Strong magnetic pair breaking in Mn-substituted MgB2 single crystals

Abstract

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 10K∕%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., 0.5K∕%Al). The magnitude of the magnetic moment (≃1.7μB 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 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.

Source-reported materials — not catalogue approval

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