Carrier localization due to local magnetic order induced by magnetic impurities in Ba(Fe${}_{1-x}$TM${}_x$)${}_2$As${}_2$ (TM = Mn and Cr) as seen via optical spectra

The charge dynamics of Ba(Fe${}_{1-x}$TM${}_x$)${}_2$As${}_2$ (TM = Mn and Cr) has been investigated by optical spectroscopy. It was found that in addition to the strong suppression of the coherent charge transport, the magnetic impurity induces a novel spectral feature related to the carrier localization in the far-infrared region above the antiferromagnetic transition temperature. We attribute it to the cooperative effect between conduction electrons and local magnetic order induced by magnetic impurities. The present results demonstrate that Mn and Cr are not conventional magnetic pair-breakers in iron pnictides.