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 $T_{\rm C} = 122$ K. In the doped CoS$_{1.9}$Se$_{0.1}$ system, the Curie temperature is lowered to $T_{\rm C} = 90$ 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 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 $0.9T_{\rm C}$, the spin-wave stiffness $D(T)$ follows the prediction of the two-magnon interaction theory, $D(T) = D(0)(1 - AT^{5/2})$, with $D(0) = 131.7 \pm 2.8$ meV-Å$^{2}$. The stiffness, however, does not collapse as $T \to T_{\rm C}$ 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$_3$.