Magnetic hysteresis

Theoretical model of magnetization $m$ against magnetic field $h$ . Starting at the origin, the upward curve is the *initial magnetization curve*. The downward curve after saturation, along with the lower return curve, form the *main loop*. The intercepts $h c$ and $m rs$ are the *coercivity* and *saturation remanence*.

Magnetic hysteresis occurs when an external magnetic field is applied to a ferromagnet such as iron and the atomic dipoles align themselves with it. Even when the field is removed, part of the alignment will be retained: the material has become magnetized. Once magnetized, the magnet will stay magnetized indefinitely. To demagnetize it requires heat or a magnetic field in the opposite direction. This is the effect that provides the element of memory in a hard disk drive.

The relationship between field strength $H$ and magnetization $M$ is not linear in such materials. If a magnet is demagnetized ( $H = M = 0$ ) and the relationship between $H$ and $M$ is plotted for increasing levels of field strength, $M$ follows the initial magnetization curve. This curve increases rapidly at first and then approaches an asymptote called magnetic saturation. If the magnetic field is now reduced monotonically, $M$ follows a different curve. At zero field strength, the magnetization is offset from the origin by an amount called the remanence. If the $H - M$ relationship is plotted for all strengths of applied magnetic field the result is a hysteresis loop called the main loop. The width of the middle section along the H axis is twice the coercivity of the material.^[1]^{: Chapter 1}

A closer look at a magnetization curve generally reveals a series of small, random jumps in magnetization called Barkhausen jumps. This effect is due to crystallographic defects such as dislocations.^[1]^{: Chapter 15}

Magnetic hysteresis loops are not exclusive to materials with ferromagnetic ordering. Other magnetic orderings, such as spin glass ordering, also exhibit this phenomenon.^[2]

^ ^a ^b Chikazumi, Sōshin (1997). Physics of ferromagnetism (2nd ed.). Oxford: Oxford University Press. ISBN 9780191569852.
^ Monod, P.; Prejean, J. J.; Tissier, B. (1979). "Magnetic hysteresis of CuMn in the spin glass state". J. Appl. Phys. 50 (B11): 7324. Bibcode:1979JAP....50.7324M. doi:10.1063/1.326943.

[Chikazumi-1] Chikazumi, Sōshin (1997). Physics of ferromagnetism (2nd ed.). Oxford: Oxford University Press. ISBN 9780191569852.

[2] Monod, P.; Prejean, J. J.; Tissier, B. (1979). "Magnetic hysteresis of CuMn in the spin glass state". J. Appl. Phys. 50 (B11): 7324. Bibcode:1979JAP....50.7324M. doi:10.1063/1.326943.

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