Why do magnets lose their magnetism over time?
Source:Author Original | Author:Mia | Published time: 2022-09-02 | 270 Views | Share:

The fridge magnets attached to the refrigerator door will fall off after a few years; the magnets on children's drawing boards will also lose their strength over time. It turns out that even the so-called "permanent magnet" is not truly permanent.


The process of degaussing is usually man-made, but it also happens naturally. Extreme temperature fluctuations, volume loss due to mechanical damage, improper storage, hysteresis losses, and corrosion can cause magnets to lose their magnetism over time. Before we understand how magnets lose their magnetism, let's first understand what causes magnetism.

Magnetism, one of the four fundamental forces of nature, is produced by the movement of charged subatomic particles, especially electrons. These negatively charged particles are constantly spinning around the nucleus, while also spinning around their own axis. The latter, formally known as electron spin, is an intrinsic property that acts primarily to create an attractive or repulsive force, which we call magnetism.


You may ask: So, every piece of matter in the world contains trillions of electrons, and every electron is a tiny magnet, isn't there a magnet everywhere? But what we see is that many objects around us are not magnetic.

This is because most electrons cancel each other's magnetism. According to the Pauli exclusion principle, electrons in the same orbital shell have opposite spins and therefore cancel each other's magnetism. And in some elements, like iron and cobalt, the final valence layer is only half filled and contains unpaired electrons. With no opposing electrons to neutralize them, these unpaired electrons together give their respective atoms their magnetic properties.


When a crystal is formed, the magnetic moments of the atoms may or may not be aligned in the same direction, depending on the result that produces the minimum internal energy. Regions where the individual magnetic moments are parallel to each other are called magnetic domains. The responses of these magnetic domains and individual atoms to external magnetic fields form the basis for various classifications of magnetic materials. Under the influence of an external magnetic field, an element or material begins to behave like a permanent magnet when each magnetic domain is oriented in the same direction.

A magnetic material cannot be called a true magnetic material unless its magnetic domains are precisely aligned. A change in the orientation of any single magnetic domain can result in a loss of net magnetic strength. Many natural factors cause these domains to randomize, but the most common and damaging is overheating.


While everything seems calm on the surface, on a microscopic level, atoms are constantly vibrating. The degree to which they vibrate depends on their energy state, which in turn depends on their temperature. Any small fluctuations in temperature can affect the strength of the atomic vibrations, and thus the overall magnetic strength. Lower temperature amplifies the magnet's magnetic force, while higher temperature adversely affects it.

When exposed to high temperatures, the atoms inside the magnet begin to vibrate at ever faster and more frantic ways. This forces some magnetic domains out of alignment, resulting in a lower net magnetic force. At sufficiently high temperatures, all the magnetic domains are pushed into dislocation and the magnetic force is completely lost. The temperature at which a magnet loses its permanent magnet ability is called the Curie point or Curie temperature.


If a magnet is heated to a temperature below the Curie point, and then cooled, the magnet will regain its magnetism, although the strength may be slightly reduced. However, heating the magnet past the Curie point and cooling it, it will not be able to regain its magnetism. In this case, an external magnetic field needs to be introduced to realign the domains and magnetize the material again.

Although high temperature is one of the main methods of demagnetization, ferrite magnets need to exceed 460°C, Alnico magnets need to exceed 860°C, samarium cobalt magnets need to exceed 750°C, and titanium magnets need to exceed 310°C. It is not uncommon for medium magnets to be exposed to such high temperature levels. Therefore, the natural loss of magnetic force is mainly due to other factors.


The first is improper storage. While this may seem trivial, proper magnet storage is crucial to ensure they don't lose strength over time. Also, try to avoid them from damaging your credit cards, hard drives, monitors, and more.

Most magnets contain moderate amounts of iron, which is known to corrode in the presence of oxygen and water. The most commonly used and strongest neodymium magnet, but also the most susceptible to corrosion, as it contains more than 60% iron. The loss of overall strength occurs as corrosion alters the underlying chemical structure that makes the material magnetic. Even though some of today's magnets have begun to use anti-corrosion coatings, you still need to be careful when storing magnets.


There's always something you don't know, a magnet placed incorrectly near another stronger magnet can also lose some or all of its magnetism. Similar poles of different magnets cannot touch or come close to each other because the stronger magnet forces the magnetic field of the weaker magnet to change direction. In some cases, a complete reversal of the poles may occur. This magnetic loss due to an external magnetic field is called hysteresis loss.


At the same time, a reduction in the size of the magnet or any structural damage can also lead to a decrease in the magnetic strength. After all, the magnetic field produced by a magnet made of the same material depends on the size of the magnet. The larger the magnet, the larger the magnetic field produced. Structural damage in the form of fragments will reduce the size of the magnet, thereby reducing its magnetic strength.


In addition, severe shocks, such as repeatedly hitting a magnet or dropping it on a hard surface, can also force the magnetic domain to deviate from its orientation, reducing the magnetism. However, this only applies to certain permanent magnets. Neodymium, samarium cobalt and ferrite magnets are very brittle and will crack if dropped on a hard surface or hammered. However, Alnico magnets are very strong and will not break or crack under mechanical stress.


If you want to prolong the life of the magnet, you'd better put it in a dry box. If you want to put multiple magnets together, you can connect the S pole of one magnet to the N pole of the other, and horseshoe magnets can also be stored this way.


Even with so many factors that can cause magnetism to fade or decrease, the decrease in magnetism is negligible even over long periods of time. For example, samarium cobalt magnets take about 700 years to naturally lose half of their magnetic strength, while neodymium magnets only lose about 5 percent of their magnetism every century.