Abstract
Muon spin rotation and relaxation studies have been performed on a “111” family of iron-based superconductors, , using single crystalline samples with Ni concentrations , 0.4, 0.6, 1.0, 1.3, and 1.5%. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For and 0.4%, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for magnetic order becomes more disordered and is completely suppressed for . The magnetic volume fraction continuously decreases with increasing . Development of superconductivity in the full volume is inferred from Meissner shielding results for . The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the phase diagram for . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting for , 1.0, and 1.3%, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below , but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant nonmagnetic state below for . The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering. In studies of superconducting properties, we find that the limit of superfluid density follows the linear trend observed in underdoped cuprates when plotted against . This paper also includes a detailed theoretical prediction of the muon stopping sites and provides comparisons with experimental results.
7 More- Received 19 January 2018
- Revised 25 April 2018
DOI:https://doi.org/10.1103/PhysRevB.97.224508
©2018 American Physical Society