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Read/Write Head Designs
As disk drive technology has evolved, so has the design of the read/write head. The earliest heads were simple iron cores with coil windings (electromagnets). By today's standards, the original head designs were enormous in physical size and operated at very low recording densities. Over the years, head designs have evolved from the first simple ferrite core designs into the several types and technologies available today. This section discusses the various types of heads found in PC hard disk drives, including the applications and relative strengths and weaknesses of each.
Five main types of heads have been used in hard disk drives over the years:
Ferrite heads, the traditional type of magnetic-head design, evolved from the original IBM 30-30 Winchester drive. These heads have an iron-oxide core wrapped with electromagnetic coils. The drive produces a magnetic field by energizing the coils or passing a magnetic field near them. This gives the heads full read/write capability. Ferrite heads are larger and heavier than thin-film heads and therefore require a larger floating height to prevent contact with the disk while it is spinning.
Manufacturers have made many refinements to the original (monolithic) ferrite head design. One type of ferrite head, called a composite ferrite head, has a smaller ferrite core bonded with glass in a ceramic housing. This design permits a smaller head gap, which enables higher track densities. These heads are less susceptible to stray magnetic fields than the older monolithic design heads.
During the 1980s, composite ferrite heads were popular in many low-end drives, such as the Seagate ST-225. As density demands grew, the competing MIG and thin-film head designs came to be used in place of ferrite heads, which are virtually obsolete today. Ferrite heads can't write to the higher coercivity media necessary for high-density disk designs and have poor frequency response with higher noise levels. The main advantage of ferrite heads is that they are the cheapest type available.
Metal-In-Gap heads are a specially enhanced version of the composite ferrite design. In MIG heads, a metal substance is applied to the head's recording gap. Two versions of MIG heads are available: single-sided and double-sided. Single-sided MIG heads are designed with a layer of magnetic alloy placed along the trailing edge of the gap. Double-sided MIG designs apply the layer to both sides of the gap. The metal alloy is applied through a vacuum-deposition process called sputtering.
This magnetic alloy has twice the magnetization capability of raw ferrite and enables the head to write to the higher coercivity thin-film media needed at the higher densities. MIG heads also produce a sharper gradient in the magnetic field for a better-defined magnetic pulse. Double-sided MIG heads offer even higher coercivity capability than the single-sided designs.
Because of these increases in capabilities through improved designs, MIG heads were for a time the most popular head design and were used in many hard disk drives in the late '80s and early '90s. They are still used today in LS-120 (SuperDisk) drives.
Thin-film heads are manufactured much the same way as a semiconductor chip—through a photolithographic process. This process creates many thousands of heads on a single circular wafer and produces a very small, high-quality product.
TF heads have an extremely narrow and controlled head gap that is created by sputtering a hard aluminum material. Because this material completely encloses the gap, the area is very well protected, minimizing the chance of damage from contact with the spinning disk. The core is a combination of iron and nickel alloy that has two to four times more magnetic power than a ferrite head core.
TF heads produce a sharply defined magnetic pulse that enables them to write at extremely high densities. Because they do not have a conventional coil, TF heads are more immune to variations in coil impedance. These small, lightweight heads can float at a much lower height than the ferrite and MIG heads; in some designs, the floating height is 2 micro-inches or less. Because the reduced height enables the heads to pick up and transmit a much stronger signal from the platters, the signal-to-noise ratio increases and improves accuracy. At the high track and linear densities of some drives, a standard ferrite head would not be capable of picking out the data signal from the background noise. Another advantage of TF heads is that their small size enables the platters to be stacked closer together, enabling more platters to fit into the same space.
Until the past few years, TF heads were relatively expensive compared with older technologies, such as ferrite and MIG. Better manufacturing techniques and the need for higher densities, however, have driven the market to TF heads. The widespread use of these heads has also made them cost-competitive with, if not cheaper than, MIG heads.
Many of the drives in the 100MB–2GB range used TF heads, especially in the smaller form factors. TF heads displaced MIG heads as the most popular head design, but they have now themselves been displaced by newer magneto-resistive heads.
