Superzone gaps

Fig. 7. Electrical resistivity (residual subtracted) versus temperature for the a- and c-axis of Tb. The dashed curves give the spin disorder component after subtraction of the phonon contribution from the total resistivity. The inset shows the anomalous rise in c-axis resistivity below I m accompanying the opening of the superzone gaps in the Fermi surface. The peak is quenched by applied magnetic fields which destroy the helical order and thereby remove the superzone gaps. (After Hegland et al. 1963.)...

The temperature dependence of the magnetic propagation vector (fig. 3) has been ascribed (Elliott and Wedgewood 1963) to the effects of the superzone gaps in modifying the stable Q. It is also suggested (Evenson and Liu 1969) that the magnetostriction may modify the wave vector. 

Since the wavevector describing the magnetic periodicity of the antiferromagnetic phases of the elements Tb-Tm is parallel to the c-axis, it transpires that the superzone energy gaps effectively remove a considerable fraction of the Fermi surface area projected normal to the c-axis, which thus produces a sharp increase in the c-axis resistivity below Tn (see fig. 6.39). The only portions of the calculated (hole) Fermi surface which have significant velocity components parallel to the c-axis are found in the network of arms located near the hexagonal faces of the Brillouin zone (see ch. 3 section 2.3.1). 

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