Homogeneous vs. inhomogeneous coexistence of magnetic order and superconductivity probed by NMR in Co- and K-doped iron pnictides

In Ba(Fe(0.95)Co(0.05))(2)As(2) all of the (75)As NMR intensity at the paramagnetic resonance position vanishes abruptly below T(SDW)(onset) = 56 K, indicating that magnetic (spin-density wave) order is present in all of the sample volume, despite bulk superconductivity below T(c) = 15 K. The two phases thus coexist homogeneously at the microscopic scale. In Ba(0.6)K(0.4)Fe(2)As(2), on the other hand, the signal loss below T(SDW) (onset) similar or equal to 75 K is not complete, revealing that magnetic order is bound to finite-size areas of the sample, while the remaining NMR signal shows a clear superconducting response below T(c) = 37 K. Thus, the two phases are not homogeneously mixed, at least for this potassium concentration. For both samples, spatial electronic and/or magnetic inhomogeneity is shown to characterize the NMR properties in the normal state. Copyright (C) EPLA, 2009