Magnetic polarons in ferromagnetic semiconductor single-electron transistors

Magnetic polaron (MP) formation is studied theoretically in a single-electron transistor (SET) consisting of a ferromagnetic semiconductor quantum dot (FSQD) coupled to nonmagnetic source, drain, and gate electrodes. Especially, using Green's-function technique we calculate the effect of the gate-voltage-dependent spin polarization of the charge-carrier spins on the magnetization and conductance of the ferromagnetic semiconductor SET in the Coulomb blockade regime. We apply the Anderson impurity model to the FSQD and the ferromagnetic subsystem inside the FSQD is treated in the mean-field approximation. By minimizing the total free energy of the FSQD we calculate the MP binding energy and the dot magnetization as a function of temperature and the gate voltage. The results show that the ferromagnetic transition temperature of the FSQD increases strongly due to the MP formation, which may contribute to the experimentally observed increase in the Curie temperature in the FSQDs. The calculated results also indicate that due to the MP formation the average magnetization of the FSQD can be controlled by the gate voltage in a wide temperature range. Furthermore, our model predicts that the conductance vs gate-voltage curve, which in nonmagnetic SETs shows a symmetric double peak structure, becomes highly asymmetric due to the MP formation.