BASICS OF MAGNETIC RECORDING
Introduction and Overview
Read-Rite's recording heads are the miniaturized hearts of disk drives and other magnetic storage devices. While they may appear to be simple components, their design and manufacture require leading-edge capabilities in device modeling, materials science, photolithography, vacuum deposition processes, ion beam etching, reliability testing, mechanical design, machining, air bearing design, tribology, and other critical skills. In general, recording heads function according to certain principles of magnetic recording which are based directly on four magnetic phenomena:
Magnetic Phenomena
| A. |
An electric current produces a magnetic field. |
| B. |
Some materials are easily magnetized when placed in a weak magnetic field. When the field is turned off, the material rapidly demagnetizes. These are called Soft Magnetic Materials. |
| C. |
In some magnetically soft materials the electrical resistance changes when the material is magnetized. The resistance goes back to its original value when the magnetizing field is turned off. This is called Magneto-Resistance or the MR Effect. Giant Magneto-Resistance, or the GMR Effect, is much larger than the MR Effect and is found in specific thin film materials systems. |
| D. |
Certain other materials are magnetized with difficulty (i.e., they require a strong magnetic field), but once magnetized, they retain their magnetization when the field is turned off. These are called Hard Magnetic Materials or Permanent Magnets. |
These four phenomena are exploited by Read-Rite in its design and manufacture of magnetic recording heads which read and write data (the source of the company's name) for storage and retrieval in computer disk drive memories, tape drives, and other magnetic storage devices.
Applications in Data Storage
Writing Heads
Heads used for writing bits of information onto a spinning magnetic disk depend on phenomena A and B to produce and control strong magnetic fields.
Reading Heads
Reading heads depend on phenomena A, B, and C, and are sensitive to the residual magnetic fields of magnetized storage media (D).
Storage Media (e.g., computer disks)
Magnetic storage media are permanently magnetized in a direction (North or South) determined by the writing field. Storage media exploit phenomenon D.
Writing Magnetic Data
Simplified sketches of a writing head are shown in Figure #1 . The view from the top of the writing head (left) shows a spiral coil wrapped between two layers of soft magnetic material; on the right is a cross-section of this head as viewed from the side. Note two things in this figure: at the lower end, there is a gap between these layers, and at their upper end these layers are joined together. The top and bottom layers of magnetic material are readily magnetized when an electric current flows in the spiral coil, so these layers become North and South magnetic poles of a tiny electromagnet. [In a real head, the distance from the gap to the top of the coil is about 30 microns (or 0.0012 inch).]
FIGURE 1: A WRITING HEAD
The N-S poles at the gap end of the writing head further concentrate the field to make this region the business end, which is the area where the writing field leaks into space outside the head. When a magnetic storage medium (a spinning computer disk, for example) is put in close proximity with the writing head, the hard magnetic material on the disk surface is permanently magnetized (written) with a polarity that matches the writing field. If the polarity of the electric current is reversed, the magnetic polarity at the gap also reverses.
Computers store data on a rotating disk in the form of binary digits, or bits transmitted to the disk drive in a corresponding time sequence of binary one and zerodigits, or bits. These bits are converted into an electric current waveform that is delivered by wires to the writing head coil. This process is sketched in Figure #2. In its simplest form, a one bit corresponds to a change in current polarity, while a zero bit corresponds to no change in polarity of the writing current. A moving disk is thus magnetized in the positive (North) direction for positive current and is magnetized in the negative (South) direction for negative current flow. In other words, the stored ones show up where reversals in magnetic direction occur on the disk and the zeroes reside between the ones.
| FIGURE 2: |
WRITING DATA ON A
STORAGE MEDIUM |
A timing clock is synchronized to the turning of the disk and bit cells exist for each tick of the clock; some of these bit cells will represent a one (a reversal in magnetic direction such as N going to S or S going to N) and others represent zeroes (constant N or constant S polarity). Once written, the bits at the disk surface are permanently magnetized in one direction or the other until new data patterns are written over the old. A fairly strong magnetic field exists directly over the location of ones and fades rapidly in strength as the recording head moves away. Moving significantly in any direction away from a one causes a dramatic loss of magnetic field strength, thus, to reliably detect data bits, it is extremely important for reading heads to fly very close to the surface of a magnetized disk.
Reading Magnetic Data
In the case of Read-Rite's leading edge products, recording heads read magnetic data with magnetically sensitive resistors called Spin Valves which exploit the GMR Effect. These GMR/Spin Valve heads are placed in close proximity to a rotating magnetized storage disk, thereby exposing the GMR element to magnetic bit fields previously written on the disk surface. If a GMR head is moved only slightly away from the disk (perhaps 2 to 3 millionths of an inch) the field strength drops below a useful level, and magnetic data cannot be faithfully retrieved.
