Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. 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 x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave 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 TN 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.
Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotation / Cheung, Sky C.; Zurab, Guguchia; Frandsen, Benjamin A.; Zizhou, Gong; Kohtaro, Yamakawa; Almeida, Dalson E.; Onuorah, IFEANYI JOHN; Bonfa', Pietro; Eduardo, Miranda; Weiyi, Wang; Tam, David W.; Song, Yu; Chongde, Cao; Yipeng, Cai; Hallas, Alannah M.; Wilson, Murray N.; Munsie, Timothy J. S.; Graeme, Luke; Bijuan, Chen; Guangyang, Dai; Changqing, Jin; Shengli, Guo; Fanlong, Ning; Fernandes, Rafael M.; DE RENZI, Roberto; Pengcheng, Dai; Uemura, Yasutomo J.. - In: PHYSICAL REVIEW. B. - ISSN 2469-9950. - 97:22(2018), p. 224508. [10.1103/PhysRevB.97.224508]
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