By Doug Leenhouts | Updated Aug 30, 2022
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Magnetism is a common yet fascinating phenomenon that permeates everyday objects—from laboratory equipment to outdoor compasses and refrigerator magnets. While many people take magnets for granted, the underlying physics involves subtle interactions at the atomic level.
Every solid material contains countless magnetic domains—tiny regions where atomic magnetic moments, or dipoles, point in the same direction. When the dipoles within a domain align, the domain itself becomes a small magnet. In some materials, such as iron, these dipoles align readily, whereas in others the alignment is limited to within a domain but not across the entire specimen. Researchers can visualize these domains with magnetic force microscopy.
When a material is exposed to a strong external magnetic field, the domains tend to align with that field, magnetizing the material. Importantly, full alignment across all domains is not necessary for a material to exhibit measurable magnetism.
Running an electric current through a conductor generates its own magnetic field. Two parallel wires carrying current in the same direction attract each other, while opposite currents repel. This principle underpins electromagnets, where a coil of wire produces a controllable magnetic field. On a planetary scale, Earth's magnetic field originates from electric currents flowing in its molten outer core, a process still being investigated by NASA scientists.
Ferromagnetic metals—iron, cobalt, and nickel—possess unpaired electrons whose spins can align parallel to one another when subjected to a sufficiently strong magnetic field. This cooperative alignment produces a pronounced magnetic moment, making these metals excellent cores for electromagnets and transformer windings. The external field from the current amplifies the material’s intrinsic magnetism, creating a powerful, localized magnetic field.
Each magnetic material has a characteristic Curie temperature. Below this threshold, the material retains magnetic order; above it, thermal agitation disrupts the alignment of magnetic domains, and the material becomes paramagnetic. The higher a material’s Curie temperature, the more energy is required to randomize its domains. When a material cooled below its Curie temperature is placed in a magnetic field, it can be magnetized again.
