Spin dynamics and magnetic ordering in mixed valence systems

Neutron scattering measurements are reported on the mixed valence compounds Ce/sub 1-x/Th/sub x/ and TmSe. The Chi''(Q,..omega..) as derived from the inelastic spectra of Ce/sub 0/./sub 74/Th/sub 0/./sub 26/ shows a peak in the ..gamma.. phase near 20.0 meV and shifts abruptly to greater than 70.0 meV at the transition to the ..cap alpha.. phase. The temperature independence of the susceptibility within the ..gamma.. phase cannot be simply reconciled with the temperature dependence of the valence within the ..gamma.. phase. TmSe is shown to order in a type I antiferromagnetic structure below T/sub N/ approx. 3.2 K. The magnetic phase diagram is understood as a successive domain reorientation and a metamagnetic phase transition for T < 3 K with increasing field. The mixed valence nature manifests itself in a reduced moment and a markedly altered crystal field. Another sample of TmSe with a lattice parameter implying 100% Tm/sup 3 +/ orders in a type II structure but never achieves long range order