Applied electric field on zigzag graphene nanoribbons: reduction of spin stiffness and appearance of spiral spin density waves.

We investigated the reduction of the spin stiffness and the appearance of the spiral spin density waves when the electric field is applied on the zigzag graphene nanoribbons for the ferromagnetic and antiferromagnetic edge states. For that purpose, we exploited the generalized Bloch theorem combined with a constraint method to keep the direction of the magnetic moment of the carbon atom at the edges. We found that the ground state of ferromagnetic configuration is unstable and leads to the spiral ground state while the ground state of the antiferromagnetic configuration is robust. We also showed that the spin stiffness in the ferromagnetic and antiferromagnetic configurations reduces as the electric field increases. Thus, we justified that not only the spin stiffness but also the ground state of the zigzag graphene nanoribbons can be controlled by the electric field.