Strong magnetic pair breaking in Mn-substituted MgB2 single crystals

Magnetic ions (Mn) were substituted in MgB2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, T-c, 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 T-c much less effectively (e.g., 0.5 K/%Al). The magnitude of the magnetic moment (similar or equal to 1.7 mu(B) per Mn), 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 T-c rapidly and lowering the upper critical field anisotropy H-c2(ab)/H-c2(c) at T=0 from 6 to 3.3 (x=0.88% Mn), while leaving the initial slope dH(c2)/dT near T-c unchanged for both field orientations.