6.7 Antiferromagnetic Materials

These are materials in which electron spins associated with magnetic atoms at particular crystallographic sites are ordered yet oriented with respect to each other in such a manner that their net magnetization is equal to zero. This is the case below a particular temperature, called as Néel temperature (T_N) above which the material behaves as a paramagnet.

Examples include metallic manganese, chromium, various transition metal oxides such as manganese oxide (MnO), forms of iron oxide (Fe₂O₃), multiferroic perovskites like bismuth ferrite (BiFeO₃). For example in MnO, since O is not a magnetic ion, the antiparallel spin arrangement of Mn²⁺, the magnetic ion, in two sites gives rise to zero magnetization. Below is the crystal structure of MnO,  drawn on (100) plane.
.

As shown below, the susceptibility of an antiferromagnetic material shows a paramagnetic (PM) behavior above (T_N) and between 0 K  and (T_N), it shows an antiferromagnetic (AFM) behavior.

Antiferromagnetic materials are not very useful as such because they do not show any magnetization whatsoever.

Some of the materials like bismuth ferrite (BiFeO₃) have been described as canted antiferromagnets where spin of atoms are not exactly antiparallel but are slightly inclined towards the so-called parallel axis which may give rise to very small magnetization.