Antiferromagnetism (redirected from Antiferrimagnetism)

Antiferromagnetism

A property possessed by some metals, alloys, and salts of transition elements in which the atomic magnetic moments, at sufficiently low temperatures, form an ordered array which alternates or spirals so as to give no net total moment in zero applied magnetic field. The most direct way of detecting such arrangements is by means of neutron diffraction. See Neutron diffraction

The transition temperature below which the spontaneous antiparallel magnetic ordering takes place is called the Néel temperature. A plot of the magnetic susceptibility of a typical antiferromagnetic powder sample versus temperature is shown in the illustration. Below the Néel point, which is characterized by the sharp kink in the susceptibility, the spontaneous ordering opposes the normal tendency of the magnetic moments to align parallel to the applied field. Above the Néel point, the substance is para-magnetic, and the susceptibility obeys the Curie-Weiss

(1) 

law, as in Eq. (1), with a negative paramagnetic Curie temperature. The Néel temperature is similar to the Curie temperature in ferromagnetism.

The cooperative transition that characterizes antiferromagnetism is thought to result from an interaction energy U of the form given in Eq. (2), where S _i and S _j are

(2) 

the spin angular momentum vectors associated with the magnetic moments of neighbor atoms i and j, and J_ij is an interaction constant. If all J_ij are positive, the lowest energy is achieved with all S _i and S _j parallel, that is, coupled ferromagnetically. Negative J_ij between nearest-neighbor pairs (i,j) may lead to simple antiparallel arrays; if the distant neighbors also have sizable negative J_ij, a spiral array may have lowest total energy. The interaction constant in Eq. (2) probably arises from superexchange coupling. This is an effective coupling between magnetic spins which is indirectly routed via nonmagnetic atoms in salts and probably via conduction electrons in metals. See Ferromagnetism, Helimagnetism

The magnetic moments are known to have preferred direction. Anisotropic effects come from magnetic dipole forces and also from spin-orbit coupling combined with superexchange. Some nearly antiparallel arrays such as Fe₂O₃ show a slight bending (called canting) and exhibit weak ferromagnetism. The anisotropy affects the susceptibility of powder samples and is of extreme importance in antiferromagnetic resonance. See Magnetic resonance

Magnetic susceptibility of powdered manganese oxide