A generally accepted viewpoint is that molecules with a spin S = 1/2 did not show any magnetic coupling when assembled into molecular dimers. However, a recent experiment successfully synthesizes covalently linked five-membered ring decorated nanographene dimers that are antiferromagnetically coupled with each other [Phys. Rev. Lett. 124 (2020) 147206]. Motivated by this work, we further study the effect of magnetic coupling engineering on spin transport properties in covalently linked nanographene dimers-based molecular devices. Our results sh...