SPIN WAVES IN THE FERROMAGNETIC HEAVY RARE EARTHS - Rare Earth Magnetism: Structures and Excitations - page 255

Fig. 5.12. Experimental and calculated neutron-scattering spectra
in Tb 90 Tm 10 for the indicated scattering vectors, which correspond to a
reduced wave-vector of 0.33 in Fig. 5.11. In the lower curve, the scattering
vector is in the c -direction, while it is close to the hexagonal plane in the
upper, where an unperturbed transverse phonon is observed. The ratio
of the impurity intensity to the magnon peak is roughly doubled when
the scattering vector moves from the c -direction to the plane, showing
that the magnetic fluctuations in the impurity mode are predominantly
along the c -axis.
the Er ions modify the two-ion coupling of the host substantially.
The theoretical results give a good account both of the excitation
energies and of the observed neutron-scattering spectra, as illustrated
in Fig. 5.12. They reveal that the difference between the interactions
of the Tb and Tm ions in this alloy has a profound influence on the
magnetic behaviour at the two types of site. The exchange forces the
Tm moment to lie in the plane at low concentrations but, according to
the calculations, the crystal fields reduce it from the saturation value of