Studies On Magnetic Properties Of Ferriet/Semiconductor Nanocomposits

With the advent of large-scale information and multimedia era, the rapid development of information technology requires electronic devices with higher storage density, faster speed, lower power consumption, smaller size and lighter weight. However, due to the quantum effects and other limitations such as Moore’s Law, the traditional process of miniaturization of electronic devices encountered insurmountable difficulties. Utilization of electronic spin in semiconductor, which is the critical materialin information technology, is considered to be a solution of current difficulties, therefore attracts tremendous attentions from researchers.As a half metallic material, ferromagnetic Fe3O4 shows great advantages such as therotically predicted 100% spin polarization at Femi level, high Curie temperature (850 K) and compatible conductivity, making it a promising candidate for spin injector, and therefore a popular material for spintronic studies.In this thesis, we focus on Fe3O4/semiconductor composite structures including different form of FesO4 (particle, thin film) and different semiconductor materials (Ⅳ Si, Ⅲ-Ⅴ GaAs and II-VI ZnS), and studies its magnetism, structure, magneto-transportation and surface/interface effect. The main innovation and achievements are as follows:1. High quality (111)-textured Fe3O4 films were successfully prepared by pulsed laser deposition method. The structure, morphology, magnetism and magnetic transport of Fe3O4 films were systematically studied. We found that the magnetic and structural property of Fe3O4 films are strongly affected by preparation process such as deposition temperature and annealing temperature. The magnetoresistance (MR) ratio of annealed Fe3O4 film is 2.6 times larger than that of in situ growth film. Moreover, the spin polarization of Fe3O4 is also increased by 60% after annealing treatment. The enhanced MR effect is ascribed to the enhanced spin dependent scattering at grain boundary and the increase of moment in ferromagnetic unities (grains of film).2.An anomalous positive MR effect was firstly observed in Fe3O4 film with strong (111) textures deposited at high temperature. The observed positive MR effect is different from the ordinary MR effect and cannot be explained by anisotropic MR effect. The unique MR behavior was intensively studied via structural, morphological and transport analysis, and a conduction mechanism including spin filter effect and grain boundary scattering/tunneling effect is proposed. The observed inverse MR effect might extend the understanding of spin dependent transport in Fe3O43.Based on the FMR analysis of (111)-textured Fe3O4 films, we found that the classical free energy model is not suitable for fitting the FMR experiment data of (111)-textured films. Hence magneto-texture anisotropy is introduced, and the expression of magneto-texture anisotropy energy is given via strict mathematical derivation including mean value theorems for integration and coordinate transformation, from the expression of cubic magnetocrystalline anisotropy energy. Thus the deduced expression can be applied in all of magnetic textured films with cubic structure such as Fe, Ni etc. Furthermore, our studies reveal the indispensability of magneto-texture anisotropy in the competing mechanism of anisotropies in textured Fe3O4 films, which would benefit its future application in spintronics.4. The spin relaxation properties of Fe3O4/GaAs single crystalline ultrathin films were studied systematically. The theoretical expressions for various contributions of ferromagnetic resonance linewidth were derived, especially the FMR linewidth contribution from effect of two magnon scattering in the plane of single crystalline films were given, by which the intrinsic FMR linewidth was separated and Gilbert damping factors of Fe3O4 single crystalline ultrathin films were obtained.It is found that the Gilbert damping factors is decreased by decreasing film thickness, indicating the enhancement of L-S coupling. The enhanced L-S coupling is ascribed to the increased orbital moment stemming from the surface symmetry breaking of Fe3O4 lattice and in turn destroyed crystal field.5. An investigations of interface effect for Fe3O4/GaAs single crystalline ultrathin films were performed.It is found that the magnetization, magnetic anisotropy and Gilbert damping factors are also decreased by covering of the Au capping layer, which is profoundly evident in thinner film (3 nm Fe3O4). Considering the small lattice mismatch between Au and Fe3O4, the decreased Gilbert damping factors is ascribed to the favor of lattice relaxation by Au capping layer.6. The core-shell structural Fe3O4/ZnS nanocomposites were well-fabricated via chemical method. Direct deposition of crystalline semiconductor on magnetic core to form a well-defined core shell structure is a challenging issue, due to the large lattice mismatch between semiconductor nanocrystals and magnetic cores. In previous studies, amorphous silica and carbon were utilized as interlayers to synthesize well defined core shell structure. In this thesis we demonstrate that core shell structural Fe3O4/ZnS bi-functional nanocomposites can be well synthesized by a facile seed mediate growth method, The magnetic and photoluminescence properties demonstrate the excellent bi-functionality of Fe3O4/ZnS nanocomposites.7. The studies on spin transport property of Ⅱ-Ⅵ semiconductor ZnS by using Fe3O4/ZnS nanocomposites system were achieved for the first time,, and an enhanced MR effect was observed on ZnS coated Fe3O4 particles, which is significant for semiconductor spintronics.