Antiferromagnetism was first discovered by Neel and Bitter and later extended by Van Vleck. Experimentally, antiferromagnetism was first observed as a property of MnO by Bizette, Squire and Tsai in 1938.
Cause of antiferromagnetism: In antiferromagnetc materials, the magnetic moments are aligned in opposite directions and are equal in magnitude.
Thus, when antiferromagnetic material is unmagnetized its net magnetisation is zero because magnetic moments of the neighbouring atoms exactly cancel when they are vectorially added.
In the presence of the strong magnetic field, antiferromagnetic materials are weakly magnetised in the direction the field. This property of the materials is called antiferromagnetism and the materials which exhibit this propert are called antiferromagnetic materials.
Examples of antiferromagnetic substances: MnO, FeO, CoO, NiO, Cr, Mn.
Reference: This article is referred from my book “electrical engineering materials” having ISBN 978-81-272-5069-0