Strong magnetic pair breaking in Mn substituted MgB_2 single crystals

TL;DR

Substituting Mn into MgB_2 single crystals introduces magnetic pair-breaking effects that rapidly suppress superconductivity, highlighting the strong electron-moment coupling and the impact of magnetic ions on superconducting properties.

Contribution

This study demonstrates the significant pair-breaking effect of Mn substitution in MgB_2 and identifies the magnetic state of Mn ions, providing new insights into magnetic impurity effects in superconductors.

Findings

Superconducting T_c drops rapidly with Mn substitution, fully suppressed at ~2% Mn.

Mn ions are divalent in a low-spin state, confirmed by susceptibility and X-ray absorption.

Mn substitution reduces the upper critical field anisotropy from 6 to 3.3 at 0.88% Mn.

Abstract

Magnetic ions (Mn) were substituted in MgB_2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, T_c, in Mg_{1-x}Mn_xB_2 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, 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 Mn^{2+} substitution for Mg^{2+} 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.