Anisotropic exchange and spin dynamics in the type-I (-IA) antiferromagnets CeAs, CeSb, and USb: A neutron study

The anomalous anisotropic magnetic interactions and the spin dynamics of the fcc type-I or -IA antiferromagnetic cerium and uranium monopnictides CeAs, CeSb, and USb have been studied by inelastic and diffuse critical neutron scattering experiments. The diffuse scattering above the antiferromagnetic ordering temperature largely corresponds to longitudinal spin fluctuations which are highly anisotropic. In the ordered state the dispersion curves of the spin-wave excitations strongly depend on the actually realized spin structure. For the antiferromagnets the spin waves split into transverse modes with different polarizations due to the exchange anisotropy. In CeAs one of these modes exhibits nearly zero energy gap and quadratic dispersion which has not previously been observed in antiferromagnets. The wave-vector-dependent susceptibility tensor has been calculated within the random-phase-approximation (RPA) by taking account of crystal-field, anisotropic bilinear exchange, and isotropic quadrupolar interactions. General expressions including all levels of the ground-state multiplet are derived for single-q and triple-q type-I as well as for type-IA antiferromagnets, and detailed formulas of the magnetic excitation spectrum are given for the particularly interesting case of effective two-level systems which are often realized in f-electron magnets. The RPA formalism consistently describes the transverse magnetic excitations for T<$T_N$ as well as the longitudinal spin fluctuations for T>$T_N$ for all compounds under study. For CeAs and CeSb the bilinear exchange interactions turn out to be similar, and evidence for important effects of higher-order magnetic interactions is found. The latter are shown to be the driving mechanism for the realization of the various magnetic phases in CeSb. For CeAs the magnetic excitation spectrum unambiguously demonstrates that a collinear single-q type-I spin structure is realized, whereas for USb a noncollinear triple-q type-I spin structure emerges from the observed magnetic excitations.