Magnets are materials that produce an invisible magnetic field, enabling them to attract or repel other magnetic materials like iron, nickel, and cobalt. They are fundamental to countless technologies and everyday items. While some magnets are considered “permanent,” their strength can diminish over time despite being designed for longevity.
Understanding Magnetic Properties
Magnetism originates at the atomic level from the motion of electric charges, specifically the spin and orbit of electrons within atoms. These movements generate tiny magnetic fields, giving each atom a magnetic moment. In most materials, these atomic magnetic moments are randomly oriented, canceling each other out, resulting in no overall magnetism.
In magnetic materials, groups of atomic magnetic moments align in the same direction, forming microscopic regions known as magnetic domains. The collective alignment of these domains creates a larger, measurable magnetic field. More consistently aligned domains result in a stronger overall magnetic field.
Magnets are broadly categorized into temporary and permanent types. Temporary magnets, like soft iron or electromagnets, exhibit magnetic properties only when exposed to an external magnetic field or electric current, quickly losing magnetism once removed. Permanent magnets, such as neodymium, ferrite, alnico, or samarium cobalt, maintain their magnetic field without continuous external stimulation, retaining domain alignment more strongly.
Factors That Reduce Magnetic Strength
Despite their “permanent” designation, magnets can lose strength due to various environmental or physical factors. This reduction in magnetic strength is known as demagnetization, where organized magnetic domains become disoriented.
Heat is a primary cause of demagnetization, as increasing temperatures cause atoms to vibrate more vigorously. This increased thermal energy disrupts the alignment of magnetic domains, reducing magnetic strength. Every magnetic material has a specific “Curie temperature,” above which it completely loses its permanent magnetic properties. For instance, iron’s Curie temperature is 769 °C, while nickel’s is 358 °C.
Physical shock or impact also contributes to a magnet’s loss of strength. Dropping or hitting a magnet can physically dislodge and disorient internal magnetic domains. This mechanical stress scrambles domains, reducing its magnetic field. Brittle magnets like hard ferrites can chip or crack from impacts, further affecting strength.
Exposure to strong external magnetic fields can also demagnetize a magnet. If a magnet is placed in an opposing magnetic field powerful enough, it can force internal magnetic domains to reorient or scramble. This disrupts established domain alignment, reducing the magnet’s inherent strength. Storing magnets with different orientations or close to stronger magnets can lead to magnetization loss.
While often negligible over human lifespans, a very slow, gradual demagnetization can occur over extremely long periods. This natural decay, sometimes called magnetic creep, happens due to subtle thermal fluctuations. Neodymium magnets, for example, typically lose less than 1% of their strength over a decade. For most practical applications, this natural decay is not the primary concern.
Can a Demagnetized Magnet Be Restored?
Magnets that have lost some strength can often be re-magnetized, though the process varies by magnet type and demagnetization extent. Temporary magnets are easily re-magnetized by being placed within a strong magnetic field. An iron nail, for example, becomes a temporary magnet near a strong permanent magnet and loses magnetism when removed.
For permanent magnets, restoration is possible but typically requires specialized equipment. This often involves using strong electromagnets or magnetizers to realign internal magnetic domains. The process applies a high DC current through a coil wrapped around the magnet to create the necessary field.
Simple methods, such as rubbing a demagnetized magnet with a stronger permanent magnet, can help restore some strength. However, complete restoration to original strength might not always be achievable, especially if the material structure was permanently altered by overheating beyond its Curie point. For many common household magnets, replacing them is often more practical and cost-effective than seeking professional re-magnetization.