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Metal-free magnetism and spin caloritronics are at the forefront of condensed-matter physics. Here, the electronic structures and thermal spin-dependent transport properties of armchair graphene nanoribbons (N-AGNRs), where N is the ribbon width (N = 5–23), are systematically studied. The results show that the indirect band gaps exhibit not only oscillatory behavior but also periodic characteristics with E3p > E3p+1 > E3p+2 (E3p, E3p+1 and E3p+2 are the band gaps energy) for a certain integer p, with increasing AGNR width. The magnetic ground states are ferromagnetic (FM) with a Curie temperatures (TC) above room temperature. Furthermore, the spin-up and spin-down currents with opposite directions, generated by a temperature gradient, are almost symmetrical, indicating the appearance of the perfect spin-dependent Seebeck effect (SDSE). Moreover, thermally driven spin currents through the nanodevices induced the spin-Seebeck diode (SSD) effect. Our calculation results indicated that AGNRs can be applied in thermal spin nanodevices.

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