Magnetoresistance Effect In Magnetic Composite Organic Nanoparticles Film

Recently, the research of spintronics has extended from inorganic materials to organic materials. Organic spintronics attracted a lot of interest and many remarkable progresses have been achieved in this field those years. Until now, spin-polarized carrier injection, transport and detection, which are the main ingredients of spintronics, can be successfully achieved usingπ-conjugated organic semiconductor (OSE). Otherwise, in the past two decades, GMR effect of the inorganic magnetic granular films which consist of single-domain ferromagnetic granules embedded in an immiscible inorganic medium have been extensively studied. In this thesis, the inorganic matrix is replaced by small molecularπ-conjugated OSE materials in granular films. The microstructures, magnetic properties and charge transport behavior in magnetic metal-organic semiconductor granular films were investigated systematically.The contents of this thesis are as follow: PARTⅠ: Magentoresistance effects in Co_x(TPD)_1-x granular films.1. The microstructure and magnetic properties in Co_x(TPD)_1-x granular films with different x have been studied. The HRTEM images show typical characteristics of granular films, roughly spherical dark cobalt granules are embedded in TPD matrix. The average size of Co granules is about 2-5 nm. With the increase of Co concentration, the mount of Co granules increase and the inter-granule space decrease accordingly. In higher x samples, Co granules begin to form network gradually. VSM results show a gradual change from superparamagnetism to ferromagnetism as x increases in these samples.2. The magnetoresistance and transport behavior in the granular film samples have been investigated at different temperature. The Co_x(TPD)_1-x granular film samples with different x were also studied, the similar phenomenon was observed. By analyzing the microstructure, magnetic property and charge transport behavior, we concluded that the MR mechanism is considered to originate from the spin-dependenttunneling process.PARTⅡ: The crossover of magentoresitance in Co-Alq₃ granualar films on Sistructures.1. The Co_0.38(Alq₃)_0.62 granular film samples were deposited on silicon substrates with the native oxide layer (p=70Ω-cm, N type), forming the Co-Alq₃/SiO₂/Si structures. A transition from negative to positive magnetoresistance effect was observed with the increase of temperature from low temperature to room temperature.2. The transport behavior were investigated at low and high temperature respectively, two kind of different origin mechanisms of the positive and negative MR was found. At low temperature, the inter-granule tunneling is believed to account for the negative MR effect. The origin of the positive MR comes from the magnetoresistance of the silicon substrate.3. By the contrastive study of Co_0.46(Alq₃)_0.54 granular films with three different resistivities silicon substrates (ρ>1000Ω-cm,ρ=70Ω-cm,ρ=2-4Ω-cm), we presented a simple charge transport model. The model explains the transition from negative to positive magnetoresistance effect. The metal concentration (x) dependence of MR and the transport behavior of the Co_x(Alq₃)_1-x granular film samples (ρ=70Ω-cm silicon as substrate) were also studied furtherly.