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

¹  Laboratoire de Spectrométrie Physique, UMR5588 CNRS and Université J. Fourier Grenoble, 38402 Saint Martin d'Hères, France, EU
²  Laboratoire National des Champs Magnétiques Intenses, UPR 3228 CNRS 25 avenue des martyrs, BP 166, 38042 Grenoble Cedex 9, France, EU
³  Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Science - P.O. Box 603, Beijing 100190, China

Corresponding author: marc-henri.julien@ujf-grenoble.fr

Received: 19 June 2009
Accepted: 23 July 2009

Abstract

In Ba(Fe_0.95Co_0.05)₂As₂ all of the ⁷⁵As NMR intensity at the paramagnetic resonance position vanishes abruptly below = 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_0.6K_0.4Fe₂As₂, on the other hand, the signal loss below 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.