Detection of Antiferromagnetism by Neutron Diffraction

T necessary conditions for the existence of ferromagnetism are: (1) the atoms must have a net magnetic moment due to an unfilled electron shell, and (2) the exchange integral / relating to the exchange of electrons between neighboring atoms must be positive. This last condition is required in order that spin states of high multiplicity, which favor ferromagnetism, have the lowest energy. It seems certain that for many of the non-ferromagnetic substances containing a high concentration of magnetic atoms the exchange integrals are negative. In such cases the lowest energy state is the one in which the maximum number of antiparallel pairs occur. An approximate theory of such substances has been developed by N6el,1 Bitter,2 and Van Vleck3 for one specific case and the results are briefly described below. Consider a crystalline structure which can be divided into two interpenetrating lattices such that atoms on one lattice have nearest neighbors only on the other lattice. Examples are simple cubic and body-centered cubic structures. Let the exchange integral for nearest neighbors be negative and consider only nearest neighbor interactions. Theory then predicts that the structure will exhibit a Curie temperature. Below the Curie temperature the spontaneous magnetization vs. temperature curve for one of the sub-lattices is that for an ordinary ferromagnetic material. However, the magnetization directions for the two lattices are antiparallel so that no net spontaneous magnetization exists. At absolute zero all of the atoms on one lattice have their electronic magnetic moments aligned in the same direction and all of the atoms on the other lattice have their moments antiparallel to the first. Above the Curie temperature the thermal energy is sufficient to overcome the tendency of the atoms to lock antiparallel and the behavior is that of a normal paramagnetic substance.