⁵⁷Fe Mössbauer and magnetic studies of Th₇Co_2.5 ⁵⁷Fe_0.5 and Th₇Ni_2.5 ⁵⁷Fe_0.5 superconductors

Abstract

The noncentrosymmetric Th 7 Co 2.5 57 Fe 0.5 and Th 7 Ni 2.5 57 Fe 0.5 superconductors were studied by means of 57 Fe transmission Mössbauer spectroscopy (TMS), ac-magnetic susceptibility and magnetization measurements. The low-temperature data showed that both compounds are type-II superconductors with transition temperature close to T c = 2 K. The collected Mössbauer spectra of the studied samples consist of one well-resolved doublet over the temperature range measured. Neither abnormal behaviour of the hyperfine parameters at or near T c nor phonon softening were found. A comparison of 57 Fe Mössbauer data of Ni-and Co-based superconductors with those of Th 7 Fe 3 indicated that with decreasing 3d-electron numbers across the series Th 7 Ni 2.5 57 Fe 0.5 → Th 7 Co 2.5 57 Fe 0.5 → Th 7 Fe 3 , the s-electron density around the Fe nucleus and as well as the electric field gradient strength increases. The latter finding is associated with increasing asymmetricity of 3d-electron distribution and this fact presumably gives arise to an exotic superconductivity in the Co-and Fe-based compounds.