What Is Magnetic Recording?

Magnetic recording is a data storage technology in which information is encoded as patterns of magnetization in a thin ferromagnetic medium, and retrieved by detecting the stray fields produced by those patterns with a sensing element. It underlies the hard disk drives that store the majority of the world's digital data, as well as magnetic tape archives and earlier technologies including floppy disks and audio cassettes. The fundamental principle has remained unchanged since Valdemar Poulsen demonstrated magnetic wire recording in 1898, but the achievable areal bit density has grown by more than seven orders of magnitude over the following century through advances in head design, medium materials, and recording geometry.

The physics of magnetic recording draws on electromagnetism, ferromagnetic materials science, and signal processing. Noise performance, linear density, and track density are the three competing dimensions of any recording system, and advancing one typically places constraints on the others.

The Recording Process and Magnetic Heads

Writing is performed by a recording head that produces a localized fringing magnetic field strong enough to switch the coercivity of the medium below it. In longitudinal recording, the magnetization of individual bit cells lies in the plane of the medium; in perpendicular recording, it is oriented normal to that plane. A thin-film inductive write head consisting of a magnetic yoke and a narrow pole tip concentrates flux to a spot comparable to the gap dimension, typically tens of nanometers.

Reading is now performed almost universally by magnetoresistive heads, which detect the stray field from bit transitions through a resistance change rather than through the induced voltage exploited by older inductive read elements. Giant magnetoresistance (GMR) spin-valve read heads, introduced commercially in 1997, were later superseded by tunneling magnetoresistance (TMR) heads whose resistance ratios exceed 100 percent, enabling reading of smaller and more closely spaced bits. A historical and technical account of this progression appears in the Computer History Museum's record of perpendicular magnetic recording.

Perpendicular and Heat-Assisted Recording

Perpendicular magnetic recording (PMR) was first demonstrated by Shun-ichi Iwasaki at Tohoku University in 1977 and became the industry standard in 2005 to 2006, enabling areal densities beyond the superparamagnetic limit that constrained longitudinal recording to roughly 100 to 200 Gbit per square inch. In PMR, a single-pole write head drives flux vertically through a high-coercivity recording layer into a soft magnetic underlayer that completes the magnetic circuit, producing a sharper write field gradient and permitting smaller, more thermally stable bit cells.

The development of this technology is documented in a review of perpendicular magnetic recording development and commercialization covering Iwasaki's foundational work through the early commercial products from Toshiba, Seagate, and Hitachi. Heat-assisted magnetic recording (HAMR), which temporarily heats the medium with a near-field laser to lower its coercivity during writing, is the successor technology under active development, targeting densities above 4 Tbit per square inch.

Magnetic Memory and Solid-State Variants

Magnetic random-access memory (MRAM) extends the principles of magnetic recording to solid-state, randomly addressable devices. In spin-transfer-torque MRAM (STT-MRAM), each memory cell is a magnetic tunnel junction whose resistance state encodes a single bit; writing is accomplished by passing a spin-polarized current through the junction rather than applying an external field. STT-MRAM combines non-volatility with the endurance and speed of static RAM, making it attractive for embedded memory in microcontrollers and as a cache replacement. IEEE Xplore hosts an extensive body of conference publications on perpendicular magnetic recording technologies that covers both hard disk and MRAM directions of the field.

Applications

Magnetic recording has applications in a wide range of fields, including:

• Enterprise and consumer hard disk drive storage
• Magnetic tape archiving for long-term data preservation
• Embedded MRAM in industrial microcontrollers and automotive electronics
• Read/write head development for advancing areal density
• Spin-transfer-torque memory for cache and working memory applications