Probing Magnetism in Cobalt-Intercalated MoTe₂

Recently, as 2D materials with long-range ferromagnetic order such as Fe₃GeTe₂, Cr₂Ge₂Te₆, CrTe₂, VSe₂, and CrI₃ have been experimentally proven, research on magnetic devices based on them has been actively conducted. On the other hand, various studies have been conducted on magnetic properties generated by defects or implanting magnetic dopants into 2D transition metal dichalcogenide (TMD) with semiconducting or metallic properties. However, from the viewpoint of magnetism, the intercalation method that inserts an additional element between the van der Waals (vdW) planes of TMD has rarely been explored. Intercalation for 2D layered materials can be applied using vapor phase methods, solid phase diffusion, and wet chemical methods. In this study, Co, the 3d transition metal, was inserted between the vdW layers of 2H-MoTe₂ by applying 2 step-intercalation, which was sequentially performed Li-intercalation and Co-intercalation based on the wet chemical method.<br/>n-Butyllithium diluted in hexane has been widely adopted as a reagent for preparing lithium intercalation complexes. Since the reaction is possible at room temperature, only Li ions can be intercalated, excluding by-products. 2H-MoTe₂ with n-type semiconducting behavior undergoes a phase transition to 1T’-MoTe₂ by Li-intercalation for 48 hours. It was confirmed that Li-intercalated MoTe₂ with metallic properties had ohmic contact with the metal electrode. As Li-ions are injected between each MoTe₂ layer, the vdW gap is also increased compared to the pristine. After Li-intercalation, the phase transition to 1T’-MoTe₂ was confirmed by measuring I-V characteristics and the transfer curve. We observed that the source-drain current increased from 20nA to 82nA at 1V due to Li-ion insertion, and current modulation was minimized when applying the back gate voltage.<br/>Next, the intercalation method of Co, a zero-valent transition metal formed by disproportionation redox reaction, was applied to Li-intercalated MoTe₂. Dicobalt octacarbonyl, Co₂(CO)₈, was used as a precursor to providing zero-valent Cobalt (Co(0)) through redox reaction. This redox reaction is carried out in acetone used as a solvent during intercalation for 30 minutes at a reaction temperature of 50^oC. Longitudinal magnetoresistance (MR) was measured in a cryogen-free probe station (Lake Shore CRX-VF) with a superconducting magnet by applying an out-of-plane magnetic field at a temperature from 200K to 300K. Our sample showed the MR ratio of 7% at 200K and 4% at 300K around ±2T. We demonstrate that MoTe₂ intercalated with Co has a ferromagnetic order in which resistance changes by an external magnetic field. This study has opened new horizons to the future field of 2D vdW-based magnetic devices.