A more recent development in magnetic recording—or more specifically, the read phase of magnetic recording—is the magneto-resistive head, sometimes also referred to as the anisotropic magneto-resistant (AMR) head. In the last few years, virtually all modern hard disk designs have shifted to using MR heads. MR heads are capable of increasing density four times or greater as compared to the previous inductive-only heads. IBM introduced the first commercially available drive with MR heads in 1991, in a 1GB 3 1/2'' model.
All heads are detectors; that is, they are designed to detect the flux transitions in the media and convert them back to electrical signals that can be interpreted as data. One problem with magnetic recording is the ever increasing desire for more and more density, which is putting more information (flux transitions) in a smaller and smaller space. As the magnetic domains on the disk get smaller, the signal from the heads during reading operations becomes weaker; distinguishing the true signal from the random noise or stray fields present becomes difficult. A more efficient read head, which is a more efficient way to detect these transitions on the disk, is therefore necessary.
Another magnetic effect that is well known today is being used in modern drives. When a wire is passed through a magnetic field, not only does the wire generate a small current, but the resistance of the wire also changes. Standard read heads use the head as a tiny generator, relying on the fact that the heads will generate a pulsed current when passed over magnetic flux transitions. A newer type of head design pioneered by IBM instead relies on the fact that the resistance in the head wires will also change.
Rather than use the head to generate tiny currents, which must then be filtered, amplified, and decoded, a magneto-resistive head uses the head as a resistor. A circuit passes a voltage through the head and watches for the voltage to change, which will occur when the resistance of the head changes as it passes through the flux reversals on the media. This mechanism for using the head results in a much stronger and clearer signal of what was on the media and enables the density to be increased.
MR heads rely on the fact that the resistance of a conductor changes slightly when an external magnetic field is present. Rather than put out a voltage by passing through a magnetic-field flux reversal—as a normal head would—the MR head senses the flux reversal and changes resistance. A small current flows through the heads, and this sense current measures the change in resistance. This design provides an output that is three or more times more powerful than a TF head during a read. In effect, MR heads are power-read heads, acting more like sensors than generators.
MR heads are more costly and complex to manufacture than other types of heads because several special features or steps must be added:
Because the MR principle can only read data and is not used for writing, MR heads are really two heads in one. The assembly includes a standard inductive TF head for writing data and an MR head for reading. Because two separate heads are built into one assembly, each head can be optimized for its task. Ferrite, MIG, and TF heads are known as single-gap heads because the same gap is used for both reading and writing, whereas the MR head uses a separate gap for each operation.
The problem with single-gap heads is that the gap length is always a compromise between what is best for reading and what is best for writing. The read function needs a thinner gap for higher resolution; the write function needs a thicker gap for deeper flux penetration to switch the medium. In a dual-gap MR head, the read and write gaps can be optimized for both functions independently. The write (TF) gap writes a wider track than the read (MR) gap reads. Thus, the read head is less likely to pick up stray magnetic information from adjacent tracks.
A typical IBM-designed MR head is shown in Figure 9.5. This figure first shows the complete MR head-and-slider assembly on the end of an actuator arm. This is the part you would see if you opened up a drive. The slider is the block device on the end of the triangular-shaped arm that carries the head. The actual head is the tiny piece shown magnified at the end of the slider, and then the MR read sensor in the head is shown further magnified.
The read element, which is the actual magneto-resistive sensor, consists of a nickel-ferrite (NiFe) film separated by a spacer from a magnetically soft layer. The NiFe film layer changes resistance in the presence of a magnetic field. Layers of shielding protect the MR sensor read element from being corrupted by adjacent or stray magnetic fields. In many designs, the second shield also functions as one pole of the write element, resulting in what is called a merged MR head. The write element is not of MR design but is instead a traditional thin film inductive head.
IBM's MR head design employs a Soft Adjacent Layer (SAL) structure, consisting of the MR NiFe film, as well as a magnetically soft alloy layer separated by a film with high electrical resistance. In this design a resistance change occurs in the NiFe layer as the MR sensor passes through a magnetic field.
