By Mark Kennan
Updated Mar 24, 2022
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Most modern magnets are engineered from advanced alloys such as aluminum‑nickel‑cobalt, neodymium‑iron‑boron, samarium‑cobalt, and strontium‑iron. To impart magnetism, the alloy is exposed to a strong external magnetic field, causing its microscopic domains to align – a process known as polarization. The result is a permanent magnetic moment that remains unless disrupted by external factors.
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Every magnetic material has a characteristic Curie temperature – the point at which thermal agitation overcomes the alignment of magnetic domains. When a magnet is heated beyond its Curie point, its polarization collapses and it becomes effectively demagnetized. Below this threshold, heat can still weaken the magnet, but the effect is usually reversible once the temperature returns to normal.
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The ability of a magnet to resist reversal by an external field is measured by its coercivity. Materials with high coercivity, such as certain neodymium alloys, retain their magnetic state even when exposed to fields of opposite polarity. Ceramic magnets, in contrast, have low coercivity and can be demagnetized more readily. Engineers sometimes counteract excess strength by pairing a magnet with an opposing field to moderate its net magnetic force.
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Demagnetization over time is typically a slow process. For instance, samarium‑cobalt magnets lose roughly 1 % of their magnetic strength per decade of use under normal conditions. This gradual decline underscores the importance of selecting the appropriate alloy for long‑term applications.
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Electromagnets differ fundamentally from permanent magnets: they generate a magnetic field only while electric current flows through the coil. Once the current is cut off, the field collapses, leaving the core material in its natural, non‑magnetic state. This property makes electromagnets ideal for applications requiring on‑demand magnetism.
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