Spin-resolved imaging of antiferromagnetic order in Fe 4 Se 5 ultrathin films on SrTiO 3 .

Unraveling the magnetic order in iron chalcogenides and pnictides at atomic scale is pivotal for understanding their unconventional superconducting pairing mechanism, but is experimentally challenging. Here, by utilizing spin-polarized scanning tunneling microscopy, we successfully resolve real-space spin contrasts exhibiting atomically unidirectional stripes in Fe 4 Se 5 ultrathin films, the plausible closely-related compound of bulk FeSe with ordered Fe-vacancies, which are grown by molecular beam epitaxy. As is substantiated with the first-principles electronic structure calculations, the spin contrast originates from a pair-checkerboard antiferromagnetic ground state with in-plane magnetization, which is modulated by a spin-lattice coupling. Our measurements further identify three types of nanoscale antiferromagnetic domains with distinguishable spin contrasts, which are subject to thermal fluctuations into short-ranged patches at elevated temperatures. Our work provides promising opportunities in understanding the emergent magnetic order and the electronic phase diagram for the FeSe-derived superconductors. This article is protected by copyright. All rights reserved.