The Modification Of Magnetite Nanoparticles And The Study On Their Magnetoresistance Properties

In addition to charge state,electron spins are exploited as a further degree of freedom in the study of spintronics,with implications in the efficiency of data storage and transfer.Read heads of magnetic hard drives based on the GMR or TMR effect has been widely used,and the magnetoresistive random-access memory(MRAM)and quantum computer based on spintronics are promising to be commercialized.The rapid development of spintronics may lead to a new round of industrial revolution.Magnetite(Fe3O4)is a ferrimagnetic half metal with high spin polarization and high Curie temperature and has been intensively investigated for room temperature spintronic applcations.Fe3O4 nanoparticles have some unique properties different from bulk materials.For instance,magnetic nanoparticles are superparamagnetic at room temperature with a high magnetic moment and limited hysteresis.And nanoparticle assembly is suitable to fabricate miniaturized and multifunctional spintronic devices with great application potentials.However,two unavoidable problems associated with particles in this size range is their modest sensitivity to applied magnetic fields and high degree of oxygen(O)-defects.In this thesis,we introduced two strategies,the zince doping and the modification with nitroxide radicals respectively to solve these two issues.By studying the relationship between the structure,magnetism and spin transport properties of the nanomaterials,our knowledge for their application in sensitive spintronic devices can be expanded.1.Enhancing Low-Field Magnetoresistance in Magnetite Nanoparticles via Zinc SubstitutionWe report a strategy to enhance room temperature low-field magnetoresistance behavior of Fe3O4 nanoparticle assembly by controlled Zn-substitution.7 nm oleylamine-coated ZnxFe3-xO4 NPs are first prepared by thermal decomposition of the corresponding metal acetylacetonates(M(acac)n,M=Fe2+,Fe3+,and Zn2+).Oleylamine is replaced by tridecanoic acid to further stabilize the ferrite NPs.Both the saturation magnetization and magnetic susceptibility are x-dependent,but their values reach the maximum at x=0.2 and 0.3 respectively due to the Zn-doping induced magnetocrystalline anisotropy change.The study of their magnetoresistance and magnetism behavior reveals a positive correlation between the low-field magnetoresistance and magnetic susceptibility-the proper Zn doping in the Fe3O4 structure increases its global magnetic moment and reduces its magnetic anisotropy,making the alignment of ZnxFe3-xO4 nanoparticles more easily and leading to the enhanced low?field magnetoresistance.As a result,the Zn0.3Fe2.7O4 NP assembly with NPs separated by tridecanoate exhibits a large magentoresistance ratio of-14.8%at 300 K under a 4.5 kOe magnetic field.The demonstrated approach to control NP substitution to enhance low-field magnetoresistance of the NP assemblies provide an attractive new startegy to the fabrication of Fe3O4-based magnetic NP assemblies with desirable transport properties for sensitive spintronic applications.2.The Impact of Different Coordination Forms of Nitroxide Radicals on Spin Based Properties of Magnetic Ferrite NanoparticlesWe report a new strategy to minimize the intrinsic symmetry breaking on the surface of magnetic nanoparticles by modifying with nitroxide radicals.5.2 nm oleylamine-coated Fe3O4 NPs are first prepared by pyrolysis method.Through the ligand exchange process,the L1-and L2-coated Fe3O4 nanoparticles were obtained.The study of the magnetism data reveals that the ligand L2 with carboxylic acid normally binds to Fe via a monodentate form and can only patch the single vacancy sites.In contrast,the ligand L1 can also employ the double-coordinated form to pathy the double vacancy sites,resulting in greatly decreasing the surface anisotropy energy and increasing the bulk anisotropy energy.The reduction of the surface symmetry breaking decreases the degree of spin scattering during the spin transport.As a result,the room temperature magnetoresistance ratio of L1-coated Fe3O4 nanoparticles is increased by 28.6%under 5.0 kOe compared to value of L2-coated Fe3O4.Unlike previous studies that only provided a simple way to compensate the O defects on the surface of nanoparticles,the ligand with flexible coordination numbers could patch the symmetry-breaking of the surface more completely.