Spin dynamics of the cerium and uranium monopnictides studied by neutron scattering (invited)

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Abstract
The spin dynamics of the cerium and uranium monopnictides studied by diffuse and inelastic neutron scattering is reviewed. The diffuse scattering above the antiferromagnetic ordering temperature largely corresponds to longitudinal spin fluctuations which are highly anisotropic. For CeAs, CeSb, and UAs multicritical behavior has been found, i.e., the symmetry of the critical scattering above TN differs from the actual type of magnetic ordering below TN. In the ordered state the magnetic excitation spectrum of UN and UAs exhibits only a broad response, whereas well defined spin-wave branches have been observed for USb and the cerium monopnictides. A very detailed study of the magnetic excitations has been performed for CeAs, where the spin-wave dispersion is split into two modes of transverse polarization due to the exchange anisotropy. One of these modes exhibits nearly zero energy gap and quadratic dispersion which has not previously been observed in antiferromangets. A generalized random-phase-approximation calculation taking into account anisotropic exchange interactions consistently describes the transverse magnetic excitations for TTN. In USb, CeSb, and CeBi the magnetic excitations display similar feature as the spin-waves in CeAs and can be understood in close analogy to the treatment applied to CeAs.