The spins and orbits of electrons in effect turn any atom into a tiny bar magnet. For most materials the magnetic moments of these atoms point in random directions and their fields cancel to produce no net magnetism.

In contrast, certain substances are ferromagnetic and their magnetic moments spontaneously align so their fields are parallel to each other and add together. This alignment is limited to a small region called a domain, with many such domains making up a ferromagnetic material.

Although they have strengthened magnetic fields, the domains themselves are randomly oriented, again resulting in no overall magnetism. An external magnetic field, however, can align the domains so their own magnetic fields reinforce each other, producing a net field throughout an object and therefore creating a magnet. This phenomenon, called ferromagnetism, is the basis of everyday magnets. At room temperature only four elements are ferromagnetic and have this behavior: iron, cobalt, nickel and gadolinium.

Soft magnetic materials like iron are easy to magnetize but the domains randomize as soon as the external field disappears; consequently, the material quickly loses its magnetism. This property is useful for electromagnets and devices like tape recording or erasing heads, which need to generate temporary or rapidly changing magnetic fields.

Hard magnetic materials like steel are more difficult to magnetize and also more difficult to demagnetize; after removal of the external field, they can retain their magnetism for a long time – sometimes for millions of years, a characteristic that aids in the geological dating of rocks. Hard magnetic materials are therefore used to make permanent magnets.

This magnetizing process has wide practical applications, with the tape recorder as only one example. Recording tape consists of a long, thin Mylar strip coated with fine particles of iron oxide or chromium dioxide. As the tape moves beneath the record head, a magnetic field aligns domains on this coating in response to the music or data signal. Afterwards the domains retain the impressed magnetic field for later replay.

Computer hard drives use essentially the same process for magnetic data storage on rapidly spinning platters.

After coming in contact with magnets or magnetic clamping tables, steel objects can become unintentionally magnetized. Machining, welding, grinding and even vibration can also magnetize steel. Undesired effects include tools that attract metal chips and shavings, a rough surface after galvanization and welds that only penetrate one side.

Similarly, constant contact with magnetic tape can impart a residual magnetism to recording equipment, which increases noise and causes inaccurate sound recording.

To be reused, an audio tape can be restored to a blank state by running the length of it past an erase head, a tedious and impractical process, especially on large scale. Discarded computer hard drives may have proprietary or sensitive data that should not be available to others. In these cases the recording medium must be demagnetized in bulk.

The nuisance of undesired magnetism has led to the development of both small and industrial demagnetizers. A demagnetizer, also known as a degausser, uses electromagnets to generate intense, high frequency AC magnetic fields. In response, individual domains realign randomly so their magnetic fields cancel or nearly cancel, eliminating or substantially reducing undesired magnetism.

Some degaussers don’t use electricity or electromagnets but have rare earth magnets instead, to provide the necessary powerful magnetic fields.

This demagnetizing principle is also used tape recorders. As the tape passes beneath an erase head, a high amplitude, high frequency magnetic field randomizes the domains in preparation for recording new sound or data. On a larger scale, bulk demagnetizers erase entire spools of magnetic tapes or hard drives in a single step.

A demagnetizer machine may have one of several common configurations, depending on the purpose. A portable demagnetizer tool would degauss drill bits, chisels or small parts resting on a flat surface or passing through a hole.

Thick materials or large solid objects might have to pass through a demagnetizing tunnel big enough to fit a standing person. The frequency, demagnetizing field strength and throughput speed must be tailored to the object and residual magnetic field being erased.