First-Order Transition in the Itinerant Ferromagnet CoS 1.9 Se 0.1

Undoped CoS 2 is an isotropic itinerant ferromagnet with a continuous or nearly continuous phase transition at T C =122 K. In the doped CoS 1 . 9 Se 0 . 1 system, the Curie temperature is lowered to T c =90 K. and the transition becomes clearly first order in nature. In particular we find a discontinuous evolution of the spin dynamics as well as strong time relaxation in the ferromagnetic Bragg intensity and small-angle neutron scattering in the vicinity of the ferromagnetic transition. In the ordered state the long-wavelength spin excitations were found to be conventional ferromagnetic spin waves with negligible spin-wave gap (<0.04 meV), indicating that this system is also an excellent isotropic (soft) ferromagnet. In a wide temperature range up to 0.9T C , the spin-wave stiffness D(T) follows the prediction of the two-magnon interaction theory, D(T) =D(0)(1-AT 5 / 2 ), with D(0)=131.7′2.8 meV A2. The stiffness, however, does not collapse as T → T C from below. Instead a quasielastic central peak abruptly develops in the excitation spectrum, quite similar to results found in the colossal magnetoresistance oxides such as (La-Ca)MnO 3 .