【作者】 米文博；

【导师】 白海力；

【作者基本信息】 天津大学 ， 材料物理与化学， 2005， 博士

【摘要】 铁磁性金属纳米颗粒薄膜系统中存在的巨磁电阻效应、巨霍尔效应、高矫顽力效应等新特性,使其在磁性传感器件、高密度记录介质、读出磁头和磁性随机存取存储器等研究领域具有广阔的应用前景。目前,具有面心四方结构的L10-FePt材料、半金属材料和铁磁性金属–半导体复合材料是凝聚态物理和材料科学领域的研究热点。本论文用磁控溅射法制备了软铁磁性金属-碳基（Fe-C, Co-C, FeN-CN）、硬铁磁性金属-碳基（FePt-C, FePtCu-C, FePtN-CN）、半金属-半导体基（Fe₃O₄-Ge）和铁磁性金属-半导体基（Fe-Ge）系列纳米颗粒薄膜,对它们的化学成份、微观结构、磁性质和输运特性进行了系统研究。通过对软铁磁性金属-碳基（Co, Fe, FeN）–C（N）颗粒薄膜的研究,发现在颗粒薄膜中,颗粒与母体间的相分离、颗粒尺寸和颗粒间的相互作用决定样品的磁性质和磁化机制。当相分离较好、颗粒间相互作用较小时,样品的矫顽力较大,并且磁化机制为单畴磁矩转动。反之,样品矫顽力较小,磁化机制为畴壁位移。用磁力显微镜直接观察到了Fe-C系统中的磁逾渗现象,为阐明颗粒间磁相互作用的变化提供了直接证据。在电子束辐照的Co-C颗粒薄膜中,观察到了Co颗粒对非晶C转变为石墨化的碳纳米结构（纳米线和纳米针）的催化作用。通过对硬铁磁性金属-碳基（FePt-C, FePtCu-C, FePtN-CN）颗粒薄膜的研究,发现适量的Cu掺杂可以促进L10-FePt合金的形成,而过量的Cu显著抑制L10-FePt相的形成。特别地,我们还发现N掺杂样品在退火过程中N的溢出和Fe-N键的断裂,可以促进L10相的形成,提高FePt合金的有序度;同时,高氮气分压可以有效控制FePt颗粒尺寸,有利于FePt颗粒薄膜在高密度磁记录介质方面的应用。通过对多晶Fe₃O₄薄膜、（Fe, Fe₃O₄）–Ge颗粒薄膜的研究,发现多晶Fe₃O₄薄膜的导电机制为隧穿导电。Fe₃O₄晶粒表面（界面）磁矩的取向对磁化强度贡献很小,但在高场下,晶粒表面（界面）磁矩的排列会导致磁电阻发生很大的变化,这就是Fe₃O₄薄膜材料中磁电阻随外加磁场呈现弱饱和现象的物理机制。在Fe_xGe_1–x颗粒薄膜中发现当x=0.5时,霍尔电阻率ρ_xy最大（126μ- cm）,为纯Fe膜的139倍;在±10 kOe的磁场范围内,ρ_xy随磁场呈线性变化关系,并且在2–300 K温度范围内,直线斜率保持不变。这一特点使Fe-Ge颗粒薄膜在微电子学器件中的实际应用具有了可能性。更多还原

【Abstract】 Novel properties such as giant magnetoresistance （GMR）, giant Hall effect （GHE） and huge coercivity have made ferromagnetic nanogranular films promising candidates for the applications of magnetic sensors, high density magnetic recording materials, read-out magnetic head and magnetic random access memory. Recently, L10 ordered FePt alloy, half-metals and ferromagnetic metal-semiconductor composite materials have become the focuses in the field of condensed matter physics and materials science.Series of ferromagnetic nanocomposite films, including soft ferromagnetic metal ?C granular films （Co-C, Fe-C, FeN-CN）, hard ferromagnetic metal–C granular films （FePt-C, FePtCu-C, FePtN-CN）, half metal–semiconductor granular films （Fe₃O₄-Ge）, ferromagnetic metal–semiconductor granular films （Fe-Ge） were fabricated using magnetron sputtering. Their chemical composition, microstructure, magnetic properties, reversal mechanism, and transport properties were studied systemically.It was found that, for the soft ferromagnetic metal–C granular films （Co, Fe, FeN）-C（N）, the phase segregation between metal particles and C matrix, particle size control and interparticle interaction determine the magnetic properties and reversal mechanism of samples. The better the phase segregation and the weaker the interparticle interaction are, the larger the coercivity is, and the reversal mechanism is single domain rotation. Contrarily, the coercivity will decrease, and the reversal mechanism becomes domain wall motion. The magnetic percolation of Fe-C system was directly observed using magnetic force microscopy, which clarifies the changes of interparticle interaction in the nanogranular system. We also found that the amorphous C in the as-deposited Co-C granular films transforms into graphitized nanostructured C （carbon nanostripes and nanoneedles） after being exposed in the electron beam for several minutes.For the hard ferromagnetic metal–C granular films （FePt-C, FePtCu-C, FePtN-CN）, we found that appropriate Cu doping can improve the transformation of ordered-L10 FePt, but excessive Cu suppresses the formation of L10 phase. Particularly, we noted that the evaporation of N and decompounding of Fe-N bonds in FePtN-CNgranular films promote the formation of L10 phase during annealing, and improves its chemical ordering. Besides, the size of FePt particles can be effectively controlled by N doping at high N₂ partial pressure, which is benefit to the practical application of FePt-based granular films in high-density magnetic recording media.The mechanism of electron transport in polycrystalline Fe₃O₄ films is tunneling. Although the moments at the surface and/or interface of Fe₃O₄ grains slightly affect the magnetization of films, the magnetoresistance of polycrystalline Fe₃O₄ films changes obviously in the high field range when a small part of the moments at the grain surface and/or interface was aligned. This observation explains the weak saturation of MR with applied field in the polycrystalline Fe₃O₄ films. For Fe-Ge composite films, when the Fe atomic fraction is 50%, the Hall resistivity (ρ_xy) reaches its maximum value of 126μΩcm, which is 139 times larger than that of pure Fe films. In the magnetic field range of -10?10 kOe,ρ_xy changes linearly with the applied magnetic field and the slope almost keeps constant at temperatures rangeing from 2 to 300 K, making the practical application of Fe-Ge composite films possible in the field of microelectronic devices.更多还原