As detailed by Britannica in their article “Magnet,” a magnet is any material capable of attracting iron and producing a magnetic field outside itself. By the end of the 19th century, all known elements and many compounds were tested for magnetism, revealing various magnetic properties. The most common property was diamagnetism, exhibited by materials showing weak repulsion by both poles of a magnet. Paramagnetism, seen in materials like chromium, involves weak induced magnetization that disappears when the magnet is removed. Only iron, nickel, and cobalt exhibit ferromagnetism, allowing them to remain permanently magnetized.
The magnetization process was characterized by William Thomson (Lord Kelvin) in 1850. The magnetic flux density (B) and the magnetizing force (H) are represented by the equation B = μH, where μ symbolizes the permeability of the material. Modern units for B are teslas (T) or webers per square meter (Wb/m²), and for H, amperes per meter (A/m). Ferromagnetic materials exhibit hysteresis, a lag in response to changing forces due to internal friction. This is represented graphically by a hysteresis loop, describing how the values of B change with H.
In 1907, it was suggested that ferromagnetic materials consist of small volumes called domains, each magnetized to saturation. This was confirmed experimentally in 1931. A ferromagnetic body appears unmagnetized when domain magnetizations are randomly distributed. The process of magnetization involves reversible and irreversible movements of domain walls, leading to remanent magnetization when the magnetizing field is removed.
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Article with all rights reserved, courtesy of britannica.com