Effect of Ni doping on magnetism and superconductivity in Eu0.5K0.5Fe2As2

The effect of Ni doping on the magnetism and superconductivity in Eu0.5K0.5Fe2As2 has been studied through a systematic investigation of magnetic and superconducting properties of Eu0.5K0.5(Fe1-xNix)(2)As-2 (x = 0, 0.03, 0.05, 0.08, and 0.12) compounds by means of dc and ac magnetic susceptibilities, electrical resistivity, and specific heat measurements. Eu0.5K0.5Fe2As2 is known to exhibit superconductivity with superconducting transition temperature T-c as high as 33 K. The Ni doping leads to a rapid decrease in T-c; T-c is reduced to 23 K with 3% Ni doping, and 8% Ni doping suppresses the superconductivity to below 1.8 K. In a 3% Ni-doped sample Eu0.5K0.5(Fe0.97Ni0.03)(2)As-2 superconductivity coexists with short-range ordering of Eu2+ magnetic moments at T-m approximate to 6 K. The suppression of superconductivity with Ni doping is accompanied by the emergence of a long-range antiferromagnetic ordering with T-N = 8.5 K and 7 K for Eu0.5K0.5(Fe0.92Ni0.08)(2)As-2 and Eu0.5K0.5(Fe0.88Ni0.12)(2)As-2, respectively. The temperature- and field-dependent magnetic measurements for x = 0.08 and 0.12 samples reflect the possibility of a helical magnetic ordering of Eu2+ moments. We suspect that the helimagnetism of Eu spins could be responsible for the destruction of superconductivity as has been observed in Co-doped EuFe2As2. The most striking feature seen in the resistivity data for x = 0.08 is the reappearance of the anomaly presumably due to spin-density wave transition at around 60 K. This could be attributed to the compensation of holes (K doping at the Eu site) by the electrons (Ni doping at the Fe site). The anomaly associated with the spin-density wave further shifts to 200 K for x = 0.12 for which the electron doping has almost compensated the holes in the system.