First-order transition in the itinerant ferromagnet CoS1.9Se0.1 -: art. no. 214411

Undoped CoS2 is an isotropic itinerant ferromagnet with a continuous or nearly continuous phase transition at T-C=122 K. In the doped CoS1.9Se0.1 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 A(2). 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)MnO3.