Antiferromagnetism Questions Asked and Answered

Friedel and Moon reply: The reference to Harry Shull in the Louis Néel obituary was an unfortunate error. It was Clifford Shull who did the first neutron diffraction experiments confirming Néel’s ideas on antiferromagnetism and ferrimagnetism.

Néel ¹ originated the name antiferromagnetism in developing a theory of systems in which the magnetic interaction between atoms on crystallographically equivalent sites favored opposite orientation of the magnetic moments on nearest neighbors. This orientation could result in ordered magnetic structures in which the net magnetization vanished because of exact cancellation of moments in opposite directions. Néel’s prediction of the behavior of the magnetic susceptibility of such systems was confirmed in 1938 in experiments on manganese oxide. The first determination of such a structure by neutron diffraction was reported by Shull and colleagues in their work on manganese oxide. ²

Ferrimagnetic materials have magnetic atoms on two inequivalent sites, or two interpenetrating sublattices, with ferromagnetic interactions between atoms on the same sublattice and antiferromagnetic interactions between atoms on different sublattices. Interactions of this type can result in ordered structures in which all the moments on one sublattice are parallel to each other but antiparallel to the moments on the other sublattice. The first prediction and theoretical description of ferrimagnetism was given by Néel; ³ he also provided a specific model for the magnetic structure of magnetite to explain its magnetic properties. This model was confirmed in the neutron diffraction work of Shull, Ernie Wollan and Wally Koehler. ⁴ Ferrimagnets exhibit a partial cancellation of moments in opposite directions, resulting in a net magnetization.

We hope these comments have clarified for Robert Mulkern the distinct but related contributions made by these two great scientists.