Magnetic recording and optical recording are the major technologies used to provide long-term storage of information in today's computer systems. Magnetic recording has been used for data storage in computer systems for over 40 years, and the advances in technology that have occurred in that time frame are nothing short of phenomenal. One might expect that after 40 years of dominance, the rate of progress in magnetic recording would be slowing down and that other technologies would be moving in to replace it. However rather than slowing down its rate of progress, magnetic recording is now advancing at a faster rate than at any time in the past. Magnetic hard-disk drives represent the largest segment of the data-storage business, and the number of hard-disk drives sold is increasing at about 20% per year. Tape drives continue to enjoy a very substantial market and are also advancing at a rapid pace while flexible disk drives continue to appear in every personal computer sold and have recently increased capacity by nearly two orders of magnitude.

Optical recording was introduced into the marketplace in 1989 and has secured a significant market. However thus far, optical recording has primarily found new market niches, rather than being directly competitive with magnetic recording. CD-ROMs are widely used for the distribution of prerecorded information—a business that is now comparable in size to the magnetic-tape-drive business. On the other hand, erasable, optical drives, which were first introduced in 1989, have not had nearly as much success and have much smaller markets than either magnetic hard drives or tape drives.

The materials used in magnetic recording heads have recently received a tremendous amount of attention. This has been the result of a fortunate set of circumstances. Ever-increasing demands for information storage, especially for graphics-intensive applications, have necessitated unprecedented increases in disk-drive areal densities. Combined with this are recent discoveries in the area of magnetoresistive materials, enabling the design and fabrication of much more sensitive recording heads. The end result is a flurry of activity that has come to dominate the field of magnetics. This article will explore choices for magnetoresistive read head materials, with an emphasis on the materials challenges.

The recording heads that are used in high-performance disk drives typically consist of separate magnetoresistive read and inductive write heads (see Figure 1) where previously a single inductive head performed both functions. Separation of the two heads allows each to be optimized for their individual function, an essential factor in enabling disk drives to contain gigabytes of storage. The write head is the simpler of the two, consisting of a U-shaped ferromagnet surrounding a set of coils. The ends of the ferromagnet are the magnetic poles defining the write gap. When current passes through the coils, a field bridges the gap, setting the orientation of the magnetization in the media. Information is stored by changing the polarity of the current in order to write a pattern of magnetic domains in the media. The materials used in write poles will be reviewed in the section, Write Head Materials.

The continued exponential growth in areal density for longitudinal magnetic-recording devices places ever more stringent demands on disk performance. The design of materials and processes must provide the required advances in technology. The magnetic properties are controlled through the choice of underlayers, magnetic alloys, and the deposition processes that control crystallographic orientation and magnetic isolation between grains. The requirement of lower head-disk spacing places increasing stress on the tribological performance of the disks, controlled by a very thin overcoat and lubricant layer. This article reviews the various materials issues relevant to magnetic-disk technology.

The major obstacle for achieving high areal density in thin-film media is transition noise. This noise arises from the zig-zag transition boundaries that occur due to cooperative switching of the magnetic grains. Both exchange coupling between grains and magnetostatic interactions cause magnetic-cluster sizes larger than the grain size. The goal is to magnetically isolate the grains and keep the grain size small. As the dimensions of the bit cell shrink, smaller grain size is required to obtain enough grains per bit cell to maintain the required signal-to-noise ratio (SNR). In the 10–40-Gbits/in.2 areal-density range, the issue of thermal stability of small (<10 nm), isolated grains needs to be addressed.

In addition to good SNR, a narrow transition width is needed in order to pack the transitions closer together. The objective is to minimize interactions between transitions to reduce nonlinear amplitude loss and superlinear noise.

Magnetic recording has been responsible for the widespread and inexpensive recording of sound and video. Despite the availability of other means of storing data, such as optical recording and semiconductor devices, flexible magnetic-recording media have advantages such as (1) low cost, (2) stable storage, (3) a relatively high data rate, (4) a relatively short seek time, and (5) high-volumetric information density.

The first commercially available magnetic-recording tapes were produced in 1947 by the 3M Company. Since that time, magnetic tapes have developed rapidly for use in audio, video, and digital-data recording systems.

