Temperature dependent Fe-57 Mossbauer spectroscopy and specific heat measurements for CaK(Fe1-xNix)(4)As-4 with x = 0, 0.017, 0.033, and 0.049 are presented. No magnetic hyperfine field (i.e., no static magnetic order) down to 5.5 K was detected for x = 0 and 0.017 in agreement with the absence of any additional feature below superconducting transition temperature T-c in the specific heat data. The evolution of magnetic hyperfine field with temperature was studied for x = 0.033 and 0.049. The long-range magnetic order in these two compounds coexists with superconductivity. The magnetic hyperfine field B-hf (ordered magnetic moment) below T-c in CaK(Fe0.967Ni0.033)(4)As-4 is continuously suppressed with the developing superconducting order parameter. The B-hf(T) data for CaK(Fe0.967Ni0.033)(4)As-4 and CaK(Fe0.951Ni0.049)(4)As-4 can be described reasonably well by Machida's model for coexistence of itinerant spin density wave magnetism and superconductivity [K. Machida, J. Phys. Soc. Jpn. 50, 2195 (1981)]. We demonstrate directly that superconductivity suppresses the spin density wave order parameter if the conditions are right, in agreement with the theoretical analysis.
Available at: http://works.bepress.com/paul_canfield/181/