energy materials and obtained an MR ratio of 40 to 50% with an RA of 2 to 3Ω·µm² in MgO barrier MTJs by using CoFeB for the pinned layer and Co₇₄Fe₂₆/NiFe for the free layer. We created Al-O barrier TMR heads that enable an areal recording density of 100 Gbit/in². Measurements using a synthetic ferrimagnetic medium indicated a large playback output signal of approximately 5000 µVpp with a bias voltage (V_b) of 150 mV. Micromagnetic simulations of head noise showed that the dominant noise was thermal fluctuation noise and that it could be reduced by strengthening the exchange coupling field H_ex between the pinned and antiferromagnetic layers. This paper describes these new TMR film and head technologies.

• CPP-GMR Technology for Future High-Density Magnetic Recording

To realize a Current-Perpendicular-to-Plane Giant Magnetoresistance (CPP-GMR) magnetic read head, we have enhanced the CPP magnetoresistance of fully metallic spin-valves. This was achieved by developing a new high-resistivity magnetoresistive material for the magnetic layers that contains a relatively high-resistivity metal and has spin-dependent scatterings. This material expands the possibility of improving the output of the CPP-GMR, which has a low resistance, and enables recording densities in excess of 300 Gbit/in². Because a high output, high signal-to-noise ratio, and low resistance are required in read sensors for high-density recording and fast data-transfer, CPP-GMR technology will be indispensable for the future system of high-density magnetic recording.

• Thermally Assisted Magnetic Recording

Thermally assisted magnetic recording can solve the fundamental problems of thermal fluctuation and write capability in magnetic recording and is regarded as the key technology for achieving densities exceeding 1 Tbit/in². In particular, the combination of optical dominant recording and Curie temperature recording is recognized as the best method for overcoming this limit. We have built a dynamic write-read tester composed of a conventional spin stand and a newly designed magneto-optical head. In this paper, we show the capability of writing small magnetic domains on TbFeCo recording media with a high coercive force and describe a technology for recording tracks having a pitch of less than 50 nm.

• Nanohole Patterned Media

An alumina nanohole array made by anodic oxidation of aluminum and filled with a magnetic nanopillar is a promising candidate for high-density patterned media because a magnetic nanopillar formed in a nanohole has a high magnetic shape anisotropy. This feature, along with the effect of a large bit volume, makes the nanopillar robust against thermal fluctuation and also makes it possible to achieve single-domain recording by separating each nanohole using non-magnetic alumina. In this paper, we describe a process for fabricating a head-flyable media on a glass substrate and show how its recording characteristics are improved by narrowing the Co nanopillar pitch and incorporating a magnetic soft-underlayer for perpendicular recording. To apply this process to bit-patterned media, the nanoholes must be aligned circumferentially so that each nanohole records a single bit. We therefore developed a new method for obtaining one-dimensionally aligned nanoholes that shows promise for realizing Tbit/in² recording.