The conversion of waste heat into spin signals at the molecular scale is a key goal in spin caloritronics. Using first-principles density functional theory (DFT) and nonequilibrium Green’s function (NEGF) method, we investigate thermally driven spin transport in a single-molecule junction based on a nickelocene (NiCp₂) bipolar magnetic molecule (BMM), where the occupied and unoccupied molecular orbitals near the Fermi energy possess opposite spin polarizations. A dual-gate architecture enables efficient electrostatic tuning of the frontier orbitals relative to the Fermi energy.
Our calculations reveal three distinct transport regimes controlled by the gate voltage V_g. At V_g = 0, the junction exhibits a strongly spin-up-polarized current (I_↑ = 29.19 nA, I_↓ = -0.71 nA at T_L = 400 K, ΔT = 60 K), giving nearly 100% spin-up filtering. At V_g = 1.8 V, the spin-up and spin-down contributions become almost equal and opposite (I_↑ = 1.83 nA, I_↓ = -1.83 nA), resulting in nearly complete suppression of the charge current (I_c ≃ 0) while a pure spin current of I_s = 3.66 nA persists. This arises from gateinduced symmetrization of the HOMO-/HOMO-1-derived (spin-up) and LUMO/LUMO+1-derived (spindown) transmission peaks relative to the Fermi energy. At V_g = 4 V, the spin-down channel dominates (I_↓ = -14.16 nA, I_↑ = 0.48 nA), reversing both the spin-polarization direction and the current flow. Quantitative analysis shows that gate voltage shifts the molecular levels with an efficiency of 13.2%- 15.9% per volt.
We further evaluate the Seebeck coefficients. At room temperature and V_g = 1.8 V, the values at the Fermi energy are S_↑ = 28.1 μV/K, S_↓ = -29.0 μV/K, S_s = 57.1 μV/K, and S_c = -0.9 μV/K. The spin Seebeck coefficient consistently exceeds the charge counterpart, confirming superior spin-dependent thermoelectric conversion. Although the absolute values are one order of magnitude lower than some reported systems—due to the relatively smooth change in transmission spectra—this points to future optimization via molecular engineering (e.g., side groups or anchor modification) to enhance the transmission slope near the Fermi energy.
In conclusion, our work demonstrates that a single BMM junction can be gate-tuned among three functional modes: spin-up filtering, pure spin-current generation, and spin-down filtering with reversed current. The generation of pure spin current without charge current, and thus without Joule heating, makes NiCp₂-based BMMs promising for ultra-low-power molecular spin caloritronic devices, such as thermal spin logic and spin-based thermoelectric converters.