Excitation Spectrum of Van-Vleck Magnets

Abstract

In this paper we study the influence of conduction electrons on the magnetic excitations in the paramagnetic state of rare earth compounds with crystalline field split 4f-states. The exchange coupling between the RE ions mediated by the conduction electrons turns the excitations of single RE ions into a spectrum of temperature dependent collective modes, one of which may become soft at the magnetic phase transition. These collective modes can be observed by inelastic neutron scattering^{1, 2} and appear as peaks in the dynamic structure factor

$$ S\left( {q\,,\,w} \right)\, = \,x\,'\,'\,\left( {q,\,w} \right)\,/\left( {1 - \,e{\,^{ - \beta w}}} \right) $$

((1))

.