The Study On Mangetic Transport Properties Of Inhomogeneous System

The discovery of giant magnetoresistance effect has attracted much attention to research of magnetic transport in inhomogeneous systems, which depends on spin of electrons. The magnetoresistance in such two inhomogeneous systems as magnetic granular alloys and doped pervoskite manganite fflms is usually considered due to spindependent scattering. Applied magnetic field, the symmetric broken between spin-up and spin-down channels gives rise to the great decrement of resistance.There are mainly two ways of theoretic study on GMR in magnetic granular alloys. One is to extended the quantum or quasi-classical treatments of GMR in multilayers to the one in magnetic granular systems. The other is a phenomenological method which adopes effective medium approxiation (EMA) and two-channel model directly. Within the framework of the latter one, the GMR dependence on temperature and volume-fraction is discussed in the paper, with consideration of different kinds of magnetic entities. EMA, assumes the components distributed in highly random fashion, is a useful method to treat the transport properties of granular composites, and the effective conductance of composites could to be derived sell-consistently by substituting the actual enviroment of the studied grain with effective medium. Nevertheless, it seems to be quite unsatisfactory for inviestigation the effective properties of systems due to ignoring the realistic structures of the medium. Actually, former researches demenstra.te that both tiny clusters formed by several magnetic atoms and big dusters formed by magnetic grains exist in films besides isolated grains. In the paper, we take the effect of tiny clusters into accunt and find that the ratio of tiny clusters to grains change in different samples. The calculated results are in good agreement with experimental data for temperature dependence. Taking the effect of big du.flers into account, we explain the volume-fraction dependence qualitatively by effective duster model.Meanwhile, we enterpret phenomenonally the rnagnetoresistance dependence onii3temperature in doped pervoskite manganite films. Due to local deviations from ferromagnetic order, tiny ferromagnetic clusters exist. On one hand, in grain regions of films, the strong inter-duster interaction, mentioned as mean field, leads to the colossal magnetoresistance effect (CMR). On the other hand, tiny clusters in the grain boundary regions are of no interactions, hence result in a temperature behavior similar to the above one of magnetic granular alloys.Yan JuDirected by Prof. Zheuya Li...