As areal densities have increased, heads have been designed with narrower and thinner MR elements. The newest heads have reduced the film width between the side contacts to as little as half a micron or less.
Giant Magneto-Resistive Heads
In the quest for even more density, IBM introduced a new type of MR head in 1997. Called giant magneto-resistive heads, they are physically smaller than standard MR heads but are so named for the GMR effect on which they are based. The design is very similar; however, additional layers replace the single NiFe layer in a conventional MR design. In MR heads, a single NiFe film changes resistance in response to a flux reversal on the disk. In GMR heads, two films (separated by a very thin copper conducting layer) perform this function.
The GMR effect was first discovered in 1988 in crystal samples exposed to high-powered magnetic fields (1,000 times the fields used in HDDs). Scientists Peter Gruenberg of Julich, Germany, and Albert Fert of Paris discovered that large resistance changes were occurring in materials comprised of alternating very thin layers of various metallic elements. The key structure in GMR materials is a spacer layer of a nonmagnetic metal between two layers of magnetic metals. One of the magnetic layers is pinned, which means it has a forced magnetic orientation. The other magnetic layer is free, which means it is free to change orientation or alignment. Magnetic materials tend to align themselves in the same direction. So if the spacer layer is thin enough, the free layer takes on the same orientation as the pinned layer. What was discovered was that the magnetic alignment of the free magnetic layer would periodically swing back and forth from being aligned in the same magnetic direction as the pinned layer to being aligned in opposite magnetic directions. The overall resistance is relatively low when the layers are in the same alignment and relatively high when in opposite magnetic alignment.
Figure 9.6 shows a GMR read element.
When a weak magnetic field, such as that from a bit on a hard disk, passes beneath a GMR head, the magnetic orientation of the free magnetic layer rotates relative to that of the other and generates a significant change in electrical resistance due to the GMR effect. Because the physical nature of the resistance change was determined to be caused by the relative spin of the electrons in the different layers, GMR heads are often referred to as spin-valve heads.
IBM announced the first commercially available drive using GMR heads (a 16.8GB 3 1/2'' drive) in December 1997. Since then, GMR heads have become the standard in most drives of 20GB and beyond. GMR enables drives to store up to 20Gb of data per square inch of disk surface, enabling drives with capacities exceeding 100GB to be produced in the standard 3 1/2'' wide, 1'' high form factor.
Virtually all hard drives and other types of magnetic media record data using longitudinal recording, which stores magnetic bits horizontally across the surface of the media. However, perpendicular recording, which aligns magnetic signals vertically on the media surface, has the potential to achieve higher data densities because vertically oriented magnetic bits use less space than longitudinally stored bits (see Figure 9.7). Professor Shun-ich Iwasaki was the first to propose this method of magnetic storage in 1976. However, until recently, the only major product to use this recording method was the short-lived 2.88MB floppy drive introduced in 1989 by Toshiba and first used in IBM PS/2 systems starting in 1991. Currently, major drive vendors such as Seagate and Maxtor are experimenting with perpendicular recording as a way to achieve signal density surpassing that achievable even with AFC pixie dust media (see the section "Increasing Areal Density with Pixie Dust," later in this chapter).
In April 2002, Read-Rite Corporation, a major maker of read/write heads, reached areal densities of 130Gb per square inch using media provided by Maxtor subsidiary MMC Technology. In November 2002, Seagate Technology announced it had achieved areal densities of more than 100Gb per square inch using this technology. According to two independent studies published in 2000, perpendicular recording could enable densities of 500Gb–1000Gb (1 terabit) per square inch.
Unlike GMR heads and AFC media (discussed later in this chapter), which can be added relatively easily to existing drive technologies, perpendicular recording requires entirely new read/write head designs. Because of the cost of such a changeover, and because GMR and AFC technologies are sufficient for the present, drives using perpendicular recording are not expected to reach the market until 2004 or later.
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