Ferromagnetism and Magnets(자석의 원리)

Ferromagnetism

Ferromagnetic objects are objects that consist of magnetic domains each with magnetic fields, arranged in opposite directions in nature, but alligned when an external magnetic field is applied. All ferromagnetic materials loose their ferromagnetivity when heated over the Curie temperature. Each ferromagnets have different Curie temperatures.

 

Condition of Ferrromagnets

All ferromagnets have a character in common. The exchange energy is higher than the dipole interaction of the material. Objects with this condition are classified as ferromagnets, and have magnetic domains under the Curie temperature.

Exchange Energy

"Exchange energy is the energy released when the anti-parallel electrons belonging to a degenerate (same energy) subshell are made to have parallel spins."

https://byjus.com/questions/what-do-you-mean-by-exchange-energy/

Dipole Interaction

When two stick magnets are positioned closely to each other, it is likely that opposite magnetic poles become alligned closely to each other. This is similar in the case of dipole interation. Two dipoles can be thought of two stick magnets, that usually allign in antiparallel formations. (image a)

https://commons.wikimedia.org/wiki/File:Dipole_interactions.png

Materials with higher exchange energy than dipole interaction energy are considered to be ferromagnets. For example, the exchange energy of iron is over 1000 times its dipole interaction energy, so it is a ferromagnetic element.

Magnetic Domains

Magnetic Domains

Magnetic domains are sections in a ferromagnetic object in which the atoms have the same magnetic direction. Each magnetic domain is surrounded, or separated by domain walls which are about 100-150 atoms in thickness.

https://en.wikipedia.org/wiki/Magnetic_domain

As you can see in the image above, magnetic domains of un-magnetized ferromagnets are positioned in a way that the magnetic fields of each domain cancel out one another, resulting in no magnetic field extending into the space outsid itself.

Domain Formation

https://en.wikipedia.org/wiki/Magnetic_domain

Magnetic domains occur naturally in ferromagnets below the Curie temperature. However, there are conditions when existing magnetic domains separate into smaller ones. The reason this happens is to reduce the magnetostatic energy required to maintain a magnetic field extending to the space outside the object. 

 

In the image above, a) shows a magnetic domain with a large magnetic field, which leads to a higher magnetostatic energy. In order to reduce this energy, the domain will split in two like shown in b). This action reduces the magnetostatic energy, but maintaining a domain wall also requries energy. By far the amount of reduced magnetostatic energy is higher than the domain wall energy, so the spllitting of magnetic domain can continue. This action lasts until the reduced enegy is the same as the increased domain wall energy, since then the total energy is at its lowest state possible.

Magnets

Forming of Magnets

When ferromagnets are applied with a strong external magnetic field, the object's magnetic domains align in a way to make the object have the same direction of magnetism. This happens by the expansion of the domain area having the same magnetic direction with the external field, in the cost of other magnetic domains which have opposite or different magnetic directions.

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/ferro.html

When a ferromagnet becomes magnetized, it will return to its original non-magnetic state within a few minutes if not for domain wall pinning. Domain wall pinning enables a magnet to stay magnetized. This is caused by antiferromagnetic atoms in the ferromagnet. These impurities stop tehh domain walls from moving. The image below describes how this happens.

https://en.wikipedia.org/wiki/Domain_wall_(magnetism)

Demagnetising Magnets

There are some ways to destroy a magnet's magnetivity. One way is to take away the material itself's ferromagnetivity. As mentioned in the start of this article, all ferromagnets lose ferromagnetism in temperatures higher than its Curie temperature. This will disable the material's ability to form magnetic domains, thus demagnetising it. Another way is to give the magnet a strong shock, or to shake it hard. These actions undoes the pinning of magnetic walls, making the magnetic domains cancel out each other again

 

 

 

thumbnail image: https://www.researchgate.net/