Phase competition and superconductivity in electron-doped BaFe2As2 and unusual magnetic order in hole-doped BaMn2As2
ABSTRACT :
In Fe-based superconductors, particularly electron doped BaFe2As2 (partial substitution of Fe by other transition metal species: Co, Ni, Cu), the stripe antiferromagnetic(AFM) structure is best characterized by spin density wave (SDW). Our neutron scattering studies show the appearance of incommensurate SDW order in Co and Ni substituted samples. But the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.
Hole doped Ba(Fe1-xMnx)2As2 displays SDW and Neel order at x=0 and at x=1 respectively. BaMn2As2 (x=1), isostructural to BaFe2As2, is a local-moment AFM insulator with a Néel temperature TN of 625 K and a large ordered moment of 3.9 µB/Mn. Remarkably, this compound can be driven metallic by the substitution of as little as 1.6% K for Ba while retaining essentially the same ordered magnetic moment and Néel temperature. Using both powder and single crystal neutron diffraction we show that the local moment AFM order in Ba1-xKxMn2As2 remains robust up to x=0.4. The ordered moment is nearly independent of x for 0 ≤x≤ 0.4 and TN decreases to 480 K at x=0.4.This provides strong evidence that the magnetic exchange interaction is relatively insensitive to the addition of charge carriers via chemical substitution for Ba. The local-moment nature of AFM order in parent and hole doped BaMn2As2 is opposed to the itinerant SDW character found for the Ba1-x KxFe2As2and Ba(Fe1-xTMx)2As2superconductor (TM=Co, Ni, Cu).