Abstract
We report nuclear magnetic resonance (NMR) measurements of single-crystalline (, 0.028, 0.033, and 0.059) annealed at for 7 days. From the observation of a characteristic shape of NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of ( K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in ( K) and ( K). Through the temperature dependence of the Knight shifts and the nuclear spin lattice relaxation rates (), although stripe-type AFM spin fluctuations are realized in the paramagnetic state as in the case of other iron pnictide superconductors, we found a gradual decrease of the AFM spin fluctuations below a crossover temperature that was nearly independent of Co-substitution concentration, and it is attributed to a pseudogaplike behavior in the spin excitation spectra of these systems. The feature finds correlation with features in the temperature-dependent interplane resistivity, , but not with the in-plane resistivity . The temperature evolution of anisotropic stripe-type AFM spin fluctuations is tracked in the paramagnetic and pseudogap phases by the data measured under magnetic fields parallel and perpendicular to the axis. Based on our NMR data, we have added a pseudogaplike phase to the magnetic and electronic phase diagram of .
5 More- Received 1 August 2015
- Revised 21 October 2015
DOI:https://doi.org/10.1103/PhysRevB.92.184504
©2015 American Physical Society