Absence of static magnetic order in non-superconducting FeSe thin films on SrTiO₃(001) revealed by the magnetism of Se vacancies

FeSe, as the simplest iron-based superconductor, invokes tremendous studies on its electronic and magnetic properties. A puzzling phenomenon revealed by the experiments is that FeSe crystal undergoes a nematic structural phase transition around 90 K but without any antiferromagnetic transition in ambient pressure. However, this is not conclusive for non-superconducting FeSe multilayer films due to the stress caused by lattice mismatch and the difficulty in measuring the magnetism in the thin film limit. Here, the absence of static magnetic order is directly revealed by the magnetism of an isolated Se vacancy in FeSe multilayer films grown on SrTiO₃(001) measured with spin-polarized scanning tunneling microscopy on the atomic scale. Symmetry analysis and first-principles calculation further confirmed the nonmagnetic ground state of FeSe. The discrepancy between local density approximation and generalized gradient approximation implies FeSe is near a magnetic critical point.