What Are Magnetic Heads?

Magnetic heads are transducers that read from and write to magnetic recording media by converting between electrical signals and localized magnetic fields. A write head generates a focused magnetic field that aligns the magnetic domains in the coating of a passing medium, encoding a bit pattern. A read head detects the stray field produced by those domains and converts it back into an electrical signal. The term encompasses inductive ring heads used in early tape recorders, composite ferrite heads common in cassette and video equipment, thin-film heads deposited on silicon wafers for hard disk drives, and the magnetoresistive and tunneling-junction read sensors that have defined storage technology since the 1990s. The engineering of magnetic heads draws on electromagnetics, materials science, lithography, and signal processing, and the performance of each successive generation has been the primary driver of exponential growth in data storage areal density.

Magnetic Recording and the Read/Write Function

A conventional hard disk write head uses a coil wound around a ferromagnetic yoke to generate a fringing field at the head gap, the narrow slot between the yoke's poles facing the media. When current flows through the coil, the field at the gap exceeds the coercivity of the recording layer, reversing the magnetization in a small region of the disk. The bit is defined by a transition between oppositely magnetized regions, not by the polarity of a single domain. The gap width and the flying height, the distance at which the head floats above the spinning disk on an air bearing, together determine the minimum transition length and thus the linear density. Perpendicular magnetic recording, which orients the easy axis of the media normal to the disk surface, extended areal densities beyond the superparamagnetic limit that constrained longitudinal recording, allowing densities past 1 terabit per square inch by 2014.

Magnetoresistive Read Sensors

Separate read and write elements became standard in the early 1990s when IBM introduced the first disk drive with a magnetoresistive (MR) read head. The MR effect, the change in electrical resistance of a ferromagnetic conductor in response to an applied field, provides far greater read sensitivity than the inductive approach it replaced. Giant magnetoresistance (GMR), discovered in 1988 and exploited in spin-valve read sensors from the late 1990s, stacks two ferromagnetic layers separated by a thin nonmagnetic spacer; resistance drops when the two layers' magnetizations align parallel and rises when they are antiparallel. Tunneling magnetoresistance (TMR) heads, which replaced GMR read elements after 2004, use a thin insulating barrier instead of a metallic spacer; spin-polarized electrons tunnel through the barrier at a rate that depends on the relative orientation of the magnetic layers, producing a much larger resistance change and thus a stronger signal. The Computer History Museum record of the 1990 magnetoresistive read head introduction documents the technology's progression from 107 Mb/in² with the first MR drive to 84 Gb/in² with early TMR heads.

Solid-State Device Evolution and Nanofabrication

Modern thin-film magnetic heads are manufactured using processes closely related to semiconductor lithography. The write element is built up layer by layer, with the pole tips shaped to nanometer tolerances using chemical mechanical polishing and ion milling. The read sensor is a stack of fewer than twenty functional layers, each deposited by ion beam or magnetron sputtering and controlled to sub-angstrom thickness. The IEEE Xplore study on spin-valve read heads for magnetic recording describes the material selection, deposition conditions, and magnetic annealing steps required to produce sensors with the defined pinned and free layer configurations that enable reliable bit detection. Heat-assisted magnetic recording (HAMR) heads add a near-field optical transducer to locally heat the media above its Curie temperature during writing, enabling even higher coercivity media to support denser bit cells. The ScienceDirect article on tunneling magnetoresistive devices as HDD read heads reviews how TMR devices are engineered for the sub-10-nm gap geometries of current-generation drives.

Applications

Magnetic heads have applications in a wide range of disciplines, including:

• Hard disk drives in enterprise and consumer data storage systems
• Magnetic tape libraries for archival data storage and backup
• Magnetic stripe card readers for payment, identification, and access control
• Industrial and laboratory magnetic field probes built on magnetoresistive elements
• Non-contact current sensors and position encoders using Hall effect and MR transducers