When a current is passed through the GMR element, changes in resistance (corresponding to changes of magnetic states arising from written N and S bits) are detected as voltage changes. These voltage fluctuations -- referred to as the signal-- are conducted to the GMR sensor terminals. Electrical noise, however, is present in all electrical circuits (GMR heads are no exception) so the combined signal and noise from a GMR reader are sent via wires to the disk-drive electronics for decoding the time sequence of pulses (and spaces between pulses) into binary ones and zeroes. The reading process, including the undesired but ubiquitous noise, is sketched in Figure #3.
| FIGURE 3: |
READING DATA FROM A
STORAGE MEDIUM |
Storing more information on a computer disk or other medium is a function of squeezing as many pulses as possible onto a data storage track. However, when pulses are very close to one another, electronic decoders suffer in their ability to separate ones from zeroes in the presence of electrical noise. This problem is alleviated somewhat by placing the GMR element between two layers of soft magnetic material to shield the element from the influence of bit fields of adjacent ones. These shields, also shown in Figure #3 , have the effect of slimming down the data pulses significantly, allowing more information to be stored and faithfully retrieved.
In creating new products for storing more data per disk, Read-Rite must design GMR elements to be extremely thin (about 15 molecular layers) and quite narrow (less than one-fifth the diameter of a red blood cell). Moreover, the GMR elements cannot short out electrically with shields, which are relatively thick, wide and tall.
Integrating the Write and Read Heads
A basic writing head is comprised of a magnetic yoke, a writing gap in the yoke, and a coil for energizing the head field. Today's reading heads have a GMR element with excitation/sensing leads, and magnetic shield layers on both sides of the sensor. While writing and reading are clearly independent functions, it is very important to place write and read heads in close proximity to the recording medium, to have the write gap and GMR element close to each other, and to maintain tight geometrical alignment between both heads. For these reasons, Read-Rite fabricates writing heads directly on top of Spin Valve/GMR reading heads using state-of-the-art photolithographic technology to ensure precise control of all critical-to-function dimensions and their mutual alignments. In this manner, the top shield of the GMR sensor becomes the bottom magnetic pole of the writing head and the result is an integrated write-read structure, or so-called merged-head design, where the GMR head and writing head share one magnetic layer. A cross-section of this merged head-shared pole concept is sketched in Figure #4.
| FIGURE 4: |
INTEGRATED WRITE-READ HEAD |
In our present manufacturing facility we use 6-inch by 6-inch substrates to build up the complex multi-layered reading and writing devices. About 30,000 integrated heads may be fabricated from a single substrate; subsequent machining and etching operations (slicing, grinding, polishing, ion milling, reactive ion beam etching) define the precise geometry of individual head sliders. The sliders are aligned and bonded to suspensions (which form HGA's), electrical connections for writing and reading are attached, and these assemblies are then cleaned, inspected and electrically tested (Figure #5).
FIGURE 5: HEAD GIMBAL ASSEMBLY
Magnetic Recording Performance
Magnetic storage devices such as disk drives are commonly ranked according to their ability to store significant quantities of computer data in a very small region. Packing data into a small disk drive may be achieved in three ways: Bit Density (the number of bits along a track) may be increased, Track Density (the number of tracks on a disk) may be increased, and the number of disks in a given stack height may be increased.
Disk drive performance is usually measured by how many bits can be stored per unit area of a storage surface. This measure of such performance, called Areal Density, is the product of Bits/inch and Tracks/inch. Sometimes this measure is translated into how many bytes of data (there are 8 bits in a byte) are stored on one surface of a disk. For example, in today's newest disk drives, areal density is between 4000 and 6000 Megabits per square inch (a Megabit is one million bits), translating into 5600 to 8400 Megabytes per surface of a 3.5-inch diameter disk. Given that a typical 300-page book requires about 750,000 bytes of storage, a single surface of a disk has the potential to store somewhere between 7500 and 11,200 books.
Continuous improvement in areal density over the history of magnetic recording - and the related leaps in the cost-effectiveness of recording head and disk technologies - is one reason for the computer industry's enormous success. The very first rotating disk drive (ca. 1957) stored data at 100 bits/inch and 20 tracks/inch (2000 bits/square inch). Since the early 1990's, industry leaders including Read-Rite have sought to increase areal density at a cumulative growth rate (CGR) of 60% per year, which is equivalent to a doubling of storage capacity every 18 months.
Addressing the Challenges of Increasing Areal Density with New Head Designs
There are significant design and materials processing challenges involved in reducing the size of writing and reading heads to conform to the higher track and bit densities that enable areal density to double every 18 months. Because output signal declines proportionally with reductions in track width, new GMR/Spin Valve materials with greater sensitivity to magnetic fields must be developed to restore signals to useful levels above the noise. Moreover, signal pulses spread out and overlap adjacent pulses along the storage track; because increased bit density exacerbates pulse overlap and loss of signal, GMR head shields must be placed even closer together to compensate for this effect. Finally, the spacing between the GMR head and the disk must be reduced to increase the strength of bit fields and restore signals to useful levels.
In each new generation of head designs, Read-Rite seeks to meet these and other challenges. Reductions of width, height, thickness or spacing must be married with improvements in process tolerances for high-volume production at acceptable prices, and new materials must be tested for long term reliability and stability in environmental conditions imposed on disk drives. These are some of the considerations shaping the evolution of our technology and our business.
