Ferromagnetism
Ferromagnetism is a type of magnetism exhibited by certain materials, such as iron, nickel, and cobalt. These materials have a magnetic moment, or a magnetic field, even without an external magnetic field. This magnetic moment is caused by the alignment of the spins of the electrons within the material, which results in a net magnetic field [American Institute of Physics].
Ferromagnetism is a common phenomenon and is the type of magnetism responsible for the behavior of magnets that are commonly used in everyday life, such as those found in speakers, electric motors, and refrigerator magnets. Ferromagnetic materials are also widely used in technology, such as hard disk drives and MRI (magnetic resonance imaging) machines. Ferromagnetism is a relatively strong type of magnetism compared to other types, such as diamagnetism and paramagnetism. Ferromagnetic materials have a significant magnetic susceptibility and a large saturation magnetization. The susceptibility of a material is a measure of how much the material's magnetization changes in response to an external magnetic field, and saturation magnetization is the limit of magnetization that a material can reach when the external magnetic field is increased to a high value. The phenomenon of ferromagnetism can be explained by the theory of quantum mechanics, specifically the theory of spin. Electrons have a property called spin, which can be thought of as tiny magnets. In a ferromagnetic material, the spins of the electrons are aligned in the same direction, resulting in a net magnetic moment. This alignment is caused by the exchange interaction between the electrons, which is a quantum mechanical effect [American Institute of Physics].
https://www.aip.org/science/physics/ferromagnetism.
https://www.magnet.fsu.edu/education/tutorials/magnetacademy/ferromagnetism/.