Antiferromagnetics

The usual antiferromagnet can be viewed as a system consisting of two interpenetrating sublattices. Each sublattice is uniformly magnetised with spins aligned parallel, but the magnetisations of the sublattices are cancelled exactly. The molecular field theory assumes that the field acting on sublattice A is 

When the magnetic field is small and the temperature large enough one can expand the Brillouin function appearing in the magnetisation as follows 

In zero external field these equations give the non-vanishing magnetisations Ma 0 and MB 0 only at or below the ordering (Neel) temperature which becomes determined by solving the above system of equations. Hence, the first equation gives the expression 

Because Wf 0, evidently, the following relationship holds valid 

the proper ordering temperature may differ from the temperature at which a maximum occurs in the susceptibility. 

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Wlien 2 g > (Eaa BB binary alloy corresponds to an Ismg ferromagnet (J> 0) and the system splits into two phases one rich in A and the other rich in component B below the critical temperature T. On the other hand, when 2s g < (Eaa+ bb > system corresponds to an antiferromagnet the ordered phase below the critical temperature has A and B atoms occupying alternate sites. 

In 1962 Heller and Benedek made aeeiirate measurements of the zero-field magnetization of the antiferromagnet MnF2 as a fiinetion of temperature and reported a p of 0.335 0.005, a result supporting an experimental parallelism between fluids and magnets. 

The development of neutron diffraction by C G Shull and coworkers [30] led to the detennination of the existence, previously only a hypothesis, of antiferromagnetism and ferrimagnetism. More recently neutron diffraction, because of its sensitivity to light elements in the presence of heavy ones, played a cmcial role in demonstrating the importance of oxygen content m high-temperature superconductors. 

Cheng H S and Wang L S 1996 Dimer growth, structure transition and antiferromagnetic ordering in small chromium clusters Phys. Rev. Lett. 77 51... 

Viitala E, Merikoski J, Manninen M and Timonen J 1997 Antiferromagnetic order and frustration in small clusters Phys. Rev. B 55 11 541... 

Antiferromagnetism Antifoam Antifoam agents Antifoaming agents... 

Parasitic ferromagnetism is a weak ferromagnetism that accompanies antiferromagnetism, eg, in a-ferric oxide [1309-37-1], a-Fe202. Possible causes include the presence of a smaU amount of ferromagnetic impurities, defects in the crystal, and slight deviations in the directions of the plus and minus spins from the original common axis. 

D. P. Landau, M. Krech. Spin dynamics simulations of ferro- and antiferromagnetic model systems comparison with theory and experiment. J Phys Condens Matter 77 R175-R213, 1999. 

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