Mark H. Kryder
A schematic of a magnetic recording system is illustrated in Fig. 2.1 . The magnetic recording medium consists of a magnetic coating on some form of substrate. (The substrate is not shown.) In the case of magnetic tape, the substrate is a flexible medium, such as Mylar, whereas in a magnetic disk drive it is typically an aluminum alloy or glass. To record and play back the information one or more magnetic recording heads are used. The recording head consists of a high-permeability magnetic core with a narrow gap cut into it and a few turns of conductor wound around it. When current flows through the conductor, magnetic flux flows through the magnetic core, emanates from the core at the gap and penetrates the magnetic medium, causing it to be magnetized to the right or the left. Binary data are encoded in the form of transitions (ones) or the absence thereof (zeroes) in the magnetization in coincidence with a clock, which is synchronized with the disk or tape motion. A similar recording head is used to sense the magnetic flux emanating from the recorded transitions in the medium during read back. In order to achieve high recording density it is imperative that the head be very close to the medium. Spacings of the order of 50 nm are used in today's disk drives. Highly sophisticated signal processing electronics are used to encode binary ones and zeroes into the write current waveforms and also to convert the waveforms sensed by the read head back into digital data. An actuator is used to servo-position the head relative to the media for accessing the desired track of data.
The rotation rates of magnetic disk drives today range from 3,600 to 10,800 rpm. With high performance actuators, it is possible to access a track on a disk in a couple of milliseconds. Hence, total access time to a random sector on the disk is only a few milliseconds, and disks provide relatively fast access to data.
Magnetic tape drives on the other hand, record data linearly over the length of the tape. Average access time is the length of time it takes to transport half the length of tape over the head and is typically many seconds. Although tape has a relatively long access time, since it is very thin and can be wound upon itself, it offers an extremely high volumetric storage density and low cost.
A relatively new technology in both disk and tape drives is magnetoresistive (MR) head technology. Previously, inductive heads, which sensed the time rate of change of magnetic flux in the head core, were used. However, inductive heads have limited sensitivity, and the amplitude of the read back signal depends upon the relative head-medium velocity. Magnetoresistive heads, on the other hand, are considerably more sensitive than inductive read heads, and since they directly detect the amount of flux flowing through the head core, the signal amplitude is independent of the head-medium velocity. Recently, IBM and Japanese manufacturers Yamaha and TDK have introduced giant magnetoresistive (GMR) head technology. GMR heads offer yet higher sensitivity than conventional MR heads.
Fig. 2.1. A schematic diagram of a magnetic recording system.