Locally generating magnetic field for spin-polarization controls of single electrons in a mesoscopic ring

Spin-polarization control with local magnetic field is one of the fundamental tasks in spin electronics. In this paper we investigate how to generate the local magnetic field for implementing the spin-polarization controls of single electrons in an open mesoscopic ring embedded by a few quantum dots. The spin-polarization electrons are input from one ferromagnetic lead, transporting along a mesoscopic ring with a few quantum dots (QDs), and then output into another ferromagnetic lead. With the standard non-equilibrium Green's function technique, we discuss how to control circular currents (CCs) of such an open mesoscopic ring and consequently the locally-produced magnetic field by manipulating the embedded QDs. It is found that, for the input electrons with sufficiently strong polarizations, e.g., the polarization strength P > 0.8, only the electrons with one of the polarizations (e.g., the spin-up electrons) can pass through the ring and almost of all the electrons with another polarization (e.g., the spin-down electrons) are effectively blocked. This is the typical spin filtering effect, originated physically from the QDs-induced quantum interferences. Second, for certain designs of the QDs the CCs in the rings present the so-called negative differential resistance behavior, i.e., the values of the CCs increase with the decreases of the biased voltages. Interestingly, we find that a sawtooth-like change of the direction of such a local magnetic field can be realized by controlling the level-splittings of the QDs in the upper-and lower arms of the ring, respectively. Therefore, both the value and direction of such a local magnetic field are adjustable, at least theoretically. The strength of such a CC-induced local magnetic field is estimated at the order of microTesla, which is sufficiently strong to implement the spin-polarization controls of the single electrons transporting along the mesoscopic structure. Finally, the temperature effect of such a local magnetic field is also discussed.