Environmental Engineering Reference
In-Depth Information
ance of higher harmonics in neutron diffraction (Koehler et al. 1966). As
illustrated in Fig. 2.4, the peak in
( q ) simultaneously moves to smaller
values of q ,andthe Q of the magnetic structure decreases correspond-
ingly. However this change does not occur uniformly with temperature,
but rather a series of commensurable wave-vectors is traversed, with ap-
parently discontinuous jumps between them (Gibbs et al. 1985). At 20
K, a second-order transition to a shallow cone structure, with an opening
angle which decreases continuously towards 80 as the temperature is
lowered, is observed. The helical component is commensurable with the
lattice, with an average turn angle of 30 , but the moments are strongly
bunched around the easy b -axes, as shown in Fig. 1.20.
To interpret this rich variety of phenomena, we will use the model
of Larsen et al. (1987). The Hamiltonian which they constructed has
J
Fig. 2.4. The Fourier transform, for wave-vectors in the c -direction,
of the indirect-exchange interaction in Ho at different temperatures, de-
duced from the magnetic excitations and used in the calculation of the
structures. The maximum in J ( q ) increases in magnitude and moves to
larger wave-vectors as the temperature is increased, leading to a decrease
in the repeat distance of the periodic structures.
Search WWH ::




Custom Search