Antiferromagnetism and Magnetic Order

When the exchange interaction favors antiparallel alignment, neighboring spins point in opposite directions, producing antiferromagnetic and ferrimagnetic order with little or no net magnetization.

Definition

Antiferromagnetism is a magnetically ordered state in which spins on interpenetrating sublattices align antiparallel so the net magnetization vanishes; ferrimagnetism is the analogous order with unequal sublattice moments that leaves a finite net magnetization, both setting in below a characteristic ordering temperature.

Scope

This topic covers magnetic orderings beyond simple ferromagnetism: antiferromagnetism with its compensating sublattices and Néel temperature, ferrimagnetism with unequal sublattices and a net moment, the two-sublattice molecular-field theory, and more complex helical and noncollinear arrangements. It treats how negative exchange and lattice geometry select the ordering pattern, the susceptibility cusp at the ordering temperature, and the role of neutron diffraction in detecting order invisible to bulk magnetization.

Core questions

• How does a negative exchange interaction lead to antiparallel sublattice ordering?
• What is the Néel temperature, and how does the susceptibility behave around it?
• How does ferrimagnetism differ from antiferromagnetism in its net moment?
• Why is neutron diffraction needed to reveal antiferromagnetic order?

Key concepts

• Antiferromagnetic sublattices
• Néel temperature and susceptibility cusp
• Ferrimagnetism and uncompensated moments
• Two-sublattice molecular-field theory
• Helical and noncollinear magnetic structures

Key theories

Néel two-sublattice theory

Néel extended molecular-field theory to two interpenetrating sublattices coupled by negative exchange, predicting antiferromagnetic order below a Néel temperature and, for unequal sublattices, ferrimagnetism with a net magnetization, explaining the magnetism of ferrites.

Clinical relevance

Antiferromagnets and ferrimagnets are central to technology: ferrites are used in transformers, inductors, and microwave devices, while antiferromagnetic order pins reference layers in magnetic-sensor spin valves and is being explored as an active medium for fast, robust antiferromagnetic spintronics.

History

Néel predicted antiferromagnetism and developed the theory of ferrimagnetism in the 1930s and 1940s, work recognized with the 1970 Nobel Prize; Shull's neutron-diffraction experiments in the late 1940s directly confirmed antiferromagnetic order that bulk magnetization could not reveal.

Key figures

• Louis Néel
• Lev Landau
• Clifford Shull

Seminal works

• neel1948
• blundell2001

Frequently asked questions

How can a material be magnetically ordered yet have no net magnetization?

In an antiferromagnet equal and opposite sublattice magnetizations cancel exactly, so there is long-range spin order but zero net moment; the order is real and detectable by neutron diffraction even though bulk magnetic measurements see almost nothing.

What is the difference between an antiferromagnet and a ferrimagnet?

Both have antiparallel sublattices, but in an antiferromagnet the opposing moments are equal and cancel, whereas in a ferrimagnet the sublattices are unequal, so a net magnetization survives, much as in a ferromagnet.