van der Waals epitaxial growth and high-temperature ferrimagnetism in ultrathin crystalline magnetite (Fe3O4) nanosheets

Abstract

Two-dimensional (2D) magnets have attracted great research interest since long-range ferromagnetic ordering has been found in few-layer Cr2Ge2Te6 and monolayer CrI3. However, most 2D magnets have low magnetic ordering temperatures, impeding their practical application. Room-temperature or high-temperature intrinsic 2D magnets are highly desired for fundamental research and applications. Here, van der Waals epitaxial growth, structure characterization, and magnetic properties of ultrathin crystalline magnetite (Fe3O4) nanosheets are reported. The Curie temperature of the as-grown ultrathin Fe3O4 nanosheets (847 K) is as high as its bulk counterpart (858 K). A large and saturated anomalous Hall effect (AHE) is observed in individual ultrathin Fe3O4 nanosheets up to 400 K. The anomalous Hall resistance increases as the thickness of the Fe3O4 nanosheets decreases to ∼10 nm. Irrespective of the thickness, the Hall angle reaches a maximum at 250 K, and the anomalous Hall conductivity σxy and longitudinal conductivity σxx obey a power-law scaling behavior of σxy ∝ σxx1.3, which slightly deviates from the universal scaling relation (σxy ∝ σxx1.6). The high Curie temperature and high stability of Fe3O4 nanosheets make them a promising candidate for spintronics and Hall sensors, as well as a building block for various van der Waals heterostructures.