  An analysis of a novel magnetic field sensor based on a graphene spin
capacitor is presented. The proposed device consists of graphene nanoribbons on
top of an insulator material connected to a ferromagnetic source/drain. The
time evolution of spin polarized electrons injected into the capacitor can be
used for an accurate determination at room temperature of external magnetic
fields. Assuming a spin relaxation time of 100 ns, magnetic fields on the order
of $\sim 10$ mOe may be detected at room temperature. The observational
accuracy of this device depends on the density of magnetic defects and spin
relaxation time that can be achieved.
