Ferrimagnetism

A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude, so a spontaneous magnetization remains.

[2] Since the discovery, numerous uses have been found for ferrimagnetic materials, such as hard-drive platters and biomedical applications.

Until the twentieth century, all naturally occurring magnetic substances were called ferromagnets.

In 1936, Louis Néel published a paper proposing the existence of a new form of cooperative magnetism he called antiferromagnetism.

[3] While working with Mn2Sb, French physicist Charles Guillaud discovered that the current theories on magnetism were not adequate to explain the behavior of the material, and made a model to explain the behavior.

[4] In 1948, Néel published a paper about a third type of cooperative magnetism, based on the assumptions in Guillaud's model.

In 1970, Néel was awarded for his work in magnetism with the Nobel Prize in Physics.

This is because at higher temperatures the thermal motion is strong enough that it exceeds the tendency of the dipoles to align.

There are various ways to describe ferrimagnets, the simplest of which is with mean-field theory.

In mean-field theory the field acting on the atoms can be written as where

is the molecular field coefficient between the i-th and k-th substructures.

as a positive quantity and write the minus sign explicitly in front of it.

These equations do not have a known analytical solution, so they must be solved numerically to find the temperature dependence of

Unlike ferromagnetism, the magnetization curves of ferrimagnetism can take many different shapes depending on the strength of the interactions and the relative abundance of atoms.

The most notable instances of this property are that the direction of magnetization can reverse while heating a ferrimagnetic material from absolute zero to its critical temperature, and that strength of magnetization can increase while heating a ferrimagnetic material to the critical temperature, both of which cannot occur for ferromagnetic materials.

This compensation point is observed easily in garnets and rare-earth–transition-metal alloys (RE-TM).

This compensation point is crucial for achieving fast magnetization reversal in magnetic-memory devices.

; this magnetization is reached when the external field is strong enough to make all the moments align in the same direction.

When this point is reached, the magnetization cannot increase, as there are no more moments to align.

[10] Ferrimagnetic materials have high resistivity and have anisotropic properties.

When this applied field aligns with the magnetic dipoles, it causes a net magnetic dipole moment and causes the magnetic dipoles to precess at a frequency controlled by the applied field, called Larmor or precession frequency.

As a particular example, a microwave signal circularly polarized in the same direction as this precession strongly interacts with the magnetic dipole moments; when it is polarized in the opposite direction, the interaction is very low.

When the interaction is strong, the microwave signal can pass through the material.

This directional property is used in the construction of microwave devices like isolators, circulators, and gyrators.

Ferrimagnetic materials are also used to produce optical isolators and circulators.

Ferrimagnetic minerals in various rock types are used to study ancient geomagnetic properties of Earth and other planets.

In addition, it has been shown that ferrimagnets such as magnetite can be used for thermal energy storage.

[11] The oldest known magnetic material, magnetite, is a ferrimagnetic substance.

The tetrahedral and octahedral sites of its crystal structure exhibit opposite spin.

Other known ferrimagnetic materials include yttrium iron garnet (YIG); cubic ferrites composed of iron oxides with other elements such as aluminum, cobalt, nickel, manganese, and zinc; and hexagonal or spinel type ferrites, including rhenium ferrite, ReFe2O4, PbFe12O19 and BaFe12O19 and pyrrhotite, Fe1−xS.