Are they Really Permanent ? How Long Do Permanent Magnets Last
How Long Do Permanent Magnets Stay Magnetic?
Demagnetizing occurs when a magnet loses the ability to attract or repel other magnets, usually as the result of heat exposure, chemical exposure or physical shock such as being dropped or hit.
Different permanent magnets will experience this phenomenon at different rates; alnico and samarium cobalt magnets will lose their magnetic strength over time when exposed to high temperatures, for instance.
They lose their magnetism over time
Whiskey, denim and your 401(K) investments may all get better with age - except magnets! In fact, their magnetic strength may even decrease over time due to heat, mechanical damage or corrosion - this phenomenon is known as demagnetization and makes magnetic materials not truly permanent.
Reasons behind some magnets gradually losing their magnetism over time can include their materials shifting alignment of atoms in different ways, due to them being made from ferromagnetic materials in which magnetic properties depend upon electron alignment; this process may be accelerated by various sources including electromagnetic fields.
Modern permanent magnets thankfully experience minimal effects from this over time; even neodymium magnets only lose a minor percentage of their strength annually, depending on storage and handling practices. Furthermore, those with higher coercivity (the resistance to changes in magnetic fields) typically experience reduced losses over time.
They lose their strength
Magnets lose strength due to many factors, including temperature, stray magnetic fields, electrical current and radiation exposure, radiation damage and damage caused by stacking other magnets with different orientations. Furthermore, their magnetic fields decrease as they cool down; this phenomenon is known as demagnetization.
Store bar, disc and horseshoe magnets so their north pole touches the south pole of another magnet to avoid their magnetic strength being depleted over time. An alternative option would be using a "keeper", an iron loop connecting their poles that helps ensure maximum magnetic retention.
Permanent magnets exposed to high temperatures may suffer irreversible magnetic strength loss. Reversible losses, which can be recovered when returning the magnet back to its original temperature, are measured as a percentage of initial flux and can vary widely among different classes of materials.
They lose their retentivity
If a magnet is dropped or exposed to strong impact, its atoms may jostle and lose their alignment, weakening its magnetic field and diminishing its power. Therefore, permanent magnets should always be carefully handled and protected.
Heat is often responsible for demagnetization. Ferromagnetic materials like nickel and iron lose their strength when heated above what's known as Curie temperature, meaning their magnetic domains become randomized and no longer attract other magnets; once this has happened, these changes cannot be reversed even after cooling the magnet down.
Other causes for magnet weakness may include exposure to alternating magnetic fields, AC electric currents, structural damage or exposure to environmental influences such as AC electric currents. Modern permanent magnets like Neodymium and Samarium Cobalt boast high retentivity which means they resist external influences effectively making them the go-to choice when choosing magnets.
They lose their coercivity
Permanent magnets also possess another property known as coercivity that measures how much magnetic energy it takes to bring back their magnetization after their initial magnetic field is removed from them. A material with high coercivity such as hard ferrite is considered hard and high coercivity permanent magnet materials like Samarium Cobalt and Nd-Fe-B possess exceptionally high coercivity values.
Coercivity of magnets is heavily determined by their microstructure, but can be enhanced through GBD. Micromagnetic simulations have revealed that core-shell structures increase coercivity of Nd60Tb20Cu20 eutectic diffusion processed magnets without allotting Dy.
As temperatures increase, magnet's atoms start vibrating and losing their fixed magnetic orientations. Once vibrations reach a temperature known as Curie Point, magnet strength and permanent weakness begin to reduce permanently; but if quickly cooled off after being heated again its magnetism may return; repeated heating-and-cooling cycles can result in permanent loss.