Strong magnetic pair breaking in Mn-substitutedMgB2single crystals

Abstract

Magnetic ions (Mn) were substituted in $\mathrm{Mg}{\mathrm{B}}_{2}$ single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, ${T}_{c}$, in ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{B}}_{2}$ has been found to be rapidly suppressed at an initial rate of $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕%\mathrm{Mn}$, 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., $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕%\mathrm{Al}$). The magnitude of the magnetic moment ($\ensuremath{\simeq}1.7\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ 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 ${\mathrm{Mn}}^{2+}$ substitution for ${\mathrm{Mg}}^{2+}$ 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 $\mathrm{d}{H}_{c2}∕\mathrm{d}T$ near ${T}_{c}$ unchanged for both field orientations.