Introduction
IBM introduced the 305 RAMAC computer
in 1956. The system included the IBM
350 magnetic disk drive, which had a storage
capacity of 4.4 Mbytes, was the size of
two large refrigerators, and weighed two
tons. Today, laptop computers commonly
have disk drives that can store 20–100
Gbytes and are the size of a pack of cards.
This represents an increase in the areal
density (number of bits per square inch of
disk surface) of almost eight orders of
magnitude: from 0.002 Mbits/in.2 in 1956
to 100 Gbits/in.2 in today’s state-of-the-art
drives. Throughout this half-century of
disk drive development, the basic recording
principle—longitudinal magnetic recording
(LMR)—has remained the same. However,
today’s high-capacity drives are nearly
the ultimate realization of this recording
scheme, and future drives will most likely
use a scheme known as perpendicular magnetic
recording (PMR), where the magnetization
in the bits is perpendicular to the
disk surface. As the recording industry
transitions from LMR to PMR, it is worth
reviewing two of the technologies that have
allowed this remarkable increase in storage
capacity, the thin-film disk and the giant
magnetoresistive (GMR) read-back head.