Field-dependent Coulomb Blockade For Intergranular System In High Spin-polarized Oxides

Recently, magnetoresistance (MR) effects have attracted great interests due to both theoretical values and applicational prospects. High spin-polarized oxides with nearly 100% spin-polarization exhibit great magnetoresistance effects in their heterogeneous structures. Among these structures, half-metallic granular systems are outstanding. Many experiments have shown that magnetotransport properties in these granular systems are dependent of the microscopic configurations and different temperature dependence of magnetoresistance has been observed in different materials. Study of MR effects in high spin-polarized oxides is not only of theoretical importance but also valuable for the technological application. The purpose of this work is to systemically study the MR behaviors in these materials and use effective medium theory (EMA) to investigate the ways to enhance magnetoresistance in the spin-polarized conductance network model. The main results of our study are listed as follows.We have concentrated on the temperature dependence of magnetoresistance in this work. At first we discuss the theoretical models describing the magnetotransport properties in half-metallic granular systems. Two famous tunneling mechanisms, the Inoue-Maekawa model and the Helman-Abeles model, are systematically compared. Then we present a conductance network to study the spin-polarized tunneling magnetoresistance based on the Inoue-Maekawa model.We exceed the traditional first-ordered approximation to systematically analyze magnetotransport properties with spin-dependent Coulomb Gap. The inversed magnetoresisitance is found at low temperature. The origin of the phenomenon is discussed. It is argued that the divergent positive MR results from the existence of the magnetic energy and the spin-dependent Coulomb Gap can serve as a new potential MR source in granular systems. At last we incorporate the two models mentioned above in a frame of conductance network and apply the theory of EMA to the network. Both the spin polarized tunneling and the magnetic energy are taken into account and the temperature dependence of magnetoresistance is discussed. It is found that the positive MR can be obviously seen especially at low temperature,high energy gap and low spin polarization, which is caused by the competition between the spin-dependent Coulomb Gap and the spin polarization.