The Susceptibility of Magnetic Hard Disk Drives to External dc Magnetic Fields

The first magnetic hard disk drive was created by IBM in 1956. Despite extreme improvements in data density and data transfer rates, the fundamental concept of operation in modern drives remains largely intact (Hayes, 2002). The drives store and retrieve data using moveable read and write heads held proximal to rotating platters. The platters are coated with a thin layer of cobalt alloy (previously an iron-based magnetic material) that is divided into magnetic domains called bit cells. A bit cell in a modern drive contains 50-100 grains of magnetic material, and the collective magnetic orientation of these grains in a single bit cell represents a logical binary "0" or "1." Through the read/write heads, the disk drive has the means to both detect previously written ones and zeroes (read) and to create or reverse magnetic polarization in bit cells to create new stored data (write). Figure 1 shows this arrangement.