Thermoelectric Charge and Spin Current Generation in Magnetic Single-Molecule Junctions: First-Principles Calculations

We apply first-principles approaches to investigate the spin (charge) Seebeck effects [S_s(c)] and spin (charge) thermoelectric figure of merits [ZT_s(c)] of manganese-phthalocyanine spin-polarized scanning tunneling microscopy (MnPc SP-STM) junctions. The magnetic tunneling junctions are N-type junctions because their S_c values are negative. Their S_s and S_c values are sufficiently large for the efficient generation of measurable spin and charge currents. ZT_s(c) values strongly depend on the competition between electron and phonon thermal conductances: ZT_s(c) ∞ S_s(c)2 for κ_ph ≥ κ_el, and ZT_s(c) ∞ S_s(c)2κ_el for κ_el ≥ κ_ph. S_s changes signs when the spin-valve junction rotates its magnetic structure from the antiparallel (AP) to the parallel (P) configuration. This behavior indicates that spin-current direction can be reversed by alternating magnetic configurations between AP and P states. Spin-current dissipation in the junctions is minimized because the sizes of the junctions are considerably smaller than the lengths of spin-flip scattering and spin dephasing. The low spin-current dissipation of the junctions suggests that they have potential applications in spintronics and renewable energy. The present finding provides a new approach to spin-current generation through the use of SP-STM based on temperature difference and to controlling spin-current direction through magnetic configurations. The integration of numerous single-molecule magnetic junctions as building blocks into a high-density device is a promising strategy for generating a considerable net spin current for applications in molecular spin caloritronics.