Antiferromagnetism and ferrimagnetism

DThe magnetic properties of matter are essentially divided into diamagnetism, paramagnetism and ferromagnetism. However, this does not fully characterise all materials. In addition to said properties, there are also antiferromagnetism and ferrimagnetism (ferrimagnetism instead of ferromagnetism).

Antiferromagnetism and ferrimagnetism are comparable to the superposition of magnetic properties of two ferromagnetic substances with different orientations in a single material. This is referred to as "two oppositely polarised ferromagnetic sublattices".

In manganese oxide (MnO), for example, neighbouring spins, the elementary magnets in materials, are aligned antiparallel. Two planes of parallel spins are formed that point in opposite directions. This is typical antiferromagnetism. The magnetic properties of two different ferromagnetic "sublattices" cancel each other out completely.

Ferrimagnetism is antiferromagnetism in which the magnetic properties of one sublattice are significantly weaker than those of the other sublattice (see illustration). It does not necessarily have to be the case that the sublattices are exactly aligned antiparallel and therefore cancel each other out completely.

Ferrimagnetism and antiferromagnetism are easy to understand if you are familiar with the basics of ferromagnetism.

In ferromagnetism, the exchange interaction of the electron spins leads to the stabilisation of the parallel alignment of neighbouring spins of the atoms. This causes a ferromagnet to become magnetic itself in a magnetic field and is known as magnetisation. If a ferromagnet is completely magnetised, all electron spins in the material are aligned in parallel. The ferromagnet itself is then maximally magnetic.

The so-called exchange interaction stabilises the parallel alignment of the spins of a certain type of atom in a ferromagnet, for example, the iron atoms in solid iron.

Properties of antiferromagnetic material

In an antiferromagnetic material, however, only some of the atomic spins stabilise with each other in parallel alignment. The remaining atoms stabilise in an opposite orientation. This is comparable to the fact that in a ferromagnetic material, the electron spins are aligned in parallel in a Weiss domain but are not parallel between different Weiss domains. The only difference is that in antiferromagnetism, the various Weiss domains overlap and form the previously mentioned sublattices. In the simplest form, two different sublattices are aligned directly antiparallel in the antiferromagnet.

A ferromagnet amplifies an external magnetic field through its own magnetisation. This often increases the external magnetic field a thousand-fold. This is not the case with antiferromagnets because the magnetic moments of the antiparallel sublattices offset each other.

Properties of ferrimagnetic material

In ferrimagnetism, the magnetic properties of the different sublattices do not fully offset each other. Ferrimagnets therefore behave like weaker ferromagnets.

The Curie temperature of ferromagnets describes the temperature at which a ferromagnet becomes paramagnetic. Above this temperature (T), the alignment of the spins is destroyed by thermal movement. Above the Curie temperature T_C, there is a simple approximation formula for the magnetic susceptibility χ of this material, namely:

\(\chi = \frac{C}{T-T_C}\)
C is the so-called Curie constant, which is different for each ferromagnetic material.

Antiferromagnets also have a characteristic temperature above which an antiferromagnet becomes paramagnetic.

This is the Néel temperature. Above the Néel temperature T_N, susceptibility is approximated using the formula

\(\chi = \frac{N}{T_N+T}\)
with the Néel constant N.