State-of-the-Art Magnetic Hard Disk Drives
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State-of-the-Art
Magnetic Hard Disk Drives
I.R. McFadyen, E.E. Fullerton, and M.J. Carey Abstract Magnetic recording has progressed dramatically over the last 50 years, with an increase of almost eight orders of magnitude in the amount of information stored per unit area of disk space. Two key enablers of this progress have been the recording medium and the read-back head. This article reviews the current state of the art in multilayer thinfilm longitudinal recording media and giant magnetoresistive (GMR) read heads, with particular emphasis on the nanostructured magnetic materials that are key to today’s high-performance hard disk drives. Keywords: film, layered, magnetic properties, magnetoresistance, memory.
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.
Magnetic Recording Figure 1 illustrates the basic concepts of longitudinal magnetic recording: writing,
MRS BULLETIN • VOLUME 31 • MAY 2006
storage, and reading. The system contains a recording head, composed of separate read and write heads, which is supported by a self-generating air bearing in close proximity to the granular magnetic medium. The head is said to “fly” over the disk. The inductive write head is a miniature electromagnet: a time-varying current in a conductor wrapped around a ferromagnetic yoke provides a time-varying magnetic field in the gap of this yoke. This field, in turn, magnetizes regions of the disk as
they pass under the gap. The data are stored as horizontal magnetization patterns, where the ones and zeros correspond to the presence or absence of magnetization reversals. It is from the in-plane, alongtrack orientation of magnetization on the disk that the term LMR is derived. The data is read back by measuring the stray fields (described in the section on GMR Heads) originating from the transitions between regions of opposite magnetization (not the magnetization directly). This analog si
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