Theory Research Of Spin-polarized Tunnelling In Ferromagnetic Metallic Granular System

In today's material physics,seeking material of high spin-polarization and large magnetoresistance near room temperature has become an important goal of the research of new materials, manganese perovskite materials provided the possibility of success. Recently, researchers found that the extrinsic magnetoresistance of the system could reached a significant effect under a low magnetic field while the intrinsic magnetoresistance needs a much higher field.This will undoubtedly increase the attractive prospect of the system.This paper studys the polycrystalline manganese perovskite materials which similar to ferromagnetic metallic granular system,And put forward a phenomenon model based on spin-polarized tunneling theory.This model divides the ferromagnetic particles into two types,one of them is particles which have ferromagnetic exchange coupling between them,another is partiles which don't have. These two currents channels were given conductivity expressions of the entire temperature range respectively .Then we use two-channel model to calculate the total resistance. This model successful fits the resistivity of polycrystalline manganese perovskite whichever at zero-magnetic field or under magnetic field.The model explain the electronic transport of the ferromagnetic metallic particle system.Each chapter is summarized below :In the first chapter,the research history and the application of the magnetoresistance materials has been introduced. We have summarized the crystal structure, electron structure, magnetic structure, and electronic transport properties of perovskite manganites.Then we introduced the electronic and magnetic transport properties of metallic granular systems in detail. In the second chapter,we introduce the basic theory of magnetic tunneling junction and several important models about spin-polarized tunneling in metallic granular system,then point out the success and shortcomings of them.In the third chapter,we put forward a phenomenon model which based on spin-polarized tunnelling theory to explain the electronic transport in the magnetic metallic granular system. The model considered the effect of grain boundary which can affect the coupling of the adjacent particles, divides the ferromagnetic particles into two types,one of them is particles which have ferromagnetic exchange coupling between them,another is partiles which don't have. These two currents channels were given conductivity expressions of the entire temperature range respectively .Then we use two-channel model to calculate the total resistance.In the forth chapter,we fit the experiment dates of the manganese perovskite sinterd at different temperature successful. The model also explain the reasons of intrinsic and extrinsic magnetoresistance,the minimum of the resistance in low temperature and the granular size effects on the magnetic character.