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 σ_xy and longitudinal conductivity σ_xx obey a power-law scaling behavior of σ_xy ∝ σ_xx^1.3, which slightly deviates from the universal scaling relation (σ_xy ∝ σ_xx^1.6). 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.