▎ 摘　　要

Spin-tunable materials are very appealing for applications in multifunctional spintronics devices because of their ability to switch between different spin states with external stimuli such as strain, light or electric-field, etc. As a promising material for spintronics, transition-metal-nitrogen-carbon (TM-N-C) materials have achieved considerable attentions. Based on density functional calculations, we systematically investigate the magnetic properties of four-nitrogen coordinated transition-metal embedded graphene (TMN4-G, TM = all 3d-elements). The embedded configurations of TMN4-G show rich tunable spin states by external strains, exhibiting two kinds of spin-state transitions: 1) Spin crossover between low- to high-spin states (TMN4-G, TM = Mn, Fe and Co); 2) Spin polarization turned up or turned off in nonmagnetic systems (ScN4-G and NiN4-G) or in magnetic CuN4-G, respectively. The spin-state transitions occur abruptly for systems with magnetic TM elements, while continuously for those with nonmagnetic elements. The origin of the spin-state transitions is explored and found related with the TM valance electrons and charge transferred. Our findings could open an avenue for the efficient tuning of spin and magnetism in graphene based TMN4 structures for application in spintronics.