Strong magnetic pair breaking in Mn substituted MgB₂ single crystals

Original · EN

Magnetic ions (Mn) were substituted in MgB₂ single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, Tc, in Mg₁₋ₓMnₓB₂ has been found to be rapidly suppressed at an initial rate of 10 K/%Mn, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the 3d local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g. with Al or C, suppress Tc much less effectively (e.g. 0.5 K/%Al). The magnitude of the magnetic moment, derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state (S = 1/2). This has been found also in X-ray absorption spectroscopy measurements. Isovalent Mn²⁺ substitution for Mg²⁺ mainly affects superconductivity through spin-flip scattering reducing Tc rapidly and lowering the upper critical field anisotropy Hc₂ab/Hc₂ᶜ at T = 0 from 6 to 3.3 (x = 0.88% Mn), while leaving the initial slope dHc₂/dT near Tc unchanged for both field orientations.