The linear-analogue technique is commonly used for most audio recorders. The magnetic tape is transported at a speed of several cm/s over a stationary head. On the other hand, helical-scanning rotary heads were developed for video recording, which afforded a high head-to-tape speed of more than several m/s and high recording-density capabilities. However high relative speed causes wear of the tape. The success of a tape in actual use depends critically on its tribological properties.

Magnetic media are divided into two groups: (1) particulate media where magnetic particles are dispersed in a polymer binder with some additives and coated onto the substrate and (2) thin-film media in which monolithic, magnetic thin films are deposited onto the substrate in vacuum. The overwhelming preponderance of media fabricated to date have been coated media. However continuous demand for increasingly higher recording density has led to thin-film media.

With significant developments in laser and semiconductor industries, optical-storage technology has successfully emerged into the consumer marketplace and more recently into the computer-based data-storage marketplace as well. For computer-based data storage and other purposes, the first generation of the International Standards Organization (ISO) standard 5-1/4-in. magneto-optic (MO) drive (325 Mbytes × 2/double-sided) was introduced in 1988 and the 3.5-in. drive (128 Mbytes) in 1991. Since then progress has been remarkable in capacity and data-transfer rate as well as cost/performance ratio, and the so-called 2X (650 Mbytes × 2 per 5-in. double-sided disk) and 3X (1 Gbyte × 2 per 5-in. double-sided disk) drives are now in the marketplace. Also a remarkable product called the MiniDisc (2.5 in.), the first recordable optical system for both the consumer and the data market, utilizes data compression and direct overwrite schemes.

The solution for increasing the capacity is by no means simple since the new technology must accommodate many factors. One of the key factors is backward compatibility. This feature is important to removable media because the customer expects new products to be compatible with their previous investments in data stored on older generation media. Another key factor is cost. The sales price ($/Mbyte) must be significantly lower than prior generations of optical-storage products and alternative storage systems (e.g., removable hard disks).

To increase the capacity in optical-storage media, several solutions are possible: short-wavelength lasers, magnetically induced super resolution (MSR), pulse width modulation (PWM), volumetric recording, land/groove recording with crosstalk cancellation, partial-response channels, banding of data, and higher numerical aperture objective lenses.

Almost all stones on a lane will become glassy if they are melted and quenched. They will become transparent and quite different in appearance from before vitrification. This visible change constitutes the recording of information. We might refer to the stone as “1 bit.” If the vitrified stone is subsequently kept at a high temperature under its melting point, it will lose its transparency and turn back to the appearance it had before melting and quenching. Thus the “1 bit” is erased. This is the simple mechanism of an erasable phase-change optical memory. In practical systems, a laser beam focused into a diffraction-limited spot is used for recording. This enables the spatial size of the “1 bit” to be very small (of submicron order) so that the recording density is very high.

Figure 1 shows a transmission-electron-microscope (TEM) photograph of an actual optical disk. The elliptical smooth areas are recording marks in the amorphous state that were formed by high-power and short-duration laser irradiation. The shortest mark length is about 0.5 μm. The area surrounding the amorphous marks is in the crystalline state and consists of small grains. The two states differ from each other in optical properties such as refractive indices and optical absorption coefficients. Accordingly when the bits are illuminated with low-intensity laser light, the reflected light from the amorphous and crystalline regions is different and may be detected as information signals.

The amorphous marks are erased by heating above the glass-transition temperature by laser irradiation, but with lower power than is used in the case of recording.

In holographic data storage, a photo-sensitive medium is exposed to the interference pattern that is generated when an object beam, with an input data page encoded in the spatial profile of the beam, is intersected by a second, coherent laser beam. The photosensitive medium replicates these interference fringes as a change in optical absorption, refractive index, or thickness. Data are retrieved from the medium by exposing it to light from just one of the beams, which is then diffracted from the stored fringe pattern to reconstruct the other beam, including all the information that had been in the input data page. For a material of sufficient thickness, a large number of interference patterns, each identified by a different grating vector, can be stored or “multiplexed” in the same volume element, with negligible crosstalk between the individual interference patterns. Multiplexing of a large number of pages in the same volume element of the recording medium can be accomplished in several ways—for example by varying the angle between object and reference beam or the wavelength of both beams. Given no other limiting factors, the number of holograms that can be multiplexed in one volume element is directly proportional to the product of the thickness of the medium and its refractive index—that is, materials with optical thicknesses of the order of several millimeters are desirable.

By its very nature, the holographic-storage mechanism distributes the stored information redundantly throughout the recording volume.