  The possible existence of a sign-changing gap symmetry in
BaFe$_{2}$As$_{2}$-derived superconductors (SC) has been an exciting topic of
research in the last few years. To further investigate this subject we combine
Electron Spin Resonance (ESR) and pressure-dependent transport measurements to
investigate magnetic pair-breaking effects on BaFe$_{1.9}M_{0.1}$As$_{2}$ ($M=$
Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence
of localized magnetic moments, is observed only for $M=$ Cu and Mn compounds,
which display very low SC transition temperature ($T_{c}$) and no SC,
respectively. From the ESR analysis assuming the absence of bottleneck effects,
the microscopic parameters are extracted to show that this reduction of $T_{c}$
cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a
sign-preserving gap function. Our results reveal an unconventional spin- and
pressure-dependent pair-breaking effect and impose strong constraints on the
pairing symmetry of these materials.
