  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.
