Network Effects On The Magnetotransport In Half-metallic Granular Systems

Recently, half-metallic granular composites have attracted great interests for their fundamental importance and the potential technological application. The complete spin-polarization in half-metals makes it possible to observe great magnetoresistance effects in their heterogeneous structures. Among these structures, half-metallic granular systems are outstanding. To enhance the magnetoresistance effects from extrinsic aspects, it is effective to use the structure in the form of polycrystals and powder compacts. Granular systems as a transport network, the observed magnetoresistance is the consequence of the network effects. In half-metallic granular systems, network effects are more crucial. Study of effects of network on the magnetoresistance is not only of theoretical importance but also valuable for the technological application.The purpose of this work is to study network effects on the magnetotransport and the main results are listed below:1. Resistor network model for the tunneling magnetoresistanceBy comparing the models of resistors in parallel and in series, we have proposed a resistor network to study the magnetotransport in half-metallic granular systems. The effects of resistor network on the characteristic features of tunneling magnetoresistance of half-metallic granular materials is studied by using a numerical simulation method. We assume the nanosized magnetic particles are well separated by insulating and nonmagnetic grain boundaries and they are distributed in a n-dimensional resistor network. The value of the tunneling magnetoresistance is found to depend sensitively on the value of n, as well as the distribution of tunneling resistors. Our findings agree with experiments well.2. Coulomb gap and magnetoresistanceWe present a self-consistent effective medium theory for the magnetotransport in nanometric manganites on the basis of the bond-disordered resistor network. The transportprocess here is assumed to be controlled by both the charging energies of grains and the spin-polarized tunneling through grain boundaries. The effects of network and band-bending at the grain boundaries on the magnetotransport are investigated. Our results are compared with the experiments on nanometric La2/3Sr1/3MnO3 and La2/3Ca1/3MnO3, good agreements are found. For Zn-ferrite granular systems, the role of grain size distribution is investigated and the intergrain correlation is found very crucial in determining the magnitude of magnetoresistance. It is found that at low temperatures, the magnetoresistance is enhanced drastically which is due to high-order tunneling. On the other hand, the decay of spin polarization will lead to the decrease of magnetoresistance at high temperatures. Our calculations agree with the experimental data well.3. Magnetotransport channels and magnetoresistanceThe temperature dependence of the intergranular magnetoresistance (EMR) of CrO2 powder compacts is studied. It is considered that there are two channels of conductance, one is spin-independent, the other is spin-dependent. The first channel conies from the higher-order inelastic hopping via localized states. The conductance increases drastically with the increase of temperature. In the second channel, the decay of polarization of electrons, which is due to surface spin-wave excitation, affects the conductance of this channel strongly. From the analysis of the temperature dependence of IMR, it is found that the contribution from these two channels to magnetoresistance varies with the different concentration of the insulating component Cr2O3 and the different magnetic property of surface layer of CrO2 grain in samples. Our calculations agree with the experimental data well. On the other hand, for the materials with distinctive intrinsic properties, the connectivity between neighboring grains is found important. To account for such connectivity, we develop a resistor network model. The result based on the network model explain the experiments well, e.g. the shoulder in the temperature dependence of magnetoresistance.