Spin-dependent transport and magnetic properties of nanosize granular materials

The transport and magnetic properties of nanosize granular materials have been studied, and attention has been paid to the Cu-Co granular systems and CrO₂ nano powders. These granular systems show significant magnetoresistance (MR) at different temperatures, which has great potential in industrial applications. The mechanisms of the giant magnetoresistance (GMR) of Cu₈₀Co₂₀ granular films and the tunneling magnetoresistance (TMR) of half-metallic CrO₂ granules have been analyzed, where the electron spin dependent transport plays an important role.; The Cu₈₀Co₂₀ granular films have been prepared by magnetron sputtering and pulsed laser deposition on silicon substrates. The microstructure is described as the cobalt nano particles being embedded in Cu matrix. The electron scattering probability strongly depends on the electron spin direction and the orientation of the magnetic granules. The temperature dependence of GMR has been analyzed and a novel experiment has been designed to demonstrate directly that the spin dependent scattering depends on the relative orientation of neighboring granules.; The interactions between magnetic particles in different granular materials are discussed. It is found that the thermal stability, MR effect and magnetic properties are significantly affected by the interparticle interactions, and the interactions can be changed with the density of particles or the thickness of granular films.; The transport properties of the CrO₂ granular system have been studied. It is found that the intergranular spin dependent tunneling is the major mechanism of conductance in the CrO₂ nano powder system at low temperature. High MR of >30% has been found which is in accordance to the half-metallic property (∼100% spin polarization) of CrO₂. The conductance can be described as the electrons tunneling through the Cr ₂O₃ interface barrier which has been characterized by electron microscopy and x-ray diffraction and spectroscopy. The temperature dependence of MR has been analyzed and is due to a spin independent channel dominant at higher temperature. A hybrid junction structure made with needle-shaped CrO₂ nano powders has been successfully made using the method of field alignment. The aligned CrO₂ powders show significantly improved characteristics and the MR ratio can reach higher than 40% at well define and relatively small fields.