Korringa-like relaxation in $A$-site ordered manganites

Half doped manganites exhibiting charge- and orbital ordering transitions are paradigm materials for studying colossal magnetoresistance, the existence of ferroelectricity or Zener polaron-type features. We report on high-temperature electron spin resonance studies of $A$-site ordered and disordered $A$BaMn$_2$O$_6$ $A=$Y,Sm,La. We find a Korringa-like spin-relaxation above the charge-ordering transition and extending up to 1000~K in the $A$-site ordered antiferromagnetic systems $A=$Y,Sm, a unique feature for a truly metallic state not having been reported in manganites before. In agreement with the ESR intensity this finding suggests that the ESR signal stems from Mn$^{4+}$ core spins which relax via the quasi-delocalized $e_g$ electrons. In contrast in the disordered AFM and the ferromagnetic samples no Korringa relaxation is observed. Hence, the conductivity does not significantly influence the spin relaxation in those compounds.