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I include this largely for completeness, but I am obliged to be brief, because it is a subject I know little about. It is my understanding that it involves materials in which the atoms or ions or molecules have a permanent dipole moment (resulting from unpaired electron spins), as in paramagnetic and ferromagnetic materials, and the crystals have domain structure, as in ferromagnetic materials, but alternating ions within a domain have their magnetic moments oriented in opposite directions, so the domain as a whole has zero magnetization, or zero susceptibility. An example of an antiferromagnetic material is manganese oxide $$MnO$$, in which the $$Mn^{2+}$$ ion has a magnetic moment. Such materials are generally antiferromagnetic at low temperatures. As the temperature is increased, the domain structure breaks down and the material becomes paramagnetic – as also happens, of course, with ferromagnetic materials. But whereas the susceptibility of a ferromagnetic material decreases dramatically with rising temperature, until it become merely paramagnetic, the susceptibility of an antiferromagnetic material starts at zero, and its transformation to a paramagnetic material results in an increase (albeit a small increase) in its susceptibility. As the temperature is raised still further, the paramagnetic susceptibility drops (as is usual for paramagnetics), so there is presumably some temperature at which the susceptibility is a maximum.