Opening the Hysteresis Loop in Ferromagnetic Fe₃GeTe₂ Nanosheets Through Functionalization with TCNQ Molecules

Ferromagnetic metal Fe₃GeTe₂ (FGT), whose structure exhibits weak van-der-Waals interactions between 5-atom thick layers, was subjected to liquid-phase exfoliation (LPE) in N-methyl pyrrolidone (NMP) to yield a suspension of nanosheets that were separated into several fractions by successive centrifugation at different speeds. Electron microscopy confirmed successful exfoliation of bulk FGT to nanosheets as thin as 6 nm. The ferromagnetic ordering temperature for the nanosheets gradually decreased with the increase in the centrifugation speed used to isolate the 2D material. These nanosheets were resuspended in NMP and treated with an organic acceptor, 7,7,8,8-tetracyano-quinodimethane (TCNQ), which led to precipitation of FGT-TCNQ composite. The formation of the composite material is accompanied by charge transfer from the FGT nanosheets to TCNQ molecules, generating TCNQ⋅^- radical anions, as revealed by experimental vibrational spectra and supported by first principles calculations. Remarkably, a substantial increase in magnetic anisotropy was observed, as manifested by the increase in the coercive field from nearly zero in bulk FGT to 1.0 kOe in the exfoliated nanosheets and then to 5.4 kOe in the FGT-TCNQ composite. The dramatic increase in coercivity of the composite suggests that functionalization with redox-active molecules provides an appealing pathway to enhancing magnetic properties of 2D materials.