  We investigate the coupling of spin and thermal currents as a means to rise
the thermoelectric efficiency of nanoscale graphene devices. We consider
nanostructures composed of overlapping graphene nanoribbons with ferromagnetic
contacts in different magnetic configurations. Our results show that the charge
Seebeck effect is greatly enhanced when the magnetic leads are in an
antiparallel configuration, due to the enlargement of the transport gap.
However, for the optimization of the charge figure of merit ZT it is better to
choose a parallel alignment of the magnetization in the leads, because the
electron-hole symmetry is broken in this magnetic configuration. We also obtain
the spin-dependent Seebeck coefficient and spin figure of merit. In fact, the
spin ZT can double its value with respect to the charge ZT for a wide
temperature range, above 300 K. These findings suggest the potential value of
graphene nanosystems as energy harvesting devices employing spin currents.
