Ferromagnetism
Ferromagnetism (along with the similar effect ferrimagnetism) is the strongest type and is responsible for the common phenomenon of everyday magnetism.
They are typically pure forms, alloys, or compounds of iron, cobalt, nickel, and certain rare-earth metals.
Ferromagnetism is vital in industrial applications and modern technologies, forming the basis for electrical and electromechanical devices such as electromagnets, electric motors, generators, transformers, magnetic storage (including tape recorders and hard disks), and nondestructive testing of ferrous materials.
Permanent magnets are made from hard ferromagnetic materials (such as alnico) and ferrimagnetic materials (such as ferrite) that are subjected to special processing in a strong magnetic field during manufacturing to align their internal microcrystalline structure, making them difficult to demagnetize.
[3] In a landmark paper in 1948, Louis Néel showed that two levels of magnetic alignment result in this behavior.
Conversely, there are non-magnetic alloys, such as types of stainless steel, composed almost exclusively of ferromagnetic metals.
The table lists a selection of ferromagnetic and ferrimagnetic compounds, along with their Curie temperature (TC), above which they cease to exhibit spontaneous magnetization.
[11] NpCo2 is a ferrimagnet below 15 K. In 2009, a team of MIT physicists demonstrated that a lithium gas cooled to less than one kelvin can exhibit ferromagnetism.
In rare circumstances, ferromagnetism can be observed in compounds consisting of only s-block and p-block elements, such as rubidium sesquioxide.
[13] In 2018, a team of University of Minnesota physicists demonstrated that body-centered tetragonal ruthenium exhibits ferromagnetism at room temperature.
[14] Recent research has shown evidence that ferromagnetism can be induced in some materials by an electric current or voltage.
In July 2020, scientists reported inducing ferromagnetism in the abundant diamagnetic material iron pyrite ("fool's gold") by an applied voltage.
The Bohr–Van Leeuwen theorem, discovered in the 1910s, showed that classical physics theories are unable to account for any form of material magnetism, including ferromagnetism; the explanation rather depends on the quantum mechanical description of atoms.
The Pauli exclusion principle, also a consequence of quantum mechanics, restricts the occupancy of electrons' spin states in atomic orbitals, generally causing the magnetic moments from an atom's electrons to largely or completely cancel.
Due to its quantum nature, the spin of the electron can be in one of only two states, with the magnetic field either pointing "up" or "down" (for any choice of up and down).
Because of Hund's rules, the first few electrons in an otherwise unoccupied shell tend to have the same spin, thereby increasing the total dipole moment.
This in turn affects the electron location and the Coulomb (electrostatic) interaction and thus the energy difference between these states.
The exchange interaction is related to the Pauli exclusion principle, which says that two electrons with the same spin cannot also be in the same spatial state (orbital).
Within each domain, the spins are aligned, but if the bulk material is in its lowest energy configuration (i.e. "unmagnetized"), the spins of separate domains point in different directions and their magnetic fields cancel out, so the bulk material has no net large-scale magnetic field.
If all the dipoles in a piece of ferromagnetic material are aligned parallel, it creates a large magnetic field extending into the space around it.
Thus, a piece of iron in its lowest energy state ("unmagnetized") generally has little or no net magnetic field.
Although this state of aligned domains found in a piece of magnetized ferromagnetic material is not a minimal-energy configuration, it is metastable, and can persist for long periods, as shown by samples of magnetite from the sea floor which have maintained their magnetization for millions of years.
As the temperature of a material increases, thermal motion, or entropy, competes with the ferromagnetic tendency for dipoles to align.
The study of ferromagnetic phase transitions, especially via the simplified Ising spin model, had an important impact on the development of statistical physics.
