Antiferromagnetic materials

The change in the Mossbauer spectrum of an antiferromagnetic material produced by an external field is considerably less dramatic than for a ferromagnetic material. Since there is no net moment per unit cell for Hext to interact with, as a first approximation in a randomly oriented antiferromagnet, Hext will increase H in about half the atoms and decrease H in the others, depending on the orientation of an individual atomic spin. The net result is to broaden the lines of the Mossbauer... 

Because the iron ions carry a magnetic moment, the Hall data are difficult to interpret. The conventional theory of the Hall effect utilizes a spin-independent resonance (transfer-energy) integral, and an adequate theory incorporating a spin-dependent resonance integral needs to be developed for antiferromagnetic materials. 

In antiferromagnetism, the spins on different nuclei interact cooperatively but in such a way as to cancel out the magnetic moments. Antiferromagnetic materials, therefore, show a drop in magnetic susceptibility at the onset of cooperative behaviour. The temperature which characterises this process is known as the Ndel temperature (Figure 9.2(c)). 

Perhaps the most direct evidence of the high effective mass in such materials comes from the work of Shapira et al. (1972) on the conductivity of EuTe and the work of Shapira and Reed (1972) on EuS, an antiferromagnetic material, with sufficient non-stoichiometry to make it a degenerate n-type semiconductor. Figure 3.5 shows the temperature variation of the resistivity for various magnetic... 

Fig. 11.58 Schematic representations of magnetic dipole arrangements in (a) paramagnetic, (b) ferromagnetic, and (antiferromagnetic materials.

NEEL TEMPERATURE. The transition temperature for an antiferromagnetic material. Maximal values of magnetic susceptibility, specific heat, and thermal expansion coefficient occur at the NEel temperature. 

Many antiferromagnetic materials at T < 7V display a magnetization a in high external fields H that varies as... 

Write down an expression for Curie s law pertaining to paramagnetic materials and for the Curie-Weiss law pertaining to ferromagnetic or antiferromagnetic materials. 

As seen from Figure 1, in this case the spins are coupled antiparallel, corresponding to the case of 7 < 0 in eqnation (3). Real antiferromagnetic materials are of course three dimensional and more complex than the one-dimensional situation presented in the figme. In three dimensions, it is convenient to subdivide the magnetic sublattice. The minimum number of sublattices is two, say A and B, with equal numbers of spins on each and with equal spin magnitudes, Sa = Sb. ... 

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