Fe Mössbauer spectroscopy*

Temperature dependent ⁵⁷Fe Mössbauer spectroscopy and specific heat measurements for CaK(Fe_1-xNi_x)₄As₄ with x = 0, 0.017, 0.033, and 0.049 are presented. No magnetic hyperfine field (i.e. no static magnetic order) was detected down to 5.5 K 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(Fe_0.967Ni_0.033)₄As₄ is continuously suppressed with the developing superconducting order parameter. The B_hf(T) data for CaK(Fe_0.967Ni_0.033)₄As₄, and CaK(Fe_0.951Ni_0.049)₄As₄ can be described by Machida's model for coexistence of itinerant spin density wave magnetism and superconductivity [K. Machida, J. Phys. Soc. Jpn. 50, 2195 (1981)].