Probing giant Zeeman shift in vanadium-doped $\mathrm{W}{\mathrm{Se}}_2$ via resonant magnetotunneling transport

Doping van der Waals layered semiconductors with magnetic atoms is a simple and effective approach to induce magnetism. However, investigation of the electrical properties of such two-dimensional semiconductors and the modulation of their magnetic order for spintronics is still lacking. Herein, we report a giant Zeeman shift from the spin-polarized state in tungsten diselenide ($\mathrm{W}{\mathrm{Se}}_2$) doped with a small amount of vanadium (V) atoms $(\sim 0.1\%)$. The Zeeman shift was measured via resonant magnetotunneling spectroscopy with a vertical graphite/V-$\mathrm{W}{\mathrm{Se}}_2$/graphite heterojunction. The $p$-type doping state near the valence band is substantially shifted under an external magnetic field by 7.8 meV/T, equivalent to a giant $g$ factor of approximately 135, an order of magnitude higher than that of other two-dimensional magnetic semiconductors. The ferromagnetic order of the spin glass state and its long-range interaction are revealed by the remanence of magnetoresistance between the zero-field cooling and field-cooling processes as well as magnetoresistance hysteresis. The ferromagnetic glass order is fully established at 50 K, whereas the long-range interaction persists at higher temperatures of up to 300 K in V-doped $\mathrm{W}{\mathrm{Se}}_2$ flakes with an approximate thickness of 5 nm. Our work sheds light on the magnetic nature of V-doped $\mathrm{W}{\mathrm{Se}}_2$ semiconductors and paves the way for future spintronics based on two-dimensional van der Waals magnetic semiconductors.

Figure 1

Device structure and detection of the doping state. (a) Schematic illustration of the ${\mathrm{Gr}}_{\mathrm{T}}$/V-$\mathrm{W}{\mathrm{Se}}_2/{\mathrm{Gr}}_{\mathrm{B}}$ tunneling junctions. (b) Band diagram of the device. (c) Comparison between the $dI/dV$ curves of V-doped and pristine $\mathrm{W}{\mathrm{Se}}_2$ devices. (d) DFT calculation of the band structure of $\mathrm{W}{\mathrm{Se}}_2$ (dotted cyan), $\mathrm{W}{\mathrm{Se}}_2$ with Se vacancy (solid red), and V-doped $\mathrm{W}{\mathrm{Se}}_2$ with Se vacancy (solid blue), including spin–orbit coupling. Here, a $10\times 10$ supercell of bilayer $\mathrm{W}{\mathrm{Se}}_2$ was used (with one V atom and one Se vacancy) for the calculation.

Figure 2

Verification of the spin polarized nature of the doping state. (a) $dI/dV$ curves and (b) $d^{2}I/d{V}^2$ curves of the V-$\mathrm{W}{\mathrm{Se}}_2$ device in the presence of a magnetic field of 0–4 T. (c) Shift in the peak position of the doping state as marked * in (b). (d) $d^{2}I/d{V}^2$ mapping against the magnetic field and bias.

Figure 3

Resistance characteristics of V-doped and pristine $\mathrm{W}{\mathrm{Se}}_2$ devices in ZFC and FC. (a) Schematic illustration of the ZFC and FC characterization processes and the changes in the spin structure. (b)–(c) Comparison between the V-doped $\mathrm{W}{\mathrm{Se}}_2$ (b) and pristine $\mathrm{W}{\mathrm{Se}}_2$ (c) devices with detailed resistance characterization during ZFC and FC processes. (b-I–III) and (c-I–III): the detailed ZFC and FC measurement processes of the corresponding V-doped and pristine $\mathrm{W}{\mathrm{Se}}_2$ devices. (b-IV) and (c-IV): zoomed-in view of the resistance changing with the oscillated magnetic field to 0 T at 3 K of the corresponding V-doped and pristine $\mathrm{W}{\mathrm{Se}}_2$ devices. For the V-doped sample, the resistance during the magnetic field oscillation to 0 T at 3 K is almost constant with the magnetic field change, maintaining a resistance remanence compared to that of ZFC. In contrast, the resistance of pristine $\mathrm{W}{\mathrm{Se}}_2$ follows the oscillated magnetic field and returns back to the resistance value of ZFC, indicating the pure magnetoresistance without remanence of pristine $\mathrm{W}{\mathrm{Se}}_2$. (b-V) and (c-V): difference between the ZFC-H and FC-H curves of the corresponding V-doped and pristine $\mathrm{W}{\mathrm{Se}}_2$ devices with the normalized resistance $(R/R_{max})$ as the $y$ axis.

Figure 4

Magnetic transition of V-doped $\mathrm{W}{\mathrm{Se}}_2$. (a) Comparison between $\left(R_{\mathrm{FC}\text{−}\mathrm{H}}\text{−}{R}_{\mathrm{ZFC}\text{−}\mathrm{H}}\right)/R_{\mathrm{ZFC}\text{−}\mathrm{H}}-T$ and $dR/dT-T$ curves of the V-doped and pristine $\mathrm{W}{\mathrm{Se}}_2$ devices to verify the magnetic transition. Inset: schematic of the change in the magnetic scattering due to the spin alignment. (b)–(c) Magnetoresistance of V-$\mathrm{W}{\mathrm{Se}}_2$ device at 2 K (b) and 50 K (c). (d) Time evolution of the resistance measured with the magnetic field scanning of V-$\mathrm{W}{\mathrm{Se}}_2$ device at 300 K.

Figure 5

Distinguishing spin-polarized states with applied voltage. (a) Spin-polarized density of states of V-doped $\mathrm{W}{\mathrm{Se}}_2$. Highly spin-polarized states are revealed within 0.5 eV below the valence band edge. (b)–(d) Band diagrams of Gr/V-$\mathrm{W}{\mathrm{Se}}_2$/Gr device at −0.1 V (b), −0.4 V (c), and −1.0 V (d) with corresponding modulation of the $R$–$T$ curves of ZFC and FC processes. Location of spin-polarized states in the bias windows can be modulated by varying the applied bias. More density of states of V-$\mathrm{W}{\mathrm{Se}}_2$ are involved in transport by increasing the bias (c), but the contribution of non-spin-polarized states become dominant in the case of a large bias (d).