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Strong magnetic pair breaking in Mn substituted MgB_2 single crystals

K. Rogacki·B. Batlogg·J. Karpinski·N. D. Zhigadlo·G. Schuck·S. M. Kazakov·P. Wagli·R. Puzniak·A. Wisniewski·F. Carbone·A. Brinkman·D. van der Marel
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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.

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K. Rogacki, B. Batlogg, J. Karpinski, N. D. Zhigadlo, G. Schuck, S. M. Kazakov, P. Wagli, R. Puzniak, A. Wisniewski, F. Carbone, A. Brinkman, D. van der Marel. 2005-10-24. Strong magnetic pair breaking in Mn substituted MgB_2 single crystals. https://doi.org/10.1103/physrevb.73.174520

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