Apparent Ferromagnetism in Exfoliated Ultrathin Pyrite Sheets

Abstract

There has been no experimental evidence for ferromagnetic ordering in isotropic atomically thin two-dimensional crystals, until a bilayer Cr2Ge2Te6 and a three-atom thick monolayer CrI3 is shown to retain ferromagnetic ordering at finite temperatures. Here, we demonstrate successful isolation of a non-van der Waals-type ultrathin nanosheet of FeS2 derived from naturally occurring pyrite mineral (FeS2) by means of liquid-phase exfoliation. Structural characterizations imply that (111)-oriented sheets are predominant and are supported theoretically by means of density functional theory surface energy calculations. Spin-polarized density theory calculations further predicted that (111)-oriented three-atom thick pyrite sheets have a stable ferromagnetic ground state different from their diamagnetic bulk counterparts. This theoretical finding is evaluated by experimentally employing low-temperature superconducting quantum interference device measurements, and an anomalous ferromagnetic kind of behavior is observed.