Magnets lose strength over time, a process known as demagnetization. The rate of loss varies drastically depending on the material and environmental conditions. This loss is a physical effect tied to the magnet’s internal atomic structure. Magnetic strength depends entirely on the uniform arrangement of microscopic regions within the material, and any disruption leads to a reduction in the magnetic field.
The Internal Structure of Magnetism
The powerful attraction of a magnet originates from the alignment of electrons within the material’s atoms. In ferromagnetic materials, such as iron or nickel, atoms group into microscopic regions called magnetic domains. Within these domains, the magnetic moments of all atoms point in the same direction. Before magnetization, these domains point randomly, canceling out the overall magnetic field.
A material is turned into a magnet by exposing it to a strong external magnetic field. This field forces the domains to shift and rotate, aligning the majority of them in a single, unified direction. This alignment creates a cumulative, large-scale magnetic field. When a magnet loses strength, the domains revert to a more chaotic, random orientation, reducing the net external field.
How Magnetic Strength Is Lost
Magnetic strength is lost when enough energy is supplied to scramble the alignment of the internal domains. One common cause is exposure to elevated temperatures. Heating a magnet causes atoms to vibrate with increased vigor, destabilizing the alignment of the magnetic domains.
If the temperature is raised high enough, the magnet reaches its Curie temperature. This threshold is where thermal energy completely overwhelms the forces holding the domains in alignment. At this point, the magnetic properties disappear entirely, and the material becomes paramagnetic. For example, a standard neodymium magnet suffers permanent loss above 80°C, while a ferrite magnet’s Curie temperature is around 450°C.
Physical impact is another destructive mechanism. A sharp blow or repeated shock introduces kinetic energy that mechanically jars the magnetic domains. This energy shifts the domain walls, causing them to move away from their unified orientation. While modern magnets are resistant to this, older or brittle materials can be demagnetized by mechanical stress.
Magnets can also lose strength through exposure to an external magnetic field that opposes their polarity. If a strong enough field is applied in the reverse direction, it can force the internal domains to flip. This opposing field must overcome the material’s magnetic resistance, neutralizing the magnet’s field.
Reversing Demagnetization
The loss of magnetic strength is often reversible, provided the material’s crystal structure has not been fundamentally altered, such as by exceeding its Curie temperature. Recovering strength requires supplying external energy to re-establish the uniform alignment of the domains. This is typically achieved by exposing the weakened material to a powerful external magnetic field.
Specialized equipment, like high-current coils or industrial magnetizers, generates a field stronger than the original magnet’s field. When the weakened magnet is placed inside, its domains are forcefully rotated back into alignment. Once the external field is removed, the material retains the strong magnetization. If the magnet was heated above its Curie point, however, the internal structure may be permanently damaged, preventing full restoration.
Permanent Versus Temporary Magnetic Materials
The ease with which a magnet loses strength is determined by its coercivity, which is the material’s resistance to demagnetization. Materials with high coercivity are known as hard or permanent magnets. These require significant energy—thermal, kinetic, or magnetic—to disrupt their domain structure.
Neodymium-iron-boron and ferrite magnets are examples of hard magnetic materials that maintain their field indefinitely under normal conditions. In contrast, soft or temporary magnetic materials, such as the iron cores used in electromagnets, have low coercivity. These materials are easily magnetized by an external field but lose alignment almost instantly once the external field is removed. This difference determines if a material is suitable for use as a permanent fixture or as a core that needs to rapidly switch its magnetic state.