What Is Demagnetization?

Demagnetization is the process by which a magnetized material loses its net magnetic moment, returning toward a state of zero or reduced macroscopic magnetization. In ferromagnetic and ferrimagnetic materials, magnetization arises from the alignment of magnetic domains, and demagnetization occurs when those domains become disordered or reoriented so their contributions cancel. The process can be intentional, as when data storage media or permanent magnets must be erased, or it can be an unwanted failure mode in motors, sensors, and magnetically stored information. Demagnetization is a central concern in magnetic materials science, electrical machine design, and precision metrology.

The physical roots of demagnetization lie in statistical mechanics and the thermodynamics of magnetic ordering. A ferromagnetic material maintains its spontaneous magnetization only below the Curie temperature, the point at which thermal energy overcomes the exchange interaction that aligns neighboring magnetic moments. Above this temperature the material becomes paramagnetic and loses all remanent magnetization. Below the Curie temperature, applied fields, mechanical stress, or alternating magnetic fields can reduce magnetization by nucleating and propagating domain walls.

Demagnetization Curves and Coercivity

The response of a magnetic material to an applied demagnetizing field is captured by the hysteresis loop, which relates the applied field H to the resulting flux density B or magnetization M. The second quadrant of this curve, called the demagnetization curve or B-H curve, is particularly important for permanent magnet design. The coercive field H_c is the magnitude of the reverse field required to reduce the net magnetization to zero, and it quantifies how resistant a material is to demagnetization. Hard magnetic materials such as neodymium-iron-boron (NdFeB) have coercivities exceeding 1 MA/m, making them resistant to demagnetization under normal operating fields. Soft magnetic materials such as silicon steel, with coercivities below 100 A/m, are easily demagnetized and re-magnetized, making them suitable for transformer cores. A detailed treatment of coercivity and its relationship to the hysteresis loop is available at the Encyclopedia Magnetica entry on coercivity.

Thermal and Mechanical Demagnetization

Heating a permanent magnet to its Curie temperature, or even to a substantially lower temperature for materials with steep temperature coefficients of coercivity, irreversibly reduces remanence. NdFeB magnets are susceptible to thermal demagnetization at temperatures above roughly 80 degrees Celsius in high-permeability magnetic circuits, a constraint that affects motor and actuator design in elevated-temperature environments. Mechanical shocks and vibrations can also displace domain walls and reduce net magnetization, which is why precision magnetic instruments such as compasses are susceptible to impact. The Hyperphysics reference on coercivity and remanence in permanent magnets provides a quantitative account of how operating temperature and applied reverse fields shift the operating point of a magnet down its demagnetization curve.

Controlled Demagnetization Techniques

Intentional demagnetization is accomplished by exposing the material to an alternating magnetic field whose amplitude starts above the coercive field and is gradually reduced to zero. As the field cycles at diminishing amplitude, domain walls oscillate with progressively smaller excursions until the domains reach a disordered equilibrium with zero net moment. This technique, called AC demagnetization or degaussing, is used to erase magnetic storage media, eliminate residual magnetism in machined steel components that could attract swarf, and neutralize the magnetic signature of naval vessels. ScienceDirect's overview of magnetic hysteresis covers how the hysteresis properties of a material determine the AC field amplitude and frequency needed for effective demagnetization.

Applications

Demagnetization has applications in a wide range of disciplines, including:

• Erasing magnetic storage media including hard disks and magnetic tape
• Degaussing of naval vessels to reduce susceptibility to magnetic mines
• Removing residual magnetism from precision machined components
• Managing flux leakage and core losses in electrical machine design
• Studying magnetic ordering transitions in materials science research