Magnetic transitions and nearly reentrant superconducting properties of HoNi2B2C

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.

Reentrant behaviour in Y-doped Ho(0.75)Y(0.25)Ni(2)B(2)C single crystal

The transport and superconducting properties of Ho(0.75)Y(0.25)Ni(2)B(2)C single crystals were investigated to study the competing effects between superconductivity and magnetism. The superconducting transition temperature T(c) is 7.6 K, determined from the resistivity transition; meanwhile, the commensurate antiferromagnetic (AFM) transition occurs at T(N) of 3.9 K, which is lower than that of pure HoNi(2)B(2)C (T(N)≈5 K). Ho(0.75)Y(0.25)Ni(2)B(2)C reentered into the normal state at T(m) (T(N)<T(m)<T(c)) when small magnetic fields were applied along the crystallographic c-axis. In contrast to the case in HoNi(2)B(2)C, the reentrant behaviour for Ho(0.75)Y(0.25)Ni(2)B(2)C only appears when the applied field H is along the c-axis, and the reentrant peak position T(P)(H) shifts to lower temperature with increasing applied field. We suggest that the disorder of magnetic structure induced by Y doping may account for the significant difference in the reentrant behaviour between Ho(0.75)Y(0.25)Ni(2)B(2)C and HoNi(2)B(2)C. Moreover, there does not exist a deep minimum in the upper critical field H(c2)(T) line at T(N) of 3.9 K for either [Formula: see text] or [Formula: see text]. The H-T phase diagram is derived and discussed.